Scientific Activities
Institute for Cosmic Ray Research, The University of Tokyo

Report to the Review Committee

20 December 2018

CONTENTS
OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
NEUTRINO AND ASTROPARTICLE DIVISION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Super-Kamiokande
T2K Experiment
XMASS ExperimentHyper-Kamiokande (R&D)
HIGH ENERGY COSMIC RAY DIVISION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
TA: Telescope Array
Tibet AS γ Project
CTA
High Energy Astrophysics Group
Ashra (R&D)
ALPACA (R&D)
ASTROPHYSICS AND GRAVITY DIVISION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
KAGRA Group
Observational Cosmology Group
Theory GroupFACILITIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Kamioka Observatory
Akeno Observatory
Norikura Observatory
KAGRA Observatory
Research Center for Cosmic Neutrinos
Low-Level Radioisotope Measurement Facility
APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
A.1: Organization
A.2: ICRR Staffs (2018)
A.3: Budget (FY2012–2017)
B.1: Agreements of International Academic Cooperation
B.2: International Meetings
B.3: Inter-University Collaborative Researches
B.4: Educational Activities
B.5: Public Relations
C: Report from the Future Planning Committee (2013 and 2017)

1

OVERVIEWThe Institute for Cosmic Ray Research (ICRR) has
its origin at a lodge for research, called Asahi Hut, built
on Mt. Norikura based on Asahi Academic Grant. In
1953, it was established as the Cosmic Ray Observatory of the University of Tokyo. This Observatory was
Japan’s first research facility for nationwide joint use.
In 1976, the Cosmic Ray Observatory was reorganized
into the Institute for Cosmic Ray Research (ICRR)
of the University of Tokyo. Since the era of Cosmic
Ray Observatory, ICRR has been functioning as “Inter University Research Institution”, where researchersin Japan in the field of cosmic rays carry out diverse
researches using various facilities in ICRR.
In 2004, the Japanese National Universities, including the University of Tokyo, became independentadministrative agencies (National University Corporations). The concept was to strengthen the Universities
by mutual competition. At that time, the concept of“independent administrative agencies” and “Inter University Research Institution” did not go along well.
This problem was recognized by the Ministry of
Education, Culture, Sports, Science and Technology(MEXT) in Japan. A new system, called “Joint Usage /Research Center”, was established. ICRR was selected
as one of these Centers in 2010. Since then, ICRR’s research style as “Inter University Research Institution”
has been officially re-recognized by the Japanese government.
In 2018 MEXT established “International Joint Usage / Research Center” which can play a central role ininternational research activity and promoto world-classresearch. ICRR was selected as one of the six International Joint Usage / Research Centers.
The external review panel of ICRR has been organized approximately every six years since 1994, the last
one having been held on 16-18 January, 2013. This report provides the material prepared for the External
Review Panel to be held on 15-17 May, 2019 to examine the research activity of the ICRR. The prime
purposes of the present Review are:(1) to give oversight to the institute’s scientific activities between 2012 and 2018, whether theymeet the international scientific standard andhave given sufficient impacts on the community,(2) to assess the scientific merits of individual research
activities and make recommendations about future support,(3) to assess technical competence of the proponents and feasibility of the R&D experiments or
projects,(4) to assess the Institute’s policy and its role in thescientific community.
At present, ICRR consists of 24 faculties, 1 project
professor1 , 1 project associate professor2 , 18 assistantprofessors, 13 project assistant professors3 , 13 postdoctoral fellows and 13 supporting technical staffs, andconsists of three divisions — Neutrino and Astroparticle Division, High-Energy Cosmic Ray Division, andAstrophysics & Gravity Division. They are further subdivided into thirteen research groups. Most of the activities receive participations by other university members that partially offset small numbers of staffs of the
ICRR. Most of the faculties are counted as members of
the Faculty of Science of the Graduate School and areengaged in the teaching of graduate students of thisuniversity, jointly with those who belong to Department of Physics and Department of Astronomy in the
Hongo campus.
ICRR runs 10 facilities (4 observatories and a center; 4 overseas) to perform its research projects. The
overseas facilities are operated jointly in collaboration
with academic institutions of respective countries undermemoranda of understanding. The Institute also supports, directly or indirectly, some of cosmic-ray related
research activities of research scientists in this country.
More information as to the structure of ICRR is given
in Appendix A.1.

Divisions

Neutrino and Astroparticle DivisionThis division is focused on particle and astroparticle
physics with primary interests in the physics of neutrinos, proton decay, and dark matter using undergroundexperimental detectors at Kamioka Observatory. The
primary detector is the Super-Kamiokande (SK), which
is a 50,000 ton water Cherenkov detector studying neutrino physics and proton decay. The Tokai-to-Kamioka(T2K) long-baseline neutrino experiment is also a focus project of this division. A liquid xenon detector(XMASS) has been constructed and aims for a direct*1 fixed-term professor*2 fixed-term associate professor*3 fixed-term assistant professors

2

detection of dark matter interactions. Further, researchand development activities towards a future megatonsized water Cherenkov detector, Hyper-Kamiokande,
are ongoing.
High Energy Cosmic Ray DivisionThis division is devoted to the study of very highenergy cosmic rays and cosmic gamma rays, consistingof three major research activities, TA, Tibet ASγ , andCTA, with one theoretical group and two R&D activities.
Telescope Array (TA) experiment explores extreme
phenomena in the universe by measuring energy spectrum, particle composition, and arrival direction distribution of cosmic rays with energies of 1018 − 1020 eV orhigher. In May 2008 this experiment started full operation of the largest detector in the northern hemisphere(Utah, USA). The TA Low-Energy extension (TALE)
studies the transition from galactic to extra-galactic
cosmic rays in the energy range between 1015.5 eV and
1018.5 eV. The TA×4 project would quadruple the TA
aperture of the highest-energy cosmic rays to accelerate
the pace of the data collection.
The Tibet ASγ Collaboration operates a surface air
shower array of scintillation detectors, underground water Cherenkov muon detectors and air shower core detectors at high altitude (4300m) in Yangbajing, Tibet,
China realizing high statistics studies of cosmic rays
with energies 1012 − 1017 eV sensitive for both charged
cosmic rays and gamma rays in the northern sky.
CTA is an international effort to construct the next
generation large scale imaging atmospheric Cherenkov
Telescope array to explore the high energy Universe
with an order of magnitude better sensitivity, higher
angular resolution and wider energy coverage. CTAJapan is leading the construction of four large size telescopes (LSTs) in La Palma, Spain.
A theoretical group, High Energy Astrophysics
group, aims at revealing violent astrophysical phenomena, in which non-thermal particles are accelerated.
The group promotes the theoretical study on the multimessenger astronomy.
In addition there are two research activities, Ashraand ALPACA. On Mauna Loa (Hawaii, USA), Ashra
operates an unconventional optical collector complex
with a very wide field of view which is to image air
showers caused by neutrinos of energies of 1015 −1018 eVor higher from violent astrophysical sources like gamma
ray bursts. ALPACA is a joint research project between
Bolivia and Japan to construct a surface air shower array and underground water Cherenkov muon detectors
at high altitude in Bolivia.

Astrophysics and Gravity DivisionThis division consists of the KAGRA Group, theObservational Cosmology Group, and the Theory
Group.
The KAGRA Group aims at direct detection ofgravitational waves. The KAGRA detector is being
built at Kamioka toward joining the next international
observation O3 with LIGO and Virgo in 2019.
The Observational Cosmology Group is conducting
an optical sky survey with prime interests in cosmology
and extragalactic astrophysics. The group participatesin the Subaru Hyper Suprime-Cam (HSC) survey under the international collaboration, which is a major
contributor of the HSC narrow band (NB) project.
The Theory group is making theoretical studies
on phenomenology-oriented particle physics, astro/cosmoparticle physics and cosmology.

Facilities

ICRR has four domestic observatories, one center,one domestic facility and four overseas observatories.
These facilities are all open to researchers, and the use
will be admitted after a proper review process.
Domestic• Kamioka Observatory.
This observatory is located 1000 m underground(2700 m.w.e.) in the Kamiokande Mine, Gifu
prefecture. The SK water Cherenkov detector is
located in this observatory. The observatory is
also used by the XMASS experiment. The observatory offers a well-developed and maintained,
low vibration and low radioactivity environment.
Therefore, it is used by a number of researchgroups both inside and outside ICRR, including
the CANDLES double beta decay experiment led
by Osaka University, the NewAGE dark matter experiment led by Kobe University, a cryogenic test experiment for gravitational wave detection (CLIO: Cryogenic Laser Interferometer
Gravitational-Wave Observatory) run by ICRR,
as well as seismic and geophysical experiments
performed by the Earthquake Research Instituteof the University of Tokyo and others.
• Akeno Observatory.
The Akeno Observatory is located in Akeno of
Hokuto in Yamanashi Prefecture. It was used
for a giant-air-shower-array experiment, AGASA.
Now it is used for small-scale observations, research and development including the multicolor imager (MITSuME) led by Tokyo Institute of Technology, large area muon telescope by

3

Chubu University et al. and a small atmospheric
Cherenkov telescope by ICRR et al. It is also
used occasionally for detector assembly including
the surface detectors of the Telescope Array extension.
• Norikura Observatory (at 2770m altitude).
This observatory, located close to the summit of
Mt. Norikura, was historically a very importantstation of ICRR. This is now used for long-termmonitors of cosmic ray muons and solar neutrons,
mainly by teams from Nagoya University and from
Shinshu University. It is occasionally used forvarious types of research (not necessarily cosmic
ray proper) that require the high altitude environment, as it now offers a unique high altitude
facility in Japan.
• KAGRA observatory.
KAGRA observatory was established in 2016 forthe operation of the large-scale cryogenic gravitational wave telescope, KAGRA, and the researchof gravitational wave physics and astronomy.
• Low-Level Radioisotope Measurement Facility.
This is located underground in the Kashiwa building, and is used for measurements of the concentration of natural radioisotope by interested researchers in the country.
• Research Center for Cosmic Neutrinos. Thiswas established in 1999 primarily to promote research of neutrino physics. This center has beenorganizing neutrino meetings and neutrino public lectures. It also acts as a body for accepting ICRR’s inter university programmes related
to Low-Level Radioisotope Measurement Facility,
and is in charge of the computer system of theinstitute.
Overseas
ICRR operates four overseas observatories, whichplay central roles to carry out its research projects,
jointly with various academic institutions in respective
countries:• The Tibet ASγ Observatory, Yangbajing, Tibet,
China• Observatory for Highest Energy Cosmic Rays,
Utah, U. S. A.
• Chacaltaya Observatory of Cosmic Physics, Mt.
Chacaltaya, Bolivia• International Observatory for
Gamma Rays, La Palma, Spain.

High

EnergyHistorical Remarks and Funding Sources

Brief histories are described for each group, including the financial view point. In general there are twochannels for funding: the use of Grant-in-Aids from theJapan Society for the Promotion of Science (JSPS) by
individual scientists, and the fund from the government
revenue directly to the Institute through the University
finance. The former applies to short term projects, sayfor 5 years with no guarantees beyond, and the latterto long-term projects that usually costs more than that
defrayed by the former. The square parentheses indicate present institute’s personnel of each group. The
current status and scientific goals of the projects are
summarized in the table.
• Super-Kamiokande
The detector was constructed from 1991 to 1996,
and data acquisition began in April 1996. Construction and operation costs are covered by funds
provided by the government. Super-Kamiokande
discovered atmospheric neutrino oscillations in1998 and solar neutrino oscillations in 2001. The
K2K (KEK to Kamioka) long-baseline accelerator experiment, operated from 1999 to 2004, confirmed the existence of atmospheric neutrino oscillations. Since full reconstruction work performed
in 2005 and 2006, the detector has been runningstably and high precision oscillation studies havebeen performed. The readout electronics and online data acquisition system were updated in 2008
to ensure stable operations over the longterm.
The detector has been refurbished in 2018 to beready for a future upgrade by doping gadolinium.
[9 faculties + 7 assistant professors + 6 project
assistant professors + 2 post-doctoral fellow]• T2K
The Tokai-to-Kamioka (T2K) long-baseline accelerator neutrino experiment started in 2009. The
SK detector functions as the far detector of theT2K experiment and ICRR is responsible for operating the far detector. Host institutes of the T2K
experiment are KEK and ICRR. Based on datataken up until March 2011, the T2K collaboration
announced an indication of electron neutrino appearance, and therefore non-zero θ13 , with a statistical significance of 2.5σ in June 2011. After the

4recovery of the J-PARC accelerator site from damage sustained in the earthquake, the T2K experiment resumed operations in March 2012. With
data taken up until June 2012, the significanceof the non-zero θ13 measurement has improved
to 3.2σ . By the end of 2017, T2K has collectedboth neutrino and anti-neutrino data. The observed number of electron neutrino candidates isnow 90 and the confidence level exceeds 7σ . Also,
they found larger number of the electron neutrino
candidates but fewer number of the electron antineutrino candidates. As a result, they found theindication of CP violation at 2σ level. [8 faculties + 7 assistant professors + 5 project assistantprofessors]• XMASSThe XMASS project started in 2000 and after its
R&D study the XMASS-I detector construction
started in 2007. It completed in 2010 and the subsequent commissioning run until 2012 enabled us
to conduct various dark matter and new physics
searches. The detector refurbishment in 2012 succeeded to reduce the impact of surface backgroundby an order of magnitude and data taking withthe lower background continues through the endof the fiscal year 2018. A planned future detector, XMASS1.5, became uncompetitive as otherlarge-scale liquid-xenon detector become operational first. Participation in a second generation
experiment started by looking for an international
cooperation to construct a more sensitive detector. The project is mainly supported by Grantin-Aids of the Ministry of Education, Culture,
Sports, Science and Technology. [4 faculties + 1
project associate professor + 2 assistant professors+ 3 project assistant professors + 1 post-doctoral
fellow]• Hyper-Kamiokande R&D
The Hyper-Kamiokande or Hyper-K will provide
major new capabilities to make new discoveries inparticle and astroparticle physics. The Hyper-K
is a priority project listed in the Roadmap 2017of the Japanese Ministry of Education, Culture,
Sports, Science and Technology (MEXT), andseed funding has been allocated within MEXT
budget request for JFY2019.
The principal
project milestones include: Construction to startin 2020, and Start data taking in 2027. The
R&D project is supported by Grant-in-Aids of the
MEXT. [9 faculties + 7 assistant professors + 6

project assistant professors + 2 post-doctoral fellow]• TA
The TA project received a fund by Grant-in-Aid
for Scientific Research in Priority Areas (2003- 2008) for its construction and commissioning. A continued fund for 5 years of operation(2009-2013) was granted as Grant-in-Aid for theSpecially Promoted Research. The TA consists ofa ground array of scintillation counters and fluorescence telescopes. The hybrid observation ofcosmic ray air showers with TA started in May
2008, and has been maintained by the contributions of Japan, USA, Korea, Russia and Belgium.
The TA collaboration confirmed GZK suppression
with 3.9σ significance. Mass composition of cosmic rays above 1018.2 eV is consistent with proton. TA found an evidence for a cluster of cosmic rays that we call the hotspot with 3.4σ . The
Grant-in-Aid for the Specially Promoted Research(2015-2019) was obtained for the TA×4 project toquadruple the aperture of the TA surface detector for its construction and commissioning. The
TA Low-Energy extension studies the transition
from galactic to extra-galactic cosmic rays. Its
telescopes were completed in 2013 and have been
operational. The deployment of the TALE surface
detectors was completed in 2017 and have been in
operation. [1.5 faculties + 2.5 assistant professors + 1 project assistant professor + 1 technicalspecialist + 2 skilled assistants]• Tibet ASγ
The Tibet air shower array was constructed with
gradual upgrades from 1988 to 2014, by the funds
from Grants-in-Aid and from the government.
As an upgrade, new air shower core detectors
were added for the next-step measurement of thechemical composition in the knee energy region,
while large (∼4,000 m2 ) muon detectors aiming
at higher-sensitivity (by a facto of ∼10 improvement) gamma-ray observation in the 100 TeV region were constructed under the Tibet air shower
array. The muon detectors also play an importantrole to measure the chemical composition of cosmic rays in the knee energy region. The hybrid
experiment (air shower array + muon detectors +air shower core detectors) started data-taking in
2014. The Tibet ASγ Collaboration found thatyearly variation in depth of the Sun shadow incosmic rays was sensitive to the coronal magnetic
field in the Sun in 2013, and that the average in-5terplanetary magnetic field employed in a popularsolar magnetic field model was underestimated byapproximately 50 % in 2018. [1.5 faculties + 1.5
assistant professors + 1 post-doctral fellow]• CTA
CTA R&D program had been financed by aGrant-in-Aid for Specially Promoted Research(2012-2016). Four CTA Large Size Telescopes(LSTs) with a 23m dish on La Palma have been
under construction with the budget for the facility development from the Japanese Government
MEXT (2016-2018), in the cooperation with Germany, Spain, France and Italy. LST1 was alreadyinaugurated in 2018 October, three more LST2-4
will be completed in the next three years. The
array of four LSTs is planned to start the scientific operation in 2022. The significant part of theoperation and maintenance of LST1-4 and CTA
North site is financed by a Grant-in-Aid for scientific research (S) (2017-2021) and specific researchbudget from MEXT since 2018. CTA-Japan isplanning the LST extension to CTA-South after
the completion of CTA-North to make CTA as
an all sky observatory. [ 4 faculties + 3 assistantprofessor + 1 post-doctral fellow]• High Energy Astrophysics
This group was established in December 2009 with
one faculty member. From April 2013 a assistantprofessor joined, but the original faculty member
retired March 2016. Until June 2017, only the assistant professor had acted as a permanent member. From July 2017, the group was reconstructedwith one faculty member. One assistant professor
joined March 2018, which stimulates the activityof the group again. The group is currently pursuing the theoretical study on high-energy astronomical phenomena based on the multi-messenger
astronomy, collaborating with the other groups in
ICRR. [1 faculty + 1 assistant professor]• Ashra R&D
The Ashra R&D and phase-1 construction were
funded by the Coordination Fund for Promoting
Science and Technology and Grant-in-Aid for Scientific Research since 2003. In 2004, University of
Hawaii (UH) joined as a local host institute. Afterthe state of Hawaii permitted the site use, the construction of light collectors in shelters on Mauna
Loa was carried out in 2007. Since 2008, optical transient observation has started. Commissioning of astronomical tau neutrino search was

performed. In 2011, the 10-year extension of thesite use permit was granted. Since 2012, efficientphysics observations for tau neutrinos, nucleons
and optical transients have been tried to continue
with budget support. [1 faculty + 1 technicalstaff]• ALPACA R&D
The ALPACA, a joint project between Bolivia
and Japan, will be composed of an 83,000 m2
surface air shower array and a 5,400 m2 underground muon detector array to make wide fieldof-view (2 sr) high-sensitivity observation of highenergy cosmic rays/cosmic gamma rays (worldbest sensitivity for gamma rays around/over 100
TeV) on the Cerro Estuqueria highland, 4,740 m
above sea level around Mount Chacaltaya, Bolivia. ALPACA-Japan will make a major contribution to construction of the surface array and themuon detector array. The R&D program is mostly
financed by TORAY science foundation. Approximately 20 % of the surface array of ALPACA will
be constructed with the grant in cooperation with
Bolivia. [1.5 faculties + 1.5 assistant professors +1 post-doctral fellow]• KAGRATAMA 300 detector had been built to detectgravitational wave (GW) from nearby our Galaxy
and operated at the highest sensitivity with thefirst stable operation in 2000. In order to realize
practical detection of GW events by more sensitive detector, the Large-scale Cryogenic Gravitational wave Telescope (LCGT, nick-named as KAGRA in 2012) was planned for GW events within
200 Mpc and its funding was requested. The first
demonstration of cryogenic thermal noise sensitivity by CLIO as of KAGRA prototype in 2010 leadthe partial funding under a new category of government revenue funding, “Leading-edge Research
Infrastructure Program”, from 2010 to 2012 andthe site excavation underground at Kamioka has
been started in May, 2012 by another funding of
MEXT. Additionally the fund for KAGRA upgrade to replace the core optics (silica mirrors)with sapphire mirrors was approved by MEXT.
This project is supported by both KEK and
NAOJ. [4 faculties + 3 assistant professor + 3
project professors + 2 project assistant professors + 7 post-doctoral fellows + 6 technical staff(2tenured + 4 project-oriented)]6• Observational Cosmology
Japanese Sloan Degital Sky Survey (SDSS) key
members moved to ICRR in 1996 with a Grantin-Aid for Specially Promoted Area, forming theobservational cosmology group. SDSS-I and II
programs were completed by 2008 with additional
supports from JSPS and RESCUE at the Department of Physics. After the completion of the
SDSS programs, the group has started new programs mainly with Subaru HSC NBs since July2010 on the arrival of a new faculty member who
wins a Grant-in-Aid for Scientific Research (A)for 2011-2014. Accordingly, the appointment of
the new assistant professor was made in April
2012. Adding the Grant-in-Aid for Scientific Research (A) for 2015-2019, the group is leading
the HSC-NB project and the related HSC galaxy
projects whose observations were started in 2014.
The HSC project observations will be completed
around 2019, and subsequently the complementary optical spectroscopic observations with PFS
will start around 2020. [1 faculty + 1 assistantprofessor + 1 project researcher + 1 JSPS researcher]• Theory
Theory group has a long history, but there was a
reset of the group in April 2001 owing to the retirement of the first group leader. A new group wasestablished in 2002 with two faulty members, andone of the member served as the Director of the Institute for three years (2001-2004). Although one
faculty member moved in June 2010, the group
had a new faculty and recovered its full activity
in February 2011. The group is currently studying theories beyond the standard model of particle
physics and applying them to the early universe
to understand the origin of matter and its fluctuations. [2 faculties + 3 post-doctoral fellows]

Summary of 2012-2018 Activities

Important events related to the ICRR’s activity in
2012-2018 since the previous external review in 2012
are summarized below.
• The 4th committee for the planning of the future
ICRR research projects was formed in 2012. The
report from this committee was released in 2013(see appendix).

• After the recovery of the J-PARC accelerator sitefrom damage sustained in the earthquake, theT2K experiment resumed operations in 2012.
• The refurbishment of the XMASS detector succeeded to reduce the impact of surface backgroundby an order of magnitude in 2012.
• The SK experiment published the appearance ofντ events in the atmospheric neutrino data in
2013.
• The TA experiment published an evidence fora cluster of ultra high energy cosmic rays (thehotspot) in 2014.
• International Observatory for High Energy
Gamma Rays, La Palma, Spain was establishedin 2015.
• The KAGRA observatory was established in 2016.
• The initial KAGRA interferometer was completed, and after commissioning work, the interferometer was operated for the first time in 2016.
• The T2K experiment presented the indication of
CP violation at 2σ level in 2017.
• The 5th committee for the planning of the future
ICRR research projects was formed in 2016. The
report from this committee was released in 2017(see appendix).
• The University of Tokyo launched NextGeneration Neutrino Science Organization(NNSO), where ICRR, Kavli IPMU, and the
School of Science cooperate to promote theproject and host the Hyper-Kamiokande construction in 2017.
• The refurbishment of the SK detector for a future
upgrade by doping gadolinium started in 2018.
• The Institute for Cosmic Ray Research was certificated as an “International Joint Usage / ResearchCenter” in 2018.
Details of above activities related to the research aredescribed in the following sections.

7

Project status and main scientific goals
Neutrino and Astroparticle Division
Project
SuperKamiokande
T2K

Statusrunning

XMASS

data
analysisplanned / R&D

HyperKamiokande

running

Main Scientific Goals
Study of neutrino oscillations, proton decay, and astrophysical phenomena such as supernova burst
Precise measurement of neutrino oscillation parameters using SK with accelerator neutrino beam
Direct search for dark matter with a liquid xenon detector
Study of neutrino CP violation, proton decays and supernova neutrinos with a megaton water Cherenkov detector

High Energy Cosmic Ray Division
Project
Telescope Array

Statusrunning

Tibet ASγ

running

CTA

under
construction
R&D
planned / R&D

Ashra
ALPACA

Main Scientific Goals
Study of extremely-high energy cosmic rays; their origin,propagation and interaction
Study of origin, acceleration mechanism and modulation ofhigh-energy cosmic rays
VHE Gamma Ray Astronomy, study of cosmic-ray origin,
particle acceleration and indirect search for dark matter
Search for astronomical high-energy neutrino sources
Study of cosmic-ray origin, Ultra-high energy gammaastronomy

Astrophysics and Gravity Division
Project name
KAGRA
Subaru-HSC-NB

Statusunderconstruction
running

Main Scientific Goals
Through detection of gravitational wave, study physics ofgravitational waves and open gravitational wave astronomy
Uncovering physical processes and history of cosmic
reionization and early galaxy formation

8

NEUTRINO AND ASTROPARTICLE
DIVISION

9SUPER-KAMIOKANDE
Introduction
Super-Kamiokande(SK, Super-K) is a 50 kton water
Cherenkov detector studying the physics of neutrinos,
proton decay, and astrophysical phenomena such as supernova. Data taking started in April 1996 and atmospheric neutrino oscillations were discovered in 1998.
Solar neutrino oscillations were discovered by a comparison of SK solar neutrino data with SNO charged
current data in 2001. The first data phase of Super-K,
SK-I, ran until July 2001. Due to an accident in 2001
about 60% of the PMTs in the detector were destroyed
and the detector was reconstructed in 2002 using theremaining PMTs. Since the accident, the PMTs havebeen encased in acrylic and FRP (fiber-reinforced plastic) shells which prevent shock-wave production even
if a single PMT implodes. With this setup the second
data phase, SK-II, ran from December 2002 to October 2005. The missing PMTs were reproduced and fullreconstruction of the detector was performed from October 2005 to July 2006. The third data phase, SK-III,lasted from July 2006 to August 2008. To ensure stable operation over the long term and to improve thesensitivity of the detector, the electronics system wasfully upgraded in September 2008. Since this upgrade
the fourth data phase, SK-IV, ran until May 2018. The
detector has been refurbished from June 2018 in orderto repair the leak of the Super-K water tank and to
improve water piping in the tank to double the waterflow rate.
As described below, high precision neutrino oscillation data were taken using both atmospheric and solar
neutrinos during these periods and significant progressin the study of proton decay, supernova physics, andother physics targets has been achieved. In addition,
the preparation for a SK-Gd phase, in which gadolinium is dissoved in tank water in order to tag neutrons,
has been conducted successfully.

Summary from 2012 to 2018
First of all, here summarized the noticeable achievements during the period of this review, i.e. after 2012.
• Measurements with atmospheric neutrinos havebeen focused on precision studies of threeflavor effects in neutrino oscillations. With a
328 kiloton-year exposure the SK data favor the
NH with a significance of between 91.5% and94.5% when constrained by T2K data and show aweak indication for δCP ∼ 3π /2. The appearanceof ντ events in the atmospheric data has been con-firmed at 4.6σ significance. Searches for exotic
oscillations have produced stringent constraints
on several extensions to the PMNS (PontecorvoMaki-Nakagawa-Sakata) mixing formalism.
• Searches for evidence of dark matter-induced neutrino signals from the galactic center, the sun andthe earth have so far yielded null results. Accordingly, limits have been placed on the WIMP selfannihilation cross section as well as on its interaction cross sections with nucleons. Currently SK’s
limits for masses below several tens GeV/c2 areamong the most stringent in the indirect searches
in the world.
• High accuracy solar neutrino measurements arecarried out in SK-IV. Improvement of the water
circulation system, lowering of energy thresholdof data acquisition system, and improvement ofanalysis software were done. Then, solar neutrino events in 3.49-3.99 MeV in electron kinetic
energy are now used in the oscillation analysis.
As a result, the global oscillation analysis of solar
neutrino experiments and KamLAND experiment
provides most accurate oscillation parameters ofθ12 and ∆m221 . However, there is about 2σ leveltension in ∆m221 between solar global analysis and
KamLAND measurement.
• The nucleon decay lifetime reached more than orclose to 1034 years for the major decay modes: p →e+ π 0 , p → µ + π 0 , and p → ν K + , with suppressing
background by new technique of neutron tagging.
These limits were original goal when the SuperK proposal was written. Searches for other decay
modes were also performed and we obtained themost stringent limits on nucleon lifetime in theworld.
• SK has been searching for galactic supernovae
with the efforts to minimize the dead time. A flux
upper limit of supernova relic neutrinos(SRN) was
obtained using all data from SK-I to III that hasreached within a factor of model predictions. In
SK-IV, SRN search by tagging neutron capture
on hydrogen was performed. The result shows
the world best limit below 16 MeV. This resultshows a high potential of neutron tagging techniques, which can be a strong tool for SRN detection.
• Preparation for the next phase of SK withGadolinium(Gd) loading (SK-Gd) is in progress.

10

phase
start
end
ID PMTs(photo coverage)OD PMTselectronics
trigger

SK-I
Apr. 1996
Jul. 2001
11,146
(40%)
1,885
ATM
hardware

SK-II
Sep. 2002
Oct. 2005
5,182
(19%)
1,885
ATM
hardware

SK-III
Jul. 2006
Aug. 2008
11,129
(40%)
1,885
ATM
hardware

SK-IV
Sep. 2008
May 2018
11,129
(40%)
1,885
QBEE
softwareTable 1. Data taking phases of Super-Kamiokande.

The primary goal for SK-Gd is the first detection of SRNs with neutron tagging. Feasibilityof loading Gadolinium into SK has been demonstrated by the EGADS (Evaluating Gadolinium’s
Action on Detector Systems) project. In addition,
we succeeded to develop Gd2 (SO4 )3 powder withlow enough radio impurity. A new water system
to dissolve Gd and purify Gd loaded water wasconstructed in a newly excavated hall near SK.
• A major refurbishment work on the SK tank,whose main purpose is to fix the leak for SK-Gd,
is underway in FY2018. We are aiming to resume
observation with pure water in January 2019, andto start Gd loading Gd within FY2019.

Atmospheric Neutrinos
Super-Kamiokande’s atmospheric neutrino data
provided the first evidence for neutrino oscillations in1998 [5] with the observation of a significant deficit of
upward-going νµ -induced interactions. This deficit isbest explained by flavor transitions among predominantly νµ and ντ . Since that discovery, subsequent
observations of neutrinos from the sun, reactors, andaccelerator neutrinos have revealed oscillations among
all three neutrino flavors and in the span of only 20
years, many aspects of the PMNS mixing paradigm
have been revealed. Currently all of its mixing angles
and mass splittings have been measured. Despite thissuccess there are several open questions in neutrino oscillation physics, including the nature of the neutrinomass hierarchy, the precise value of the atmosphericmixing angle θ23 , and whether or not neutrino oscillations violate CP. The atmospheric neutrino data set at
Super-K is sensitive to effects that can resolve theseissues but also to exotic oscillation effects predicted
models of physics beyond the Standard Model. Overthe last six years the Super-K has been focused on detailed studies of the atmospheric neutrino flux and its
oscillations to both address these open questions andfully establish its role as a background to searches fordark matter and proton decay.
With the discovery of mixing between the first andthird neutrino generations via the mixing angle θ13

matter-induced oscillation effects are expected to appear in the flux of atmospheric νe and νµ . The largestof these effects is a predicted resonant enhancement ofthe νµ → νe oscillation probability of neutrinos traversing the Earth’s core at multi-GeV energies. Fortunately this effect is present either for neutrinos if themass hierarchy is normal or antineutrinos if it is inverted. This enhancement appears as an increase in
the upward-going electron-like data at SK, but also affects the spectrum of the muon-like data. The size of
this effect is modulated by the precise value of θ23 andof the PMNS mixing matrix’s CP-violating phase δCP .
These two parameters are also expected to impact theoscillations in the low energy ( Eν < 1 GeV ) data suchthat the entire atmospheric neutrino data set can beused to constrain the most important open questionsin oscillation physics.
In order to maximize sensitivity to these effects andthose of non-standard oscillation scenarios the atmospheric neutrino data spanning neutrino energies from
100 MeV to TeV and above are analyzed together withina unified framework. Data are divided into three broad
categories based on the energy deposition of eventsin the SK inner and outer detectors. The fully contained (FC) sample features events with primary vertices within the 22.5 kiloton fiducial volume of the inner detector and no particles exiting the inner detector.
Events with fiducial vertices but with one or more particles exiting (typically muons) into the OD are termed
partially contained (PC). Upward-going muons withvertices within or outside of the outer detector and penetrating into the inner detector form the Up-µ sample.
The FC sample is further divided into 14 subsamples based on visible energy, particle type, andnumber of rings to improve sensitivity to a wide variety
of oscillation phenomena. Though it is not possible toidentify neutrino and antineutrino interactions on an
event-by-event basis, sensitivity to the mass hierarchy
and CP-violating effects is enhanced through statisticalseparation of these populations. Further, additional insitu constraints on non-oscillating neutral current andντ background are provided by an additional hierarchysensitive multi-ring e-like (other) sample that was introduced into since 2012. Both the PC and Up-µ samples

11

have average energies above 10 GeV, their lower energy constituents posses hierarchy sensitivity and theirhigher energy events have exquisite sensitivity to exotic oscillation effects. By 2018 atmospheric neutrino
data representing a combined exposure of 328 kilotonyear exposure of the detector have been analyzed. Figure 1 shows the 19 event samples used in these analyses
together with the best prediction from the oscillation
study presented below.
PMNS Oscillation Analysis
The event selection presented above has been optimized for sensitivity to oscillations characteristic of
θ13 -induced matter effects. To test for the presence of
these effects the SK atmospheric analysis is divided into
three parts, each incorporating and increasing amountof external information. Each analysis test both hierarchy hypothesis and the full range of values of the θ23 ,∆m232 , and δCP mixing parameters.
The first analysis is a fully unconstrained test of
PMNS mixing and includes a search for the value of
θ13 itself and relies only on the atmospheric neutrino
data. In the second analysis the value of sin2 θ13 is
fixed to 0.0219 according to the average value provided
by the Particle Data Group. With this constraint theatmospheric neutrino sensitivity is expected to increaseas the data can be used to constrain the other mixing
parameters controlling the size of the matter effects.
Finally, since it is known that the size of these effects
varies with θ23 , in particular, a model of the T2K experiment is introduced in the third fit to incorporate theirprecise constraints on this parameter. To construct themodel atmospheric neutrino MC are reweighted to theT2K flux and fit against publicly available binned data
to directly incorporate constraints from T2K into the
SK analysis.
Table 2 lists the result of each analysis. All fits favora normal mass hierarchy with θ23 in the second octant
and δCP near 3π /2. Constraints on the mixing parameters from the SK only analysis with the θ13 constraint
appear in Figure 2. The significance of the mass hierarchy result varies based on the assumed values of themixing parameters and on the analysis under consideration. Over the range of values allowed at 90% C.L.
the inverted hierarchy hypothesis is rejected by between
91.5% and 94.5% for analysis constrained by the T2K
model and between 80.6% and 96.7% otherwise. Preferences for θ23 and δCP are consistent with other resultsin the community, with the constraint on the latter improving noticeably when SK is analyzed together with
the T2K model [85]. Despite these successes the data
are as-yet insufficient to provide solutions for the open
questions posed above.
Tau Neutrino Physics
Atmospheric neutrino oscillations are primarily between νµ and ντ , but the latter is not found in the

primary atmospheric flux at energies where such transitions occur. As a result searching for oscillationinduced ντ interactions within Super-K is not only important for validation of PMNS-style mixing but canalso be used to establish a means of studying its properties. However, the search for ντ events is complicatedby the large threshold energy for the charged current
production of the τ lepton as well as its propensity to
decay into several light-producing hadrons. Above the
3.5 GeV production threshold not only is atmospheric
neutrino flux reduced by orders of magnitude relative
to its peak, but single- and multiple-pion production
process become increasingly important at these energies. The latter fact makes charged current νe and νµinteractions of similar energies become backgrounds.
A neural network-based analysis is used to extractthe ντ signal from the various backgrounds. However,no cut by neural network analysis is applied and instead it is used with zenith angle information to form
probability density functions (PDFs) as part of a twodimensional unbinned maximum likelihood fit. The
PDFs for each event are combinations of that for background and signal:
PDFs = PDFsbg + α × PDFstau + ∑ εi · PDFssys ,(1)iwhere the last term represents the change of the PDFs
due to systematic error effects. Unlike previous analyses, this likelihood simultaneously incorporates shapeand systematic error information to extract the signal.
The fit has been applied to the full detector exposure and indicates the ντ normalization factor α is1.47 ± 0.32(stat+syst.). This corresponds to a statistical significance 4.6σ to reject the no-tau-appearance
hypothesis, up from 3.8σ from the previous result. In
addition this data has been used for the first time to
produce a flux-averaged measurement of the total tau
neutrino cross section. For energies between 3.5 GeV
and 70 GeV the result is 0.94 ± 0.20 × 10−38 cm2 asshown in Figure 3. This result is in agreement withboth lower-statistics measurements at accelerators andwith models at the 1.5σ level.
Exotic Oscillations
Despite the enormous success of the PMNS mixing
paradigm in explaining the results of most oscillation
experiments there remains room to explore oscillation
effects predicted in theories that extend the Standard
Model. Indeed, the results of short-baseline oscillation
searches do not fit well within the standard three neutrino mixing framework and hint at the possible existence of one or more sterile neutrinos. Further, as aninterferrometric effect neutrino oscillations are a sensitive probe of oscillations that deviate from the characteristic pathlength over energy, L/E, dependence ofstandard oscillations. The SK atmospheric neutrino
data have been successfully utilized to address both ofthese topics.

12

400

Sub-GeV e-like 1-dcy e

Sub-GeV e-like 0-dcy e

Sub-GeV µ-like 0-dcy e

200

200

500

0
1.3

0
1.3

0
1.3

0
1.3

0
1.5

0.7

0.7

0.7

0.7

0.5

Sub-GeV π0-like Single-Ring

Multi-GeV e-like ν e

400Up-µ Stopping

1000

1000
200

400

Sub-GeV µ-like 1-dcy e

400

Multi-GeV e-like ν e

PC Stop

Up-µ Non-showering

100

1000

50

500

100

200

200

0
2

0
1.5

0
1.3

0
1.5

0
1.3

0

0.5

0.7

0.5

0.7

Sub-GeV µ-like 2-dcy e

200

Multi-Ring e-like ν eMulti-Ring e-like ν e

Up-µ Showering

PC Thru

200

500
200

100

100
100

0
1.3

0
1.3

0
1.5

0
1.3

0
1.5

0.7

0.7

0.5

0.7

0.5

Sub-GeV π0-like Two-Ring

Multi-GeV µ-like

400

Multi-Ring µ-like

-1

-0.5

0

Multi-Ring Other

400
200

500

200

200

0
1.3

0.7

0
1.3

500

1000

momentum (MeV)

0.7

-1

0
1.3

0

1

0.7

-1

cos zenith

0
1.3

0

1

cos zenith

0.7

-1

0

1

cos zenith

cos zenith

Fig. 1. Zenith angle and lepton momentum distributions for atmospheric neutrino data corresponding to a 328 kiloton-year exposureof Super-Kamiokande spanning its four run periods. The blue histogram denotes the best fit MC expectation and error bars on thedata points are statistical. Sub-panels beneath each distribution show the ratio relative to the MC prediction. Upward-going anddownward-going events correspond to cosine zenith of −1 and +1, respectively.

Fit
SK θ13 FreeSK θ13 Constrained
SK+T2K

Hierarchy
NH
IH
NH
IH
NH
IH

sin2 θ13
0.018+0.029−0.0130.008+0.017−0.007————sin2 θ23
0.587+0.036−0.0690.551+0.044−0.0750.588+0.031−0.0640.575+0.036−0.0730.550+0.039−0.0570.550+0.035−0.051|∆m232,31 | [×10−3 eV2 ]2.50+0.13−0.31
2.20+0.33−0.13
2.50+0.13−0.20
2.50+0.08−0.37
2.50+0.05−0.12
2.40+0.13
−0.05

δCP
4.18+1.45−1.66
3.84+2.38−2.12
4.18+1.41−1.61
4.18+1.52−1.66
4.88+0.81−1.48
4.54+1.05−0.97

Table 2. Best fit oscillation parameters obtained by the three flavor oscillation analysis. Fits are conducted for both the normal (NH)and inverted (IH) hierarchy assumptions for the atmospheric neutrino data (“SK only”) and including constraints from the T2K
experiment(“SK+T2K”). All fits are performed assuming sin2 θ13 =0.0219, which is taken from PDG.

Short-baseline accelerator experiments suggest that
if oscillations into a sterile neutrino exist the characteristic mass splitting is 400 times greater than that seen inatmospheric neutrino oscillations: ∆m214 ∼ 1 eV2 . This
splitting would induce an oscillation frequency in theatmospheric neutrino data that is fast enough to produce an oscillation effect that is sensitive only to thesize of the mixing angle between the active and sterile
states. That is, the SK data are largely insensitive tothe details of the mass splitting as well as to the numberof sterile neutrino states. Using a 273 kiloton-year subset of the atmospheric neutrino data SK found no evidence for sterile neutrino and established limits on theirmixing parameters at the 90% C.L. of |Uµ 4 |2 < 0.041
and |Uτ 4 |2 < 0.18 [72]. Figure 4 shows the limits fromSuper-K in comparison with other experiments.
Lorentz invariance violating neutrino oscillations are
expected to produce exotic oscillation effects between

13
20

20

20

Inverted Hierarchy

10

99%

5

10

99%

5

95%
90%
68%

0
0.001

∆ χ2

10

99%

5

95%
90%
68%

0.002

0.003

0.004

0.005

Normal Hierarchy

15

15

∆ χ2

∆ χ2

15

0
0.2

95%
90%
68%

0.4

0.6

0.8

0
0

2

4

sin2 θ23| ∆ m232 | , | ∆ m231 | eV2

6

δCP

102
10

1

10-1

NEUT CC ν τ cross section

NEUT CC ν τ cross section

DONUT CC ν τ cross section

DONUT CC ν τ cross section

10-2
SK measured CC tau cross section

10-3
0

20

40

∆m241 (eV2)

Cross Section (10-38 cm2)

Fig. 2. Constraints on neutrino oscillation parameters from the Super-K atmospheric neutrino data fit assuming sin2 θ13 = 0.0219 ± 0.0012. Orange lines denote the inverted hierarchy result, which has been offset from the normal hierarchy result, shown in cyan, by thedifference in their minimum χ 2 values.

10

1

60

Neutrino Energy (GeV)
Fig. 3. Flux averaged neutrino cross section measurement fromSuper-K. Solid lines denote the expected ντ (red) and ν̄τ crosssections as a function of energy. Dashed lines denote the crosssection based on measurements by the DONuT experiment.

all neutrino flavors proportional to L or L × E. Dueto the wide range of both pathlengths and energies
available in the atmospheric sample it is possible toprobe such effects in detail. In particular, the high energy upward-going subsets of the atmospheric neutrino
data would be subject to large spectral and angulardistortions beyond what is seen from PMNS oscillations. Super-K has produced an analysis of atmospheric
neutrino data which for the first time makes use of
a complete framework for studying Lorentz invarianceviolation together with standard neutrino oscillations
without any simplifying assumptions. This enabled theanalysis to make full use of the entire atmospheric neutrino data sample and though no significant departurefrom the expectations of standard oscillations was observed, new limits have been placed on the presence ofLorentz violating effects and existing limits have been

10-1 -3
10

10-2

|U |2

10-1

µ4

Fig. 4. Super-K limits on mixing into a sterile neutrino in comparison
with a combined analysis of MiniBooNE and SciBooNE (dotted)
and CCFR (dash-dot). The shaded region shows the 90% C.L.
limit from SK.

strengthened by the range between three and seven orders of magnitude [71].
Indirect Searches for Dark Matter
The existence of dark matter in the universe is supported by cosmological and astronomical observations,but to date there is no definitive explanation of its nature. One promising hypothesis is that it is a weaklyinteracting massive particle (WIMP), which can arise
in supersymmetic extensions of the Standard Model.
If such a particle decays or annihilates into Standard
Model particles, their subsequent decay chains would

14

10−17
10−18
10−19

<σV> [cm3s-1 ]10−20

SK bb +SK W W
SK µ+µSK ν ν
IceCube86, 2012-14, bb +IceCube86, 2012-14, W W
IceCube86, 2012-14, µ+µIceCube86, 2012-14, ν ν
Eur.Phys.J.C 77:627 (2017)

ANTARES, 2007-2015, bb +ANTARES, 2007-2015, W W
ANTARES, 2007-2015, µ+µANTARES, 2007-2015, ν ν
Phys.Lett. B769 (2017)

10−21

10−22
10−23

10−24
10−25

expectation for thermal relic scenario

−26

10

10−1

1

10

102

Mχ [GeV/c2]

103

104

Fig. 5. Super-K limits on the WIMP velocity-averaged selfannihilation cross section in comparison with results from the
ANTARES and IceCube experiments.

also produce neutrinos that can be detected at Super-K.
Since WIMPs are massive they are expected to accumulate within large gravitational potential wells, such neutrinos would be observed from the center of the galaxy,
the sun, or even the Earth. In the last six years Super-K
has undergone searches for these neutrinos as a directional excess coming from each of these locations.
While the precise mass of WIMPs is unknown it
is possible to search for evidence of such a directionalsource by assuming a particular mass and that WIMPs
decay or annihilate directly into quarks or leptons
which subsequently produce a neutrino spectrum basedon known Standard Model processes. The Super-K
searches use the full data set outlined above, but analyzed in coordinates where the event direction is reconstructed relative to the galactic center or the center of the Earth. Both the reconstructed direction andmomentum are used in the analysis, with the latter
helping to constrain the WIMP mass via its spectral
information. For all searches no evidence of a dark
matter signal is found in the Super-K data. Figure 5
shows the derived limit on the velocity-averaged WIMP
self-annihilation cross section as a function of massin comparison with results from other experiments.
The power of the Super-K data lies in its low analysis threshold, which has resulted in the most stringent
limits below masses of several tens GeV/c2 regardless ofassumed annihilation channel. Similarly, the left panelof Figure 6 shows limits on the WIMP-nucleon spinindependent cross section from the search for eventsfrom the center of the Earth. The right panel shows
limits on the spin-dependent cross section of WIMPs
coming from the sun [73]. In both cases Super-K’s constraints are the strongest among the indirect search experiments, though less constraining than limits fromdirect search experiments.

Other Topics
Over the last six years the Super-K atmosphericanalysis has made several other measurements, which
support those above or others in the field. As an example of the former, the Super-K produced its first evermeasurement of the atmospheric neutrino flux [80] in
both the electron and muon channels and demonstrated
it is consistent with the expectations of standard models in the field. Regarding the latter, the first direct
detection of gravitational waves by the LIGO experiment and several other detections have been made sincethen. Super-K produced follow-up searches for neutrinos coincident with the first detection of a blackholeblackhole merger [81] and with a neutron star-neutronstar merger [86]. In addition, a search was performed
for neutrinos coincident with the astrophysical neutrino
data observed at IceCube [84]. Though each of thesesearches yielded a null result, it is clear that further
searches and subsequent detections will advance multimessenger astronomy and are therefore an importantavenue of research at Super-K going forward.
Looking Toward the Future
The pursuit of oscillation physics is among the mostimportant goals for the atmospheric neutrino physics
program at Super-Kamiokande and indeed in the community in general. Super-K, especially with constraints
from the T2K experiment, has excellent sensitivity to
the mass hierarchy and can thereby make contributions
to the study of δCP as well. Currently we aim to introduce several new features into the analysis to further
improve this sensitivity, including the introduction ofneutron-tagged analysis samples for better neutrinoantineutrino separation, the implementation of an improved reconstruction algorithm that will enable theexpansion of the fiducial volume, as well as techniques
to improve background suppression and systematic uncertainties. Not only will these be important for addressing the physics goals of the atmospheric sample
itself, but will also guide the development of the expected sensitivity of and the future analysis at HyperKamiokande.

Solar Neutrinos
Solar neutrinos are produced by the nuclear fusion
reaction, 4p → α + 2e+ + 2νe , in the core of the Sun.
SK detects solar neutrinos through neutrino-electron
elastic scattering, νx + e → νx + e, where the energy, direction, and time of the recoil electron are measured.
Thanks to its large (22.5 kiloton) fiducial mass, SK
makes precise measurements of 8 B solar neutrinos, including precision information on their energy spectrum
and its time variation.
Solar neutrino experiments (SK, SNO, Borexino andradio chemical experiments) have provided direct evidence for solar neutrino flavor conversion. Considering10−36−37

10

SK I-IV, bb
SK I-IV, τ+τ+ SK I-IV, W W

10−38

WIMP-proton SD cross section [cm2]

WIMP-nucleon SI cross section [cm2]15

DAMA/LIBRA (2008)
ANTARES, 2007-2012, bb
ANTARES, 2007-2012, τ+τ- +ANTARES, 2007-2012, W W +IceCube, 2011-2012, τ+τ-/W W

10−39
10−40
10−41
10−42
10−43
10−44
10−45

10

102

3

10

2

Mχ [GeV/c ]

10-34
10-35

SK I-IV, bb
SK I-IV, t+ t+ SK I-IV, W W

DAMA/LIBRA (2008)
LUX, WS2013+WS2014-16
PICO-60, 2016-2017 _IceCube, 2011-2014, bb
IceCube, 2011-2014, t+t+- IceCube, 2011-2014, W W

10-36
10-37
10-38
10-39
10-40
10-41

10

102

Mc [GeV/c2]

Fig. 6. Limits on the spin-independent WIMP-nucleon interaction cross section for the Super-K search for WIMPS from the center of the
Earth (left) and on the spin-dependent cross section for WIMPs from the sun (right). Both analyses assume 100% branching ratios into
each of the channels shown.

with the KamLAND reactor experiment, a solar neutrino oscillation scenario is almost established. However, there is still a room for non-standard possibilities
in the current experimental results. For example, there
is about 2σ tension on ∆m221 oscillation parameter between a global oscillation analysis from solar neutrino
experiments and an oscillation analysis from the reactor experiment. Another possible room for new physics
would be that the expected transition between vacuum
oscillation and matter oscillation in solar neutrino energy spectrum has not been confirmed yet.
The next step of solar neutrino measurements is theprecise determination of the oscillation parameters andthe role of matter therein. For that purpose, SK has
been collecting high statistics solar neutrino data toattempt measurements of a day-night flux difference
and a distortion of the energy spectrum.
In this term (after 2012), many efforts continuously
made to reduce backgrounds and to increase the precision of the detector calibrations, both of which are
crucial for solar neutrino measurements.
The most serious background comes from the beta
decay of 214 Bi, which is produced in the decays of radon(222 Rn) in the air and detector materials as well as fromradium in the purified water. In order to reduce the214 Bi background, the SK water system was upgradedagain. In 2013-2014, a new heat exchanger was installedto achieve precise control of the water temperature in
the detector. This system controls water temperature
with 0.1 degree accuracy. It confines high radon remaining water to the very bottom of the SK detector,
and then keeps lower radon concentration in the central
fiducial volume region.
In order to study energy spectrum in the mattervacuum transition region, we have lowered the trigger
threshold of the data acquisition system from 34 observed PMT hits within 200 nsec to 31 hits since May
2015. The 3 hits corresponds to about 0.5 MeV electronenergy. After May 2015, the trigger efficiency between
3.49 MeV and 3.99 MeV electron kinetic energy was
improved from ∼86% to ∼100%.
In the analysis software, the energy reconstruction
method is improved in April 2018 to consider PMT
gain and dark rate variations as a function of time.
Preliminary results with this new analysis tools werereleased at the Neutrino conference held in June 2018.
In 2016, we released a paper about the solar neutrino
analysis results until February 2014 (SK-IV 1664 days
data set) [79]. Up to now, SK-IV finished data taking.
In this report, the updated preliminary results using
the data until the end of December 2017 (SK-IV 2860
days data set) are shown. Table 3 shows a summary ofsolar neutrino observation in each SK phase. The total
live time of solar neutrino measurements throughout allphases of SK is 5695 days.
Since SK has observed solar neutrinos for more than
20 years, this long term observation covers nearly 2
solar activity cycles. Figure 7 shows SK yearly flux
measurements since SK-I until SK-IV with the sun spot
numbers in the same period. From the yearly data in
Fig. 7, the χ 2 value is calculated as χ 2 = 21.57/21 d.o.f.,which corresponds to a probability of 41%. The SK
solar rate measurements are consistent with a constantsolar neutrino flux emitted by the Sun.
During the SK-IV 2860 days data set, preliminary number of the observed solar neutrino events are
55810+363−361 (stat) ± 949(syst) in 3.49-19.5 MeV electron
kinetic energy region. This event rate corresponds
to a 8 B solar neutrino flux of (2.289 ± 0.015(stat) ±0.039(syst)) × 106 /cm2 /sec, assuming a pure νe flavor content. A statistically combined flux from all
SK phases is (2.327 ± 0.044(stat + syst)) × 106 /cm2 /sec
(preliminary).
Figure 8 shows the solar angle distribution in thelowest energy bin of the SK-IV 2860 days data set. The
observed number of the solar neutrino events between

16PeriodSK-I
SK-II
SK-III
SK-IV

April, 1996 - July, 2001
October, 2002 - October, 2005
July, 2006 - August, 2008
September, 2008 - May, 2018

0.55

250
SK data(Stat. only)
Systematic error

0.5

SK combined flux

200

Sun spot number

0.45

150

0.4

100

0.35

50

Events/day/kton/bin

Phase

Sun spot number

Data/MC(Unoscillated)

Table 3. Summary of the solar neutrino observation in SK. The energy threshold is in electron kinetic energy. The SK-IV information ispreliminary.

Live time(days)
1496
791
548
2860

Energythr. (MeV)
4.49
6.49
4.49
3.49

0.35
Preliminary
0.3
0.25
0.2
0.15
0.1
0.05

0.3 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 0
8

6MC B Flux = 5.25×10 /cm2/sec

Date

Fig. 7. The measured 8 B solar neutrino flux in SK since beginningof SK-I(1996) until end of SK-IV(2018). The vertical axis shows
the ratio of the observed flux to the expectation from the unoscillated MC simulation assuming a 8 B flux of 5.25×106 /cm2 /sec.
The red points show the yearly flux measured by SK (statisticaluncertainty only). The gray bands show the systematic uncertainties in each SK phase. The black-horizontal line shows the
combined measured flux value with uncertainty drawn as the red
horizontal band. The back points show the sun spot number
provided from the Wold Data Center SILSO, Royal Observatoryof Belgium, Brussels (http://www.sidc.be/silso/datafiles/). The
results during SK-IV are preliminary.

3.49 and 3.99 MeV is 1797+169−167 (stat) (preliminary).
The measured energy spectrum of the SK-IV 2860
days data set is shown in Fig. 9 (top). The verticalaxis shows the ratio of the observed energy spectrum
to the expectation from the unoscillated MC simulation
assuming the 8 B flux of 5.25×106 /cm2 /sec. Figure 9(bottom) shows a statistical combination of SK spectrafrom all phases. The combined energy spectrum is consistent with the standard neutrino oscillation analysis
at solar global best-fit ∆m221 parameter within 1σ level,
but there are ∼ 2σ tension with the standard neutrino
oscillation analysis at KamLAND best-fit ∆m221 parameter.
Concerning differences in the day and night fluxes,
the expected flux asymmetry, defined as ADN = (day −night)/ 12 (day + night), is about 2% based on current
understanding of neutrino oscillation parameters. Although this is not a large effect, long term observations

0
-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8
1
cosθsunFig. 8. Solar angle distribution of SK-IV 2860 days data set forevents with electron kinetic energies between 3.49 and 3.99 MeV.
The black points are data while the red histogram is the best fit to
the data. The blue histogram shows the background componentof this fit.

by SK enable discussion of a finite value of the daynight asymmetry. In 2016, we summarized the daynight asymmetry analysis using all SK data up to SKIV 1664 days data set. Figure 10 shows the solar zenith
angle dependence of the solar neutrino fluxes from theevents in 4.49-19.5 MeV from SK-IV 1664 days data
f it,SK
set. The obtained ADN
value using the amplitude fit
method was −3.3 ± 1.0(stat) ± 0.5(syst)%, which was a
2.9σ difference from zero [79]. Currently, the day-night
analysis up to SK-IV 2860 days data set is ongoing.
A global solar neutrino oscillation analysis has been
performed including all SK data (SK-I [38], SK-II [47],
SK-III [54], day-night amplitude fitting from SK-IV
1664 days data set [79], and preliminary energy spectrum from SK-IV 2860 days data set) as well as datafrom other solar neutrino and reactor experiments,
same as Ref. [79].
Figure 11 (top) shows the allowed regions from SK
data with the 8 B flux constraint of (5.25 ± 0.20) ×
106 /cm2 /sec as well as those from KamLAND data.
SK data with a flux constraint is significantly contributes to the measurement of θ12 and ∆m221 parameters among solar neutrino experiments.
Figure 11 (bottom) shows the allowed regions from2

Blue:Total uncertainty

Preliminary

Red:Energy-uncor. Sys.
: Energy-cor. Sys.

0.5

1σ

-5
2
2
in 10 eV
21

0.55

8
6
4 2σ
2

∆χ

0.6

sin2(Θ12)=0.316+0.034-0.026sin2(Θ12)=0.332+0.027-0.022sin2(Θ12)=0.324+0.021-0.018-52
sin2(Θ13)=0.0219±0.0014∆m221=(7.54+0.19-0.18) 10 eV-52∆m221=(4.73+1.32-0.80) 10 eV-52∆m221=(7.49+0.19-0.17) 10 eV

15

0.45

∆m

DATA/MC(unoscillated)

17
3σ

0.4

10
0.35

5
Very Preliminary
1σ

0.1

0.2

0.3

SK I/II/III/IV LMA Spectrum
Preliminary

0.58

1σ

-5
2
2
in 10 eV
21

0.52

3σ

8 3σ
6
4 2σ
2

0.56
0.54

2σ

0.4
0.5 2 4 6 822sin (θ )
∆χ
12

2

0.6

∆χ

Data/MC (unoscillated)

0.3 SK-IV 2860days
MC 8B Flux = 5.25x106/cm2/sec
0.25
4
6
8
10
12
14
16
18
Recoil electrons kinetic energy[MeV]

sin2(Θ12)=0.316+0.034-0.026sin2(Θ12)=0.310±0.014
sin2(Θ12)=0.310±0.012-52
sin2(Θ13)=0.0219±0.0014∆m221=(7.54+0.19-0.18) 10 eV-52∆m221=(4.82+1.20-0.60) 10 eV-52∆m221=(7.49+0.19-0.17) 10 eV

15

∆m

0.5
0.48

10

0.46
0.44

5

0.42
0.4

4

6

8

10

12

14

16

18
Ekin in MeV

Very Preliminary
1σData/MC (Unoscillated)

Fig. 9. Solar neutrino energy spectrum in SK-IV (top) and a statistical combination from all phases (bottom). Each point shows theratio of the data to the expected flux using an unoscillated 8 B
solar neutrino spectrum. In the bottom plot, blue and green lines
are predictions from solar + KamLAND best-fit point and solar
global best-fit point. The black and brown lines are best-fit of
SK spectrum data with quadratic and exponential functions [79].

0.5

0.1

0.2

0.3

2σ

3σ

0.4
0.5 2 4 6 822sin (θ )∆χ
12Fig. 11.
Allowed regions of neutrino oscillation parameters in the ∆m221 and sin2 θ12 plane with a constraint ofsin2 θ13 = 0.0219 ± 0.0014. The green, light blue, and red results of the upper figure are obtained from SK data, KamLAND,
and SK+KamLAND, respectively. Those of the lower figure are
obtained from solar global, KamLAND, and solar global+KamLAND. The curves are drawn for each 1σ step of 1-5σ for SK/solar, and of 1-3σ for the KamLAND and SK/solar+KamLAND
results. Contours at 3σ are filled with their colors.

0.48
0.46
0.44

0.4

All
Day
Night

0.42

-1

0

1
cosθz

Fig. 10. The solar zenith angle dependence of the solar neutrino
flux from the events in 4.49-19.5 MeV from SK-IV 1664 days
data set. The black points are observed flux. The red and bluelines are expected flux values at solar global and solar global +KamLAND best-fit parameters, respectively.

the solar global analysis and solar+KamLAND global
analysis. The obtained parameters from the solar−5 eV2 andglobal analysis are ∆m221 = (4.82+1.20−0.60 ) × 10
2sin θ12 = 0.310 ± 0.014. Comparing these values with
−5 eV2
those from KamLAND (∆m221 = (7.54+0.19−0.18 ) × 10
+0.0342and sin θ12 = 0.316−0.026 ), there is a 2σ level tension in the ∆m221 results, which is evident in the figure. Combining the solar global analysis with KamLAND analysis, the oscillation parameters become−5 eV2 and sin2 θ∆m221 = (7.49+0.19
12 = 0.310 ±−0.17 ) × 10
0.012. These are current most accurate oscillation parameters of θ12 and ∆m221 .
Summary and future prospects
High accuracy solar neutrino measurements are carried out in SK-IV. Improvement of the water circulation
system, lowering of energy threshold of data acquisi-

18tion system, and improvement of analysis software were
done. Then, solar neutrino events in 3.49-3.99 MeV inelectron kinetic energy are now used in the oscillation
analysis.
The combined energy spectrum is consistent withthe solar global best-fit within 1σ , but ∼ 2σ tension
with the KamLAND measurement.
The combined day-night asymmetry value until SKIV 1664 days data set is −3.3 ± 1.0(stat) ± 0.5(syst)%,which is a 2.9σ difference from zero. Currently, The
day-night analysis up to SK-IV 2860 days data set isongoing.
The global oscillation analysis of solar neutrino experiments and KamLAND experiment provides mostaccurate oscillation parameters of θ12 and ∆m221 . However, there is about 2σ level tension in ∆m221 between
solar global analysis and KamLAND measurement.
The energy spectrum and day-night asymmetry arequite important for understanding the matter effect of
the neutrino oscillation. The solar neutrino measurement at SK will continue, and those measurements will
be discussed with meaningful statistical significancewith further increase of the SK data.

Nucleon Decay Searches
Nucleon decay search gives us a unique window totest Grand Unified Theories (GUTs) of elementary particles. SK is the world’s largest detector to search fornucleon decays and it has accumulated data of 91.7
kt·yr (SK-I), 49.2 kt·yr (SK-II), 31.9 kt·yr (SK-III),and 192.1 kt·yr (SK-IV until January 2018), resulting in
365kt·yrs of data in total. Various nucleon decay modes
have been searched for in SK, but no significant signal
has been seen so far, as is shown in Table 4. The lifetime
limits reached more than or close to 1034 years in important decay modes, especially, the limits for p → e+ π 0
and p → µ + π 0 have exceeded the original goals written
in the SK proposal. The new electronics introduced
from SK-IV made improvement in selection efficiencyof p → µ + π 0 and p → ν̄ K + [69] by increasing tagging
efficiency of Michel electron from muon. The new electronics also enabled to tag neutrons which are accompanied with atmospheric neutrino background[82]. We
describe the detail of these important modes.
The proton decays into one charged lepton and one
neutral pion (p → e+ π 0 and p → µ + π 0 ) are the ones
that occur in a large number of theoretical models. This
decay mode is mediated by super-heavy gauge bosons.
Discovery of a signal would give us information about
the mass of the gauge bosons, MX , because the proton
lifetime is predicted to be proportional to MX4 . To discriminate the signal from atmospheric neutrino background, we reconstruct the total visible energy and total momentum corresponding to the parent proton massand the proton’s Fermi momentum in oxygen, respectively.

In SK-IV period, we updated the electronics andit enables tagging of neutrons to separate proton decay signal from atmospheric neutrino background. The
signature of the neutron, 2.2 MeV gamma ray emission
from the neutron capture on hydrogen with a mean capture time of 200 µ sec, is too faint to be triggered by thedata acquisition system used in SK-I through SK-III.
The upgraded electronics in SK-IV adopt a trigger-lessreadout scheme to record every hit, including all hits by
2.2 MeV gamma rays. A software trigger is then issued
after every fully-contained event to save all hits within
a 500 µ sec timing window for physics analyses. Neutron tagging can benefit proton decay analyses, providing an additional handle for rejecting the atmospheric
neutrino interactions that are the main background to
proton decays. Atmospheric neutrino interactions areoften accompanied by neutrons, while the probabilityof a neutron being emitted by deexcitation of a nucleus after proton decay is rather small, and no neutron is produced in the decay of a free proton. Furthermore, these proton decay modes do not produce neutrons in secondary interactions in water because thefinal state particles in these decay modes are leptonand gammas. Almost a half of atmospheric neutrino
background could be rejected by requiring no neutron.
To find 2.2 MeV γ candidates, we search for hit clusters with ≥ 7 hits within a 10 ns sliding time window,
after time-of-flight (TOF) subtraction using the vertexof the prompt neutrino interaction. Sixteen variables
described in a reference1 are input to a neural network
to distinguish the 2.2 MeV γ signal from background.
The efficiency of neutron tagging by this method is estimated to be 20.5±2.1% with a mis-tagging probabilityof 1.8%. The performance of the neutron tagging was
confirmed by introducing a neutron source (AmericiumBeryllium) into the SK tank.
Figure 12 shows the number of tagged neutrons for
p → e+ π 0 (upper) and p → µ + π 0 (lower) analyses excluding the signal region defined by reconstructed total
mass and momentum. These figures show that the neutron multiplicity in the atmospheric neutrino MC (solid
histogram) agrees well with data (dots). The dashedhistograms show the true number of captured neutrons
in the MC, indicating that additional background rejection can be achieved if the neutron tagging efficiency
is improved in the future when gadolinium is dissolvedin the SK water.
A new analysis technique is applied in these analysis. The signal region is divided into two regions:Ptot < 100 MeV/c and 100 ≤ Ptot < 250 MeV/c. The
region below 100 MeV/c is dominated by free protons, and the region 100 ≤ Ptot < 250 MeV/c is dominated by bound protons. A reduced systematic error
for < 100 MeV/c can be achieved because the initial*1 Irvine, Tristan James, “Development of Neutron-Tagging Techniques and Application to Atmospheric Neutrino Oscillation Analysis in Super-Kamiokande”, Ph. D. Thesis, University of Tokyo,
2014.

19

mode

p → e+ π 0
p → µ +π 0
p → ν̄ K +− µ + spec.
− prompt γ− π +π 0

exposure(kt · yr)
365
365
365

B.G.

38-39
30-38

observed
event
0
1

31-37
7-9
7-10

–
0
0

–
0.4
0.6

ε Bm
(%)

0.6
0.7

τ /B
limit
(1034 yrs)
2.0
1.2
0.8

Table 4. Summary of nucleon decay search results in Super-Kamiokande.

1200
1000
800

Number of events

600
400
200
0
450
400
350
300
250
200
150
100
50
0

0

2

4

6

Neutron multiplicity

810

Fig. 12. Distribution of the number of tagged neutrons for p → e+ π 0(top) and p → µ + π 0 (bottom) analyses excluding the signal region defined by total mass and momentum. Dots show SK-IV
data with 133.5 kton·years exposure, the solid histogram shows
the multiplicity of tagged neutrons per event in the atmosphericν MC, and the dashed histogram shows the true multiplicity ofneutrons per event.

protons and the products of the proton decay do not
suffer from any of the various nuclear interactions. In
addition, background contamination from atmospheric
neutrinos is concentrated in the 100 ≤ Ptot < 250 MeV/c,
while the region below 100 MeV/c is nearly backgroundfree. Figure 13 shows the reconstructed proton mass
vs. total momentum distribution for signal and atmospheric ν MC, combining all data from SK-I through
SK-IV. In the signal MC plots, the light blue dots show
the free proton case and the dark blue dots show boundprotons.
The new two-box analysis achieves better discovery
reach. For example, the 3σ discovery reach in the proton lifetime for p → e+ π 0 is 13.3% higher than the conventional single-box analysis for the current exposure,and 20.9% higher for an exposure of 1 megaton·year,
which may be achieved by the next generation of detectors.

We analyzed data from SK-I through SK-IV, in total
365kt·years until January 2018. As shown in Table 4,
there were no candidate events observed for p → e+ π 0with 0.6 expected background. One event observed forp → µ + π 0 but it is consistent with expected background(0.7 events). Thus we set lower limit of proton lifetime
with 90% confidence level as 2.0×1034 and 1.2×1034
years for p → e+ π 0 and p → µ + π 0 , respectively.
Not only π 0 , we studied nucleon decay into
charged anti-lepton and other mesons, p → (e+ , µ + ) +(η , ρ 0 , ω ) and n → (e+ , µ + ) + (π − , ρ − ) systematicallywith 316kton·year data. Some candidates were observed but they were consistent with expected background and we got lower limits of nucleon lifetime inrange from 3×1031 to 1×1034 years for these modes.
p → ν̄ K + is also important because it is a dominantmode in SUSY-GUTs. In this mode, most of K + stops
in water and decays into two particles, ν µ + or π + π 0 ,which have monochromatic momentum and are backto-back each other. Thus we looked for single muonring events with 236 MeV/c. Another important feature is that the remaining nuclei may emit de-excited
gamma rays after the proton decay and this γ shouldbe emitted before K + → ν µ + (lifetime: 12 nsec). Thus
we looked for the prompt gamma rays in the single
muon ring sample. Charged current quasi-elastic scattering (nνµ → pµ − ) with nuclear gamma ray, muon below Cherenkov threshold and proton above threshold
was one of the dominant background for this search.
because the proton ring was reconstructed as muon byshifting vertex largely due to difference of their opening angle of Cherenkov ring. It makes fake time difference between muon and gamma ray. To reject such
events, new algorithm which separates proton ring frommuon ring by maximum likelihood method based on
Cherenkov opening angle and hit pattern. We achieved
about 50% background rejection with this method.
For K + → π + π 0 , the momentum of charged pion isjust above the Cherenkov threshold and it doesn’t makeclear Cherenkov ring. We searched for monochromatic
π 0 s which have PMT activities in the backward. π 0
decays into two γ s and this decay mode should have
two e-like rings in many cases and we selected two ring
sample in the conventional method . However, if energy

20

1000
800

p→e+π0 MC

Atmospheric ν MC

Data

p→µ+π0 MC

Atmospheric ν MC

Data

Total momentum (MeV/c)

600
400
200
0
1000
800
600
400
200
0

0

500

1000

0

500

1000

0

500

1000

2

Total mass (MeV/c )Fig. 13. Reconstructed proton mass vs. total momentum for p → e+ π 0 (top) and p → µ + π 0 (bottom). The left panels show signal MC,
where light blue corresponds to free protons and dark blue is bound protons. The middle panels show atmospheric ν MC corresponding
to 500 years live time of SK, and the right panels show SK-I to SK-IV data. The dot size is enlarged in the signal box. Signal
regions defined by total momentum are shown by black boxes. The lower box (Ptot <100 MeV/c) is dominated by free protons withless systematic uncertainty in signal MC, while most of events in higher box (100 <= Ptot < 250 MeV/c) are bound protons affectedby nuclear effect. Remaining background events concentrate in the higher box and the lower box is almost background free. Better
sensitivity is achieved by separating the signal regions.

of one γ is too small or opening angle of two γ is small,
it is identified as one ring sample. To find out one-ring
π 0 events, we employed special routine which is used forrejecting background for T2K νe analysis. By adding
one ring sample, detection efficiency of K + → π + π 0 was
improved by 20%.
Search for p → ν̄ K + mode was carried out with improvement described above and neutron tagging only
for SK-IV by using 365kt·yrs data. We didn’t findany proton decay candidates and calculated partial lifetime limits, taking into account systematic uncertainties. The obtained limits are 8.0 × 1033 years at 90%
confidence level.
Super-Kamiokande also looked for the other various
nucleon decay modes and Figure 14 shows summaryof the lower limit of the nucleon lifetime. We haveobtained the world’s strongest nucleon lifetime limits
for most of the modes.

Future prospect
As seen in this section, the new electronics installedin SK-IV contributes to reduce atmospheric neutrino
background by neutron tagging. We achieved to reduce background by almost half for several nucleon decay modes in pure water case, in which neutron tagging
efficiency is rather small (about 20%). Figure 15 shows
background reduction of p → e+ π 0 as a function of efficiency of neutron tagging. We are going to load Gd into
water in SK tank and we expect 80% neutron tagging
efficiency with 0.1% Gd concentration. In this setup,
we are able to achieve further 50% background reduction from SK-IV case. Super-Kamiokande can keep
good discovery potential of proton decay with longer
exposure by suppressing background low enough.

Supernova Neutrinos

In 1987, the observation of supernova SN1987A by
Kamiokande, IMB and BAKSAN opened the neutrino
astronomy. This observation confirmed that the en-

21

BKG fraction

Fig. 14. Summary of lower limits of nucleon with 90% CL studied
by Super-Kamiokande. Red and green color shows proton decay
modes which were analyzed with 365kt· and 350kt· years data,
respectively. Modes in blue color were results of SK I-III data.

1

0.8

0.6

0.4

0.2

0

0

0.25

0.5

0.75

1

Tagging efficiency
Fig. 15. Background reduction for p → e+ π 0 as a function of neutron
tagging efficiency. Vertical axis is normalized without neutron
tagging case.

ergy released by neutrinos is about several×1053 ergs.
However, the core collapse supernova (ccSN) mechanism is not yet fully understood. SK would be able
to detect several thousand neutrino events if a ccSN
happened near the center of our galaxy. Such an observation would enable us to investigate in detail themechanics of the ccSN explosion.
On average, 1-2 ccSNe per century are expected inour galaxy. We must be prepared for these events because neutrinos arrive earlier than optical signals. A
real-time supernova neutrino burst monitor searches
for time clustered events [78]. This online programis called SNWATCH. Events with total energy greaterthan 7 MeV and vertex position within the 22.5-kton
fiducial volume in SK are selected. Cosmic ray muonsand their subsequent decay electron events are removed. For each selected event, a 20-second time window is opened backwards in time, and the number of
selected events in the window, Nclus , is counted. A variable D that identifies the dimension of the vertex distribution is computed. It is an integer number from 0 to
3, corresponding to point-, line-, plane- and volume-like
distributions, respectively.
When Nclus ≥ 60 and D = 3 a prompt SN warning is generated including automatic phone-calling andemails to experts. Then, the experts check whether it
is a real supernova signal or not by looking at various
plots which are uploaded to a secured site accessible
from the Internet. These alarms are usually due to theaccidental coincidence of two cosmic ray induced clusters. So far, no real supernova neutrino burst signal
has been observed at SK.
When SNWATCH issues such a warning, first SK
shifts call SNWATCH conveners and report the status of SK. Then, SNWATCH conveners and executive
committee members meet via TV conference system,and discuss about a prompt announcement to outside researchers and the press. The decision is taken
within one hour after the SN warning. In order totrain people and test SNWATCH system, there aredrills few times in every month. In a drill, a laserdiode light-source flashes to mimic a galactic supernova,
and SNWATCH warning notifies SK shifts. Shift takers then call SNWATCH conveners and give a report,
and converners actually have to have a meeting. By
doing such drills, SK collaboration is ready for a real
supernova.
Other searches conducted at SK are those for neutrinos from Supernova Relic Neutrinos (SRNs). The SRN
signal is the diffuse supernova neutrino backgroundfrom all the core collapse supernovae in the past. Thissignal is expected to be detectable in the 16-30 MeV
energy region, which corresponds to the gap between
the spectrum of solar and atmospheric neutrinos, however it has never been detected and only upper limit
was set by SK [58].
In SK-IV, a new result of the SRN search using theneutron tagging technique was also published [70]. In

9
8
7
6

νe flux upper limit (cm-2 s-1 MeV-1)

Entries / 25µs

Entries / 1 MeV

22
6
5
4
3
2

5

1

4
100

200

300

400

3

500

∆ T(µs)2

Efficiency(%)

1
16
15
14
13
12
11
10
12

102

10

1

10-1
Electron energy (MeV)

14

16

18

20

22

24

26

28

30

Electron energy (MeV)
Fig. 16. Energy spectrum of prompt signals (points). The red histogram shows the expected accidental background. The plot
embedded in the upper right shows the timing difference for thedelayed candidates. The bottom figure shows the detection efficiency of SRN for each energy bin; the jumps at 18 MeV and
24 MeV are due to energy-dependent spallation cuts. Errors are
statistical only.

this analysis, the neutrons captured on hydrogen from
SRN reactions (ν̄e , p → e+ , n) are searched. After a neutron is captured, a single 2.2 MeV gamma is emitted.
Thus, by detecting the prompt positron signal and thedelayed 2.2 MeV gamma signal, we can reduce backgrounds, most of which are not accompanied by neutrons.
Figure 16 shows the energy spectrum of prompt signal, the time difference between a prompt signal anda delayed signal, and the detection efficiency of SRN
for each energy bin. Figure 17 shows the obtained flux
limit comparing with other results. The neutron detection efficiency is very low because of the low energyof the gamma from the neutron capture on hydrogen(compare the 2.2 MeV gamma with the kinetic energy
threshold of 3.5 MeV for solar neutrino analysis). However, with this method we could obtain the world best
limit below 16 MeV. This result shows a high potentialof neutron tagging techniques, which can be a strong
tool for SRN detection.

The Super-Kamiokande Gadolinium Project(SK-Gd)Supernova relic neutrinos
As described in the previous section, SK obtained
flux upper limit of SRNs. The obtained limit is getting
closer to the predictions and SRN signals would be detected in the SK-Gd phase. The predicted spectra of

10-2

14

16

18

20

22

24 26 28 30
Neutrino energy (MeV)

Fig. 17. Model-independent SRN 90% C.L. upper limits as a function of neutrino energy for SK-IV (solid circle). For comparison,both KamLAND result (open square) and previous SK result(solid triangle) are also shown.

SRN2 are shown in Figure 18.
Since reactor neutrinos and atmospheric neutrinos
are dominant background below 10 MeV and above 30
MeV respectively, 10–30 MeV is an open window for the
SRN measurement. SK has been searching for SRN as
described in the previous section, recently with 2.2MeV
gamma tagging. However, it is still limited by background because the detection efficiency of the 2.2 MeV
gamma with Cherenkov light is low. By adding 0.2%
gadolinium sulfate into the 50 kton water tank, thissituation will be significantly improved3 as shown in
Figure 19. Gadolinium has a thermal neutron capture
cross section of 49,000 barns (about 5 orders of magnitude larger than that of protons) and emits a gamma
cascade of 8 MeV that can be easily detected using
Cherenkov light. In order to obtain a 90% efficiency
for neutron capture, the Gd concentration should be
0.1% (0.2% if Gd2 (SO4 )3 ) as illustrated in Figure 19.
Figure 20 shows expected signal spectra for SRN
models and conservative background estimation of atmospheric neutrinos with its component. Table 5 showsthe expected number of events and background with 10
years observation. Here, we assume the νµ CC background events are 1/4 of those of SK, νe CC background
events are 2/3, and NC background events are 1/3 withneutron tagging, respectively. Topological informationof events should allow to reduce the background further, but it is not included in this estimation.
EGADS
In order to study the effect of dissolving Gd in the
SK tank, an R&D project called EGADS has been run*2 S. Horiuchi et al., Phys. Rev. D79, 083013(2009).
*3 J. Beacom and M. Vagins, Phys. Rev. Lett. 93, 171101 (2004).

23
Table 5. Expected numbers of SRN signals and backgrounds through SK-Gd 10 years observation

10

7

10

6

10

5

10

4

10

3

10

2

reactor

10-16 MeV
11.3
11.3
7.7
5.1
10

16-28 MeV
19.9
13.5
4.8
6.8
24

–
e

QH

SRN (T =8MeV)
SRN (T =6MeV)

Total
31.2
24.8
12.5
11.9
34

significance
5.3σ
4.3σ
2.5σ
2.1σ
−

Q
S


H

*G
Ȗ

SRN (T =4MeV)0H9

SRN (T =SN1987A)

Ʃ7a ǋV9HUWLFHVZLWKLQFP

10&DSWXUHVRQ*G

2

Neutrino Flux (/cm /sec/MeV)

Model
Teff = 8 MeV
Teff = 6 MeV
Teff = 4 MeV
Teff =SN1987a
BG




*GJLYHVa
HIILFLHQF\IRU
QFDSWXUH





 



*GLQ:DWHU



1
10
10

-1
-2

atmospheric

–
e

-3

10
-4

Fig. 19. Neutron tagging with gadolinium (left), and capture
efficiency as a function of Gd concentration (right) (M. Vagins’ talk in NEUTRINO2010, June 2010, Athens, https://indico.cern.ch/event/73981).

10
-5

10
10

-6

0

10

20

30

40

50

60
70
80
Neutrino Energy (MeV)

Fig. 18. Expected energy spectra of supernova relic neutrinos (fromS. Horiuchi et al., Phys. Rev. D79, 083013(2009)). Each color
corresponds to an effective neutrino temperature, and each range
shows astrophysical uncertainties.

ning in the Kamioka mine since 2014. A new hall was
excavated near SK and a 200 m3 stainless steel tankwith ancillary equipment was constructed. The idea
is to mimic the SK conditions inside the 200 m3 tank.
The tank is equipped with a selective water filtration
system that removes impurities while retaining the Gd
and a 15 m3 Gd premixing and pretreatment plastic
tank. It is also instrumented a device to measure thewater attenuation length, UDEAL (Underground Device Evaluating Attenuation Length).
Figure 21 shows the history of the measured watertransparency in the 200 m3 tank since October, 2014.
The four loadings are clearly identified. It demonstrates that transparencies can be comparable to thoseachieved at the pure-water SK’s phases III and IV,
and that in steady operation the measured transparencies are nicely stable at values very appropriate forphysics analyses. The Gd2 (SO4 )3 concentration in the
200 m3 tank is monitored by using an Atomic Absorption Spectrometer (AAS). Water samples are collectedby a pump from three points in the detector, at threedifferent height, using the same pipe as the UDEAL

sampling. The measured concentration since November
2014 is also shown in Figure 21. The four loadings areclearly seen as steps in the concentration. The concentrations at all three sampling points are: 1) very close
to each other, indicating that there is a homogeneous
solution in the 200 m3 tank, and 2) stable between loadings or any other external intervention, demonstratingthat there is no significant Gd loss during continuous
water recirculation and purification.
In order to see the positive effects of the Gd loading,i.e. the neutron tagging capability, an Am/Be neutron
source with BGO crystal was deployed in the EGADS
detector. Through the process; 241 Am →237 Np + α ,9 Be + α →12 C + γ (4.4 MeV) + n, the scintillation lightfrom BGO by 4.4 MeV γ can be used as the prompt
signal which mimics the Cerenkov light. Figure22(a) shows the distribution of time differences between
prompt and delayed (neutron captured) events after
application of several simple analysis cuts. The exponential fit of the distribution indicates a characteristicof 31.32 ± 0.76 (stat.) µ s, which is consistent with the
expectation for 0.1% Gd concentration. The flat component of the fit function indicates the BG rate in thedata. In order to remove the remaining background(BG), a statistical background subtraction method wasapplied. This method uses data of delayed events rejected by the ∆tprompt−delayed < 500 µ s cut as a background sample. Figure 22 (b) shows the spectrum of
the number of hits from a calibration run with the
Am/Be source deployed in the center of the detector
after background subtraction. The same distribution
from a GEANT4 MC simulation is shown. The effi-

24

EntriesFig. 20. Expected background components of SK-Gd (left), and expected SRN spectra of some models (right) (from S. Horiuchi et al.,Phys. Rev. D79, 083013(2009)).

700
600

χ2 / ndf = 98.57 / 95
p0
= 294.7e+05 ± 5.2e+05
p1= 31.32e+03 ± 0.76e+03 ns
p2= 42.37 ± 0.79

D

Preliminary

500

AmBe run400

exp(-x/p1)
p0*
+p2
p1

300
200
100
0

0

100

150

200

250

300

350

400

×103
500

450

∆tPrompt-Delayed [ns]EntriesFig. 21. Percentage of Cherenkov light remaining for water inside theinstrumented 200 m3 EGADS tank for increasing concentrationsof gadolinium sulfate. The green, red, and blue lines correspondto data taken from the top, center, and bottom sample pointsof the 200 m3 tank, respectively, while the light blue band shows
the range of ultra-pure water transparencies during SK-III and
SK-IV. The inset is the gadolinium sulfate concentration history,
as measured by the AAS, for the three sampling points in the top,
center and bottom of the EGADS detector (A. Renshaw, Physics
Procedia 37, 1249 (2012); L. Magro, EPJ Web of Conferences
95, 04041 (2015)).

50

800
700

E

AmBe 2017 Data
MC600
Preliminary
500
400
300
200
100
0
0

ciency of the Gd neutron capture selection was measured to be 85.3 ± 0.9% (stat) in data and 84.4 ± 0.3%
(stat) in MC. These results indicate the expected neutron tagging in 0.2% Gd2 (SO4 )3 water.
In summer 2017, the ATM-based electronics thatwere installed in EGADS were replaced with the frontend electronics used in the current SK. This allows the
EGADS DAQ to stand high rates as in case of a close
SN. For example, in case of Betelgeuse (about 200 pc)
we expect about 3×104 events and according to ourtests our DAQ could withstand SN bursts up to about
106 events. On the other hand, a SN in the opposite sideof our galaxy would produce very few events in EGADS.
However, because of the clear signature of neutron capture on Gd, the detection of a few inverse beta decay
events within 10 seconds would unmistakably allow thedetection of such a SN.

10

20

30

40

50

60

70

80

90

100

nhits

Fig. 22. (a) Distribution of the difference of time between promptand delayed events in Am/Be run with Am/Be source deployedin the center of the detector. The blue line indicates the fit of the
distribution with an exponential + flat component function. (b)Distribution of the number of PMT hits for Am/Be data (black)and MC (red) with Am/Be source deployed in the center of thedetector. Statistical background subtraction was applied.

Loading Gd into Super-Kamiokande
In order to realize the Gd implementation to SK,
we needed additional developments from various view
points, such as environmental safety and low energy
physics impacts.
New water system
Based on the EGADS results, we designed and constructed “Gd dissolution and purification system” to be
used in SK. The system consists of a gadolinium sulfate

25

SK-Gd water system
Gd2(SO4)3 powder in
1 ton flexible container

Powder transport“Massager”
Weighing hopper

ReceivinghopperCircle feeder

Circle feeder㼁㼂

Dissolving system㻭㻶㻌 㻭㻶㻌 㼀㻻㻯㻠㻠㻜㻜 㻝㻜㻞㻜㻾㼑㼠㼡㼞㼚㻌㼣㼍㼠㼑㼞㻌㼒㼕㼘㼠㼑㼞㻌

Pretreatment system
Band-Pass system

㼜㼡㼙㼜㻌
㻴㻱

㼀㻻㻯 㻭㻶㻌㻠㻠㻜㻜㻜㻚㻞P㼙

Fast-Recirc system㼀㻻㻯 㻴㻱

㻭㻶 㻝P㼙 㼁㼂
㻠㻠㻜㻜

㻜㻚㻞P㼙

㼀㼑㼙㼜㻚㻌㼏㼛㼚㼠㼞㼛㼘㻌㼡㼚㼕㼠㻌㻭
㼟㼡㼜㼜㼘㼥㻌
㼜㼡㼙㼜㻌

Current SK
water system

Fig. 24. The water piping modification㼀㼑㼙㼜㻚㻌㼏㼛㼚㼠㼞㼛㼘㻌㼡㼚㼕㼠㻌㻮

one third of that emanated from all the PMTs of theinner detector. Moreover, the sealing material will bepresent only in the outer detector, so the Rn emanationfrom the sealing material should be acceptable.
The tank repair work started on May 31st in thisyear (2018) and is expected to be finished in January
2019.

㻴㻱
㻹㼑㼙㼎㼞㼍㼚㼑㻌㼐㼑㼓㼍㼟㼕㼒㼕㼑㼞㻵㻰㻿㻷㻌㼠㼍㼚㼗 㼎㼛㼠㼠㼛㼙㼞㼑㼠㼡㼞㼚㻌
㼜㼡㼙㼜㻌

Fig. 23. The SK-Gd water systemtransportation unit, a dissolving unit, a pre-treatment
unit and a main purification unit in the recirculation
loop. The system is installed in Lab. G located approximately 160 m away from the SK tank. The flow
diagram of the system is shown in Fig. 23. The system
dissolves gadolinium in the SK tank in 17 days (when it
is operated with the circulation flow rate of 120 m3 /h)or 35 days ( when it is operated with the circulation flowrate of 60 m3 /h) and circulates and purifies gadolinium
sulfate solution with the flow rate of 120 m3 /h.
Stopping the leakage
The water tank of SK has a leak that is estimated to
be about 1 ton/day. The impact of Gd leakage to theenvironment is not regulated, therefore it is assumed
that the impact is the same as Hg leakage which has
the most stringent regulations. Thus, the leakage has
to be fixed and the goal is to stop the leak or reduce it
by a factor 30 at least. According to previous studies,
the main leakage is considered to be at the bottom of
the tank. However, to be safer, it was decided to apply
sealants to all the welding part of the inner wall of the
SK tank that is made of stainless steel (SUS) panelswith a thickness of 3-4 mm, height of 2 m and a lengthof 6 m. The total extension of the weld line is about
6000 m.
MineGuardTM had been considered for the overcoat
sealing, however it was found that the original formulation with poly-urethane undergoes hydrolysis. Then a
new formulation with poly-urea was developed. It wasalso found that CaCO3 which is added to MineGuardTM
in order to increase its viscosity is contaminated with
238 U. To improve the situation, a pure SiO instead of2
CaCO3 was selected. The newly developed material isnamed MineGuard CTM . It is estimated that the total
Rn emanation from the sealing material will be about

Upgrade of the water piping in the tankIn this work, the water piping inside the tank is also
improved. When dissolving Gd2 (SO4 )3 with the newwater system, Gd2 (SO4 )3 is added to the water coming back from the SK tank, and its aqueous solution
is supplied to the tank. It is necessary to achieve a
uniform Gd concentration inside the tank as soon as
all the dissolving process is completed and also to keepa good water transparency. If we scale the flow rateof the EGADS system to SK-Gd system, at least 120
m3 /h of flow is necessary, while SK-IV has circulated
and purified water at a flow rate of 60 m3 /h. As alreadymentioned above, the new water system was designed
to process the dissolving and the purification with therecirculation of 120 m3 /h. As a consequence, the piping in the tank should be upgraded for doubled flow
rate. In addition, the water flow in the tank has been
restricted by the so-called “segmentation Tyvek” which
was designed to separate the ID and OD optically but
also separate the ID and OD water flow completely.
Therefore, to improve the water replacement efficiency,
the piping will be modified as shown in Fig. 24
Purification of Gd2 (SO4 )3The usual Gd2 (SO4 )3 on the market contains radioactive impurities. Since these impurities would bepresent in the whole detector volume and could mimic
several signals, these would be potential backgroundsources for low energy neutrinos including SRN andsolar neutrinos. For example, with the 238 U contamination in Gd2 (SO4 )3 , spontaneous fission would leadto a sizable background for SRN and the solar neutrino analysis would be affected by betas/gammas coming from 226 Ra daughters. In order to reduce the radioactive impurities in the Gd2 (SO4 )3 , we are currently

26

working in cooperation with several companies to produce high purity Gd2 (SO4 )3 that could meet the requirements for low energy physics. While these companies provide us with new cleaner samples, we measure them with Ge detectors and an ICP-MS, to determine the U, Th and Ra concentrations. The measurements with Ge detectors are performed in the Canfrancand Boulby laboratories, in Spain and the UK respectively, and the ICP-MS measurements are done in the
Kamioka observatory. The results of the measurementsof the impurities in Gd2 (SO4 )3 is summarized in Table 6. The company B product achieved the required
purity. In FY2018, 1.5 tons of Gd2 (SO4 )3 will be purchased.
Schedule of the SK-Gd
The tank refurbishment work is being carried out for
4 months up to the middle of October, 2018. The purewater supply will be carried out in about two and a halfmonths from the middle of October until the beginningof January 2019 and the neutrino observation and T2K
experiment will be resumed with ultra pure water.
Then, it is planned to start measuring the SRN
by dissolving 10 tons of Gd2 (SO4 )3 (corresponding to
0.01% of Gd concentration) with adjustment of theschedule of T2K experiment in FY2019. Although the
Gd concentration of 0.01% is 1/10 of the final target of
0.1% Gd concentration, the neutron capture efficiency
is already 50%. The SK detector will be run at thisconcentration, then, the remaining Gd2 (SO4 )3 will be
added to achieve our full target loading.

27

Table 6. Physics-based requirements for radioactive impurities in Gd2 (SO4 )3 and the current status of the purification.

Chain
Typical Gd2 (SO4 )3Requirements
Detector
Company A
Company B
Company C

238 U
238 U

50
< 50
Ge ICP-MS−< 0.04< 11 < 0.04
< 10 < 0.04

232 Th226 Ra

5
< 0.5
Ge
−
< 0.2
< 0.2

232 Th100< 0.05
ICP-MS
0.09
0.02
0.06

228 Ra10< 0.05
Ge
−
−
< 0.2

unit: mBq/kg(Gd2 (SO4 )3 )235 U
228 Th235 U
227 Ac /227 Th
100
30
300
< 0.05 < 3
<3
Ge
Ge
Ge
−
−
−
< 0.3 < 0.4< 1.7< 0.3 < 0.3
< 1.2

28

Super-Kamiokande Collaboration
Spokesperson : Masayuki Nakahata (Kamioka Observatory, ICRR, The University of Tokyo)

Institute
ICRR, Univ. of Tokyo
Boston Univ.
California State Univ.
Chonnam Univ.
Duke Univ.
Ecole PolytechniqueFukuoka Institute of Technology
Gifu Univ.
Gwangiu Institute of Scienceand Technology
Imperial College London
INFN Sezione di Bari andUniversite Politecnico di Bari
INFN Sezione di Napoli andUniversite di Napoli
INFN Sezione di Padova andUniversite di Padova
INFN Sezione di Roma andUniversite La Sapienza
KEK
Kobe Univ.
Kyoto Univ.
Miyagi Kyoiku Univ.
Nagoya Univ.
National Centre for Nuclear
Research
Okayama Univ.
Oxford Univ.
Osaka Univ.
Queen Mary University of
LondonSeoul National Univ.
Shizuoka Univ. of Welfare
Stony Brook Univ.
SungKyunKwan Univ.
Tokai Univ.
Tokyo Institute of Technology
Tokyo Univ. of Science
TRIUMF.
Tsinghua University
Univ. of Autonoma Madrid
Univ. of British Columbia
Univ. of California, Irvine
Univ. of Hawaii
Univ. of Liverpool
Univ. of Sheffield
Univ. of Tokyo
Univ. of Tokyo, Kavli IPMU
Univ. of Toront
Univ. of Winnipeg
Yokohama National Univ.
Total(∗ ) Number of collaborators.

Country
Japan
USA
USA
Korea
USA
France
Japan

(∗ )
35
7
2
3
3
5
1

Japan
Korea

1
1

UK
Italy

3
4

Italy

3

Italy

1

Italy

3

Japan
Japan
Japan
Japan
Japan
Poland

11
8
8
1
4
2

Japan
UK
Japan
UK

9
2
1
3

Korea
Japan
USA
Korea
Japan
Japan
Japan
CanadaChina
Spain
Canada
USA
USA
UK
UK
Japan
Japan
CanadaCanada
Japan

1
1
7
1
1
3
3
1
2
2
1
9
2
3
2
2
7
4
1
1
175

29

Members
Staff
Kamioka Observatory
Yoichiro Suzuki, Professor (till Mar. 2014)
Masayuki Nakahata, Professor
Shigetaka Moriyama, Professor
Masato Shiozawa, Professor
Yoshinari Hayato, Assoc. Professor
Yasuhiro Kishimoto, Assoc. Professor
Hiroyuki Sekiya, Assoc. Professor
Shoei Nakayama, Assoc. Professor
Makoto Miura, Research Associate
Yusuke Koshio, Research Associate (till Mar. 2013)
June Kameda, Research Associate
Atsushi Takeda, Research Associate
Kou Abe, Research Associate
Roger Wendell, Research Associate (till Dec. 2015)
Motoyasu Ikeda, Research Associate (from Sep. 2013)
Yasuhiro Nakajima, Research Associate (from Aug.
2016)
Tomonobu Tomura, Project Research Associate (tillMar.2016)
Hidekazu Tanaka, Project Research Associate (fromJun.2012)
Lluis Magro Marti, Project Research Associate (fromMay, 2016)
Yo Kato, Project Research Associate (from Sep.2016)
Christophe Bronner, Project Research Associate (fromMay 2017)
Yosuke Kataoka, Project Research Associate (fromDec. 2017)
Takatomi Yano, Project Research Associate (fromDec. 2017)
Research Center for Cosmic Neutrinos
Takaaki Kajita, Professor
Kimihiro Okumura, Assoc. Professor
Yasuhiro Nishimura, Research Associate (from Sep.
2016)
Yasuhiro Nishimura, Project Research Associate(Apr.2012 - Sep. 2016)
Senior Fellows
Kamioka Observatory
Shigeki Tasaka (Aug. 2015 - Mar. 2017)
Postdoctorial Fellows
Kamioka Observatory
Lluis Magro Marti (Apr. 2011 - Mar. 2013)
Yo Kato (Apr. 2016 - Sep. 2016)
Guillaume Pronost (from Mar. 2016)
Yuuki Nakano (from Apr. 2017)

Graduate students
Six students were awarded doctor degrees and six
students earned master degrees during 2012–2018, supervised by ICRR staff members.

List of Publications
Before Year 2012
[1] “Measurement of a small atmospheric νµ /νe ratio”,
Super-Kamiokande Collaboration (Y. Fukuda etal.), Phys. Lett. B 433, 9 (1998), Cited 938
times.
[2] “Study of the atmospheric neutrino flux in themulti-GeV energy range”, Super-Kamiokande Collaboration (Y. Fukuda et al.), Phys. Lett. B 436,
33 (1998), Cited 882 times.
[3] “Measurements of the solar neutrino flux fromSuper-Kamiokande’s first 300 days”, SuperKamiokande Collaboration (Y. Fukuda et al.),Phys. Rev. Lett. 81, 1158 (1998) [Erratum-ibid.
81, 4279 (1998)], Cited 890 times.
[4] “Search for proton decay via p → e+ π 0 in a large
water Cherenkov detector”, Super-Kamiokande
Collaboration (M. Shiozawa et al.), Phys. Rev.
Lett. 132, 3319 (1998), Cited 190 times.
[5] “Evidence for oscillation of atmospheric neutrinos”,
Super-Kamiokande
Collaboration(Y. Fukuda et al.), Phys. Rev. Lett. 81, 1562
(1998), Cited 5519 times.
[6] “Calibration of Super-Kamiokande using an electron linac”, Super-Kamiokande Collaboration(M. Nakahata et al.), Nucl. Instrum. Meth. A 421,
113 (1999), Cited 161 times.
[7] “Constraints on neutrino oscillation parametersfrom the measurement of day-night solar neutrino
fluxes at Super-Kamiokande”, Super-Kamiokande
Collaboration (Y. Fukuda et al.), Phys. Rev. Lett.
82, 1810 (1999), Cited 507 times.
[8] “Measurement of the solar neutrino energy spectrum using neutrino electron scattering”, SuperKamiokande Collaboration (Y. Fukuda et al.),Phys. Rev. Lett. 82, 2430 (1999), Cited 513
times.
[9] “Measurement of the flux and zenith-angle distribution of upward through-going muons by SuperKamiokande”, Super-Kamiokande Collaboration(Y. Fukuda et al.), Phys. Rev. Lett. 82, 2644
(1999), Cited 827 times.
[10] “Observation of the east-west anisotropy of the atmospheric neutrino flux”, Super-Kamiokande Collaboration (Y. Futagami et al.), Phys. Rev. Lett.
82, 5194 (1999), Cited 111 times.

30[11] “Search for proton decay through p → ν̄ K +in a large water Cherenkov detector”, SuperKamiokande Collaboration (Y. Hayato et al.),Phys. Rev. Lett. 83, 1529 (1999), Cited 176
times.
[12] “Neutrino-induced upward stopping muons inSuper-Kamiokande”, Super-Kamiokande Collaboration (Y. Fukuda et al.), Phys. Lett. B 467, 185(1999), Cited 299 times.
[13] “N-16 as a calibration source for SuperKamiokande”, Super-Kamiokande Collaboration(E. Blaufuss et al.), Nucl. Instrum. Meth. A 458,638 (2001), Cited 38 times.
[14] “Tau neutrinos favored over sterile neutrinos inatmospheric muon-neutrino oscillations”, SuperKamiokande Collaboration (S. Fukuda et al.),Phys. Rev. Lett. 85, 3999 (2000), Cited 935
times.

[22] “Search for anti-nu(e) from the sun at SuperKamiokande I”, Super-Kamiokande Collaboration(Y. Gando et al.), Phys. Rev. Lett. 90, 171302(2003), Cited 67 times.
[23] “The Super-Kamiokande detector”,
SuperKamiokandecollaboration (Y. Fukuda et al.),Nucl. Instrum. Meth. A 501, 418 (2003), Cited
629 times.
[24] “Development of super-high sensitivity radon detector for the Super-Kamiokande detector”, C.
Mitsuda, T. Kajita, K. Miyano, S. Moriyama, M.
Nakahata, Y. Takeuchi, S. Tasaka, Nucl. Instrum.
Meth. A 497, 414 (2003).
[25] “A search for periodic modulations of the solar neutrino flux in Super-Kamiokande I”, SuperKamiokande Collaboration (J. Yoo et al.), Phys.
Rev. D 68, 092002 (2003), Cited 74 times.

[15] “Detection of accelerator produced neutrinos at a
distance of 250-km”, K2K Collaboration (S.H. Ahn
et al.), Phys. Lett. B 511, 178 (2001), Cited 296
times.

[26] “Precise measurement of the solar neutrino
day / night and seasonal variation in SuperKamiokande-1”, Super-Kamiokande Collaboration (M.B. Smy et al.), Phys. Rev. D 69, 011104(2004), Cited 282 times.

[16] “Solar B-8 and hep neutrino measurements from1258 days of Super-Kamiokande data”, SuperKamiokande Collaboration (S. Fukuda et al.),Phys. Rev. Lett. 86, 5651 (2001), Cited 1283
times.

[27] “Limits on the neutrino magnetic moment using
1496 days of Super-Kamiokande-I solar neutrinodata”, Super-Kamiokande Collaboration (D.W.
Liu et al.), Phys. Rev. Lett. 93, 021802 (2004),
Cited 79 times.

[17] “Constraints on neutrino oscillations using 1258
days of Super-kamiokande solar neutrino data”,
Super-Kamiokande Collaboration (S. Fukuda etal.), Phys. Rev. Lett. 86, 5656 (2001), Cited 824
times.

[28] “Search for electron neutrino appearance in a 250
km long baseline experiment”, K2K Collaboration(M.H. Ahn et al.), Phys. Rev. Lett. 93, 051801(2004), Cited 69 times.

[18] “Search for neutrinos from gamma-ray bursts using
Super-Kamiokande”, Super-Kamiokande Collaboration (S. Fukuda et al.), Astrophys. J. 578, 317(2002), Cited 35 times.
[19] “Determination of solar neutrino oscillation parameters using 1496 days of Super-Kamiokande
I data”, Super-Kamiokande Collaboration (S.
Fukuda et al.), Phys. Lett. B 539, 179 (2002),
Cited 995 times.

[29] “Search for dark matter wimps using upward
through-going muons in Super-Kamiokande”,
Super-Kamiokande Collaboration (S. Desai et al.),Phys. Rev. D 70 083523 (2004) [Erratum-ibid. D
70 109901 (2004)], Cited 382 times.
[30] “Evidence for an oscillatory signature in atmospheric neutrino oscillation”, Super-Kamiokande
Collaboration (Y. Ashie et al.), Phys. Rev. Lett.
93: 101801, 2004, Cited 619 times.

[20] “Search for supernova relic neutrinos at SuperKamiokande”, Super-Kamiokande Collaboration(M. Malek et al.) , Phys. Rev. Lett. 90, 061101
2003), Cited 226 times.

[31] “Measurement of single pi0 production in neutral current neutrino interactions with water by
a 1.3-GeV wide band muon neutrino beam”, K2K
Collaboration (S. Nakayama et al.), Phys. Lett. B
61, 255 (2005), Cited 67 times.

[21] “Indications of neutrino oscillation in a 250 kmlong baseline experiment”, K2K Collaboration(M.H. Ahn et al.), Phys. Rev. Lett. 90, 041801(2003), Cited 928 times.

[32] “Evidence for muon neutrino oscillation in an
accelerator-based experiment”, K2K Collaboration (E. Aliu et al.), Phys. Rev. Lett. 94 081802(2005), Cited 897 times.

31[33] “A measurement of atmospheric neutrino oscillation parameters by Super-Kamiokande I”, SuperKamiokande Collaboration (Y. Ashie et al.) etal., Phys. Rev. D71: 112005, 2005, Cited 1060
times.
[34] “Search for nucleon decay via modes favored by supersymmetric grand unification models in SuperKamiokande-I”, Super-Kamiokande Collaboration(K. Kobayashi et al.), Phys. Rev. D 72, 052007(2005), Cited 137 times.
[35] “An improved search for νµ → νe oscillation in along-baseline accelerator experiment”, K2K Collaboration (S. Yamamoto et al.), Phys. Rev. Lett.
96, 181801 (2006), Cited 54 times.
[36] “Measurement of Neutrino Oscillation by the K2K
Experiment”, K2K Collaboration (M. H. Ahn etal.), Phys. Rev. D 74 072003 (2006), Cited 466
times.
[37] “Observation of the Anisotropy of 10 TeV Primary Cosmic Ray Nuclei Flux with the SuperKamiokande-I Detector”, The Super-Kamiokande
Collaboration (G. Guillian et al.), Phys. Rev. D
75, 062003 (2007), Cited 135 times.
[38] “Solar neutrino measurements in SuperKamiokande-I”, The Super-Kamiokande Collaboration (J. Hosaka et al.), Phys. Rev. D 73,
112001 (2006), Cited 551 times.
[39] “Three flavor neutrino oscillation analysis of atmospheric neutrinos in Super-Kamiokande”, The
Super-Kamiokande Collaboration (J. Hosaka etal.), Phys. Rev. D 74, 032002 (2006), Cited 245
times.
[40] “Search for Diffuse Astrophysical Neutrino Flux
Using Ultra-High Energy Upward-Going Muons
in Super-Kamiokande I”, The Super-Kamiokande
Collaboration (M. E. C. Swanson et al.), Astrophys.J. 652 206 (2006), Cited 18 times.
[41] “High energy neutrino astronomy using upwardgoing muons in Super-Kamiokande-I”, The SuperKamiokande Collaboration (K. Abe et al.), Astrophys.J. 652, 198 (2006), Cited 30 times.
[42] “A Measurement of Atmospheric Neutrino Flux
Consistent with Tau Neutrino Appearance”, The
Super-Kamiokande Collaboration (K. Abe et al.),Phys. Rev. Lett. 97, 171801 (2006), Cited 163
times.
[43] “Search for Neutral Q-balls in Super-Kamiokande
II”, The Super-Kamiokande Collaboration(T. Takenaga et al.), Phys. Lett. B 647, 18(2007), Cited 44 times.

[44] “Search for Supernova Neutrino Bursts at SuperKamiokande”, The Super-Kamiokande Collaboration (M. Ikeda et al.), Astrophys J. 669, (2007)
519, Cited 104 times.
[45] “Study of TeV Neutrinos with Upward Showering Muons in Super-Kamiokande”, The SuperKamiokande Collaboration (S. Desai et al.), Astropart. Phys. 29, 42 (2008), Cited 70 times.
[46] “Search for Matter-Dependent Atmospheric Neutrino Oscillations in Super-Kamiokande”, The
Super-Kamiokande Collaboration (K. Abe et al.),Phys. Rev. D 77, 052001 (2008), Cited 23 times.
[47] “Solar neutrino measurements in SuperKamiokande-II”, The Super-Kamiokande Collaboration (J. P. Cravens), Phys. Rev. D 78, 032002(2008), Cited 294 times.
[48] “First Study of Neutron Tagging with a Water Cherenkov Detector”, The Super-Kamiokande
Collaboration (H. Watanabe et al.), Astropart.
Phys. 31, 320 (2009), Cited 74 times.
[49] “Search for Proton Decay via p → e+ π 0 and p →µ + π 0 in a Large Water Cherenkov Detector”, The
Super-Kamiokande Collaboration (H. Nishino etal.), Phys. Rev. Lett. 102, 141801 (2009), Cited
197 times.
[50] “Kinematic reconstruction of atmospheric neutrino events in a large water Cherenkov detector with proton identification”, The SuperKamiokande Collaboration (M. Fechner et al.),Phys. Rev. D 79, 112010 (2009), Cited 23 times.
[51] “Search for Neutrinos from GRB 080319B at
Super-Kamiokande”, The Super-Kamiokande Collaboration (E. Thrane et al.), Astophys. J. 697,
730 (2009), Cited 6 times.
[52] “Search for Astrophysical Neutrino Point Sources
at Super-Kamiokande”, The Super-Kamiokande
Collaboration (E. Thrane et al.), Astrophys. J.
704, 503 (2009), Cited 37 times.
[53] “Atmospheric neutrino oscillation analysis with
sub-leading effects in Super-Kamiokande I, II
and III”, The Super-Kamiokande Collaboration(R. Wendell et al.), Phys. Rev. D 81, 092004(2010), Cited 320 times.
[54] “Solar neutrino results in Super-Kamiokande-III”,
The Super-Kamiokande Collaboration (K. Abe etal.), Phys. Rev. D 83, 052010 (2011), Cited 334
times.
[55] “Study of Non-Standard Neutrino Interactions
with Atmospheric Neutrino Data in SuperKamiokande I and II”, The Super-Kamiokande32

Collaboration ( G. Mitsuka et al.), Phys. Rev. D
84, 113008 (2011), Cited 74 times.
[56] “An Indirect Search for WIMPs in the Sun using 3109.6 days of upward-going muons in SuperKamiokande”, The Super-Kamiokande Collaboration (T. Tanaka et al.), Astrophys. J. 742, 78(2011), Cited 167 times.
[57] “Search for Differences in Oscillation Parameters
for Atmospheric Neutrinos and Antineutrinos at
Super-Kamiokande”, The Super-Kamiokande Collaboration (K. Abe et al.), Phys. Rev. Lett. 107,241801 (2011), Cited 95 times.
[58] “Supernova Relic Neutrino Search at SuperKamiokande”, Super-Kamiokande Collaboration(K. Bays et al.), Phys. Rev. D. 85, 052007 (2012),
Cited 101 times.
[59] “Search for Nucleon Decay into Charged Antilepton plus Meson in Super-Kamiokande I and II”,
The Super-Kamiokande Collaboration (H. Nishino
et al.), Phys. Rev. D. 85, 112001 (2012), Cited 66
times.
[60] “Search for GUT monopoles at SuperKamiokande”, The Super-Kamiokande Collaboration ( K. Ueno et al.), Astropart. Phys. 36,
121 (2012), Cited 22 times.
After Year 2012[61] “Search for proton decay via p → µ + K 0 in Super-

Kamiokande I, II, and III”, Super-Kamiokande
Collaboration (C. Regis et al.), Phys. Rev. D. 86,
012006 (2012), Cited 51 times.

[67] “Search for Nucleon Decay via n → νπ 0 and p →νπ 0 in Super-Kamiokande”, Super-Kamiokande
Collaboration (K. Abe et al.), Phys. Rev. Lett.
113, 121802 (2014), Cited 41 times.
[68] “Search for Trilepton Nucleon Decay via p →e + νν and p → µ + νν in the Super-Kamiokande
Experiment”, Super-Kamiokande Collaboration(V. Takhistov et al.), Phys. Rev. Lett. 113, 101801(2014), Cited 14 times.
[69] “Search for proton decay via p → ν K + using using 260 kiloton · year data of Super-Kamiokande”,
Super-Kamiokande Collaboration (K. Abe et al.),Phys. Rev. D 90, 072005 (2014), Cited 88 times.
[70] “Supernova Relic Neutrino Search with Neutron Tagging at Super-Kamiokande-IV”, SuperKamiokande Collaboration (H. Zhang et al.), Astropart. Phys. 60, 41 (2015), Cited 45 times.
[71] “Test of Lorentz invariance with atmospheric neutrinos”, Super-Kamiokande Collaboration (K. Abe
et al.), Phys. Rev. D 91, 052003 (2015), Cited 33
times.
[72] “Limits on sterile neutrino mixing using atmospheric neutrinos in Super-Kamiokande”, SuperKamiokande Collaboration (K. Abe et al.), Phys.
Rev. D 91, 052019 (2015), Cited 76 times.
[73] “Search for Neutrinos from Annihilation of Captured Low-Mass Dark Matter Particles in the Sun
by Super-Kamiokande”, Super-Kamiokande Collaboration (K. Choi et al.), Phys. Rev. Lett. 114,141301 (2015), Cited 172 times.

[62] “Search for Nucleon Decay into Charged Antilepton plus Meson in Super-Kamiokande I and II(H. Nishino et al.)”, Phys. Rev. D 85, 112001(2012), Cited 123 times.

[74] “Search for n - nbar oscillation in SuperKamiokande”, Super-Kamiokande Collaboration(K. Abe et al.), Phys. Rev. D 91, 072006 (2015),
Cited 76 times.

[63] “Evidence for the Appearance of Atmospheric
Tau Neutrinos in Super-Kamiokande”, SuperKamiokande Collaboration (K. Abe et al.), Phys.
Rev. Lett. 110, 181802 (2013), Cited 101 times.

[75] “Search for dinucleon decay into pions at SuperKamiokande”, Super-Kamiokande Collaboration(J. Gustafson et al.), Phys. Rev. D 91, 072009(2015), Cited 18 times.

[64] “Calibration of the Super-Kamiokande Detector”,
Super-Kamiokande Collaboration (K. Abe et al.),Nucl. Instrum. Meth. A 737, 253 (2014), Cited
75 times.

[76] “Search for Nucleon and DiNucleon Decays withan Invisible Particle and a Charged Lepton
in the Final State at the Super-Kamiokande
Experiment”, Super-Kamiokande Collaboration(V. Takhistov et al.), Phys. Rev. Lett. 115, 121803(2015), Cited 11 times.

[65] “First Indication of Terrestrial Matter Effects on
Solar Neutrino Oscillation”, Super-Kamiokande
Collaboration (A. Renshaw et al.), Phys. Rev.
Lett. 112, 091805 (2014), Cited 76 times.
[66] “Search for Dinucleon Decay into Kaons in SuperKamiokande”, Super-Kamiokande Collaboration(M. Litos et al.), Phys. Rev. Lett. 112, 131803(2014), Cited 14 times.

[77] “First measurement of radioactive isotope production through cosmic-ray muon spallation in SuperKamiokande IV”, Super-Kamiokande Collaboration (Y. Zhang et al.), Phys. Rev. D 93, 012004(2016), Cited 15 times.

33[78] “Real-Time Supernova Neutrino Burst Monitor at
Super-Kamiokande”, Super-Kamiokande Collaboration (K. Abe et al.), Astropart. Phys. 81, 39(2016), Cited 12 times.
[79] “Solar Neutrino Measurements in SuperKamiokande-IV”, Super-Kamiokande Collaboration (K. Abe et al.), Phys. Rev. D 94, 052010(2016), Cited 50 times.
[80] “Measurements of the atmospheric neutrino flux
by Super-Kamiokande: Energy spectra, geomagnetic effects, and solar modulation”, SuperKamiokande Collaboration (E. Richard et al.),Phys. Rev. D 94, 052001 (2016), Cited 30 times.
[81] “Search for Neutrinos in Super-Kamiokande associated with Gravitational Wave Events GW150914
and GW151226”, Super-Kamiokande Collaboration (K. Abe et al.), Astrophys. J. Lett. 830, 1(2016), Cited 18 times.
[82] “Search for proton decay via p → e + π 0 and
p → µ + π 0 in 0.31Megaton·years exposure of theSuper-Kamiokande water Cherenkov detector”,
Super-Kamiokande Collaboration (K. Abe et al.),Phys. Rev. D 95, 012004 (2017), Cited 51 times.
[83] “Search for nucleon decay into charged antilepton plus meson in 0.316 megaton·years exposureof the Super-Kamiokande water Cherenkov detector”, Super-Kamiokande Collaboration (K. Abe etal.), Phys. Rev. D 96, 012003 (2017), Cited 9times.
[84] “Search for an Excess of Events in the SuperKamiokande Detector in the Directions of the Astrophysical Neutrinos Reported by the IceCube
Collaboration”, Super-Kamiokande Collaboration(K. Abe et al.), Astrophys. J. 850, 166 (2017),
Cited 1 times.
[85] “Atmospheric neutrino oscillation analysis withexternal constraints in Super-Kamiokande I-IV”,
Super-Kamiokande Collaboration (K. Abe et al.),Phys. Rev. D 97, 072001 (2018), Cited 15 times.
[86] “Search for Neutrinos in Super-Kamiokande associated with the GW170817 neutron-star merger”,
Super-Kamiokande Collaboration (K. Abe et al.),Astrophys. J. Lett. 857, L4 (2018), Cited 2
times.
[87] “Search for Boosted Dark Matter Interacting
With Electrons in Super-Kamiokande”, SuperKamiokande Collaboration (C. Kachulis et al.),Phys. Rev. Lett. 120, 221301 (2018), Cited 7
times.

34

T2K EXPERIMENT
Summary from 2012 to 2018

We first summarize the results and achievements obtained during the review period. The most significant
ones will be described in more details in the following
sections.
• Using the electron candidate events from the first
four years of data taking, T2K observed the appearance of electron neutrinos from the oscillationof muon neutrinos with 7.3σ significance [3]. In
combination with the measurement of θ13 from thedisappearance of ν̄e by reactor experiments, thosedata also allowed a first evaluation of the allowed
regions for δCP in T2K.
• After that, T2K started analyzing simultaneously
all its samples to measure more precisely δCP and
the atmospheric parameters θ23 and ∆m232 whileproperly accounting for the correlations between
the measurements. We first realized a joint analysis of the electron and muon candidate events [4],then a joint analysis of the ν -mode and ν̄ -mode
data [5] and finally added a new appearance sample, enriched in resonant interactions, to the analysis [6].
• T2K moved to a new, more precise reconstruction algorithm for the far detector for the resultspresented during the summer 2017 1 . The additional precision of this new algorithm allowed
both to use a new, larger fiducial volume, and to
add a new likelihood cut to reduce the main background in disappearance samples. As a result, thenumber of events in the appearance samples increased by 12% without decreasing purity, and themain background for disappearance samples was
reduced by 50%.
• Using the results of the measurement of θ13 byreactor experiments, T2K presented more andmore precise results on δCP . Initially only able
to exclude some values of δCP at 90% CL, T2K
could later report the exclusion of the conservation of CP symmetry in neutrino oscillations
at 90% CL [6] and then with 2σ significance 2 .
The precision of the measurement of the atmospheric parameters also increased with the different improvements and additional data, the most
*1 M. Hartz (on behalf of the T2K collaboration), T2K Neutrino Oscillation Results with Data up to 2017 Summer, KEK special seminar
Aug. 4th 2017
*2 ibid.

recent results 3 yielding sin2 (θ23 ) = 0.536+0.031−0.046 and2−3
2
4|∆m32 | = (2.434 ± 0.064) × 10 eV /c .
• Using data collected in ν̄ -mode, T2K could also
search for CP violation without using the resultsof the reactor experiments by comparing directly
the oscillations of neutrinos and anti-neutrinos.
The favored values of δCP in this case are similar tothe ones obtained with constraints for the reactor
experiments, although with lower statistical significance. The anti-neutrino data were also used
to compare the disappearance of νµ and ν̄µ [7, 8]
to search for new physics, and to look for the appearance of ν̄e from the oscillations of ν̄µ .
• During this period, many cross-section measurements, important for the modelization of neutrino
interactions used in oscillation analyses, were doneusing the near detectors, both on and off-axis. In
particular, T2K reported the first measurementof νe cross-section at the GeV scale differential
as a function of electron kinematics [9]. T2K alsopresented cross-section measurements for νµ , bothinclusive and exclusive, for interactions on scintillator (CH) [10, 12, 14, 16, 17], iron [11, 15], andwater [13, 18, 19]. T2K presented a first antineutrino cross-section measurement [20], and using this time the far detector, a measurement of
the neutral current cross-section on oxygen [21].
• T2K also performed searches for physics beyond
the standard model: search for oscillations due tosterile neutrinos [22] and violation of Lorentz invariance [23] in the near detectors, as well as measurement of the neutrino time of flight between
J-PARC and Super-K [24]. No evidence for new
physics was found.

Introduction

The Tokai to Kamioka (T2K) experiment [1] is along baseline neutrino oscillation experiment: a manmade beam of neutrinos is used to do precise studiesof the oscillations of neutrinos. Accelerated protons areused to produce the neutrino beam in the J-PARC center in the Ibaraki prefecture, which then travel 295 km
to reach the Super-Kamiokande (Super-K) detector in
the Gifu prefecture where neutrinos can be detected
after oscillations. A complex of near detectors located
280 meters away from the proton target is used to monitor the neutrino beam, and constrain systematic uncertainties on the neutrino fluxes and interactions. T2K*3 M. Wascko (on behalf of the T2K collaboration), T2K Status, Results, and Plans, Presentation at the Neutrino 2018 conference

35

was the first long baseline experiment to use the off-axis
beam technique 4 : the beam is not aimed directly at
Super-K, but in a direction making a 2.5◦ angle with thefar detector direction. This gives increased sensitivity
to neutrino oscillations while reducing the backgrounds
by producing a narrow band neutrino beam centered on
the energy corresponding to the first maximum of theνµ → νe oscillation probability.
The initial physics goal of the experiment is to observe this νµ → νe oscillation, by detecting electron neutrinos in a beam of neutrinos produced in the muon flavor. This observation would confirm the paradigm ofneutrino oscillations by observing for the first time theappearance of neutrinos of a certain flavor through oscillation, and allow to measure the last unknown mixing angle at the time of the start of the experiment,
θ13 . At the same time, T2K can produce precise measurements of the atmospheric parameters θ23 and ∆m232
through precise study of the pattern of disappearanceof the muon neutrinos.
After the appearance of electron neutrinos νµ → νe
has been observed, the experiment can use its ability
to produce a beam of either neutrinos or anti-neutrinos
to compare the oscillations of neutrinos and their antiparticles. This allows to study the main remaining
open questions in neutrino oscillations (CP symmetry
and ordering of the neutrino mass states) by looking
at the differences between the oscillations νµ → νe andν̄µ → ν̄e . The experiment can also perform searches forphysics beyond the standard model (sterile neutrinos,
CPT violation). Finally, the near detectors allow to doa wide range of neutrino and anti-neutrino cross-section
measurements.
Experimental apparatus
In J-PARC, protons are accelerated to 30 GeV by
a series of three accelerators, and hit a 1.9 interactionlength graphite target. The collisions produce hadrons,in particular charged pions and kaons, which are focused by three electromagnetic horns. The hadrons
then go through a 96m long decay tunnel where they
decay in flight into neutrinos. A beam dump at theend of the decay tunnel stops the remaining hadrons,
while high energy muons (5 GeV/c or higher) can passthrough this beam dump and are measured to provide a first, indirect monitoring of the neutrino beam.
The horns can be used either with a positive current(ν -mode), in which case the beam is mainly made ofνµ , or with a negative current (ν̄ -mode) which gives amainly ν̄µ beam.
The near detectors are separated into two groups.
On the axis of the beam, the INGRID detector, made offourteen identical modules is used to monitor the beam
direction and rate stabilities. Each module is made*4 D. Beavis, A. Carroll, I. Chiang, et al.,Long Baseline Neutrino Oscillation Experiment at the AGS (Proposal E889),1995. Physics
Design Report, BNL 52459.

of a succession of iron plates to provide large target
mass (∼7.1 tons per module) and scintillator planes for
detection. Using the number of events reconstructedin each module, the beam direction can be measured
daily with better than 0.4 mrad accuracy. Located in
the direction of Super-K, the off-axis detector ND280 is
made of several detectors located inside a 0.2T magnet.
The higher precision of those off-axis detectors allow
to do more detailed measurements of the unoscillated
neutrino beam. In neutrino oscillation analyses, theND280 is used to provide information on the νµ andν̄µ unoscillated spectra directed at SK, constrain thedominant backgrounds, and constrain the combinationof flux and interaction cross sections.
The far detector, Super-K, is a 50 kton water
Cherenkov detector, shielded from atmospheric muonsby 1000 m of rock, or 2700 meters-water-equivalent(m.w.e.) mean overburden. To select events corresponding to the T2K beam, Super-K is synchronized
via GPS to the J-PARC beamline. Hit information
within ±500µ s from the beam arrival timing are used
for T2K data analysis. Events where only one ring
was reconstructed (corresponding to one charged particle above Cherenkov threshold) are used in oscillation
analysis. Those events are separated into muon-like andelectron-like events based on the light pattern of thisring, and additional selection cuts are applied to produce samples enriched in charged-current quasi-elastic
interactions for which the energy of the neutrino canbe better reconstructed.

Experiment status

T2K started collecting physics data in January 2010,and has now completed its 9th run, accumulating a total of 1.51 × 1021 protons on target (POT) in ν -modeand 1.65 × 1021 POT in ν̄ -mode. The experiment ran
for the first time using anti-neutrino beam in 2014, andhas been alternating since between runs in ν -mode andin ν̄ -mode. The details of the data used in the far detector analysis (slightly lower due to data quality cuts)
can be found in Tab. 1. Over this period, the event
rates and the beam direction were found to be consistent with the expectations and stable by the measurements of the muon monitor and the on-axis neardetector. In particular, the beam direction remained
stable well within the ±1mrad target.
The intensity of the proton beam has been increasing progressively over time, from a few dozens kW in
the first runs to stable operation at 485 kW at the endof Run 9. Operation at more than 500 kW was also
successfully demonstrated, and the beam intensity is
expected to continue increasing in the future. This increase of the beam intensity coupled with improved reliability of the accelerator has allowed to progressively
collect larger amounts of data over shorter periods oftime as can be seen on Fig. 1.

36
Table 1. T2K data taking periods and integrated numbers of protons
on target (POT) used in the far detector analysis.

Accumulated POT

Run
Period
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Run 8
Run 9
Total

10

Dates
Jan.2010 - Jun.2010
Nov.2010 - Mar.2011
Mar.2012 - Jun.2012
Oct.2012 - May.2013
May.2014 - Jun.2014
Oct.2014 - Jun.2015
Feb.2016 - May.2016
Oct.2016 - Apr.2017
Oct.2017 - May.2018
Jan.2010 - May.2018

×1020 POT
ν
ν
0.32
–
1.11
–
1.60
–
3.60
–
0.24
0.51
0.19
3.55
0.48
3.50
7.17
–
0.20
8.79
14.94 16.35

× 10202017/188

2016/176
2015/16
2012/132014/15422012
2010/11

2010

0
Nov

Jan

Mar

May

2014

Jul

Month
Fig. 1. Protons on target accumulated as a function of time for thenine T2K data taking periods.

Status at the beginning of the review period

At the beginning of the review period in 2012, T2K
was fully built and had been collecting data for 3 years(run 1-3), collecting a total of 3.01 × 1020 POT inν -mode. The experiment had been able to recover in
9 months after the big earthquake on March 11, 2011
and the intensity of the beam had reached 200 kW.
An analysis of the electron-like events collected during this period produced the first evidence of νµ → νeappearance driven by nonzero θ13 , with a statistical significance of 3.2σ 5 . Using the muon-like events recorded
during runs 1 and 2 (corresponding to 1.43 × 1020 POT
in ν -mode), T2K had presented the first νµ disappearance analysis and measurements of θ23 and |∆m232 |by the off-axis beam technique [2]. Assuming twoflavor νµ → ντ oscillations, the best fit values obtained
were sin2 (2θ23 ) = 0.98 and |∆m232 | = 2.65 × 10−3 eV2 /c4 .
The boundary of the 90% confidence region included*5 T. Nakaya (on behalf of the T2K collaboration), “New Resultsfrom T2K”, presentation at the XXV International Conference on
Neutrino Physics and Astrophysics (NEUTRINO 2012).

the points (sin2 (2θ23 ),|∆m232 |) = (1.0, 3.1 × 10−3 eV2 ),(1.0, 2.2 × 10−3 eV2 ),and (0.84, 2.65 × 10−3 eV2 ), whichcorrespond to the largest of (|∆m232 |), the smallest of(|∆m232 |),and the smallest of (sin2 (2θ23 )) points, respectively.

Neutrino oscillation results

In this section, we describe the general method bywhich neutrino oscillations are studied in T2K, thenpresent the main results obtained during the review
period, and finally look at perspectives for the future.
Analysis method The data observed at the far detector are compared to the predictions of the threeflavor oscillation model to make statistical inferences.
To be able to make those predictions, a model of the experiment is constructed using a simulation of the flux ofneutrinos reaching the detectors and a model describing
the interactions of neutrinos. The predictions from thismodel are compared to the data observed in the neardetectors to tune the predictions for the far detector by
constraining the model parameters.
The fluxes of the different flavors of neutrinos reaching the detectors are predicted by a series of simulations. The flux and properties of the proton beamreaching the target are measured by the proton beamline monitors, and used as inputs for the simulations.
Interactions of the protons in the graphite target andproduction of secondary hadrons are then simulated using the FLUKA package 6 . Measurements from hadronproduction experiments, in particular NA61/SHINE 7 ,are used to tune this part of the simulation and theout-of-target interactions. The propagation and decay
in flight of the hadrons in the decay tunnel are thensimulated using the GEANT3 and GCALOR 8 packages. Interactions of ν and ν̄ are modeled using the
NEUT Monte Carlo event generator 9 .
A binned likelihood fit of the events selected as
charged current interactions in the near detectors isused to constrain the flux and neutrino interaction uncertainties. Those events are binned as a function of
the momentum and angle of the particle reconstructed
as a µ − or a µ + with respect to the axis of the detector,
and arranged in different samples based on the topology of the event observed in the detector. The result of
this fit provides the initial values and uncertainties ofthe flux and interaction model parameters used in thefar detector analysis to measure parameters describing
neutrino oscillations.
*6 T. Bhlen et al., Nucl. Data Sheets 120, 211 (2014)*7 N. Abgrall et al. (NA61/SHINE Collaboration), Eur. Phys. J. C
76, 84 (2016).
*8 C. Zeitnitz and T. A. Gabriel, Proceedings of International Conference on Calorimetry in High Energy Physics (World Scientific,
Corpus Christi, Texas, 1992), ISBN 9789810213039, pp. 394–404.
*9 Y. Hayato, Acta Phys. Pol. B 40, 2477 (2009)

37
Data
Best fit
Background component

10

Angle (degrees)

5

180
1

150

Data
Best fit

120

0.8

90

0.6

60

0.4

30

0.2

0

5

0

0
500
1000
1500
Momentum (MeV/c)

Fig. 2. Distribution of the electron momentum and angle of the νecandidate events from T2K run 1-4 with MC predictions at thebest fit.

Observation of electron neutrino appearance
T2K reported the first observation of the explicit appearance of a different neutrino flavor from neutrinos ofanother flavor through neutrino oscillations in 2014 [3].
The analysis used the electron-like events observed at
the far detector during the runs 1 to 4 to exclude theθ13 = 0 hypothesis. After all cuts, the total number ofcandidate νe events selected in the data was 28, which
is significantly larger than the 4.92 ± 0.55 expected
events for θ13 = 0.
The neutrino oscillation parameters were evaluatedusing a binned extended maximum-likelihood fit. The
likelihood consisted of four components: a normalization term (calculated from a Poisson distribution using the mean value from the predicted number of MC
events), a term for the spectrum shape (momentum
and angle of the particle reconstructed as an electronin the main analysis, reconstructed neutrino energy inthe alternative analysis), a systematics term, and a constraint term (present only when results of external measurements are used to constrain the values of some ofthe oscillation parameters). In the fit, the likelihood
was integrated over the nuisance parameters to obtaina marginalized likelihood for the parameters of interest.
The following oscillation parameters were fixed inthis analysis: sin2 (θ12 )=0.306, ∆m221 = 7.6 × 10−5 eV
2 /c4 10 , sin2 (θ ) = 0.5, ∆m2 = 2.4 × 10−3 eV2 /c4 (from23
32
T2K run 1-3 νµ disappearance result [25]), and δCP = 0.
For the normal (inverted) hierarchy case, the bestfit value obtained with a 68% C.L. is sin2 (2θ13 ) =+0.0450.140+0.038−0.032 (0.170−0.037 ). Fig. 2 shows the best-fit result, and the 28 observed νe candidate events. The
alternative analysis using reconstructed energy and a
profile likelihood method produced consistent best-fit*10 G. L. Fogli, E. Lisi, A. Marrone, A. Palazzo, and A. M. Rotunno,
Phys. Rev. D 84, 053007 (2011)

values and nearly identical confidence regions. The significance for a nonzero θ13 was calculated to be 7.3σ ,using the difference of log likelihood values between thebest-fit θ13 value and θ13 = 0. An alternative methodof calculating the significance, by generating a large
number of toy MC experiments assuming θ13 = 0, also
returned a value of 7.3σ . These significance were calculated using fixed values for θ23 and δCP . For any valuesof these parameters consistent with their uncertainties,
the significance remained above 7σ .
Comparison of the disappearance of muon neutrinos and anti-neutrinos In the standard threeflavor oscillation picture, the survival probability invacuum is identical for muon neutrinos and antineutrinos. For the neutrino energies used by T2K, matter
effects do not significantly affect this symmetry. Any
difference in the oscillations could therefore be interpreted as possible CPT violation and/or evidence ofnonstandard interactions 11 12 . To look for such effects,
we measured separately the values of the parametersdescribing muon neutrino and anti-neutrino disappearance using the muon-like events collected in ν -modeand ν̄ -mode.
In practice, the analysis allowed the antineutrino oscillation parameters for ν µ disappearance to vary independently from those describing neutrino oscillations,
i.e., θ23 6= θ 23 and ∆m232 6= ∆m232 , where the barred parameters govern antineutrino oscillations. All other parameters were assumed to be the same for neutrinos andantineutrinos since the muon like samples cannot constrain them. δCP was fixed to 0 as it has a negligible
impact on the disappearance spectra at T2K, and θ13
value was constrained using the measurements by thereactor experiments. Finally, the data from ν -modeand ν̄ -mode are analyzed together in a simultaneous
fit, as the main uncertainty on the background to measure θ 23 and ∆m232 in ν̄ -mode is the size of the neutrino
background, which depends on the values of θ23 and∆m232 .
Using data corresponding to an exposure of 7.48 ×1020 and 7.47 × 1020 protons on target (POT) for neutrinos and antineutrinos respectively, the best fit values
obtained for the parameters describing neutrino oscillations assuming normal hierarchy are sin2 (θ23 ) = 0.51
and ∆m232 = 2.53 × 10−3 eV2 /c4 with 68% confidence intervals of 0.44 – 0.59 and 2.40 – 2.68 (×10−3 eV2 /c4 )respectively. For the antineutrino parameters, thebest fit values are sin2 (θ 23 ) = 0.42 and ∆m232 = 2.55 ×
10−3 eV2 /c4 with 68% confidence intervals of 0.35 – 0.67
and 2.28 – 2.88 (×10−3 eV2 /c4 ) respectively. In Fig. 3,
the 90% confidence regions obtained for the parameters describing the disappearance of muon neutrinos
and antineutrinos are compared. The confidence regions obtained in the two cases are compatible, and no*11 A. Kostelecky and M. Mewes, Phys. Rev. D 85, 096005 (2012)*12 O. G. Miranda and H. Nunokawa, New J. Phys. 17, 095002 (2015)

38−3

5.5 ×102∆ m32 or ∆ m232 (eV2)

5
4.5
4

MINOS ν best fit
Super-K ν best fit
T2K ν best fit
T2K ν best fit

MINOS ν 90% CL
Super-K ν 90% CL
T2K ν 90% CL
T2K ν 90% CL

T2K ν best fit, sin2(θ23) > 0.5

3.5
3
2.5
2
1.5
1
0.2

0.3

0.4

0.5
2

0.6

0.7

0.8

2

sin θ23 or sin θ23
Fig. 3. 90% confidence regions for sin2 (θ23 ) and ∆m232 in ν mode
(corresponding to 7.482×1020 POT) and ν -mode (corresponding
to 7.471×1020 POT). Normal hierarchy is assumed. 90% confidence regions obtained by Super-K and MINOS for ν are also
shown. The best fit in the case sin2 (θ23 ) > 0.5 is also displayedfor comparison with the MINOS result.

evidence for new physics was found in this comparison.
This measurement of θ 23 and ∆m232 is also consistent
with the results obtained by the Super-K 13 and MINOS 14 collaborations.
Exclusion of the conservation of CP symmetry
in neutrino oscillations, first at 90% CL and then2σ In 2017, T2K published results where for the firsttime the conservation of CP symmetry in neutrino oscillations was excluded with some noticeable statistical significance. First, a joint analysis of the one ring
electron-like and muon like events collected in ν -modeand ν̄ -mode over the runs 1 to 7 (corresponding to
7.482×1020 POT in ν -mode and 7.471×1020 POT inν̄ -mode) found a one dimensional 90% CL range forδCP of (-3.13, -0.39) [5], which did not include the CPconserving values of 0 and π .
An improved result was then obtained by adding
an additional sample to the far detector analysis whilestill using the data collected from run 1 to 7 [6]. Until
that point, for the appearance channel, only the singlering e-like interactions without additional activity inthe detector were used for the oscillation analysis. The
new analysis included an additional sample enriched in
νe interactions in which the e-like ring is accompanied
by a delayed Michel electron due to the decay chain
π + → µ + → e+ of π + ’s produced in the neutrino interactions. This sample was only used in the ν mode
and increased the statistics of the νe sample in Super-K*13 K. Abe et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett.
107, 241801 (2011)*14 P. Adamson et al. (MINOS Collaboration), Phys. Rev. Lett. 108,
191801 (2012)by roughly 10%. With this additional sample, the one
dimensional 90% CL range for δCP reduced to (-2.95;-0.44).
In the summer 2017, we presented new results 15
with the additional data collected during run 8 (for a total exposure of 14.7×1020 POT in ν -mode and 7.6×1020
POT in ν̄ -mode) and a number of improvements in theanalysis. First, the model describing neutrino interactions was improved both in terms of modelization of
the interactions, and of parameterization of their uncertainties. This was in particular the case for the interaction of a neutrino with a pair of correlated nucleons which can lead to significant mis-reconstruction of
the neutrino energy in the disappearance analysis. The
model was also improved for the nuclear effects affecting
charged current quasi-elastic interactions, which is themain interaction type in T2K, and for pion production.
The second significant change is a major update of the
far detector analysis, with the use of a new event reconstruction algorithm, giving better particle identification
capabilities, and improved vertex and momentum resolution. Taking advantage of the increased precisionof this new algorithm, the selection cuts for the fiducial volume were optimized to increase sensitivity, anda new likelihood cut was introduced for the disappearance samples to reduce the background coming fromneutral current single pion interactions. The expected
number of events in the different samples for the newand previous analyses can be seen in Tab. 2. The number of expected events for the e-like samples increased
by about 12%, providing better sensitivity to δCP . In
the case of the µ -like samples, the number of expected
events slightly decreased, but the purity increased: thenumber of events for the signal CCQE interaction decreased by 15%, while the number for the single pion
neutral current background decreased by 50%, allowing
more precise measurements of the atmospheric parameters. With those improvements and additional data,
we found a 2σ CL interval of (-2.98, -0.60) for δCP : theconservation of CP symmetry in neutrino oscillation
was excluded with 2σ significance.
At the beginning of the summer 2018, we presented
updated results 16 including data collected during thefirst half of the run 9. The 2σ CL interval for δCP wasfound to be of similar size, and slightly shifted: (-2.914,-0.642), confirming the 2σ exclusion of the conservationof CP symmetry obtained the summer before. The results of the data fit for δCP and the atmospheric parameters can be seen on Fig. 4 and 5 respectively.
Proposal for an extended run of T2K to 20 × 1021
POT (T2K-II) Following the discovery of νe appearance and indications that CP symmetry might be*15 M. Hartz (on behalf of the T2K collaboration), T2K Neutrino Oscillation Results with Data up to 2017 Summer, KEK special seminar
Aug. 4th 2017*16 M. Wascko (on behalf of the T2K collaboration), T2K Status, Results, and Plans, Presentation at the Neutrino 2018 conferenceTable 2. Comparison of the expected number of events in each sample for the run 1-8 exposure, between the new analysis introduced
for the summer 2017 results and the previous one.

Sampleν mode 1R e-likeν mode 1R µ -likeν mode CC1π +ν̄ mode 1R e-likeν̄ mode 1R µ -like

New analysis
69.5
261.6
6.9
7.6
62.0

Previous analysis
56.5
268.7
5.6
6.1
65.4

∆m232 (NH),∆m213 (IH) (eV2c-4)

39
2.8

T2K Run 1-9c Preliminary

×10−3Best fit

2.7
2.6
2.5
2.4
2.3
2.2
0.3

0.35

0.4

0.45

Inverted

15
10
5
−3

−2

−1

0

1

2

3

δCP
Fig. 4. Results of the fit of T2K run 1-9c data for δCP with 2σ confidence intervals obtained using the Feldman-Cousins method.
The value of θ13 was constrained using measurements by reactor
experiments.

maximally violated in neutrino oscillations, we started
studying the future potential of the experiment for the
study of CP violation, both with the approved POT
and possible extensions of the data taking. Those studies were done with the 2016 version of the analysis,
which does not include the improvements of the far detector analysis described in the previous paragraph. We
assumed 50% effective statistical improvements (coming from beamline and analysis improvements such as
the ones described previously for the far detector) andthat the systematic uncertainties could be reduced by
1/3 from their value at the time to reach 4%. We thenfound that for an exposure of 20 × 1021 POT, the sensitivity to CP violation reached 3σ or higher for the theoscillation parameter region favored by the T2K results
at the time (currently favored region is narrower andincluded in this one): δCP = −π /2, 0.43< sin2 (θ23 ) <0.6
and the mass hierarchy being normal (Fig. 6). In addition the atmospheric parameters, θ23 and ∆m232 , can bemeasured with a precision of 1.7◦ and 1%, respectively.
As a result we proposed an extension of the T2K
run to 20 × 1021 POT, T2K phase 2 [26]. A progressive
increase of the intensity of the J-PARC beam, from the∆ χ2 to exclude sinδCP=0-2∆ln(L)

20

0

0.55

0.6

0.65

0.7

Fig. 5. 68% and 90% CL regions obtained for the atmosphericparameters using T2K run 1-9c data. The value of θ13 wasconstrained using measurements by reactor experiments.

Normal250.5

sin2(θ23)

T2K Run1-9c Preliminary
30

Normal - 68CL
Normal - 90CL
Inverted - 68CL
Inverted - 90CL

20
20x1021 POT w/ eff. stat. improvements & 2016 sys. errs.
7.8x1021 POT w/ 2016 sys. errs.
True sin2θ23=0.43
True sin2θ23=0.50
True sin2θ23=0.60

15
10

3σ C.L.
99% C.L.

5
90% C.L.

0
−200

−100

0

100

200

True δCP(°)
Fig. 6. Sensitivity to CP violation as function of true δCP , for theapproved T2K statistics and a proposed extension of the datataking with analysis improvements. The mass hierarchy is assumed to be normal, and to be known. Sensitivities for different
true values of sin2 (θ23 ) (0.43, 0.5, 0.63) are compared.

current ∼500kW to 1.3MW would allow to collect thisamount of data by 2026, when the next generation experiments DUNE and Hyper-Kamiokande are expected
to start. An upgrade the off-axis near detector ND280
would allow to achieve the reduction of systematic uncertainties needed. A working group was established
for this upgrade with members from Europe (including
CERN neutrino group), Japan and the US, and produced a reference design for this detector with the goalof installing it in 2021. The proposal for this upgrade
was submitted to the CERN SPS committee at the beginning of 2018 17 and several beam tests are scheduledat CERN during the summer 2018, while a Technical
Design Report is expected by the end of the year. In
Japan, the proposal for an extended T2K run received*17 A. Blondel et al., The T2K-ND280 upgrade proposal, CERN-SPSC2018-001

40stage 1 status at the summer 2016 J-PARC PAC.

Summary

During the review period, T2K has been taking datawith an increasingly intense neutrino beam, reachingstable operation at 485 kW and collecting a total of
1.51 × 1021 POT in ν -mode and 1.65 × 1021 POT inν̄ -mode. Using those data, T2K presented the first observation of the appearance of a neutrino flavor throughoscillations, the first exclusion of the conservation of CP
symmetry in neutrino oscillations with 2σ significanceand precise measurements of the atmospheric parameters θ23 and ∆m232 . The data recorded in the near detectors were also used to perform cross-section measurements on different target materials, as well as searches
for new physics. An extension of the T2K run, with thepotential to exclude the conservation of CP symmetry
with 3σ significance before the start of the next generation experiments DUNE and Hyper-Kamiokande, wasproposed. Work has started on an upgrade of the neardetector that would allow to reduce the systematic uncertainties to the level needed to reach this goal.
The Super-K detector is currently being refurbished
to allow the Super-Kamiokande experiment to move to
its next phase, SK-Gd, where gadolinium sulfate will
be dissolved in the ultra pure water to increase theability of the experiment to detect neutrons. Studies
have been performed to evaluate the impact of this upgrade on the T2K operations, and confirmed that theexperiment can continue after the upgrade. The good
performance of the event reconstruction in the presenceof gadolinium was confirmed by simulation based studies, and the change in event selection efficiency will besmall. It is also anticipated that the increase in neutron
detection efficiency will bring better understanding of
the interactions of neutrinos and anti-neutrinos.

T2K collaboration

Spokesperson: Tsuyoshi Nakaya (Kyoto University)
Host institutes of the T2K experiment are the HighEnergy Accelerator Research Organization(KEK) and
ICRR. One of the members of the Executive Committeeof the experiment is from ICRR: first Masato Shiozawa
until 2017, and Yoshinari Hayato since.

Institute
ICRR
KEK
Kavli IPMU (WPI), University ofTokyo
Kobe University
Kyoto University
Miyagi University of Education
Okayama University
Osaka City University
Tokyo Institute of Technology
Tokyo Metropolitan University
Tokyo University of Science
University of Tokyo
Yokohama National University
TRIUMF
University of British Columbia
University of Regina

Country
Japan
Japan
Japan

(*)
25
26
4

Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
CanadaCanadaCanada

3
14
1
4
6
3
2
2
8
4
19
10
3

41

Institute
University of Toronto
University of Victoria
University of Winnipeg
York University
CEA/DAPNIA Saclay
LLR Ecole polytechnique (IN2P3)
LPNHE-Paris
RWTH Aachen University
INFN Bari
INFN Roma
Napoli University and INFN
Padova University and INFNIFJ PAN, Cracow
NCBJ, Warsaw
Technical University, Warsaw
University of Silesia, Katowice
Warsaw University
Wroclaw University
INR
IFAE, Barcelona
IFIC, Valencia
UAM, Madrid
Bern
ETHZ
University of Geneva
Imperial College London
Lancaster University
Oxford University
Queen Mary, University of London
Royal Holloway, Univ. of London
STFC/RAL/Daresbury Laboratory
University of Glasgow
University of Liverpool
University of Sheffield
University of Warwick
Boston University
Colorado State University
Duke University
Louisiana State University
Michigan State University
SLAC
Stony Brook University
University of California, Irvine
University of Colorado
University of Pittsburgh
University of Rochester
University of Washington
IFIRSE, Quy Nhon/IOP, Hanoi(*): Number of collaborators

Country
CanadaCanadaCanadaCanadaFranceFranceFranceGermany
Italy
Italy
Italy
Italy
PolandPolandPolandPolandPolandPolandRussia
Spain
Spain
Spain
Swiss
Swiss
Swiss
UK
UK
UK
UK
UK
UK
UK
UK
UK
UK
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
Vietnam

(*)
4
1
4
5
18
8
9
4
5
3
7
8
9
5
5
3
2
3
26
8
5
2
6
9
12
15
12
9
8
2
13
4
17
7
9
2
8
1
5
5
1
19
3
5
3
2
6
1

Members
Staff
Kamioka Observatory
Yoichiro Suzuki, Professor (till Mar. 2014)
Masayuki Nakahata, Professor
Shigetaka Moriyama, Professor
Masato Shiozawa, Professor
Yoshinari Hayato, Assoc. Professor
Yasuhiro Kishimoto, Assoc. Professor
Hiroyuki Sekiya, Assoc. Professor
Shoei Nakayama, Assoc. Professor
Makoto Miura, Research Associate
Yusuke Koshio, Research Associate (till Mar. 2013)
June Kameda, Research Associate
Atsushi Takeda, Research Associate
Kou Abe, Research Associate
Roger Wendell, Research Associate (till Dec. 2015)
Motoyasu Ikeda, Research Associate (from Sep. 2013)
Yasuhiro Nakajima, Research Associate (from Aug.
2016)
Tomonobu Tomura, Project Research Associate (tillMar.2016)
Hidekazu Tanaka, Project Research Associate (fromJun.2012)
Lluis Magro Marti, Project Research Associate (fromMay, 2016)
Yo Kato, Project Research Associate (from Sep.2016)
Christophe Bronner, Project Research Associate (fromMay 2017)
Yosuke Kataoka, Project Research Associate (fromDec. 2017)
Takatomi Yano, Project Research Associate (fromDec. 2017)
Research Center for Cosmic Neutrinos
Takaaki Kajita, Professor
Kimihiro Okumura, Assoc. Professor
Yasuhiro Nishimura, Research Associate (from Sep.
2016)
Yasuhiro Nishimura, Project Research Associate(Apr.2012 - Sep. 2016)
Senior Fellows
Kamioka Observatory
Shigeki Tasaka (Aug. 2015 - Mar. 2017)
Postdoctorial Fellows
Kamioka Observatory
Lluis Magro Marti (Apr. 2011 - Mar. 2013)
Yo Kato (Apr. 2016 - Sep. 2016)
Guillaume Pronost (from Mar. 2016)
Yuuki Nakano (from Apr. 2017)
Graduate students
One student was awarded doctor degree and three42

students earned master degrees during 2006–2012, supervised by ICRR staff members.

List of Publications
Before Year 2012
[1] K. Abe et al. (T2K Collaboration), “The T2K
Experiment,” Nucl. Instrum. Meth. A 659, 106(2011) doi:10.1016/j.nima.2011.06.067
From Year 2012
[2] K. Abe et al. (T2K Collaboration), “First
Muon-Neutrino Disappearance Study with an
Off-Axis Beam,” Phys. Rev. D 85, 031103(2012) doi:10.1103/PhysRevD.85.031103. (152 citations)[3] K. Abe et al. (T2K Collaboration), “Observationof Electron Neutrino Appearance in a Muon Neutrino Beam,” Phys. Rev. Lett. 112, 061802 (2014).
doi:10.1103/PhysRevLett.112.061802. (487 citations)[4] K. Abe et al. (T2K Collaboration), “Measurements of neutrino oscillation in appearance anddisappearance channels by the T2K experimentwith 6.6x1020 protons on target,” Phys. Rev.
D 91, no. 7, 072010 (2015). doi:10.1103/PhysRevD.91.072010. (281 citations)[9] K. Abe et al. (T2K Collaboration), “Measurementof the Inclusive Electron Neutrino Charged Current Cross Section on Carbon with the T2K Near
Detector,” Phys. Rev. Lett. 113, no. 24, 241803(2014) doi:10.1103/PhysRevLett.113.241803. (68citations)[10] K. Abe et al. (T2K Collaboration), “Measurement of the inclusive νµ charged current cross section on carbon in the near detector of the T2K
experiment,” Phys. Rev. D 87, no. 9, 092003(2013) doi:10.1103/PhysRevD.87.092003 (125 citations)[11] K. Abe et al. (T2K Collaboration), “Measurementof the inclusive νµ charged current cross section
on iron and hydrocarbon in the T2K on-axis neutrino beam,” Phys. Rev. D 90, no. 5, 052010 (2014)
doi:10.1103/PhysRevD.90.052010. (49 citations)[12] K. Abe et al. (T2K Collaboration), “Measurementof the νµ charged current quasielastic cross section on carbon with the T2K on-axis neutrino
beam,” Phys. Rev. D 91, no. 11, 112002 (2015)
doi:10.1103/PhysRevD.91.112002. (40 citations)[13] K. Abe et al. (T2K Collaboration), “Measurement of the electron neutrino charged-current interaction rate on water with the T2K ND280
π 0 detector,” Phys. Rev. D 91, 112010 (2015)
doi:10.1103/PhysRevD.91.112010. (10 citations)[5] K. Abe et al. (T2K Collaboration), “Combined
Analysis of Neutrino and Antineutrino Oscillationsat T2K,” Phys. Rev. Lett. 118, no. 15, 151801(2017) doi:10.1103/PhysRevLett.118.151801. (115
citations)[14] K. Abe et al. (T2K Collaboration), “Measurement of the νµ charged-current quasielastic crosssection on carbon with the ND280 detector at
T2K,” Phys. Rev. D 92, no. 11, 112003 (2015)
doi:10.1103/PhysRevD.92.112003. (33 citations)[6] K. Abe et al. (T2K Collaboration), “Measurement of neutrino and antineutrino oscillations by
the T2K experiment including a new additional
sample of νe interactions at the far detector,”Phys. Rev. D 96, no. 9, 092006 (2017) Erratum: [Phys. Rev. D 98, no. 1, 019902 (2018)]
doi:10.1103/PhysRevD.96.092006, 10.1103/PhysRevD.98.019902. (63 citations)[15] K. Abe et al. (T2K Collaboration), “Measurement of the muon neutrino inclusive chargedcurrent cross section in the energy range of 1-3
GeV with the T2K INGRID detector,” Phys. Rev.
D 93, no. 7, 072002 (2016) doi:10.1103/PhysRevD.93.072002. (13 citations)[7] K. Abe et al. (T2K Collaboration), “Precise Measurement of the Neutrino Mixing Parameter θ23
from Muon Neutrino Disappearance in an OffAxis Beam,” Phys. Rev. Lett. 112, no. 18, 181801(2014) doi:10.1103/PhysRevLett.112.181801. (255
citations)[8] K. Abe et al. (T2K Collaboration), “Updated
T2K measurements of muon neutrino and antineutrino disappearance using 1.5×1021 protons on
target,” Phys. Rev. D 96, no. 1, 011102 (2017)
doi:10.1103/PhysRevD.96.011102. (23 citations)[16] K. Abe et al. (T2K Collaboration), “Measurement of double-differential muon neutrino
charged-current interactions on C8 H8 without pions in the final state using the T2K off-axis
beam,” Phys. Rev. D 93, no. 11, 112012 (2016)
doi:10.1103/PhysRevD.93.112012. (43 citations)[17] K. Abe et al. (T2K Collaboration), “Measurement of Coherent π + roduction in Low Energy
Neutrino-Carbon Scattering,” Phys. Rev. Lett.
117, no. 19, 192501 (2016) doi:10.1103/PhysRevLett.117.192501. (16 citations)[18] K. Abe et al. (T2K Collaboration), “First measurement of the muon neutrino charged current

43single pion production cross section on water with
the T2K near detector,” Phys. Rev. D 95, no. 1,
012010 (2017) doi:10.1103/PhysRevD.95.012010.
(19 citations)[19] K. Abe et al. (T2K Collaboration), “First measurement of the νµ charged-current cross section on a water target without pions in the final state,” Phys. Rev. D 97, no. 1, 012001 (2018)
doi:10.1103/PhysRevD.97.012001. (7 citations)[20] K. Abe et al. (T2K Collaboration), “Measurementof ν̄µ and νµ charged current inclusive cross sections and their ratio with the T2K off-axis neardetector,” Phys. Rev. D 96, no. 5, 052001 (2017)
doi:10.1103/PhysRevD.96.052001 (4 citations)[21] K. Abe et al. (T2K Collaboration), “Measurementof the neutrino-oxygen neutral-current interaction
cross section by observing nuclear deexcitation γrays,” Phys. Rev. D 90, no. 7, 072012 (2014)
doi:10.1103/PhysRevD.90.072012. (21 citations)[22] K. Abe et al. (T2K Collaboration), “Search forshort baseline νe disappearance with the T2K
near detector,” Phys. Rev. D 91, 051102 (2015)
doi:10.1103/PhysRevD.91.051102. (18 citations)[23] K. Abe et al. (T2K Collaboration), “Search for
Lorentz and CPT violation using sidereal time dependence of neutrino flavor transitions over a shortbaseline,” Phys. Rev. D 95, no. 11, 111101 (2017)
doi:10.1103/PhysRevD.95.111101. (6 citations)[24] K. Abe et al. (T2K Collaboration), “Upper boundon neutrino mass based on T2K neutrino timing
measurements,” Phys. Rev. D 93, no. 1, 012006(2016) doi:10.1103/PhysRevD.93.012006. (4 citations)[25] K. Abe et al. (T2K Collaboration), “Measurementof Neutrino Oscillation Parameters from Muon
Neutrino Disappearance with an Off-axis Beam,”
Phys. Rev. Lett. 111, no. 21, 211803 (2013)
doi:10.1103/PhysRevLett.111.211803. (130 citations)[26] K. Abe et al., “Proposal for an Extended Run ofT2K to 20×1021 POT”, arXiv:1609.04111 [hep-ex]

44

XMASS EXPERIMENT
Introduction

XMASS is a multi-purpose experiment using liquid
xenon which aims at the detection of cold dark matter, the search for neutrinoless double beta decay, andthe detection of low energy solar neutrinos. The nameXMASS stands for these three goals: Xenon detector
for weakly interacting MASSive particles, Xenon neutrino MASS detector for double beta decay, and Xenon
MASsive detector for Solar neutrino.
Astronomical observations suggest that there is dark
matter (non-luminous particles with mass) in the universe. One of the most likely candidates for dark matter is a weakly interacting massive particle (WIMP),for example the lightest supersymmetric particle. A
recoil of a xenon nucleus from an interaction with dark
matter will produce scintillation light in liquid xenon.
The Super-Kamiokande experiment shows that neutrinos have mass. However, we do not yet know the absolute mass of the neutrinos and whether neutrinos are
Majorana type or Dirac type. Xenon nuclei with mass
number 136 are one of the double beta decay nuclei
which are best suited for this research.
The energy spectrum of the higher energy solar neutrinos is measured by Super-Kamiokande, SNO, andBorexino, and that of low energy solar neutrinos (ppneutrinos, etc.) remains important to be measured
and monitored. With 10 ton of liquid xenon, one
would be able to detect pp neutrinos and 7 Be neutrinos by ν +e scattering with a rate of 10 events/day and5 events/day, respectively.
For all these purposes, background caused by
gamma rays which come from outside of the liquid
xenon needs to be suppressed. The key idea to reduce
this background is that gamma rays can be absorbed byliquid xenon itself (self-shielding) [1]. A sphere of liquid
xenon absorbs low energy gamma rays from the outside
within 10-20 cm and thus provides for low backgroundin the central volume. WIMPs and neutrinos, however,interact throughout the detector. Therefore, if the vertices of the events can be reconstructed, WIMPs andneutrinos can be observed in a low background environment by extracting only events observed deep insidethe detector. Event reconstruction can be accomplishedby observing photons with photo multipliers mounted
surrounding the liquid xenon. Liquid xenon has thefollowing advantages to realize this idea:• With the high atomic number (Z = 54) and thehigh density (∼ 3 g/cm3 ) of liquid xenon, external
gamma-rays can be absorbed in a short distancefrom the detector wall (self-shielding).
• Its large light yield of 42,000 photons/MeV, whichis as good as that of NaI(Tl) scintillator, enables
good event reconstruction as well as the detectionof small energy signals like those from dark matterrecoil.
• 175 nm scintillation light of liquid xenon can beread out by photomultiplier tubes (PMTs) with abi-alkaline photocathode and a quartz window.
• Purification is easier than for other materials (e.g.
distillation is possible).
• Isotope separation is possible. It is possible toenrich 136 Xe for double beta decay or deplete 136 Xe
for solar neutrino measurements.
We designed and started to construct the first
XMASS detector (100 kg FV) for dark matter searches
from 2007. The detector started commissioning in 2010
for a commissioning phase. The data consist of calibration data, test runs to understand the detector performance, and normal runs for dark matter searches.
Based on the data we found that we could improvethe background in the detector by covering parts of its
inner surface structures, where natural radioactive contaminations were found. This motivated refurbishment
work which started in 2013. After the refurbishment,
we started to take data with the improved detector for
more than four years, until now.

Summary from 2012 to 2018

Notable achievements in the past six years were:• Accumulating the largest exposure of a liquid
xenon detector for dark matter searches. With
the large and stable XMASS-I detector, a total
exposure of more than 800 days × 830 kg is obtained. This is particularly useful to search forannual modulation caused by dark matter particle interactions.
• The highest photoelectron yield among liquid
xenon dark matter detectors, ∼14 PE/keV electron equivalent. It enables a very low energy
threshold necessary for low mass WIMP searches.
• The lowest radon background. A low level ofradon background is crucial for future dark matter searches as well as detecting pp-solar neutrino.
At ∼ 9 µ Bq/kg-LXe just one order of magnitude improvement on this background would enable XMASS to observe pp-solar neutrinos.

45• Particle identification (PID) discriminating between electrons and gammas has been achieved
utilizing the difference in scintillation waveform
at various electron energies. This is most effective
around 60 keV and was used for several importantsearches of new physics signatures in this energy
range.
In addition to improving the detector performance as
described above, notable physics results in dark matter, nuclear physics, and astroparticle physics were obtained:• Dark matter physics– We conducted a search for light dark matter with masses below 20 GeV/c2 using lowthreshold data [9] and a search for dark matter with masses in the range from 20 GeV/c2
to 10 TeV/c2 using event vertex reconstruction [10]. Our limit 2.2 × 10−44 cm2 for a
60 GeV/c2 WIMP. While these results are not
the most stringent LXe results, they are theonly ones from a single-phase detector andthus complementary to the mainstream ones.
– Searches for inelastic scattering of dark matter particles on the isotope 129 Xe were carriedout [11, 12]. We significantly improved thelimit to 4.1×10−39 cm2 .
– A searches for dark matter related annual modulation due to the Earth’s rotation around the Sun was first published on
1 year of data [13] and improved on 2.5
years of data [14] to examine the signal
claimed by DAMA/LIBRA. We did not findany significant modulation amplitude in thedata for periodicity with periods between 50
and 600 days. The obtained positive upper limit for the amplitude is (1.3–3.2)×10−3/day/kg/keVee (2–6 keVee ), and is the moststringent above 3.0 keVee .
– Bosonic superweakly interacting massive particles (super-WIMPs) are warm dark mattercandidate. They are much lighter than usual
WIMPs and may help with some issues in the
CDM senario. We published the first experimental search [15] for super-WIMPs and excluded the possibility that such particles constitute all of dark matter. A improved searchcarried out later set the most stringent upper
limit [16].
• Nuclear physics– We searched for 124 Xe double electroncapture and found no signal [17, 18] . This
allowed us to set the most stringent limit todate. Our lifetime limit made some theoretical predictions untenable.

• Astorparticle physics– We searched for solar axions produced via e+
γ → e + a [19]. No axion signal was observed
and we set the best direct experimental limit
on gaee .
– Axions predicted in some theories with large
extra dimensions can be produced in the Sun.
Such KK axions were searched for by annual
modulation with our distance to the Sun. We
thus set the first experimental constraint on
KK axion-photon coupling [20].
– We investigated the possibility to detectgalactic supernova neutrinos via coherent
elastic neutrino-neucleus scatting in the
XMASS-I detector [21]. The total numberof events integrated over about 18 s after the
explosion of a supernova10 kpc away from the
Earth is expected to be 3.5–21.1, while background in the same time window would be
negligible.
In the following sections, a description of the
XMASS-I detector, its refurbishment, more details of
the physics results, and ongoing activities towards future direct dark matter searches are given.

The XMASS-I detector

XMASS-I is a single phase LXe scintillator detector
located underground (2700 m water equivalent) at the
Kamioka Observatory [7]. Fig. 1 shows a picture of
the central part of the XMASS-I detector. It contains∼830 kg of LXe in its active volume. This volume isviewed by 630 hexagonal and 12 cylindrical Hamamatsu
R10789 PMTs arranged on an 80 cm diameter pentakisdodecahedral OFHC PMT holder. A total photocathode coverage of more than 62% is achieved. This almost PMT holder is contained in a double-walled vessel also made of oxygen free high conductivity (OFHC)
copper. The waveforms in each PMT are recorded by
CAEN V1751 waveform digitizers with 1 GHz sampling rate and 10 bit resolution. The detector is calibrated regularly with a 57 Co source [8] inserted along
the central vertical axis of the detector. By the datataken with the 57 Co source at the center of the detector
volume, the photoelectron (PE) yield was determined
to be ∼14 PE/keV. Two different energy scales wereused: keVee represents an electron equivalent energy,
and keVnr denotes the nuclear recoil energy. In orderto shield the LXe detector from external gammas, neutrons, and muon-induced BG, the copper vessel wasplaced at the center of a φ 10 m × 10.5 m cylindrical tank filled with pure water (Fig. 2). The watertank is equipped with 72 Hamamatsu R3600 20-inch
PMTs to provide both an active muon veto and passive
shielding against those BG sources. XMASS-I was thefirst direct detection dark matter experiment equipped

46

Fig. 3. The PMTs with the copper ring. Right figure shows the edgeof PMT window with a half of the copper ring. The aluminum
seal of the PMT window is covered by the ring to stop scintillation
lights and radiations from the contaminations in the seal.

Fig. 1. PMT holders and backside of PMTs
Fig. 4. The inner surface of the detector before (left) and after(right) of the refurbishment. The thin plates are added to covers
the gaps made by the rings. Shadowing effect by gaps aroundthe aluminum seal was the cause of the BGs.

Fig. 2. Detector with copper vessel and an inner view of the water
shieldwith such an active water Cherenkov shield. The LXeand water Cherenkov detectors are henceforth called
the Inner Detector (ID) and the Outer Detector (OD),
respectively.

Refurbishment of the detector

During the commissioning data-taking, we foundthat the majority of events at low energy originatedfrom radioactive contamination in the aluminum sealof the photomultiplier tube (PMT) window. In orderto reduce the impact of this BG, and also to understand the origins of the background correctly for future
detector design, the detector was refurbished. As wewill see in the following section, the background in thefiducial volume caused by the aluminum seal was successfully suppressed and found to be minor after thisrefurbishment.
In this refurbishment, the contaminated parts of the
PMTs were covered with copper rings and plates in order to stop scintillation lights and radiation caused bythis contamination. The PMTs were cleaned with acidand the copper parts were electropolished in order toremove possible surface contamination. Fig. 3 showsa PMT with a copper ring. The two photos in Fig. 4
show the inner surface of the detector before and after
the refurbishment. After a year of refurbishing the detector, data-taking resumed in November 2013. Fig. 5
shows the background energy spectrum before and after refurbishment. The background level was reduced
by one order of magnitude as expected from a study
before refurbishment.

Search for WIMP dark matter

WIMPs are one of the well-motivated dark mattercandidates and are thought to be observable throughnuclear recoils produced in their elastic scattering interactions with target nuclei. Although many theories

Fig. 5. The observed energy spectrum of background befor (black)and after (red) the refurbishment. One order of magnitude reduction was achieved by the refurbishment.

Data
The best fit BG MC
1 σ error for the best fit

0.004
0.003
0.002
0
0

WIMP

60 GeV/c2, 2.2 × 10-44 (90% CL)

5

10

15

20

25
30
Energy [keVee]

Fig. 6. Observed energy spectrum from 705.9 days of the XMASS
fiducial volume data overlaid with the best-fit BG estimate.
The expected 60 GeV/c2 WIMP spectrum at a spin-independent
WIMP-nucleon cross section of 2.2 × 10−44 cm2 is also shown.

beyond the Standard Model of particle physics predict
WIMPs with mass larger than 100 GeV/c2 , some experiments indicate a possible WIMP signal with a lighter
mass of ∼10 GeV/c2 . Therefore, we first performed asearch for low-mass WIMPs using the 6.7 live days ofdata taken in February 2012 [9]. In order to achieve a
low energy threshold of 0.3 keVee , the full LXe volume
was analyzed with a simple event selection and without discriminating between nuclear-recoil and electron
recoil events. The observed spectrum did not have any
feature consistent with WIMP signals, and hence we
set a 90% CL upper limits on the spin-independent
WIMP-nucleon cross section for WIMPs with masses
below 20 GeV/c2 . This result excluded part of the parameter space allowed by other experiments.
We also conducted a search for WIMPs with masses
between 20 GeV/c2 and 10 TeV/c2 using 705.9 live
days of data collected between November 2013 andMarch 2016 [10]. The event vertex was reconstructedbased on the number of PEs in each PMT, and a fiducial volume containing 97 kg of xenon was establishedby requiring a radial distance from the center of the
LXe detector within 20 cm. To search for the WIMP
dark matter signal in our fiducial volume data, we performed a chi-square fit of the observed energy spectrum with the expected signal and BG spectra in the
2–15 keVee energy range. Fig. 6 shows the energy spectrum of the data and the best-fit BG estimate. The
event rate in the fiducial volume after data reduction
was (4.2 ± 0.2) × 10−3 /day/kg/keVee at 5 keVee , witha signal efficiency of 20%. All remaining events are
consistent with our background evaluation, most of the“mis-reconstructed events” originated from 210 Pb in thecopper plates lining the detector’s inner surface, but
not from the aluminum seal of the PMTs identified
before. Hence, the 90% CL upper limit on the spinindependent WIMP-nucleon cross section was derived
as a function of mass as shown in Fig. 7. Our lowest limit is 2.2 × 10−44 cm2 for a 60 GeV/c2 WIMP, andthis is the most stringent constraint from a single-phase
LXe detector.
Inelastic scattering that excites low-lying nuclear

-39

10

10-40
10-41
10-42
10-43
-44

10

-45

10

-46

10

10-47

DAMA/LIBRA (Na)CoGeNT (2013)
CDMS-Si (2014)

XMASS (2013)

ork

101

018)
-50 (2
kSide 017)
r
a
0
D
0
36
(2
rCDM
LUX
DEAP
S
17)
-II (20
X
Panda
017)
N1T (2XENOThis w

Supe

.

0.001

WIMP-nucleon cross section [cm2]

0.006
0.005

d
mo
S )
AS017
XM (2

[/day/keVee/kg]

47

102
103
2
WIMP mass [GeV/c ]104

Fig. 7. Spin-independent WIMP-nucleon cross section as a functionof the WIMP mass. The red solid curve represents the 90% CL
limit obtained from the XMASS fiducial volume data. Our limits
derived from the full volume spectrum (black dashed) as well asfrom the annual modulation search (red dash-dot-dot) are also
shown.

states in suitable target nuclei provides another avenue
to probe WIMP dark matter. The observation of inelastic scattering of WIMPs on nuclei would imply aspin-dependent interaction mechanism as well as that
WIMPs have spin. 129 Xe has the lowest-lying excitednuclear state at 39.58 keV and almost the highest natural abundance of 26.4% among the xenon isotopes.
We searched for such inelastic scattering of WIMPs on
129 Xe using 132.0 live days of data collected between
December 2010 and May 2012 with a restricted target mass of 41 kg [11]. No significant excess of events
was observed, and hence a 90% CL upper limit on the
spin-dependent WIMP-neutron cross section was derived (blue dashed in Fig. 8). This limit was the first
derived exclusively from data on inelastic scattering. A
further search was conducted using new data collected
between November 2013 and July 2016 [12]. The total live time in this data set amounts to 800.0 days
and the fiducial xenon mass was enlarged to 327 kg.
The preliminary result indicated no significant signal
for inelastic scattering of WIMPs, and we set a 90% CL
upper limit on the cross section with the best limit of
4.1×10−39 cm2 at 200 GeV/c2 (red solid in Fig. 8). This
limit is the most stringent among the WIMP searches
via inelastic scattering.

Search for annual modulation

The count rate for a dark matter signal is expected
to exhibit annual modulation due to the Earth’s motion
around the Sun, and this annual modulation would bea strong signature for dark matter. The DAMA/LIBRA experiment observed annual modulation of its
event rate at 9.3σ significance in data collected over
14 annual cycles with 100–250 kg of NaI(Tl). However, the interpretation of the result as a dark mattersignature is in question because of tension with other10-35!"#$%&'()&$#!"#*%&'()&$#!"#+%&,-.&"#

-02

10-36
10

XEN

s

10-38

Thi
100

ON

10-39

XEN

10-40

)
stic

ela
k (In
wor
)

ic
last

(E

LUX

)
stic
(Ela

)

stic

Ela

-II (

aX
and

tic)

elas

0 (In

10
ON

XM

P

10-41

10

2

10

3

1.0-1.5 keVee

-01

la

, Ine

014

(2
ASS

-37

)
stic

6"78$9:';*7<+="#+>?+>8@88A

2

SD WIMP-neutron cross section [cm ]

48

-0/

1.5-2.0 keVee

-034
-0,4
2.0-2.5 keVee

-0,

-0.4

2.5-3.0 keVee

4

10
10
WIMP mass [GeV]

-0.-

,--

/--

5--

2--

.---

!"#$%&'($)"*+,-./

direct detection experiments. We conducted a search
for an annual modulation signal in the full 832 kg LXevolume of XMASS-I using data taken after the detector refurbishment. We analyzed 359.2 live days of datacollected between November 2013 and March 2015 in
the first publication [13], and we published the second
result including more than one year of additional datacollected until July 2016, which resulted in a total of
800.0 live days [14].
The observed count rate after cuts as a function oftime in the energy region between 1.0 and 3.0 keVee
is shown in Fig. 9. In the figure, the data points arecorrected for relative efficiency based on the best-fit
parameters. To account for changing BG rates due tolong-lived isotopes such as 60 Co and 210 Pb, we modeled
the BG time variation using a linear function.
Fig. 10 shows the best-fit amplitudes as a functionof energy after correcting for efficiency. A hypothesis test was done using the χ 2 difference (χ02 − χ12 )between the modulation hypothesis and the null hypothesis. It gave a p-value of 0.11 for the null hypothesis in the 1.0–20 keVee energy range. DAMA/LIBRA obtained the modulation amplitude of ∼ 2 × 10−2
/day/kg/keVee in the 2.0–3.5 keVee energy range, and
XENON100 reported the amplitude of (1.67 ± 0.73) ×
10−3 /day/kg/keVee in the 2.0–5.8 keVee energy range.
We obtained a positive upper limits of (1.3–3.2)×10−3/day/kg/keVee in the same energy region and obtained
the most stringent constraint above 3.0 keVee .
A frequency analysis was also conducted to find any
periodicity in the 1–6 keVee energy range where significant amplitudes were observed by DAMA/LIBRA. We
used the χ 2 difference (χ02 − χ12 ) as a test statistics to
calculate the significance of the signal for each periodicity, Fig. 11 shows the result from the observed data
together with the expected ±1σ and ±2σ local sig-

Fig. 9. Observed count rate as a function of time in the 1.0-3.0 keVeeenergy range after correcting for relative efficiency. The error bars
show the statistical uncertainty. The red solid curves represent
the best-fit result for a model-independent analysis.
E)9&"0F5@"9G'&57-"F315+2"4-F!

#$%&'()*"+","-(./*01/23/2"4 !
""Fig. 8. Spin-dependent WIMP-neutron cross section as a function of
the WIMP mass. The blue dashed represents the 90% upper limit
using 132.0 days of data, while the red solid shows the preliminary
90%CL upper limits using the 800.0 days of data. The otherexperimental constraints are derived from elastic scattering data.

H

6B 6H :B :H CB
78%"9("*5!56+2+3

78%"9("*5!5:+2+,;#<#/=>?@#5A:B6CD5

+2+0
+

1+2+0

1+2+,

1+2+3
+

,

-

.

/

0+

0,

0-

0. 0/ ,+#$"%&'()"*""!

Fig. 10. Observed modulation amplitude as a function of energy
for the model independent analysis (circle). The solid lines represent the XMASS 90% positive (negative) upper limits on themodulation amplitude. The ±1σ and ±2σ bands represent the
expected sensitivity on the amplitude.

nificance bands from the dummy samples without any
modulation signal. Taking into account the “look elsewhere effect,” we calculated the global significance as
indicated in the figure. No significant periodicity was
found between 50 and 600 days.

Search for bosonic super-WIMP dark matter

Although WIMP dark matter is a well-motivatedmodel and fits the cold dark matter paradigm, simulations based on this cold dark matter scenario expect
a richer structure on galactic scales than is observed.
Furthermore, there is so far no evidence of supersymmetric particles at the LHC, and therefore, it is important to also investigate other dark matter candidates.
This leads to an interest in considering lighter and more

0,
/,

=(<(

. 2345%6(57

9:;"8<"&2! 2- 235%8(57

10-11

g

" "
#
$ !

1,

Ae

49

(ALPs)

rana
Majo

9:;"8<"&2! 2. 235%8(57Demo.
XENON100- 2345%6(5710-12

prev. work

.,

LUX

-,

10

-13

PandaX-II

this work
,

1,

-,,

.,,

1,,

_’/_

!"#$%&'&()*+
10-14
10-22

Fig. 11. The χ 2 difference, χ02 − χ12 , as a function of period forthe observed data in the 1–6 keVee energy range. The expectedbands for the local ±1σ and ±2σ significance and lines for the
global 1σ and 2σ significance are also shown.

10

10

-25

weakly interacting particles such as super-WIMPs, awarm dark matter candidate. Bosonic super-WIMPs,
namely hidden photons and axion-like particles, are experimentally interesting since their absorption in a target material would deposit an energy essentially equivalent to the super-WIMP’s rest mass.
We conducted a search for vector and pseudoscalarsuper-WIMPs in the 40–120 keV/c2 mass range firstwith a 41 kg×132.0 days exposure [15]. No significant
excess above background was observed, and hence a
90% CL upper limit on coupling constants for pseudoscalar bosons and vector bosons was obtained (blacksolid in Fig. 12). This was the first direct detection
experiment exploring the vector super-WIMPs and theobtained limit for the vector super-WIMPs excludes thepossibility that such particles constitute all of the dark
matter. The absence of the signal also provides themost stringent direct constraint on the coupling constant of pseudoscalar dark matter to electrons. This
result was published in Physical Review Letters, selected as an Editors’ Suggestion. An improved search
was then performed with a 327 kg×800.0 days exposure [16]. As a result of fitting the energy spectrumwith the expected signal and background, no significant signal was found and the obtained upper limits(red solid in Fig. 12) are the most stringent in both
direct and indirect searches to date.

10

-26

-23

10-24

10

ork
v. w
pre

RG

10-27

HB

!

Natural xenon contains 124 Xe (abundance 0.095%)
which undergoes double electron capture: the simultaneous capture of two orbital electrons of the atomin the nucleus. Measurement of its two-neutrino mode
would provide a new reference for the calculation of
nuclear matrix elements while observation of its neutrinoless mode would be evidence for the Majorana nature of the neutrino. In the case of two-neutrino double K-shell electron capture (2ν 2K) on 124 Xe, the totalenergy deposition is 63.6 keV provided by atomic X-

o.
a
an
jor
XENON100
Ma

2m
ALPs mass
De [keV/c ]

se
ffu
di

Search for 124 Xe double electron capture

102

(HP)

this work

-28

102

HP / ALPs mass [keV/c 2]

Fig. 12. Obtained 90% C.L. upper limits on coupling constants forelectrons and pseudoscalar super-WIMPs (top) and for electrons
and vector super-WIMPs (bottom).

rays and/or Auger electrons. The predicted half-live of
124 Xe against 2ν 2K is spread over a wide range between
1020 and 1024 years.
We performed a search for 2ν 2K on 124 Xe with afiducial xenon mass of 41 kg (containing 39 g of 124 Xe)using 132.0 live days of data [17]. After all cuts, 5
events remained in the signal region while the expectedbackground was 5.3 ± 0.5 events. This allowed us to
set the most stringent lower limit on this half-life of2ν 2K > 4.7 × 1021 years at 90% CL (magenta dashedT1/2
in Fig. 13). An improved search was further conductedwith an enlarged fiducial xenon mass of 327 kg (containing 311 g of 124 Xe) using 800.0 live days of data [18].
We also developed a novel method for discriminating
the 2ν 2K signal from the β -ray background using LXescintillation time profiles. As a result of the energy
spectrum fitting with the expected signal and background spectra in the 30–200 keVee energy range, nosignificant signal was found and we set a 90% CL lower
2ν 2K of 2.1 × 1022 years. As shown in Fig. 13,
limit on T1/2
the present result gives a lower limit that is stronger bya factor 4.5 over our previous result, and gives the moststringent experimental constraint reported to date.

Search for solar axions

Axion would be produced in the Sun through various
mechanisms; here, we focus on Compton scattering ofphotons on electrons, e + γ → e + a, and bremsstrahlung

50

Xe 2ν2K half-life (years)

Other important studies
1024

1023
XMASS(This work)

1022

124

XMASS(2016)
Gavrilyuk et al.

1021

1020

XENON100

QRPA
Suhonen

QRPA
HirschSU(4)στRumyantsev

PHFB
Singh

PHFB
ShuklaMCM
AunolaFig. 13. 90% CL exclusion limits on the 124 Xe 2ν 2K half-life overlaid
with the theoretical calculations. For the theoretical predictions,the reported 2ν ECEC half-lives are converted to 2ν 2K half-lives,
divided by the branching ratio for the two electrons being captured from the K-shell, P2K =0.767. The lower and upper edgesof the bands correspond to gA = 1.26 and gA = 1, respectively.

of axions from electrons, e + Z → e + a + Z. We searched
for such axions through the axio-electric effect, an analog of the photo-electric effect, using the 6.7 live daysof the full LXe volume data [19]. No prominent featureof an axion signal was observed above background anda 90% CL upper limit on the axion-electron coupling
constant was derived as gaee < 5.4 × 10−11 for axions
with masses below 1 keV/c2 . This was the best direct
experimental limit on gaee at that time, and the limit
was close to the one obtained theoretically based on
consistency between the observed and expected solar
neutrino fluxes.
In theories with large extra dimensions, axions
could propagate in the extra dimensions beyond the
4-dimensional spacetime and would acquire KaluzaKlein (KK) excitations, which could be observed as
particles with heavier masses in the standard spacetime. The KK axions can be produced in the Sun via
the Primakoff effect (γ Z → aZ) and the photon coalescence mechanism (γγ → a). While most of the produced
KK axions escape from the solar system, a small fraction can be gravitationally trapped in orbits around the
Sun and decay into two photons inside the detector.
The event rate of the KK axion decays would modulate annually since the expected number density of thetrapped KK axions falls off as r−4 , where r is the distance from the Sun. We looked for the decay of these
solar KK axions using 832 kg×359.2 days of data by annual modulation [20]. No significant amplitude excess
was found, and therefore a 90% confidence level exclusion limit was set on the KK axion-photon coupling
constant gaγγ as a function of the KK axion number
density. This is the first experimental constraint on
KK axions.

Detectability of galactic supernova neutrinos
Coherent elastic neutrino-nucleus scattering(CEvNS) is a process in which a neutrino with anenergy on the order of MeV interacts with all nucleonsin a nucleus coherently, resulting in a large crosssection approximately proportional to the square of
the number of neutrons in the target nucleus. This
interaction is sensitive to all flavors of neutrinos. We
investigated the possibility to detect galactic supernova
neutrinos via CEvNS in the XMASS-I detector [21].
The total number of events integrated over about 18 s
after the explosion of a supernova 10 kpc away fromthe Earth was expected to be 3.5–21.1, depending on
the supernova model used to predict the neutrino flux,
while the number of background events in the sametime window was measured to be negligible. In case of
a supernova explosion as close as Betelgeuse, the total
number of observable events can be more than ∼104 ,making it possible to distinguish different supernova
models by examining the evolution of the neutrino
event rate in XMASS.
Measurement of the LXe scintillation time profile
The time profile of LXe scintillation could potentially be used for particle identification and vertex reconstruction. We measured the scintillation time profile for gamma-ray induced events with 55 Fe, 241 Am,
and 57 Co sources [22]. In this analysis two decay components, τ1 and τ2 , were assumed. The decay constantτ2 increased from 27.9 ns to 37.0 ns as the gamma-ray
energy increased from 5.9 keV to 122 keV. A fast decay
component with τ1 ∼ 2 ns was necessary to reproduce
the data. Energy dependencies of τ2 and the fractionof the fast decay component were studied as a functionof the kinetic energy of the electrons induced by the
gamma-rays. The obtained results were almost consistent with previously reported results and extended
them to the lower energy region relevant for direct dark
matter searches.
A measurement of the scintillation time profile fornuclear recoil events was also conducted using a 252 Cf
neutron source [23]. With τ1 = 4.3 ± 0.6 ns taken froma prior research, τ2 was measured to be 26.9+0.8−1.2 ns with
+0.028the fast decay componet fraction of 0.252−0.020 . We also
evaluated the performance of pulse shape discrimination between nuclear recoil and electronic events withthe aim of reducing the electromagnetic background
in WIMP searches. For a 50% nuclear recoil acceptance, the electronic event acceptance was 13.7±1.0%
and 4.1±0.7% in the energy ranges of 5–10 keVee and
10–15 keVee , respectively.
Measurement of 210 Pb and 210 Po in the copper bulk
Oxygen-free copper (OFC) is a readily available
commercial material of low radioisotope (RI) content,

51

and thus it is often used in recent low-background experiments. There are multiple methods to measure the
RI contaminations in materials, however, it is difficult
to measure the 210 Pb contamination with high precision below O(100) mBq/kg. We established a new
method to measure the 210 Po and 210 Pb contaminations in the copper bulk with a low-background alpharay counter [24]. The energy distribution of alpharays emerging from the bulk becomes continuous since
they lose part of their energy before reaching the surface. In order to determine the 210 Pb contamination,
we measured 210 Po several times over different periods
since the 210 Pb and 210 Po decay equilibrium may be
broken. Owing to our measurements’s high sensitivityof a few mBq/kg, the 210 Pb and 210 Po contaminationsin the OFC bulk were identified and measured for the
first time. The obtained 210 Pb contaminations of various OFC samples were 17–40 mBq/kg, though 210 Po
contamination varied widely. This 210 Pb was understood to be a small residual from the electrolysis process based on the investigation of copper samples atvarious stages in production.
Development of low RI PMT with a dome shape photocathodeFor a future detector with a larger volume andsmaller background, we developed a new type of threeinch PMT; the R13111. To reduce the impact of thedetector’s inner surface background, which is the maincause of the background for the XMASS-I detector,
the photocathode geometry is changed to dome-shaped
from flat. As shown in Fig. 14, PMTs with domeshaped photocathode are more efficient in detecting
scintillation photons originating on the surface of thedetector than flat PMTs. Our simulation shows thatthree PMTs surrounding the event vertices on the surface should detect 40%-50% of all photoelectrons. Using this information the background originating fromevents at the surface can be reduced by three order ofmagnitudes.
From the performance check of the new PMTs, apartfrom basic properties such as low dark rate and stable
operation inside liquid xenon, the sensitivity for light
impinging on the PMT from the side, which is the mostimportant for suppression of the surface background,
was confirmed to be as desired: more than 80% of thesensitivity near the center of the photocathode.
Due to careful screening of all components, radioactive
impurities in the PMT are drastically reduced. Compared to the R10789 used in the XMASS-I detector, theR13111 manufactured in 2015 achieved a reduction of
69% for the U chain, 59% for the Th chain, larger than
86% for 40K and 84% for 60 Co. Fig. 15 summarizes the
reduction of the radioactive impurities for from R10789
to R13111.

Fig. 14. The schematic view of difference of effectiveness of photon
detection for the detector surface event between flat photocathode and dome-shaped photocathode.

Fig. 15. The radioactivies of R13111 and R10789 (used in the
XMASS-I detector).

Activity towards the future direct dark mattersearchBased on the experience with the XMASS-I detector, we planned to construct a larger detector, XMASS1.5, with five tons fiducial mass of liquid xenon. It
was expected to reach a sensitivity of around 10−47 cm2for WIMPs with a mass of 100 GeV. In 2016, we reevaluated the plan and concluded that XMASS-1.5
was not competitive any more once one-ton-scale dualphase liquid-xenon detectors become operational. The
primary reason for this decision was that the background due to electron scattering by low energy solar
neutrinos is difficult to distinguish from a WIMP signal in single-phase liquid xenon detectors. Though wehave some discrimination power between electron andnuclear recoil signals using the scintillation waveforms,
it is hard to complete with the particle identification indual-phase detectors. In addition, dual-phase detectors
with similar target size (second generation, G2) werealready approved for construction. For these reasons,rather than trying to construct a uncompetitive detector in Kamioka, we aimed for a future, more sensitive,
third generation detector (G3) and in the meantime to52collaborate with competitors building their multi-ton
G2 detectors.
This future plan was submitted to the future projectcommittee in ICRR and evaluated as shown in a
separated report. The committee agreed that the
XMASS-1.5 was not competitive with other contemporary projects and accepted this change of our planfor the future. It also recognized participation in theone of G2 experiments, in particular the XENONnT
experiment, as appropriate. The XENON collaboration plans to construct the XENONnT detector with ∼five ton liquid xenon target in 2019.
XMASS and the Kamioka Observatory are providing substantial know-how to achieve XENONnT’s ambitions. Two areas stand out: xenon purification in theliquid phase, and a newly added neutron veto surrounding the LXe TPC. These address the two of the most serious problems arising with the transition to multi-ton,dual- phase TPC based G2 dark matter direct detection
experiments: suppressing electronegative contaminantsin the liquid xenon, and tagging fast neutrons emerging
in the TPC. These stem from alpha-neutron reactions
and in rare cases scatter only once before leaving the
TPC, thus mimicking the nuclear recoil signature of a
WIMP interaction. From XMASS the new, Japanese
members of XENON have expertise with the circulation of LXe and use it in supporting the new liquid purification system for XENONnT. As members also of
the Super-Kamiokande collaboration a significant portion of the new Japanese XENON collaborators haveknowledge of and access to the water Cherenkov based
neutron detection pioneered in Kamioka by EGADS.
There is currently a very high chance that we will use
this Kamioka technology at Laboratori Nazionali Del
Gran Sasso (LNGS) to convert the water Cherenkov
muon veto shielding for the TPC into a highly efficientneutron veto with our technology.
Having found our place in our new dark matter detection experiment, we are still planning to use Hall-C
for the development of new low background techniques.
On our Japanese side this will be directed towards thethird generation of LXe based dark matter detectors
beyond the upcoming G2 experiments.

Summary

We have achieved following results in the last six
years.
• Refurbishment of the XMASS-I detector and reducing the background more than one order ofmagnitude.
• Operation of the XMASS-I detector for more thanfour calendar years. This enables us to assess possible annual modulation caused by dark matterparticles.
• Understanding the background of the detector inside the fidicual volume well and improve dark

matter searches on the basis of this understanding.
• Development of particle discrimination between
electrons and gamma rays.
• Various searches and studies of new physics (elastic scattering and inelastic scattering by dark
matter particles, and solar KK axions), nuclear
physics (double electron captures), and astrophysics (supernovae neutrinos).
• Development of the world’s most sensitive alpharay counter.
The achievements above already expanded beyond ouroriginal physics scope and we developed our own uniqueapproach to new physics. We will conduct more studies
using existing large exposure data set.

XMASS Collaboration
Spokesperson : Shigetaka Moriyama (from Dec. 2016)(Kamioka Observatory, ICRR, The University of Tokyo),
Yoichiro Suzuki (till Dec. 2016) (Kavli Institute for the
Physics and Mathematics of the Universe (WPI), The University of Tokyo)
Institute
ICRR, Univ. of Tokyo
Kavli IPMU, Univ. of Tokyo
Kobe Univ.
Tokai Univ.
Gifu Univ. (till Mar. 2016)
Yokohama National Univ.
Miyagi Kyoiku Univ.
Nagoya Univ.
Tokushima Univ.
(from Oct. 2014)
Nihon Univ. (from Apr. 2018)
Institute for Basic Science
Korea Research Instituteof Standards and Science(KRISS)
Total(∗ ) Number of collaborators.

Country
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan

(∗ )
16
4
2
1
1
1
1
3

Japan
Japan
Korea
Korea

1
1
3
2

Members
Staff
Yoichiro Suzuki, Professor (till Mar. 2014)
Masayuki Nakahata, Professor
Shigetaka Moriyama, Professor
Yasuhiro Kishimoto, Assoc. Professor
Hiroyuki Sekiya, Assoc. Professor
Atsushi Takeda, Research Associate
Kou Abe, Research Associate
Masaki Yamashita, Project Assoc. Professor

36

53

Hiroshi Ogawa, Project Research Associate (till Mar.
2018)
Kazuyoshi Kobayashi, Project Research Associate
Katsuki Hiraide, Project Research Associate
Byeongsu Yang, Project Research Associate (till Apr.
2018)
Koichi Ichimura, Project Research Associate (fromAug. 2013)
Postdoctorial Fellows
Kazufumi Sato, PD Researcher (from Jun. 2015)
Senior Fellows
Shigeki Tasaka (Aug. 2015 - Mar. 2017)
Graduate students
Two students were awarded doctor degrees and six
students earned master degrees during 2003–2018, supervised by ICRR staff members.

List of Publications
Before Year 2012[1] Y. Suzuki, “Low-energy solar neutrino detection
by using liquid xenon,” hep-ph/0008296.
[2] K. Ueshima et al. (XMASS Collaboration), “Scintillation yield of liquid xenon at room temperature,” Nucl. Instrum. Meth. A 594 (2008) 148.
[3] K. Abe et al. (XMASS Collaboration), “Distillation of Liquid Xenon to Remove Krypton,” Astropart. Phys. 31 (2009) 290.
[4] K. Ueshima et al. (XMASS Collaboration),“Scintillation-only Based Pulse Shape Discrimination for Nuclear and Electron Recoils in Liquid
Xenon,” Nucl. Instrum. Meth. A 659 (2011) 161.
[5] A. Minamino et al. (XMASS Collaboration), “Selfshielding effect of a single phase liquid xenon detector for direct dark matter search,” Astropart.
Phys. 35 (2012) 609.
[6] K. Abe et al. (XMASS Collaboration), “Radon Removal from Gaseous Xenon with Activated Charcoal”, Nucl. Instrum. Meth. A 661 (2012) 50.
Since Year 2012
[7] K. Abe et al. (XMASS Collaboration), “XMASS
detector,” Nucl. Instrum. Meth. A 716 (2013) 78.
[8] N. Y. Kim et al. (XMASS Collaboration), “Microsource development for XMASS experiment,”
Nucl. Instrum. Meth. A 784 (2015) 499.

[9] K. Abe et al. (XMASS Collaboration), “Light
WIMP search in XMASS,” Phys. Lett. B 719(2013) 78.
[10] K. Abe et al. (XMASS Collaboration), “A
direct dark matter search in XMASS-I,”
arXiv:1804.02180 [astro-ph.CO], accepted for
publication in Phys. Lett. B.
[11] H. Uchida et al. (XMASS Collaboration), “Search
for inelastic WIMP nucleus scattering on 129 Xe indata from the XMASS-I experiment,” Prog. Theor.
Exp. Phys. 2014 (2014) 063C01.
[12] T. Suzuki et al. (XMASS Collaboration), “Search
for WIMP-129 Xe inelastic scattering with particle identification in XMASS-I,” arXiv:1809.05358[astro-ph.CO].
[13] K. Abe et al. (XMASS Collaboration), “Direct
dark matter search by annual modulation in
XMASS-I,” Phys. Lett. B 759 (2016) 272.
[14] K. Abe et al. (XMASS Collaboration), “Direct
dark matter search by annual modulation with 2.7
years of XMASS-I data,” Phys. Rev. D 97 (2018)
102006.
[15] K. Abe et al. (XMASS Collaboration), “Searchfor bosonic superweakly interacting massive
dark matter particles with the XMASS-I detector,” Phys. Rev. Lett. 113 (2014) 121301(Editors’ Suggestion).
[16] K. Abe et al. (XMASS Collaboration), “Searchfor dark matter in the form of hidden photons
and axion-like particles in the XMASS detector,”arXiv:1807.08516 [astro-ph.CO], accepted for publication in Phys. Lett. B.
[17] K. Abe et al. (XMASS Collaboration), “Search
for two-neutrino double electron capture on 124 Xe
with the XMASS-I detector,” Phys. Lett. B 759(2016) 64.
[18] K. Abe et al. (XMASS Collaboration), “Improved
search for two-neutrino double electron captureon 124 Xe and 126 Xe using particle identification in
XMASS-I,” Prog. Theor. Exp. Phys. 2018 (2018)
053D03.
[19] K. Abe et al. (XMASS Collaboration), “Search forsolar axions in XMASS, a large liquid-xenon detector,” Phys. Lett. B 724 (2013) 46.
[20] N. Oka et al. (XMASS Collaboration), “Search forsolar Kaluza-Klein axions by annual modulation
with the XMASS-I detector,” Prog. Theor. Exp.
Phys. 2017 (2017) 103C01.

54[21] K. Abe et al. (XMASS Collaboration), “Detectability of galactic supernova neutrinos coherently scattered on xenon nuclei in XMASS,” Astropart. Phys. 89 (2017) 51.
[22] H. Takiya et al. (XMASS Collaboration), “A measurement of the time profile of scintillation induced
by low energy gamma-rays in liquid xenon with the
XMASS-I detector,” Nucl. Instrum. Meth. A 834(2016) 192.
[23] K. Abe et al. (XMASS Collaboration), “A measurement of the scintillation decay time constantof nuclear recoils in liquid xenon with the XMASSI detector,” arXiv:1809.05988 [physics.ins-det].
[24] K. Abe et al. (XMASS Collaboration), “Identification of 210 Pb and 210 Po in the bulk of copper samples with a low-background alpha particle
counter,” Nucl. Instrum. Meth. A 884 (2018) 157.

55

HYPER-KAMIOKANDE (R&D)
Introduction

The Hyper-Kamiokande or Hyper-K [1, 7] is the
ICRR’s next main project. The detector, as a straightforward extension of the Super-Kamiokande (Super-K),
will provide major new capabilities to make new discoveries in particle and astroparticle physics thanks toan order of magnitude increase in detector mass andimprovements in photon-detection system along withthe envisioned J-PARC Megawatt-class neutrino beam.
Figure 1 shows a schematic view of the Hyper-K cylindrical detector with 60 m in height and 74 m in diameter. The water mass is 0.258 million metric tons, withan order of magnitude larger fiducial mass of 0.187
million metric tons than Super-K. The Hyper-K and
J-PARC neutrino beam measurement of neutrino oscillation is more likely to provide a 5-sigma discoveryof CP violation than any other existing experiment.
Hyper-K will also be the world leader for nucleon decay
searches. The sensitivity to the partial lifetime of protons for the decay modes of p → e+ + π 0 is expected to
exceed 1035 years. The astrophysical neutrino program
involves precision measurements of solar neutrinos andtheir matter effects, high-statistical Supernova burst
and Supernova relic neutrinos. The Hyper-K is a priority project listed in the Roadmap2017 of the Japanese
Ministry of Education, Culture, Sports, Science andTechnology (MEXT), and seed funding has been allocated within MEXT budget request for JFY2019. The
principal project milestones include: Construction tostart in 2020, and Start data taking in ∼ 2027.

Summary from 2012 to 2018

List of milestones which we have achieved during
this review period through April 2012 to the summer
2018 is shown below.

• 2013.3 Selected as a important large-scale projectin Masterplan2013 by Science Council of Japan(SCJ) 1 .
• 2015.1 International proto-collaboration has been
formed.
• 2015.1 ICRR and the Institute of Particle and Nuclear Studies (IPNS) of KEK signed a memorandum of understanding (MoU) for cooperation on
the Hyper-K project.
• 2016.2 Hyper-K Advisory Committee (HKAC)
meeting (1st round)• 2016.2 Hyper-K Design Report [5]• 2016.6 KEK’s Project Implementation Plan
(KEK-PIP) set top priority to the J-PARC upgrade for Hyper-Kamiokande 2 .
• 2017.2 Selected as a important large-scale projectin Masterplan2017 by SCJ 3 .
• 2017.3 Recommended as a ICRR’s next projectby the ICRR future project review committee’sreport 4 .
• 2017.7 Selected in Roadmap2017 by MEXT 5 .
• 2017.9 HKAC meeting (2nd round)• 2017.10 The University of Tokyo launched
Next-Generation Neutrino Science Organization(NNSO) 6 , where ICRR, Kavli IPMU, and the
School of Science cooperate to promote the projectand host the detector construction.
• 2018.5 Revised Hyper-K Design Report [7]• 2018.8 Seed funding has been allocated within
MEXT budget request for JFY2019.

Fig. 1. Schematic view of the Hyper-K water tank.

56

6LJQLILFDQFH IRU VLQ]CP=0 exclusion
Normal mass hierarchyτ/ β [years]

m 3r

10

HK

8

DUNE

6

DUNE 40 kton, staged , 3σ
SK 22.5 kton , 3σ
HK 186 kton HD , 3σ
35

10

k

T2K-II

4

k

2

1034

T2K+NOvA(~2020)0p → e+ π0

-150

-100

-50

0

50

100
150
£CP [degree]

1033
2020

2030

2040
Year

Fig. 2. Significance of the CP violation (nonzero sin δCP ) discovery
in lepton sector with a 10-year observation in Hyper-K (HK) as
a function of the unknown CP phase. The normal hierarchy ofneutrino masses is assumed. Ongoing and planned long baseline
experiments are superimposed.

&39 VLJQLILFDQFH IRU ]CP=-90°, normal hierarchy

Fig. 4. The p→e+ π 0 discovery reach in proton lifetime with 3σsignificance as a function of year. It shows Hyper-K (HK) planing
to start in 2026, superimposed with the ongoing Super-K (SK)and planned DUNE experiments. The DUNE project assumes
10 kton operation from 2024, toward full 40 kton by increasing
10 kton every year.

6LJQLILFDQFH >k@

10
8

HK
DUNE

6

T2K-II

4

T2K
2

NOvA
0

2022

2026

2030

2034

2038

Fig. 3. Discovery potential of the CP violation (nonzero sin δCP )in lepton sector as a function of year. The normal hierarchy ofneutrino masses is assumed. Ongoing and planned long baseline
experiments are superimposed.

Physics potentials

Hyper-K will be able to measure the magnitude ofthe leptonic CP violation with high precision, which
could explain the baryon asymmetry in the Universe.
Figure 2 shows an expected significance of the CP violation (nonzero sin δCP ) discovery by ten years operation. Hyper-K covers the 76% of δCP parameter spacewith 3σ or more significance, and 57% of the parameter
space with 5σ or more. Hyper-K will go beyond 7σ significance if δCP = −90◦ as suggested by T2K 7 results.
*1 Master plan 2014 (in Japanese), http://www.scj.go.jp/ja/info/kohyo/pdf/kohyo-22-t188-1.pdf*2 KEK
Roadmap(Project
ImplementationPlan),
https://www.kek.jp/ja/About/OrganizationOverview/Assessment/Roadmap/KEK-PIP.pdf (2016).
*3 Master plan 2017 (in Japanese), http://www.scj.go.jp/ja/info/kohyo/kohyo-23-t241-1.html*4 Report by ICRR future project committee (in Japanese),
http://www.icrr.u-tokyo.ac.jp/public_relation/blog/171026.pdf*5 MEXT
Roadmap2017,
http://www.mext.go.jp/component/b_menu/shingi/
toushin/__icsFiles/afieldfile/2017/10/18/1388523_002.pdf*6 Next-Generation
Neutrino
ScienceOrganization,
http://www.nnso.u-tokyo.ac.jp/index-e.html*7 K. Abe et al. [T2K Collaboration], Phys. Rev. Lett. 118, 151801.

Figure 3 shows the discovery significance for the caseof δCP = −90◦ as a function of year. The US-based program LBNF/DUNE 8 plans to start its beam in 2026
and it is critical for Hyper-K to start its construction
and operation on time.
In Grand Unified Theories, which unify quarks andleptons and unify three forces other than gravity, thedecay of proton is one of the most dramatic predictions.
A proton decay p→e+ π 0 is dominant decay mode mediated by heavy gauge bosons in various models with aprediction close to the current limit of the proton decay
life time. Figure 4 shows the 3σ discovery potential forthe p→e+ π 0 mode as a function of year. Hyper-K is anonly realistic proposal which can go beyond the proton lifetime of 1 × 1035 years. Hyper-K will also extend
reach to many other decay modes such as SUSY favoredp → ν K + , decays into heavier mesons, and di-nucleon
decays.
High statistics observations of neutrinos from acore collapsed Supernova neutrinos (CCSN) along withgravitational waves are the only way to obtain precious
inside information on the dynamics of the CCSN central
engine and the explosion mechanism. If a CCSN explosion were to take place near the center of our Galaxy,
Hyper-K would observe as many as tens of thousands ofneutrino interactions. Furthermore, Hyper-K will havethe ability to precisely determine the arrival time ofsupernova neutrinos, which will help contribute to theunderstandings of both neutrino and CCSN properties.
Moreover the continuous flux of relic supernova neutrinos from all past CCSN explosions in the observable
universe will guarantee a steady accumulation of valuable astrophysical data. Figure 5 shows the expected
number of relic supernova neutrino events as a functionof year.
*8 R. Acciarri et al. [The DUNE Collaboration], arXiv:1601.05471[physics.ins-det].

Number of SRN events in FV

57

HK
SK-Gd
JUNO

400
350
300

HK (BH 30%)
SK-Gd (BH 30%)
JUNO (BH 30%)

250
200
150
100
50
0

2020

2025

2030

2035

2040

2045
Year

Fig. 5. The accumulated number of supernova relic neutrino events
as a function of year. The JUNO experiment and SK-Gd are alsoplotted. Effective temperature of neutrinos inside supernova isassumed to be 6 MeV. The solid line assumes no black hole formation while the dashed line assumes that 30% of core-collapsestars form a black hole.

Cavern excavation

In order to pin down the candidate place, seismic
prospecting by using artificially generated elastic wave
was performed at the large area of (400 m)3 . The top
panel in Fig. 6 shows the results of reflection imaging
overlaid with known faults and fracture zones. The
study identified the known faults as expected, while
there was no new faults found at the central region.
The bottom panel is rock class distribution made bycombining the results of seismic tomography, reflection
imaging, and the past geological survey. In both figures, the red dashed rectangle denotes a region which
has the best rock quality and the least uneven rockover the entire Hyper-K candidate site. We successfully
identified the best candidate place that is suitable forthe gigantic Hyper-K cavern construction. The cavern
excavation is found to be feasible by known technology.

Photosensors

A Cherenkov light in a ultra pure water is detected by 40,000 newly developed photomultiplier tubes(PMTs) shown in Fig. 7 giving 40% photo-coverage ofthe inner detector surface. Its peak quantum efficiency
is about 30%, that is 1.4 times higher than that of theSuper-K PMT. By combining with the improved photoelectron collection efficiency the new PMT achieved
twice higher single photon detection efficiency. The
timing and charge resolutions at single photo-electron
also becomes much better as 1.1 ns and 35% which
can be compared with 2.1 ns and 53% of the Super-K
PMT, respectively. Moreover, pressure tolerance was
improved up to the 125 m water depth in order to fit
the 60 m depth of the detector water.
To prevent a chain reaction of imploding PMTs
caused by single PMT implosion, every PMT in theHyper-K water tank will be housed in a shock-wave
prevention cover. The prototype cover consists of anacrylic front window and a stainless steel backside coverFig. 6. (Top) Top view of reflection imaging by seismic prospecting
at altitude of 483 m above sea level. Asterisks (∗) show identified
reflection positions indicating existence of faults, fracture zone,
and open cracks in the bedrock. Blue dashed lines indicate thelocation of known faults. The red dashed rectangles denotes a
selected candidate region for Hyper-K cavern and dashed circle
indicates the size of the Hyper-K cavern. (Bottom) Rock class
distribution obtained by combining the results of seismic tomography, reflection imaging and the geological survey results with
bore-hole and the existing tunnels.

with conical shape. A mock test simulating the eventof a photo-sensor implosion in Hyper-K was carried out
in a deep vertical shaft. It has been demonstrated thatthe peak amplitude of the pressure shock-wave is significantly reduced and thus does not cause a chain reaction
up to 80 m depth. In addition, studies are going on for
two alternative designs aiming to reduce weights and /or production costs: one is a stainless steel tube-likeshaped cover, and another is a resin-made monolithic
cover.

58

Fig. 7. New 50 cm photomultiplier tube with a box-and-line dynode.

Hyper-Kamiokande Proto-Collaboration
Project Leader : Masato Shiozawa (Kamioka Observatory,
ICRR, The University of Tokyo)
Institute
Kamioka Observatory, ICRR,
Univ. of Tokyo
Boston Univ.
California State Univ.
Chonnam Univ.
Dongshin Univ.
Ecole Polytechnique
GIST College
Imperial College London
INFN Sezione di Bari andUniversità e Politecnico diBari
INFN Sezione di Napoli andUniversità di Napoli
INFN Sezione di Padova andUniversità di Padova
INFN Sezione di Roma andUniversità La Sapienza
Institute for Nuclear Researchof the Russian Academy of
Sciences
Iowa State Univ.
IRFU
KEK
Kobe Univ.
Kyoto Univ.
Kyoto Sangyo Univ.
Laboratory Nuclear and HighEnergy Physics

Country
Japan

(∗ )
22

USA
USA
Korea
Korea
France
Korea
UK
Italy

4
3
4
2
5
1
6
7

Italy

6

Italy

6

Italy

2

Russia10

USAFrance
Japan
Japan
Japan
Japan
France

2
5
8
3
7
1
3

Lancaster Univ.
Los Alamos National Laboratory
Louisiana State Univ
Michigan State Univ
Miyagi Kyoiku Univ.
Nagoya Univ.
Nagoya Univ.
KobayashiMaskawa Institute for theOrigin of Particles and the
Universe
Nagoya Univ.
Institutefor Space-Earth Environmental Research
National Centre for Nuclear
Research
Okayama Univ.
Osaka City Univ.
Oxford Univ.
Pennsylvania State Univ.
Pontificia
Universidade
Catolica do Rio de Janeiro
Queen Mary University of
LondonRutherford Appleton Laboratory
Royal Holloway University of
LondonSeoul National Univ.
Seoyeong Univ.
Stockholm Univ.
Stony Brook Univ.
SungKyunKwan Univ.
Tohoku Univ.
Tokyo Institute of Technology
Tokyo Univ. of Science
TRIUMF.
Univ. of Washington
Univ. of Autonoma Madrid
Univ. of British Columbia
Univ. of California, Davis
Univ. of California, Irvine
Univ. of Edinburgh
Univ. of Geneva
Univ. of Hawaii
Univ. of Liverpool
Univ. of Pittsburgh

UK
USA

6
1

USA
USA
Japan
Japan
Japan

3
1
1
1
2

Japan

2

Poland

8

Japan
Japan
Japan
USA
Brazil

4
2
4
1
2

UK

12

UK

8

UK

7

Korea
Korea
Sweden
USA
Korea
Japan
Japan
Japan
CanadaUSA
Spain
CanadaUSA
USA
UK
Switzerland
USA
UK
USA

4
1
1
5
4
3
4
1
7
1
3
3
2
5
7
7
3
9
1

59

Univ. of Regina
Univ. of Rochester
Univ. of Sheffield
Univ. of Tokyo
Univ. of Tokyo, Earthquake
Research InstituteUniv. of Tokyo, ICRR, RCCN
Univ. of Tokyo, Kavli IPMU
Univ. of Tokyo, NNSO
Univ. of Toront
Univ. of Warwick
Univ. of Winnipeg
Warsaw Univ.
Warsaw Univ. of Technology
Wroclaw Univ.
Virginia Tech
Yerevan Physics InstituteYork Univ.
Kyiv National Univ.
Yokohama National Univ.
Total

Canada
USA
UK
Japan
Japan

2
1
8
4
2

Japan
Japan
Japan
CanadaUK
CanadaPolandPolandPolandUSA
Armenia
CanadaUkraine
Japan

6
8

(∗∗ )
4
6
2
1
6
1
4
1
5
2
1
307

(∗ ) Number of collaborators.
(∗∗ ) Members also affiliatedwith ICRR, Kavli IPMU, andthe School of Science, University of Tokyo.

Hyper-Kamiokande Working Group
Project leader : Masato Shiozawa(Kamioka Observatory, ICRR, The University of Tokyo)

Mar.2016)
Hidekazu Tanaka, Project Research Associate (fromJun.2012)
Lluis Magro Marti, Project Research Associate (fromMay, 2016)
Yo Kato, Project Research Associate (from Sep.2016)
Christophe Bronner, Project Research Associate (fromMay 2017)
Yosuke Kataoka, Project Research Associate (fromDec. 2017)
Takatomi Yano, Project Research Associate (fromDec. 2017)
Research Center for Cosmic Neutrinos
Takaaki Kajita, Professor
Kimihiro Okumura, Assoc. Professor
Yasuhiro Nishimura, Research Associate (from Sep.
2016)
Yasuhiro Nishimura, Project Research Associate(Apr.2012 - Sep. 2016)
Postdoctorial Fellows
Kamioka Observatory
Lluis Magro Marti (Apr. 2011 - Mar. 2013)
Yo Kato (Apr. 2016 - Sep. 2016)
Guillaume Pronost (from Mar. 2016)
Yuuki Nakano (from Apr. 2017)
Graduate students
One student earned master degrees during 2012–2018, supervised by ICRR staff members.

List of Publications
Members at ICRR

Before Year 2012

Staff
Kamioka Observatory
Yoichiro Suzuki, Professor (till Mar. 2014)
Masayuki Nakahata, Professor
Shigetaka Moriyama, Professor
Masato Shiozawa, Professor
Yoshinari Hayato, Assoc. Professor
Yasuhiro Kishimoto, Assoc. Professor
Hiroyuki Sekiya, Assoc. Professor
Shoei Nakayama, Assoc. Professor
Makoto Miura, Research Associate
Yusuke Koshio, Research Associate (till Mar. 2013)
June Kameda, Research Associate
Atsushi Takeda, Research Associate
Kou Abe, Research Associate
Roger Wendell, Research Associate (till Dec. 2015)
Motoyasu Ikeda, Research Associate (from Sep. 2013)
Yasuhiro Nakajima, Research Associate (from Aug.
2016)
Tomonobu Tomura, Project Research Associate (till[1] K. Abe et al., “Letter of Intent: The HyperKamiokande Experiment — Detector Design andPhysics Potential —,” arXiv:1109.3262 [hep-ex],
Cited 487 times.
After Year 2012[2] E. Kearns et al. [Hyper-Kamiokande Working
Group], “Hyper-Kamiokande Physics Opportunities,” arXiv:1309.0184 [hep-ex], Cited 65 times.
[3] K. Abe et al. [Hyper-Kamiokande Working
Group], “A Long Baseline Neutrino Oscillation Experiment Using J-PARC Neutrino
Beam and Hyper-Kamiokande,” arXiv:1412.4673[physics.ins-det], Cited 91 times.
[4] K. Abe et al. [Hyper-Kamiokande ProtoCollaboration], “Physics potential of a longbaseline neutrino oscillation experiment using a J-PARC neutrino beam and HyperKamiokande,” PTEP 2015, 053C02 (2015)

60

doi:10.1093/ptep/ptv061[hep-ex]], Cited 160 times.

[arXiv:1502.05199[5] [Hyper-Kamiokande Proto-Collaboration],“Hyper-Kamiokande Design Report,”
KEK Preprint 2016-21 / ICRR-Report-701-20161, Cited 40 times.
[6] K. Abe et al. [Hyper-Kamiokande Collaboration], “Physics potentials with the second HyperKamiokande detector in Korea,” PTEP 2018,
no. 6, 063C01 (2018) doi:10.1093/ptep/pty044[arXiv:1611.06118 [hep-ex]], Cited 62 times.
[7] [Hyper-Kamiokande Proto-Collaboration],“Hyper-Kamiokande Design Report,”
arXiv: 1805.04163, Cited 4 times.

61

HIGH ENERGY COSMIC RAYDIVISION

62

TA: TELESCOPE ARRAY
Introduction of TA and TALE

The Telescope Array (TA) is the largest UltraHigh Energy Cosmic Ray (UHECR) observatory in thenorthern hemisphere. The main aim of TA is to explore the origin and nature of UHECRs by measuring the energy spectrum, arrival direction distribution
and mass composition. The TA collaboration consistsof researchers from USA, Russia, Korea, Belgium andJapan.
The TA detector consists of a surface array of 507
plastic scintillator detectors (SDs) and three stationsof fluorescence detectors (FDs). It is located in thedesert of Utah in USA. The SDs were deployed on asqure grid with 1.2-km spacing, and the SD array covers approximately 700 km2 . Each SD has two layers of
1.2-cm-thick scintillator of 3 m2 . The full operation ofSDs started in March 2008. The duty cycle is greaterthan 95%. The FDs view 3◦ - 31◦ or 33◦ above horizon. All three FD stations started the observation in
November 2007, and have duty cycles of approximately
10%.
The TA Low-Energy extension (TALE) enables detailed studies of the energy spectrum and composition
from ∼1016 eV upwards. The main aim of TALE is toclarify the expected transition from galactic cosmic rays
to extragalactic cosmic rays and the comparison of thedata with Monte Carlo simulation that takes into account the results of the LHC experiments. The TALE
detector is located north of the TA site. The TALE
FD views 31◦ - 59◦ in elevation angle. The TALE SD
is described later.

Summary from 2012 through 2018

Here we summarize the TA results from 2012
through 2018.
We confirmed the ankle at 1018.7 eV and the flux
suppression above 1019.8 eV.
The TA Xmax measurement above 1018.2 eV is consistent with light composition. We need more statistics
to clarify the feature above 1019 eV.
We found an indication of a cluster of cosmic rays
above 5.7×1019 eV (TA hotspot) by oversampling using
20◦ -radius circles for the first five years of SD data. In
order to confirm the TA hotspot and understand its
feature, we proposed TAx4, which would quadruple the
TA SD aperture with additional two FD sites. We areconstructing TAx4, which is in addition expected to
explore spectrum anisotropy that TA currently sees at
a level of 3σ . And we will measure energy spectrum
and Xmax around the cutoff with more events.
We confirmed the breaks at 1016.2 eV and 1017.0 eV
with the TALE FD. For the precise Xmax measurement,

the TALE SDs were fully deployed at the TALE site.

Results

Here we present the results more in detail about
energy measurement, composition, anisotropy and interdiscplinary research.
Energy Measurement
TA energy spectrum
We published the cosmic-ray energy spectrum for
four years of the TA SD data [6]. The preliminary result of cosmic-ray spectrum for nine years of the TA SD
data is shown in Fig. 1 [72]. The spectrum shows theankle at an energy of 1018.7 eV and the flux suppression
above 1019.8 eV. The statistical significance of having
the same spectral index above the ankle (no suppression) is rejected at ∼7σ .

Fig. 1. The preliminary TA cosmic-ray flux multiplied by E3 for TA
SD nine-year data. The solid line shows the fit of the TA data
to a broken power law.

Fig. 2 shows the TA and Pierre Auger Observatory(Auger) energy spectra with energy shifts by -5.2% and+5.2%, respectively. Significant discrepancy is seen
above around 1019.5 eV, whereas good agreement isseen below 1019.5 eV. The TA and Auger collaborations formed a working group, and compared the TA
and Auger spectra in the common declination (δ ) band(from -15.7◦ to +24.8◦ ) as shown in Fig. 3a. The cutoff
energies agree at 0.5σ level. Based on this study, TA
compared TA energy spectra for δ > 24.8◦ and δ <24.8◦ and found the difference of the cutoff energies asshown in Fig. 3b. The chance probability of obtained
this difference is 3.5 σ [30].
TALE energy spectrum
The energy spectrum using the TALE FD data isshown between ∼2 PeV and 100 PeV as shown in
Fig. 4 [28]. The events observed with the TALE FD are

63

Fig. 2. The TA and Auger energy spectra with energy shifts by-5.2% and +5.2%, respectively.
(a)

(b)

Fig. 3. a) The TA (black) and Auger (red) energy spectra of UHECR
events in the common declination band after energy shifts. b)
The TA energy spectra for the declination angles below 24.8◦ in
red and above 24.8◦ in black

We checked the shape of Xmax distributions, too. After allowing for systematic shfting of the data Xmax distributions and performing the likelihood test on thedata and MC distributions, we find that we fail to reject QGSJet II-04 protons as being compatible with thedata for all energy bins at the 95% confidence level.
QGSJet II-04 helium, nitrogen and iron are rejected
for log10 (E) < 19.0, 19.2, and 19.4, respectively. For
log10 (E) > 19.0, TA has insufficient statistics to distinguish the difference between different composition.
The TA and Auger composition working group reported on a comparison of Xmax distributions measured
by the Auger and TA observatories [78]. A direct comparison of the measured Xmax distributions is not correct
due to different detector acceptances and resolutions aswell as different analsysis techniques. A set of showers
compatible to the composition measured by the Auger
detectors was simulated and reconstructed using theofficial TA software chain. The TA data is within thesystematic uncertainties compatible to a mixed composition as the one measured by the Auger detectors.

placed into three subsets: Cherenkov dominated events,
fluoresence dominated events, and mixed signal events.
And we see two clear breaks at 1016.22 eV and 1017.04 eV
in the energy region measured with the TALE FD. We
possibly see the knee feature at around 1015.6 eV. It becomes of great importance to measure the composition
precisely using hybrid events by adding the TALE surface detector array to understand this spectral feature.
The TALE SD is described later.
E3J(E) [eV2 /m 2 /s/sr]

TALE Energy Spectrum (Monocular)

Fig. 5. The average reconstructed Xmax as a function of energy. The
black, red, magenta, orange and blue colors denote the data,
pure proton, helium, nitrogen and iron QGSJet II-04 predictions,
respectively. The shaded area indicates the size of the total
systematic uncertainty.

5×1024break point 17.04 ± 0.03
4×10243×1024break point 16.22 ± 0.02
2×1024slope:-3.12 ± 0.01

-2.92 ± 0.01

-3.19 ± 0.02

fit χ2 / ndf = 31.6 / 39
1024
15.5

16

16.5

17

17.5

18

18.5

log E [EeV]10

Fig. 4. a) TALE cosmic-ray energy spectrum. The gray band indicates the size of the systematic uncertainties.

Mass Composition
Xmax
The result of the depth of shower maximum Xmax result using the first 8.5 years of hybrid events was published [24]. Fig. 5 shows the evolution of the average
Xmax with energy together with MC expectations. The
result is in agreement with light composition within
systematic uncertainty.

TA SD composition
The result on UHECR mass composition obtainedwith the TA SD is presented [31] using the boosted
decision tree multivariate analysis based on 14 observables sensitive to the properties of the shower front andthe lateral distribution function. The average atomic
mass corresponds to < lnA >= 1.52 ± 0.08 (stat.) ±0.36 (syst.). The obtained composition is qualitatively
consistent with the TA hybrid results.
TA muon studies
The number of muons in inclined air showers observed with water Cherenkov surface detectors at
the Pierre Auger Observatory is approximately 1.8
times that of the proton prediction with QGSJet II-03

64

hadronic model at 1019 eV 1 . We studied muons in air
showers using seven years of the TA SD data [25, 83].
Air shower events are classified using θ , φ and R parameters to search for the condition of high purity ofmuons. Here θ is the zenith angle of the shower axis, φis the azimuthal angle of the location of a surface detector around the shower core on the ground, and R is the
distance of the location of the surface detector from theshower axis. The direction of zero degrees of φ is opposite to the cosmic-ray incident direction projected onto
the ground. The counterclockwise direction is positive.
The condition ( 30◦ < θ < 45◦ and 150◦ < |φ | < 180◦
(the older shower side in an inclined shower), 2000 m< R < 4000 m) gives muon purity of ∼65% from theMC expectation at E ∼1019 eV. Typical ratios of charge
densities of the data to those of the MC are 1.72 ± 0.10
(stat) ± 0.40 (syst) at 1910 m < R < 2160 m and 3.14
± 0.36 (stat) ± 0.72 (syst) at 2760 m < R < 3120 m asshown in Fig. 6. This result is qualitatively consistent
with that by Auger. As described above, Xmax measured
with the TA FD is consistent with light composition.
So the results of muon studies by Auger and TA makean inquiries about hadronic interaction models used in
air shower MC simulations.

Fig. 6. The ratios of the average charge densities of the data to
MC simulations as a function of core distance for 30◦ < θ <45◦ , 150◦ < |φ | < 180◦ and 1500 m < R < 4500 m. The red,
blue , green and yellow represent QGSJET II-03, QGSJET II-04,
EPOS1.99 and SIBYLL2.1, respectively.

Search for EeV photons
We present the search for photons with the first nine
years of the TA SD data, employing multivariate analysis with the classifier based on the Boosted Decision
Tree [79]. There are no photon candidates found in thedata set for 1018.0 , 1018.5 , 1019.0 and 1019.5 eV, and the
diffuse flux limits for photons are obtained.
*1 A. Aab et al., (Pierre Auger Collaboration), Phys. Rev. D 91
032003 (2015).

Arrival Directions of UHECRs
TA hotspot of the highest-energy cosmic rays
We have searched for intermediate-scale anisotropyof 72 cosmic-ray events above 5.7×1019 eV using five
years of the TA SD data [10]. Fig. 7a shows a sky mapof those cosmic rays. The angular resolution of arrival
directions of cosmic rays in this energy range is 1.0◦
to 1.7◦ . A cluster of events appears in this map centered near right ascension of approximately 150◦ anddeclination of approximately 40◦ with a diameter ofaprproximately 30◦ to 40◦ . We found a cluster of events
that we call the hotspot by oversampling using circles
20◦ in radius. The Li-Ma significance (pre-trial significance) plot is shown in Fig. 7b. The hotspot has a
Li-Ma statistical significance of 5.1σ (the number ofobserved events = 19 and the number of expected in
an isotropic cosmic-ray sky = 4.5). The probabilityof such a hotspot appearing by chance in an isotropic
cosmic-ray sky is estimated to be 3.4σ (post-trial significance). There are no known specific sources behind thehotspot. The hotspot is located near the supergalactic
plane, which contains local galaxy clusters. The closest major cluster is the Ursa Major cluster (20 Mpc
from Earth). The angular distance between the hotspot
the supergalactic plane is approximately 19◦ . For the
first nine years of the TA SD data, 143 events above
5.7×1019 eV were observed. The hotspot seems to belarger than originally thought. So we scanned by circles
with different radii and obtained the maximum Li-Ma
significance of 5σ with oversmpling using circles with aradius of 25◦ and its chance probability was 3σ [72].
Hot/cold spot
An energy dependent intermediate-scale anisotropy
was studied using UHECRs above 1019.2 eV for sevenyears of the TA SD data [26]. The energy distributions
inside oversampled circles are compared to that outsideusing the Poisson Likelihood Ratio test. The maximum
pre-trial significance was obtained to be 6.17σ at rightascension of 139◦ and declination of 45◦ . The energy
distribution within the circle at the center of maximum
significance shows a deficit of events below 1019.75 eV
and an excess above 1019.75 eV as shown in Fig. 8. The
post-trial probability of this energy anisotropy, appearing by chance anywhere on isotropic sky, is found to be
9×10−5 (3.74σ ) by MC simulation.
Spectrum anisotropy with respect to the SuperGalactic Plane (SGP)In the cosmic-ray energy region above 1019 eV, theshape of cosmic-ray energy spectrum may carry an imprint of the distribution of the cosmic-ray source density. The SGP consists of local galactic clusters containing more nearby objects. By considering the effectof the loss of UHECRs due to attenuation during the
propagation in the universe, the energy spectrum of

65

(a)

(b)

data as shown in Fig. 9. We found that the Off-source
spectrum has a break at lower energy than the Onsource spectrum. The chance probability of obtaining
such or larger difference in statistically equivalent distributions is estimated to be 6.2±1.1×10−4 (3.2σ ) bya MC simulation [29]. The observed difference in the
spectra is in a reasonable quantitative agreement witha simplified model assuming that the UHECR sources
trace the galaxy distribution from the 2MRS catalogue,
primary particles are protons and the magnetic deflections are neglected.

Fig. 7. Preliminary aitoff projection of the UHECR maps for nine
years of the TA SD data in equatorial coordinates. (a) The blue
data points show the arrival directions of the UHECRs with E> 5.7×1019 eV. The closed and open stars denote the Galactic Center (GC) and the anti-Galactic Center (Anti-GC), respectively. The solid curves indicate the Super-Galactic Plane (SGP)and the Galactic Plane (GP). (b) Li-Ma significance map. Our
FoV is defined as the region above the dashed curve at decl. =-10◦ .

Fig. 9. The energy distributions of UHECR events observed with
the TA SDs. The histogram shows the distribution of all events.
Closed and open symbols show energy distributions observed in
On- and Off-source areas respectively.

Fig. 8. The histogram of energies of events inside the spherical cap
bin of radius 28.43◦ (red) at the maximum pre-trial significance.
It’s compared to the histogram of expected (normalized outside)
energies (blue).

UHECRs in the area near the SGP (On-source area) is
expected to be different from that in the area far fromthe SGP (Off-source area) containg less nearby objects.
Here the On-source area is defined as the area within±30◦ from the SGP latitude of 0◦ and the Off-sourcearea is defined as the area that is the rest in the sky.
The dotted broken power law fit line for open squares
denotes the Off-source spectrum and the solid broken
power law fit line for closed squares denotes the Onsource spectrum for the first five years of the TA SD

Search for EeV protons of galactic origin
Galactic EeV cosmic rays may be seen mostly along
the galactic plane, and there may be a shortage of
events coming from directions near the galactic anticenter. Guided by models of the galactic magnetic fieldthat indicate that the enhancement along the galactic
plane should have a standard deviation of about 20◦
in galactic latitude, and the deficit in the anti-Galactic
Center (Anti-GC) direction should have a standard deviation of about 50◦ , we use the TA data to search forthese effects [20]. Neither an enhancement along thegalactic plane nor a deficit in the Anti-GC direction isfound. We obtain an upper limit on the fraction of EeV
cosmic rays of galactic origin at 1.3% at 95% confidence
level.
Search for point-like sources of EeV neutral particles
We searched for steady point-like sources of EeV
neutral particles for the first five years of the TA SD
data [14]. We found no significant point-like excess

66

above 0.5 EeV. We also serached for coincidence with
the Fermi bright Galactic sources. No significant coincidence was found within the statistical uncertainty.
Interdisciplinary research
TA SD burst events in coincidence with lightning
The TA SD observed several short-time bursts of
air shower like events for the first five years. The expectation of chance coincidence is less than 10−4 . We
found evidence for correlations between these bursts of
the TA SD events and powerful lightning data obtainedwith the National Lightning Detection Network in timing and position [19]. After installting a 3-D lightning
mapping array and electric field change instrument at
the TA SD site, detailed features were observed [27].

TA extension

TAx4
As mentioned above, TA found evidence forintermediate-scale anisotropy of arrival directions ofcosmic rays with greater than 5.7×1019 eV. With enhanced statistics, we expect to observe the structureof the hotspot along with other possible excesses andpoint sources along with the correlations with extreme
phenomena in the nearby universe. We proposed toquadruple the area of the TA SD aperture by installing
additional 500 counters of the current TA SD design
including the existing TA SD array, which will be deployed on a square grid with wider, 2.08-km spacing
between each [36]. The new array would need two FD
stations overlooking it to increase the number of hybrid
events for the confirmation of the energy scale and themeasurement of Xmax . These FDs will be formed using
additional refurbished HiRes telescopes. The layout offour times TA, which we call TAx4, is shown in Fig. 10
together with TALE.
The proposal of the SD part of TAx4 was approved
for the Japan Society for the Promotion of Sciencethrough Grant-in-Aid for Scientific Research on Specially Promoted Research in Japan in 2015 as a fiveyear project. The SD deployment would start in early2019. The FD part of TAx4 was approved by NSF
in the USA in 2016. The TAx4 FD at the northernsite was completed and started the stable operation in
February of 2018. The TAx4 FD at the southern site
is under construction as of summer in 2018.
TALE
The TALE FD operation was commenced in thespring of 2013. We saw two clear breaks in the energy spectrum measured with the TALE FD as shownin Fig. 4. It is of importance to measure Xmax precisely
adding timing information of surface detectors near theshower core on the ground. Therefore we proposed tocomplete the full TALE surface detector array. The

Fig. 10. The layout of the proposed TAx4. The array of 507 SDs(red filled circles on the left) is the current TA SD array. There
are three TA FD stations (Middle Drum (MD) to the north, Long
Ridge (LR) to the west, and Black Rock (BR) to the east of the
TA SD array). The array of surface detectors to the north of the
TA SD array is the TALE SD array. Additional two sub-arraysof 500 surface detectors in total (green filled circles) for TAx4
are located to the northeast and southeast of the TA SD array.
Additional two FD stations with refurbished HiRes telescopes for
the TAx4 are located at the MD and BR FD sites and view tothe northeast and southeast as denoted each by the black frameof the fan.

proposal of adding remaining SDs in the TALE SD array was approved for the Japan Society for the Promotion of Science through Grant-in-Aid for Scientific
Research (S) in 2015 as a five-year project. The TALE
SD array consists of 103 plastic scintillation counters,
which are identical to those of the TA SD array. These
counters have graded spacings, ranging from 400 m near
the TALE FD to 600 m further away, which merge into
the TA SD array with 1200-m spacing at its northwestern corner. The 35 TALE surface detectors were firstly
deployed in 2013. The full TALE SDs were deployedin February of 2017. The TALE SD array is in stable
operation.
The observation of cosmic rays with energies
down to 1015 eV, called the Non-Imaging CHErenkov(NICHE) array [80], was proposed. The plan is to install an array of simple Cherenkov counters of PMTs

67

each three inches in diameter on the ground looking
upwards within the TALE SD array. The part of the
NICHE with 15 PMTs, called jNICHE, was constructedby the Japan Society for the Promotion of Sciencethrough Grant-in-Aid for Young Scientists (A) that wasapproved in 2014.

Associate experiments and prospects

The near future plan is to complete TAx4 and continue data taking of TALE as described above.
The 60 times TA consisting of 10,000 SDs with 2 km
spacing (TA2) was proposed to elucidate the origin of
UHECRs as a next-generation air-shower ground arrayexperiment [32] in the UHECR2012 symposium. The
proposal was reported several times at the Town Meeting hosted by the Cosmic-ray Researchers Congress(CRC) and the ICRR Future Plan Discussion Committee.
The TA site is used worldwidely for R&D of future
detectors. The studies of muon detection in air showers are under way with scintillator with absorber andAuger water tanks as an associate experiment at theTA site. Tests of cosmic-ray observations using new
types of fluorescence telescopes, which aim at future
large and low-cost observations of UHECRs, are being
performed as associate experiments at the TA site. One
is FAST, which consists of a segmented mirror and four
PMTs. Another is CRAFFT, which consists of a 1-m2
Fresnel lens and an 8-inch PMT.
The JEM-EUSO is the space project to investigate
the highest-energy cosmic rays by a wide view UV telescope that will be mounted on the International Space
Station (ISS). The prototype (EUSO-TA) of JEMEUSO has been tested occasionally at the TA site since
December, 2012 2 . K-EUSO consists of Schmidt optics
and a multi-anode-photomultiplier array focal surface,covering a field of view of 40 degrees. The advantage is
its rather uniform and large aperture (annual exposure
is four times TAx4) in the whole sky. It is planned tolaunch in 2022.

Spokespersons
After December 2011 and before December 2017
Hiroyuki Sagawa, ICRR, Univ. of Tokyo
Gordon Thomson, Dept. of Physics, Univ. of Utah
After December 2017
Shoichi Ogio, Dept. of Physics, Osaka City Univ.
Charles Jui, Dept. of Physics, Univ. of Utah
*2 G. Abdellaoui et al., JEM-EUSO Collaboration, “EUSO-TA - First
results from a ground-based EUSO telescope”, Astrop. Phys. 102(2018) 98-111.

Collaboration Name
Institute
Country (∗ )Univ. of Utah
USA
26
Univ. of Yamanashi
Japan
5
Tokyo Insititute of TechnolJapan7
ogy
Hanyang Univ.
Korea
2
Tokyo Univ. of ScienceJapan
3
KindaiUniv.
Japan
1
Yonsei Univ.
Korea
1
KEK
Japan
2
Osaka City Univ.
Japan
22
ICRR, Univ. of Tokyo
Japan
17
Kanagawa Univ.
Japan
3
Saitama Univ.
Japan
5
Rutgers Univ.
USA
2
Tokyo City Univ.
Japan
1
Institute for Nuclear Research Russia
9of the Russian Academy of
Sciences
Waseda Univ.
Japan
2
Chiba Univ.
Japan
2
UNIST
Korea
2
Ewha Womans Univ.
Korea
1
IPMU, Univ. of Tokyo
Japan
1
RIKEN
Japan
3
Kochi Univ.
Japan
1
Ritsumeikan Univ.
Japan
1
Univ. Libre de Bruxelles
Belgium
2
Earthquake Research InstiJapan2tute, Univ. of Tokyo
Hiroshima City Univ.
Japan
1National Institute of RadioJapan1logical Science
Ehime Univ.
Japan
1
Sungkyunkwan Univ.
Korea
6
Ritsumeikan Univ.
Japan
1
Osaka
ElectroJapan
1
Communication Univ.
Shinshu University
Japan
5
Institute of Physics, Czech
Czech
1
Academy of Sciences
Total
139
∗
( ) Number of participants as of June 2018.

Members
StaffsHiroyuki Sagawa, Assoc. Prof., Feb. 2004 to Nov. 2016;
Prof., Nov. 2016 to the present
Takashi Sako, Assoc. Prof. Oct. 2017 to the present
Masahiro Takeda, Assist. Prof., to the present
Toshiyuki Nonaka, Assist. Prof., to the present

68

Kazumasa Kawata, Project Assist. Prof., Apr. 2012
to Mar. 2014; contd TA in Tibet group, Apr. 2014 toMar. 2018; Assist. Prof., Apr. 2018 to the present
Masaki Fukushima, Prof., to Sep. 2015

List of Publications

Postdoctoral Fellows
Tatsunobu Shibata, Apr. 2005 to Mar. 2014
Yuichiro Tameda, Apr. 2010 to Mar. 2013
Daisuke Ikeda, Apr. 2010 to Mar. 2018
Eiji Kido, Apr. 2012 to the present
Naoto Sakaki, Apr. 2016 to Mar. 2018
Toshihiro Fujii, Apr. 2016 to the present[1] H. Tokuno et al., “New air fluorescence detectors employed in the Telescope Array experiment”,
Nucl. Instr. Meth. A676 (2012) 54-65.

Graduate Students
One student was awarded doctor degrees and one student earned master degrees during 2012–2018, supervised by ICRR staff members.

[3] T. Abu-Zayyad et al., “The Energy Spectrum of
Telescope Array’s Middle Drum Detector and the
Direct Comparision to the High Resolution Fly’s
Eye Experiment”, Astropart. Phys. 39-40 (2012)
109.

After Year 2012
Papers in Refereed Journals[2] S. Kawana et al., “Calibration of photomultipliertubes for the fluorescence detector of telescope array experiment”, Nucl. Instr. Meth. A681 (2012)
68-77.

[4] T. Abu-Zayyad et al., “The surface detector arrayof the Telescope Array experiment”, Nucl. Instr.
Meth. 689 (2012) 109.
[5] T. Abu-Zayyad et al., ”Search for anisotropy ofultra-high enenrgy cosmic rays with the Telescope
Array experiment”, Astrophys. J. 757:26 (2012).
[6] T. Abu-Zayyad et al., “The Cosmic Ray Energy
Spectrum Observed with the Surface Detector of
the Telescope Array Experiment”, Astrophys. J.
Lett. 768:L1 (2013).
[7] T. Abu-Zayyad et al., “The energy spectrum of
ultra-high-energy cosmic rays measured by theTelescope Array FADC fluorescence detectors inmonocular mode”, Astropart. Phys. 48 (2013)16-24.
[8] T. Abu-Zayyad et al., “CORRELATIONS OF
THE ARRIVAL DIRECTIONS OF ULTRAHIGH ENERGY COSMIC RAYS WITH EXTRAGALACTIC OBJECTS AS OBSERVED BY THE
TELESCOPE ARRAY EXPERIMENT”, Astrophys. J. 777:88 (8pp), 2013.
[9] T. Abu-Zayyad et al., “Upper limit on the flux ofphotons with energies above 1019 using the Telescope Array surface detector”, Phys. Rev. D88,
112005 (2013).
[10] R.U. Abbasi et al., “INDICATIONS OF
INTERMEDIATE-SCALEANISOTROPY
OF COSMIC RAYS WITH ENERGY GREATER
THAN 57 EeV IN THE NORTHERN SKY MEASURED WITH THE SURFACE DETECTOR OF
THE TELESCOPE ARRAY EXPERIMENT”,
Astrophys. J. Lett. 790:L21 (2014).
[11] P. Tinyakov for the Telescope Array Collaboration,“Latest results from the telescope array”, Nucl. Instr. Meth. A742 (2014) 29-34.

69[12] B.K. Shin et al., “Gain monitoring of telescope array photomultiplier cameras for the first 4 years ofoperation”, Nucl. Instr. Meth. A768 (2014) 96-103.

Ridge FADC Fluorescence Detectors and Surace
Array in Hybrid Mode”, Astrophys. J., 858:76(2018).

[13] A. Aab et al., (Pierre Auger and Telescope Array Collaborations), “SEARCHES FOR LARGESCALE ANISOTROPY IN THE ARRIVAL DIRECTIONS OF COSMIC RAYS DETECTEDABOVE ENERGY OF 1019 eV AT THE PIERREAUGER OBSERVATORY AND THE TELESCOPE ARRAY”, Astrophys. J., 794:172 (2014).

[25] R.U. Abbasi et al., “Study of muons from ultrahigh energy cosmic ray air showers measured with
the Telescope Array experiment”, Phys. Rev. D 98,
022002 (2018).

[14] R.U. Abbasi et al., “A NORTHERN SKY SURVEY FOR POINT-LIKE SOURCES OF EeV
NEUTRAL PARTICLES WITH THE TELESCOPE ARRAY EXPERIMENT”, Astrophys. J.
804:133 (2015).
[15] R.U. Abbasi et al., “Study of Ultra-High Energy
Cosmic Ray composition using Telescope Array’s
Middle Drum detector and surface array in hybridmode”, Astropart. Phys. 64 (2015) 49-62.
[16] R.U. Abbasi et al., “Measurement of the proton-air
cross section with Telescope Array’s Middle Drumdetector and surface array in hybrid mode”, Phys.
Rev. D 92, 032007 (2015).
[17] R.U. Abbasi et al., “The energy spectrum of cosmic rays above 1017.2 eV measured by the fluorescence detectors of the Telescope Array experiment in seven years”, Astropart. Phys. 80 (2016)
131-140.
[18] R.U. Abbasi et al., “First upper limits on the radarcross section of cosmic-ray induced extensive air
showers”, Astropart. Phys. 87 (2017) 1-17.
[19] R.U. Abbasi et al. “The bursts of high energy
events observed by the telescope array surface detector”, Phys. Lett., A381 (2017) 2565-2572.
[20] R.U. Abbasi et al., “Search for EeV protons ofgalactic origin”, Astropart. Phys. 86 (2017) 21-26.
[21] B.R. Dawson, M. Fukushima, P. Sokolsky, “Past,
present, and future of UHECR observations”,
Prog. of Theor. and Exp. Phys., 2017, 12A101.
[22] V. Verzi, D. Ivanov, and Y. Tsunesada, “Measurement of energy spectrum of ultra-high energy cosmic rays”, Prog. of Theor. and Exp. Phys., 2017,
12A103.
[23] O. Deligny, K. Kawata, P. Tinyakov, “Measurement of anisotropy and the search for ultra highenergy cosmic ray sources”, Prog. of Theor. andExp. Phys. 2017, 12A104.
[24] R.U. Abbasi et al., “Depth of Ultra HIgh Energy
Cosmic Ray Induced Air Shower Maxima Measured by the Telescope Array Black Rock and Long

[26] R.U. Abbasi. et al., “Evidence of Intermediatescale Energy Spectrum Anisotropy of Cosmic Rays
E ≥ 1019.2 eV with the Telescope Array Surface
Detector”, Astrophys. J. 862:91 (2018).
[27] R.U. Abbasi et al., “Gamma Ray Showers Observed at Ground Level in Coincidence With
Downward Lightning Leaders”, J. Geophys. Res:
Atmos., 123 (2018).
[28] R.U. Abbasi et al. “Cosmic-Ray Energy Spectrum
between 2 PeV and 2 EeV Observed with the TALE
detector in monocular mode”, accepted for Astrophys. J., arXiv:1803.01288.
[29] R.U. Abbasi et al., “Search for Anisotropy in the
Ultra High Energy Cosmic Ray Spectrum using
the Telescope Array Surface Detector”, submitted
to Phys. Rev. Lett., arXiv:1707.04967.
[30] R.U. Abbasi et al., “Evidence for Declination Dependence of Ultrahigh Energy Cosmic Ray Spectrum in the Northern Hemisphere”, submitted to
Astrophys. J., arXiv:1801.07820.
[31] R.U. Abassi et al., “Mass composition of ultrahigh-energy cosmic rays with the Telescope Array
Surface Detector Data”, submitted to Phys. Rev.
D., arXiv:1808.03680.
Papers in Conference Proceedings
for ICRR TA members and others referred in this report[32] S. Ogio, “Future plans of the Telescope Arrayexperiment”, International Symposium on Future Directions in UHECR Physics (UHECR2012),
CERN, Geneva, Switserland, 2012/2/16 (oral).
[33] D. Ikeda, “Recent Results of Energy Spectrum and
Mass Composition from Telescope Array Fluorescence Detec”, 23rd European Cosmic Ray Symposium (ECRS-2012), Moscow, Russia, 2012/7/3-7/7(poster).
[34] H. Sagawa, “Detection of the Atmospheric Showers
with Telescope Array (TA) experiment”, Conference on Calorimetry for the High Energy Frontier(CHEF2014), Paris, France, 2013/4/25 (oral).

70[35] H. Sagawa, “Highlights from the Telescope Array Experiment”, 33rd International Cosmic Ray
Conference (ICRC2013), Rio de Janeiro, Brazil,
2013/7/4 (invited).
[36] H. Sagawa, “The plan of the Telescope Array Experiment for the Next Five Years”, ICRC2013,2013/7/2-7/9 (poster).
[37] K. Kawata, “Search for EeV Neutral Particles
from the Point-like Sources with the Telescope Array Surface Detector”, ICRC2013, 2013/7/2-7/9(poster).
[38] K. Kawata, “Search for the Large-Scale CosmicRay Anisotropy at 1018 eV with the Telescope Array Surface Detector”, ICRC2013, 2013/7/2-7/9(poster).
[39] E. Kido, “Constraining UHECR source models
by the TA SD energy spectrum”, ICRC2013,2013/7/4 (oral).
[40] D. Ikeda, ‘Ultra-High Energy Cosmic Ray Spectrum Measured by the Hybrid Analysis in the Telescope Array‘”, ICRC2013, 2013/7/2-7/9 (poster).
[41] T. Shibata, “Absolute energy calibration of
the Telescope Array Fluorescence Detector with
an Electron Linear Accelerator”, ICRC2013,2013/7/3 (oral).
[42] D. Ikeda, “Test of Radar Echo Detection using theElectron Beam from the ELS at the Telescope Array Site: A Test for Future Large Scale Extension of the Air Shower Observatory”, ICRC2013,2013/7/2-7/9 (poster).
[43] T. Nonaka, “Design and prospect of a surface
Muon Detector for Telescope Array experiment”,
ICRC2013, 2013/7/2-7/9 (poster).
[44] M. Fukushima, “Observation of UHECRs - status and prospects -”, Symposium on Cosmology
and Particle Astrophysics (CosPA 2013), Honolulu, USA, , 2013/11/15 (oral).
[45] E. Kido, “Recent Results from the Telescope Arrayexperiment”, High Energy Messengers: Connecting the Non-Thermal Extragalactic Backgrounds(HEM 2014), Chicago, USA, 2014/6/9 (oral).

(ISVHECRI 2014), CERN, Geneva, 2014/8/19(oral).
[48] M. Fukushima, “Observation of UHECRs in thenext decade”, 24th European Cosmic Ray Symposium (ECRS 2014), Keil, Germany, 2014/9/1-5(oral).
[49] H. Sagawa, “TAx4”, Ultra-High-Energy CosmicRay Conference 2014 (UHECR 2014), Springdale,Utah, USA, 2014/10/14 (oral).
[50] E. Kido, “Simulations of Ultra High Energy Cosmic Rays propagation”, UHECR2014, 2014/10/13(oral).
[51] T.
Nonaka,“TA-muon”,
2014/10/14 (oral).

UHECR2014,[52] H. Sagawa, “Recent results from the Telescope Array - studies of ultra-high energy cosmic rays andthe prospect -”, International Conference on the
Structure and the Interactions of the Phton including the 21th International Wrokshop on PhotonPhoton Collisions and the International Workshop
on High Energy Photon Colliders (Photon2015),
Novosibirsk, Russia, 2015/6/15 (invited).
[53] K. Kawata, “Ultra-High-Energy Cosmic Ray Conference, 34th International Cosmic Ray Conference (ICRC2015), The Hauge, The Netherlands,
2015/7/30 (oral).
[54] R. Takeishi for the Auger and TA Collaborations,“Initial results of a direct comparison between the
Surface Detectors of the Pierre Auger Observatory
and the Telescope Array”, ICRC2015, 2015/7/30(poster).
[55] T. Nonaka, “Performance and Operational Statusof Muon Detectors in the Telescope Array Experiment”, ICRC2015, 2015/7/30. (poster).
[56] D. Ikeda, “Energy Spectrum and Mass Composition of Ultra-High Energy Cosmic Rays Measured by the hybrid technique in Telescope Array”,
ICRC2015, 2015/7/30 (poster).
[57] T. Nonaka, “Anisotropy search in the Ultra HighEnergy Cosmic Ray Spectrum in the Northern
Hemisphere using the Telescope Array surface detector”, ICRC2015, 2015/7/30 (poster).

[46] K. Kawata, “Telescope Array Experiment: Recent Results and Future Plans”, Joint TeV Particle
Astrophysics / IDM Conference (TEVPA/IDM),
Amsterdam, The Netherlands, 2014/6/23-24(oral).

[58] T. Fujii, “Energy Spectrum and Mass Compositionof Ultra-High Energy Cosmic Rays Measured with
the Telescope Array Fluorescence Detector Using a Monocular Analysis”, ICRC2015, 2015/7/30(poster).

[47] M. Fukushima, “TA Recent Results andProspects”, The XVIII International Symposium
on Very High Energy Cosmic Ray Interactons[59] E. Kido, ”Interpretation of the energy spectrum
observed with the Telescope Array surface detectors”, ICRC2015, 2015/8/4 (oral).

71[60] H. Sagawa, “Telescope Array extension: TAx4”,
ICRC2015, 2015/8/5 (oral).
[61] H. Sagawa, “Telescope Array Extension”, Cosmic
Ray International Seminar 2015 (CRIS2015), Gallipoli, Italy, 2015/9/14 (invited).
[62] T. Fujii, “Energy spectrum measured by theTelescope Array experiment in 1015.8 eV to1020.3 eV range”, TeV Particle Astrophysics 2015
(TEVPA2015), Kashiwa-no-ha Conference Center,
Kashiwa, Chiba, 2015/10/26 (oral).
[63] T. Nonaka, “Results from the Telescope Array experiment, Hotspot and anisotropy”, TEVPA2015,
2015/10/26 (invited).
[64] T. Nonaka, “Surface detector for TAx4 expansion
and status of Muon measurement at TA site”,
Next-Generation Techniques for UHE Astroparticle Physics, Chicago, IL, USA, 2016/2/29 (invited).

[73] D. Ikeda, “Hybrid Measurement of the Energy
Spectrum and Composition of Ultra-High Energy
Cosmic Rays by the Telescope Array”, ICRC2017,2017/7/18 (poster).
[74] T. Fujii,“A systematic uncertainty on the energy
scale of the Telescope Array fluorescence detectors”, ICRC2017, 2017/7/18 (poster).
[75] T. Nonaka, “Anisotropy search in Energy distribution in Northern hemisphere using Telescope Array Surface Detector data”, ICRC2017, 2017/7/18(poster).
[76] R. Takeishi, “Study of muon from ultra-highenergy cosmic ray air showers measured with
the Telescope Array experiment”, ICRC2017,2017/7/18 (poster).
[77] E. Kido, “Interpretation of the energy spectrum
observed with the Telescope Array detectors”,
ICRC2017, 2017/7/18 (poster).

[65] D. Ikeda, “Recent Results and Future Prospects
from the Telescope Array experiment”, 10th Cosmic Ray International Seminar (CRIS2016), Ischia/Naples, Italy, 2015/7/6 (invited).

[78] V. de Souza, “Testing the agreement between the
Xmax distributions measured by the Pierre Auger
and Telescope Array Observatories”, ICRC2017,2017/7/18 (oral).

[66] E. Kido, “The extension of the Telescope Array experiment”, 38th International Conference on HighEnergy Physics (ICHEP2016), Chicago, IL, USA,
2016/8/8 (poster).

[79] . G. Rubtsov, “Telescope Array search for EeV
photons and neutrinos”, ICRC2017, 2017/7/17(oral).

[67] T. Nonaka,“Anisotropy Search in Energy Distribution in the Northern Hemisphere Using the Telescope Array Surface Detector Data”, 2016 International Conference on Ultra-High Energy Cosmic
Rays (UHECR2016), Kyoto Research Park, Kyoto, Kyoto, Japan, 2016/10/11 (oral).
[68] R. Takeishi, “Studies of Muons in Extensive AirShowers from Ultra-High Energy Cosmic Rays Observed with the Telescope Array Surface Detector”,
UHECR2016, 2016/10/12 (poster).
[69] E. Kido, “The TAx4 experiment”, UHECR2016,2016/10/14 (oral).
[70] D. Ikeda, “Recent Results from the Telescope Array Experiment”, Astroparticle Physics YachayTech, Quito/Urcuqui,Ecuador, 2016/12/6 (invited).
[71] E. Kido, “The TAx4 experiment”, The 35th International Cosmic Ray Conference (ICRC2017),
Busan, Korea, 2017/7/15 (oral).
[72] J.N. Matthews, “Highlights from the Telescope Array”, ICRC2017, 2017/7/17 (invited).

[80] D. Bergman, “j-NICHE: Prototype detectors of a
non-imaging Cherenkov array at the Telescope Array site”, ICRC2017, 2017/7/17 (oral)[81] H. Sagawa, “Latest results of the Telescope Array experiment on ultra-high energy cosmic rays”,
Second KIAA Workshop on Astroparticle Physics,
Beijing, China, 2017/8/17 (invited).
[82] R. Takeishi, “Observation of ultra-high energy cosmic rays with the Telescope Array experiment”,
6th International Conference on New Frontiers in
Physics (ICNFP2017), Grete, Greek, 2017/8/21(oral).
Doctor theses[83] R. Takeishi, ”Study of muons from ultra-high energy cosmic ray air showers measured with theTelescope Array experiment”, University of Tokyo(2016).

72

Tibet ASγ PROJECT
Introduction

The Tibet air shower experiment has been successfully operated at Yangbajing (90◦ 31’ E, 30◦ 06’ N;
4300 m above sea level) in Tibet, China since 1990.
It has continuously made a wide field-of-view (approximately 2 steradian) observation of cosmic rays andgamma rays in the northern sky.

Summary from 2012 to 2018

Summarized here are the noticeable achievements
during the period of this review, i.e., between 2012 and2018.
• The 2-dimensional anisotropy (0.1 % level) at
sidereal time frame in the multi-TeV energy range
is characterized by the “Tail-in” (hump) and“Loss-cone” (dip) structures. The Milagro experiment claimed the loss cone ampliture showed a
yearly variation. However, we found no such variation in the observed anisotropy.
We measured the cosmic-ray anisotropy from a
10 TeV to 1 PeV. Above 100 TeV, we found theanisotropy distinct from that at multi-TeV region.
The Loss cone and the Tail-in structure becomesless significant and a new component appeared.
This result obtained in the northern hemisphereis consistent with the IceCube observation in thesouthern hemisphere.
• A clear solar-cycle variation of the Sun’s shadow
in 10 TeV cosmic rays are observed by the Tibet
air shower array covering a full solar cycle from1996 to 2009. Numerical simulations of the Sun’sshadow are developed, employing the Potential
Field Source Surface (PFSS) model and the Current Sheet Source Surface (CSSS) model for thecoronal magnetic field, to interpret the physicalimplications of the observed solar cycle variation.
It is found that the intensity deficit in the simulated Sun’s shadow is very sensitive to the coronalmagnetic field structure, and the observed variation of the Sun’s shadow is better reproduced by
the CSSS model than the PFSS model. This is
the first successful attempt to evaluate the coronal magnetic field models by means of the Sun’sshadow observed in 10 TeV cosmic rays.
We analyzed the Sun shadow in cosmic rays andfound that the shadow center displaced northward(southward) from the optical center of the Sun inthe ”away” (”toward” ) interplanetary magnetic
field (IMF) sector. We found that the average

IMF strength in the away (toward) sector is 1.54
± 0.21stat ± 0.20sys (1.62 ± 0.15stat ± 0.22sys ) times
larger than the expected from the numerical simulation assuming the solar magnetic field model.
This demonstrates that the obsrvation of the Sun
shadow in cosmic rays enables us to make a quantitative evaluation of the average solar magnetic
field between the Sun and the Earch, which is difficult by satallite experiments.
• Based on data collected by the Tibet air shower
array and the 100 m2 underground muon detector(MD) during 438 live days, we search for continuous gamma-ray emission from the Crab Nebula
above 100 TeV. No significant excess was found,and the most stringent upper limit is obtained
above 140 TeV, even with the small 100 m2 MD.
• As an upgrade, new air shower core detectors(YAC-II) were constructed and started datataking in 2014 for the next-step measurement of
the chemical composition in the knee energy region, while large (∼4,000 m2 ) muon detectors(MD) aiming at higher-sensitivity (by a facto of∼10) gamma-ray observation in the 100 TeV region was constructed under the Tibet air shower
array and started data-taking in 2014. The MD
also plays an important role to measure the chemical composition of cosmic rays in the knee energy
region.

Experiment

The Tibet I array was constructed in 1990 and it
was gradually upgraded to the Tibet II by 1994 which
consisted of 185 fast-timing (FT) scintillation counters
placed on a 15 m square grid covering 36,900 m2 , and36 density (D) counters around the FT-counter array.
Each counter has a plastic scintillator plate of 0.5 m2
in area and 3 cm in thickness. All the FT counters
are equipped with a fast-timing 2-inch-in-diameter photomultiplier tube (FT-PMT), and 52 out of 185 FT
counters are also equipped with a wide dynamic range
1.5-inch-in-diameter PMT (D-PMT) by which we measure up to 500 particles which saturates FT-PMT output, and all the D-counters have a D-PMT. A 0.5 cm
thick lead plate is put on the top of each counter inorder to increase the counter sensitivity by converting
gamma rays into electron-positron pairs in an electromagnetic shower. The mode energy of the triggered
events in Tibet II is 10 TeV.
In 1996, we added 77 FT counters with a 7.5 m lattice interval to a 5,200 m2 area inside the northern part73

the energy scale uncertainty at ±12 % level.
Tibet III Air Shower Array (2003)

15 m

36,900 m2

FT Detector (512)FT Detector w/ D−PMT(249)
Density Detector (28)789 detectors

Fig. 1. Schematic view of Tibet III.

of the Tibet II array. We called this high-density array
Tibet HD. The mode energy of the triggered events in
Tibet HD is a few TeV.
In the late fall of 1999, the array was further upgraded by adding 235 FT-counters so as to enlarge thehigh-density area from 5,200 m2 to 22,050 m2 , and wecall this array and further upgraded one Tibet III. In
2002, all of the 36,900 m2 area was covered by the highdensity array by adding 200 FT-counters more. Finally
we set up 56 FT-counters around the 36,900 m2 highdensity array and equipped 8 D-counters with FT-PMT
in 2003. At present, the Tibet air shower array consistsof 761 FT-counters (249 of which have a D-PMT) and
28 D-counters as in Fig. 1.
The performance of the Tibet air shower array has
been well examined by observing the Moon’s shadow(approximately 0.5 degrees in diameter) in cosmic rays.
The deficit map of cosmic rays around the Moon
demonstrates the angular resolution to be around 0.9◦
at a few TeV for the Tibet III array. The pointing error
is estimated to be better than ∼0.01◦ by the displacement of the shadow’s center from the apparent centerin the north-south direction, as the east-west component of the geomagnetic field is very small at the experimental site. On the other hand, the shadow center
displacement in the east-west direction due to the geomagnetic field enables us to spectroscopically estimate

Physics Results

TeV gamma-ray astrophysics
Gamma rays at TeV energies from the Crab Nebula were first detected by the Whipple collaboration in
1989. Since the detection, the energy spectrum of the
Crab Nebula has been measured by many imaging air
Cherenkov telescopes as well as air shower arrays; theTibet ASγ collaboration was the first air shower array
which detected gamma rays from the Crab Nebula atmulti-TeV energies. The Crab Nebula is now commonlyused as a standard candle for gamma-ray experiments.
The energy spectrum is reproduced with a mechanism
based on the synchrotron self-Compton (SSC) emissionof high energy electrons. None of the experiments has
detected gamma rays above 100 TeV from the Crab
Nebula, and the best upper limits have been given by
the CASA-MIA experiment. The observation of theenergy spectrum of the Crab Nebula around 100 TeV
with higher sensitivity is very important in order toconfirm the leptonic origin of the Crab’s TeV gammaray emission. Therefore, a search is made for gamma
rays above ∼100 TeV from the Crab Nebula, using the
Tibet air shower (AS) array combined with a 100 m2
muon detector (MD).
Using the data collected by the 100 m2 MD fromMarch 2008 to February 2010 (438 live days), we searchfor continuous gamma-ray emission from the Crab Nebula above 100 TeV. No significant excess is found,and the most stringent upper limit is obtained above
140 TeV, as shown in Fig. 2, even with the small 100 m2
MD [3].
Cosmic-ray anisotropy in the multi-TeV region
with high precision
Galactic cosmic rays at TeV energies reach the Earthalmost isotropically, due to strength of the galacticmagnetic field disturbing the trajectory of cosmic rays.
The Larmor radius of a 1 TeV proton in a lG magnetic
field is 0.001 pc, many orders of magnitude shorter thanthe distance to any potential source of cosmic rays. Numerous experiments including the Tibet air-shower experiment, on the other hand, reported the existenceof a small (∼0.1%) galactic cosmic- ray anisotropy at
sidereal time frame. They consistently found that there
are two distinct large-scale structures in the anisotropy;
one is a deficit region in the cosmic-ray flux called the”loss-cone”, distributed between 150 degrees and 240
degrees in right ascension, and the other an excess region called the ”tail-in”, distributed between 40 degrees
and 90 degrees in right ascension. The common understanding of the origin of the anisotropy has not been
established until now.
The Milagro experiment reported a steady increasein the amplitude of the loss-cone at 6 TeV from July

74

Amplitude of Loss Cone (%)

0.5

0.4

2000

2002

2004

2006

Tibet 4.4TeV
6.2TeV
12TeV
Milagro median 6TeV

0.3

0.2

0.1

0
52000

52500

53000

53500

54000

MJD

Fig. 3. From Ref. [1]. Time dependence of the loss-cone amplitude
observed by the Tibet experiment at 4.4 TeV (open squares), 6.2
TeV (filled circles) and 11 TeV (open diamonds), along with Milagro’s data (open inverse triangles) and the best-fit linear function
to Milagro’s data. The MJDs of our data points correspond to
the times at the middle of each period. The upper horizontal
axis represents years. All the error bars are the linear sums of thestatistical and systematic errors.

Fig. 2. From Ref. [3] (references therein) . (a) Upper limits on
the Crab Nebula’s differential flux at the 90% confidence levelobtained by this work (thick solid lines with downward arrows).
The upper limits obtained by this work assuming zero excess
counts are drawn by thin solid lines with downward arrows. The
flux points measured by our previous work (closed circles) as well
as by HEGRA (open inverse triangles), CANGAROO (filled andopen triangles), MAGIC (open pentagons), and H.E.S.S. (filledinverse triangles) are also shown. (b) Upper limits on the Crab
Nebula’s integral flux at the 90% confidence level obtained bythis work (filled circles with downward arrows), along with theupper limits at the 90% confidence level by CASA-MIA (opensquares with downward arrows). The upper limits obtained bythis work assuming zero excess counts are drawn by open circles
with downward arrows.

2000 to July 2007, in the latter half of the 23rd solar cycle. Milagro was a water Cherenkov detector located in
New Mexico at the altitude of 2630 m above sea level,composed of an 80 m × 60 m × 8 m pond surrounded
by a 200 m × 200 m array of 175 water tanks. Usinga data set consisting of 96 billion cosmic-ray events,
which is unprecedentedly large at TeV energies, Milagro significantly claimed the detection of the increase in
the loss-cone amplitude at 6 TeV. The Tibet air-shower
experiment would be the only experiment that operated during the corresponding period and is capable ofexamining Milagro’s results with comparable statistics
at TeV energies. We examined Milagro’s claim using

the data collected by the Tibet air-shower experiment
during 1916 live days from November 1999 through December 2008.
Figure 3 shows the time dependence of the loss-cone
amplitude measured at 4.4, 6.2, and 11 TeV by the Tibet experiment in comparison with that measured at 6
TeV by Milagro. It can be clearly seen that, contrary to
Milagro’s results, the loss-cone amplitude measured by
the Tibet experiment is quite stable at 4.4, 6.2, and 11
TeV during the period from 2000 to 2008 [1]. Milagro
argued that variations in the heliosphere in relation tosolar activities could cause the time dependence of thesidereal cosmic-ray anisotropy. In that case, the sametendency would be seen at sub-TeV energies, where theanisotropy is expected to be much more sensitive tosolar activities. Matsushiro underground muon observatory, however, reports that the loss-cone amplitude
shows no significant increase at 0.6 TeV during the corresponding period, suggesting that the steady increasein the loss-cone amplitude observed by Milagro at 6
TeV is not caused by the solar modulation effects.
Recently, IceCube reported the anisotropy observedin the southern sky, showing a new phenomenon different from that obtained by EAS-TOP in 2012. A clear
deficit with a post-trial significance of -6.3σ at 400 TeV
was detected, which was then confirmed by the result
from Ice-Top in 2013. The Ice-Top data further revealed the existence of anisotropy at energies up to 1
PeV in 2013. On the other hand, the anisotropy feature above ∼300 TeV observed by the Tibet air shower
array in 2006 was found to be different from those inlower energy regions and in agreement with IceCube’s
result at 400 TeV.
By improving the reconstruction of primary energy,
we will also extend the analyzed energy range to two

75

Fig. 4. From Ref. [6]. 2D anisotropy maps in five energy samples (15, 50, 100, 300, and 1000 TeV, from top to bottom). Left panels show
the relative intensity maps (with 30◦ smoothing), while right panels show the 1D projections. The blue curve shows the first harmonic
fitting to the data.

decades between 10 TeV and 1 PeV, which is also thewidest coverage of such works. Analysis of the 10 - 1000
TeV large-scale sidereal anisotropy of gaalactic cosmic
rays was made with the data collected by the Tibet
air shower array from 1995 October to 2010 February[6]. In this analysis, we improve the energy estimate
and extend the declination range down to -30◦ . Our
result is shown in Fig. 4. We find that the anisotropy
maps above 100 TeV are distinct from that at a multiTeV band. The so-called tail-in and loss-cone features
identified at low energies get less significant, and a newcomponent appears at ∼100 TeV. The spatial distribution of the galactic cosmic-ray intensity with an excess(7.2σ pre-trial, 5.2σ post-trial) and a deficit (-5.8σ pretrial) are observed in the 300 TeV anisotropy map, consistent with IceCube’s results at 400 TeV. We further
find that the amplitude of the first order anisotropy
increases sharply above ∼100 TeV, indicating a newcomponent of the anisotropy. These results will makesignificant contribution to understanding the origin andpropagation of galactic cosmic rays.
Global 3-dimensional structure of the solar andinterplanetary magnetic fields by observing theSun’s shadow in cosmic rays
An understanding of the structure of the Sun’s magnetic field between it and the Earth is essential if hu-

manity is to venture into space. However, it is extremely difficult to directly observe the coronal magnetic structure within the Earth’s orbit, and while
spacecraft such as Voyager and Ulysses have carriedout some observations, the solar neighborhood is an extremely harsh environment characterized by high temperatures and high levels of radiation. Even the latest
spacecraft cannot get close to the Sun, and as a result
our understanding of this region is limited.
The Tibet AS γ experiment observed the Sun’sshadow of cosmic rays at 10 TeV energies from 1996
to 2009, created when it shields galactic cosmic rays
reaching the Earth.
The coronal magnetic field deflects these charged
particles as they pass near the Sun, causing variationin the form of the Sun’s shadow. The analyzed data
were recorded over the 14-year period from 1996 to
2009, and an anti-correlation was discovered between
the 11-year solar cycle and this variation, as shown in
Fig. 5. The stable size of the Moon’s shadow in cosmic rays over the same period demonstrates that thesystematic uncertainty is negligible.
We have also developed and performed numericalsimulations of the Sun’s shadow in galactive cosmic
rays, based on two major coronal magnetic field models - the Potential Field Source Surface (PFSS), andthe Current Sheet Source Surface (CSSS). The PFSS
model leaves out the effect of the electric current in theSunspot number

76
180

a

140
100
60
20

Moon’s shadow
Deficit (%)

Sun’s shadow
Deficit (%)

−1

b

−2
−3
−4
PFSS Rss=2.5R
CSSS Rss=2.5R
CSSS Rss=10R

−5
−6
−1

c

−2
−3
−4
−5
−6
1996

1998

2000

2002

2004

2006

2008

2010

Year

Fig. 5. From Ref. [2]. Temporal variations of (a) the monthly
mean sunspot number, (b) the deficit intensity due to the Sun’sshadow and (c) the deficit intensity due to the Moon’s shadow.
The open squares in the panel (b) are the observed central deficit(Dobs ). The blue triangles, green squares and red circles indicate
the central deficits (DMC ) by the MC simulations assuming the
PFSS (Rss = 2.5RJ ), the CSSS (Rss = 2.5RJ ) and the CSSS
(Rss = 10.0RJ ) models, respectively. RJ is the solar radius,
while Rss is the radius of the source surface (SS) beyond which
the magnetic field lines become radial. The dashed lines in thepanels (b) and (c) are the deficits expected from the apparent
angular size of the Sun and the Moon.

vicinity of the Sun on the structure of the coronal magnetic field, while the CSSS model assumes the effect of
the electric current.
The simulation of the trajectories of galactic cosmic
rays based on the two models found that the CSSS
model better describes the size variation of the Sun’sshadow, as shown in Fig. 5.
This is the world’s first and most successful analysis of the coronal magnetic structure using the dataobtained by the observation of the cosmic ray shadowof the Sun. This research provides a new approach to
exploring the Sun’s magnetic structure and, with future increased in accuracy, should provide even moredetailed knowledge of the magnetic filed between the
Sun and the Earth. This research employed the phenomenon that the path of charged cosmic rays is bent
when it passes through a magnetic field, and was madepossible by the long-term accumulation of cosmic radiation data over a period of 14 years, and is a major
step forward in the long-term observation of cosmic radiation. The paper from this result was highlighted as
one of the ”Editor’s Suggestions” in Physical Review
Letters (PRL) in 2013[2].
The strong coronal magnetic field affects the cosmicray deficit in the shadow, while the interplanetary magnetic field (IMF) between the Sun and the Earth deflects trajectories of TeV cosmic rays a little. The
small deflection has actually been observed by some air
shower experiments as the North-South displacement of
the Sun’s shadow center from the optical center of theSun. We can utilize the observed features of the Sun’sshadow as a probe of the solar magnetic field. The solar
magnetic field on the photosphere has constantly beenmonitored by optical measurements employing the Zeeman effect, while the local IMF at the Earth has been
directly observed by an near-Earth satellite. However,
the observation of the average IMF between the Sun
and the Earth is still difficult. As the deflection angle of cosmic rays is proportional to the magnetic fieldstrength, the observed Sun’s shadow can be used forevaluating the IMF strength averaged over the distance
between the Sun and the Earth.
We use cosmic-ray data recorded during the period
between the year 2000 and the year 2009 by the TibetIII air shower array for the analysis of the Sun’s shadow.
The observed air shower events are divided into sevenenergy bins (representative energies: 4.9, 7.7, 13, 22,
43, 90, and 240 TeV), according to the shower size ΣρFT
which is the sum of the number of particles per 1 m2 asa measure of energy of a primary cosmic-ray particle.
The number of on-source events (Non ) is defined as
the number of air shower events coming from the direction within a circle of ∆d radius centered at a given
point on the celestial sphere. The number of background, namely, off-source events (Noff ) is calculated byaveraging the number of events within each of the eightoff-source windows located at the same zenith angle as
the on-source one. Subsequently, the flux deficit relative to the number of background events is estimatedas Dobs = (Non - Noff )/Noff at every 0.1◦ grid of the Geocentric Solar Ecliptic (GSE) longitude and latitude.
The IMF sector polarity is allocated to each day inreference to the daily-averaged GSE-x and GSE-y components of the IMF (Bx , By ) observed by the near-Earthsatellite and the average Dobs in ”Away” and ”Toward”
sectors is separately calculated. The ”Away” (”Toward”) sector polarity is allocated to a day when the
IMF observed two days later satisfies Bx <0 and By >0(Bx >0 and By <0).
The average N-S and E-W displacement angles in”Away” and ”Toward” sectors are calculated for eachenergy bin and plotted as functions of cosmic ray rigidity in Fig. 6. We convert the representative energy ofeach energy bin to the average rigidity of the Sun’sshadow by considering the energy spectra and chemical composition of primary cosmic rays based on the
direct measurements. As expected from the magnetic
deflection of charged particles, the observed displacement angles displayed by black solid circles are reasonably fitted by a function α /(R/10TV) of rigidity R in
TV with a fitting parameter α standing for the deflection angle at 10 TV.
In the N-S displacement in Fig. 6(a) and 6(b), we
see that the magnitudes of the expected shift (broken curves) are smaller than the magnitudes of theobserved one (solid curves) in both sectors, implying
an underestimation by the MC simulations. On theother hand, the expected E-W shifts in Figure 2(c) and2(d), are consistent with the observation, which indi-

77
E-W Displacement(b) Toward0

1.4
ObservedMC
Observed Best Fit
MC Best Fit

1.2
1
0.8

−0.2
−0.4

α Obs = 0.303 ± 0.053 −0.6(± 0.038stat ± 0.036sys)

0.6(± 0.034stat ± 0.049sys)

−0.8

0.4

−1

0.2

−1.2

0

α Obs = -0.407 ± 0.060

−1.4
10

100

GSE Longitude (deg)
West
East

GSE Latitude (deg)
South
North

N-S Displacement(a) Away(c) Away(d) Toward

0

0

−0.2

−0.2

−0.4

−0.4

−0.6

α Obs = -0.242 ± 0.046 −0.6(± 0.034stat ± 0.029sys)

−0.8
−1

−1

−1.2

−1.2

−1.4

10

100

α Obs = -0.205 ± 0.038(± 0.029stat ± 0.025sys)

−0.8

−1.4
10

100

10

100

Rigidity R (TV)Fig. 6. From Ref. [8]. The rigidity dependence of the N-S [(a), (b)] and E-W [(c), (d)] displacements of the center of the Sun’s shadow
in away and toward sectors. Black solid circles and red open triangles in each panel show the observed and simulated displacements,
respectively, each as a function of the rigidity (R) on the horizontal axis. The error bar of each solid circle indicates the statisticalerror. Black solid and red broken curves display the function of α /(R/10 TV) best fitting to black solid circles and red open triangles,
respectively. The best-fit parameter α to the observed data is indicated in each panel with systematic errors estimated from thesystematic error of the primary energy in our analyses of the Moon’s shadow.

cates the E-W shifts are mainly caused by the deflectionof cosmic-ray trajectories in the geomagnetic field. It is
cofirmed that α in the E-W shifts averaged over ”Away”
and ”Toward” sectors is -0.212 ± 0.034 (±0.023stat ±0.025sys ) and consistent with the E-W shiftss in the
Moon’s shadow. Hence, it is suggested that the underestimation of the N-S shifts in Fig. 6 should be causedby the underestimation of the average field strength
between the Sun and the Earth.
For the purpose quantitatively estimating the underestimation, the expected magnetic field strength issimply multiplied by a constant factor f uniformly inthe space outside the geomagnetic field. Then, we repeat the MC simulations by changing f and calculateα ( f ) best fit to each expected shift. The f best fiting the observed shift is evaluated to be 1.54 ± 0.21stat± 0.20sys (1.62 ± 0.15stat ± 0.22sys ) in the ”Away”(”Toward”) sector, indicating that the model underestimates the IMF strength. This is the first demonstration in the world that the Sun’s shadow in cosmic
rays allows us to qualitatively evaluate the average IMF
strength between the Sun and the Earth[8].

Ongoing Plans

Chemical composition of primary cosmic rays
making the knee in the all-particle energy spectrum
We have measured the energy spectra of primarycosmic-ray protons, helium, all particles around theknee energy region. The main component responsible for making the knee structure in the all particle
energy spectrum is heavier nuclei than helium. The
next step is to identify the chemical component making
the knee in the all particle energy spectrum. For the
purpose, we constructed Yangbajing Air shower Core
detectors II (YAC-II composed of 124 scintillation detectors over ∼500m2 in area), and started data-taking
in 2014. YAC-II aims at mainly studying the energy
spectra of proton and helium components in the knee

energy region.
Gamma-ray astronomy in the 100 TeV region
For the purpose of detecting high-energy cosmic
gamma rays with an air shower array, a large underground muon detecter is very effective to reduce cosmicray background.
We decided to add a large (∼4,000 m2 ×1.5m deep)
underground (∼ 2.5 m soil+concrete overburden) water
Cherenkov muon detector array (Tibet MD) under thepresent Tibet air shower array (Tibet AS). By Tibet AS+ MD, we aim at background-free detection of celestial point-source gamma rays around 100 TeV with theworld-best sensitivity and at locating the origins (PeVatrons) of cosmic rays accelerated up to the knee (PeV)energy region in the northern sky. The measurementof cut off energies in the energy spectra of such gamma
rays in the 100 TeV region may contribute significantly
to understanding of the cosmic- ray acceleration limit atSupernova Remnants (SNRs). Search for extremely diffuse gamma-ray sources by Tibet AS + MD, for example, from the galactic plane or from the Cygnus region
may be very intriguing as well. Above 100 TeV, theangular resolution of Tibet AS with 2-steradian wide
field of view is 0.2◦ and the hadron rejection power ofTibet MD is 1/1000.
In 2014, construction of the ∼4,000 m2 MD was successfully completed and data-taking started. We haveaccumulated approximately three-year data. Development of Monte Carlo simulation is under way for comparison with real data.Various analysis tools are also
extensively being developed. According to the simulation, the sensitivity of the current configuration (Tibet
AS + MD) is demonstrated in Fig. 7.

Summary

During the period from 2012 to 2018, the Tibet ASγ
experiment contributed significantly to the cosmic-ray
physics field in various aspects, thanks to unique features of the Tibet air shower array located at 4300 m

78
10-9
10-10
Integral Flux (photons cm-2 s-1)

THE TIBET ASγ COLLABORATION

Tibet AS
Tibet AS+MD 1/3 scale
Tibet AS+MD full scaleMAGIC
VERITAS
HESS
HAWC
CTA
HEGRA Crab

10-11
-12

10

10-13
10-14
10-15
10-16 -1
10

100

101
Energy (TeV)

102

103

Fig. 7. Sensitivity to point-like gamma-ray sources with Tibet
AS+MD (see,Tibet AS+MD 1/3 scale) by pink curve.

above sea level, Yangbajing, Tibet, in China. We
are looking forward to new results from the Tibet
AS+MD+YAC-II experiment during the forthcoming
6 years.

Institute
Country (∗ )ICRR, Univ. of TokyoJapan
4
Hirosaki Univ.
Japan
2
National Astronmomical ObChina
2servatories
Institute of High Energy
China
24
Physics
Hebei Nornal Univ.
China
2
Tibet Univ.
China
9Shandong Univ.
China
4SouthWest Jiaotong Univ.
China
3
Kanagawa Univ.
Japan
4
Utsunomiya Univ.
Japan
1
Konan Univ.
Japan
3
Waseda Univ.
Japan
3
Yokohama National Univ.
Japan
3
Shinshu Univ.
Japan
6
JAXA, ISASJapan
1China Meteorological AdminChina
1
istration
Shandon Arigiculture Univ.
China
1
Saitoma Univ.
Japan
1National Institute of InforJapan1
maticsSakushin Gakuin Univ.
Japan
1
China Univ. of Petroleum
China
1
Tokyo Metropolitan College
Japan
1of Industrial Technology
Max-Planck-Institut
für Germany
1
PhysikYachay Tech
Ecuador1
Nihon Univ.
Japan
1
Shonan Institute of TechnolJapan
1
ogy
JAEA
Japan
1
Total
77
(∗ ) Number of participants as of March 2018.

Members
StaffsMasato Takita, Associate Professor (February 2001
to November 2016), Professor (November 2016 to thepresent)
Takashi Sako, Associate Professer (TA/Tibet), October 2017 to the present
Munehiro Ohnishi, Assistant Professor, April 1993 to
the present
Kazumasa Kawata, Specially Appointed AssistantProfessor (April 2013 to March 2018), Assistant Professor (Tibet/TA, April 2018 to the present) Takahide
Kobayashi, Technical Staff, from 1967 to March 2013

79

Postdoctoral Fellows
Takashi Sako, from April 2010 to March 2017 andfrom June 2018 to the present

List of Publications

Graduate Students
Three graduate students obtained master degrees during 2012–2018, supervised by ICRR staff members.

[1] ”Is the large-scale sidereal anisotropy of the galactic cosmic-ray intensity really instable at TeV
energies?”, M. Amenomori et al., Astroparticle
Physics, 36 , 237-241 (2012).

After Year 2012
Papers in Refereed Journals[2] ”Probe of the Solar Magnetic Field Using the“Cosmic-Ray Shadow of the Sun”, M. Amenomori
et al., Physical Review Letters, 111 , 011101-1011101-5 (2013).
[3] ”SEARCHE FOR GAMMA RAYS ABOVE 100
TeV FROM THE CRAB NEBULA WITH THE
TIBET AIR SHOWER ARRAY AND THE 100m2
MUON DETECTOR”, M. Amenomori et al.,THE ASTROPHYSICAL JOURNAL, 813 , 98-15 (2015).
[4] ”Performance of the Tibet hybrid experiment(YAC-II + Tibet-III + MD) to measure the energy spectra of the light primary cosmic rays atenergies 50 - 10,000 TeV”, J. Huang et al., Astroparticle Physics, 66 , 18-30 (2015).
[5] ”Sensitivity of YAC to measure the lightcomponent spectrum of primary cosmic rays at the’knee’ energies”, L. M. . Zhai et al., J. Phys. G
Nucl. Part. Phys. , 42 , 045201-1-14 (2015).
[6] ”Northern sky Galactic Cosmic Ray anisotropy between 10-1000 TeV with the Tibet Air Shower Array”, M. Amenomori et al., THE ASTROPHYSICAL JOURNAL, 836 , 153-1-7 (2016).
[7] ”Energy Determination of Gamma-Ray Induced
Air Showers Observed by An Extensive Air Shower
Array”, K. Kawata et al., Experimental Astronomy, 44 , 1-9 (2017).
[8] ”Evaluation of the Interplanetary Magnetic Field
Strength Using the Cosmic-Ray Shadow of theSun”, M. Amenomori et al., Physical Review Letters, 120 , 031101-1-7 (2018).
Papers in Internatinal Conference Proceedings[9] ”Cosmic-ray shadow” of the Sun at 3 TeV observedby the Tibet Air Shower Array”, M. Amenomori etal., Proceedings of The 33rd International Cosmic
Ray Conference, Rio de Janeiro, Brazil, July 2 July 9, 2013, ID=0340.
[10] ”A Northern Sky Survey for TeV gamma-ray
steady point sources using the Tibet-III air shower
array”, M. Amenomori et al., Proceedings of The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=0498.

80[11] ”Average mass of primary cosmic rays in theknee energy region inferred from Tibet experiment”, M. Amenomori et al., Proceedings of The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=0525.
[12] ”Observation of Multi-TeV Gamma Rays from
MGRO J2019+37 and MGRO J2031+41 with the
Tibet Air Shower Array”, M. Amenomori et al.,Proceedings of The 33rd International Cosmic Ray
Conference, Rio de Janeiro, Brazil, July 2 - July
9, 2013, ID=0513.
[13] ”Observation of thundercloud-related charged particles in Tibet”, M. Amenomori et al., Proceedings of The 33rd International Cosmic Ray Conference, Rio de Janeiro, Brazil, July 2 - July 9,2013, ID=0505.
[14] ”The TIBET AS+MD Project; progress report2013”, M. Amenomori et al., Proceedings of The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=0508.
[15] ”A Monte Carlo study to measure heavycomponent spectra of the primary cosmic-rays
at the knee by a new hybrid experiment (YACII+Tibet-III+MD)”, L. M. Zhai et al., Proceedings of The 33rd International Cosmic Ray Conference, Rio de Janeiro, Brazil, July 2 - July 9,2013, ID=1049.
[16] ”Hadronic interaction and EAS muon investigatedwith the (YAC + Tibet-III + MD) hybrid experiment”, M. Amenomori et al., Proceedings of The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=1051.
[17] ”Observation of the large-scale sidereal anisotropyof the galactic cosmic ray at 300 TeV with the
Tibet Air shower Array”, M. Amenomori et al.,Proceedings of The 33rd International Cosmic Ray
Conference, Rio de Janeiro, Brazil, July 2 - July
9, 2013, ID=0256.
[18] ”Primary proton and helium spectra at energy range from 50 TeV to 1015 eV observed with (YAC+Tibet-III ) hybrid experiment”,
M. Amenomori et al., Proceedings of The 33rd
International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=1047.
[19] ”Progress Report on the MD-A under TIBET III
array”, M. Amenomori et al., Proceedings of The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=1018.
[20] ”Study on the primary mass sensitivity of muon
multiplicity measured with (YAC-II +Tibet-III +MD) experiment”, J. S. Liu et al., Proceedings of

The 33rd International Cosmic Ray Conference,
Rio de Janeiro, Brazil, July 2 - July 9, 2013,
ID=1057.
[21] ”Test of the hadronic interaction model EPOSLHC and QGSJETII-04 with Tibet EAS coredata”, M. Amenomori et al., Proceedings of The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013, ID=1056.
[22] ”Progress report on the TIBET AS+MD Project”,
M. Amenomori et al., Proceedings of the 12th
Asia Pacific Physics Conference, Makuhari Messe,
Chiba, Japan, July 14 - July 19, 2013, JPS Conf.
Proc. 1, 2014, 013124-1-4.
[23] ”Observation of intense fluxes of charged particles in association with thundercloud in Tibet”, M. Amenomori et al., Proceedings of The
34th International Cosmic Ray Conference, The
Hague, The Netherlands, July 30 - August 6, 2015,
ID=0246.
[24] ”Sidereal anisotropy of Galactic cosmic ray observed by the Tibet Air Shower experiment andthe IceCube experiment”, M. Amenomori et al.,Proceedings of The 34th International Cosmic Ray
Conference, The Hague, The Netherlands, July 30- August 6, 2015, ID=0279.
[25] ”Observation of primary cosmic rays with the new
Tibet hybrid experiment(YAC-II + Tibet-III +MD)”, M. Amenomori et al., Proceedings of The
34th International Cosmic Ray Conference, The
Hague, The Netherlands, July 30 - August 6, 2015,
ID=0421.
[26] ”YAC sensitivity for measuring the lightcomponent spectrum of primary cosmic rays atthe ”knee energies”, L. M. Zai et al., Proceedingsof The 34th International Cosmic Ray Conference,
The Hague, The Netherlands, July 30 - August 6,
2015, ID=0424.
[27] ”Shower reconstruction performance of the new Tibet hybrid experiment consisting of YAC-II, TibetIII and MD arrays”, D. Chen et al., Proceedingsof The 34th International Cosmic Ray Conference,
The Hague, The Netherlands, July 30 - August 6,
2015, ID=0430.
[28] ”Investigation of hadronic interaction models
from 10TeV to 1 PeV with the Tibet AS-core”,
M. Amenomori et al., Proceedings of The 34th International Cosmic Ray Conference, The Hague,
The Netherlands, July 30 - August 6, 2015,
ID=0431.
[29] ”Simulation Study On High Energy Electron andGamma-ray Detection With the Newly Upgraded81

Tibet ASgamma Experiment”, Xu Chen et al.,Proceedings of The 34th International Cosmic Ray
Conference, The Hague, The Netherlands, July 30- August 6, 2015, ID=0432.
[30] ”On the primary model to explain the relation between a rigidity-dependent spectral hardening of
proton and helium spectra and a sharp knee of the
all-particle spectrum”, Ying Zhang et al., Proceedings of The 34th International Cosmic Ray Conference, The Hague, The Netherlands, July 30 August 6, 2015, ID=0546.
[31] ”Search for gamma rays above 100 TeV from the
Crab Nebula using the Tibet air shower array andthe 100 m2 muon detector”, M. Amenomori et al.,Proceedings of The 34th International Cosmic Ray
Conference, The Hague, The Netherlands, July 30- August 6, 2015, ID=0833.
[32] ”Energy Determination and Gamma/Hadron Separation using the Lateral Distribution of EAS forthe 100 TeV Gamma-Ray Astronomy”, K. Kawata
et al., Proceedings of The 34th International Cosmic Ray Conference, The Hague, The Netherlands,
July 30 - August 6, 2015, ID=0935.
[33] ”The TIBET AS+MD Project; progress report2015”, M. Amenomori et al., Proceedings of The
34th International Cosmic Ray Conference, The
Hague, The Netherlands, July 30 - August 6, 2015,
ID=0969.
[34] ”Long term stability analysis on the MD-A under
TIBET III array”, M. Amenomori et al., Proceedings of The 34th International Cosmic Ray Conference, The Hague, The Netherlands, July 30 August 6, 2015, ID=1001.
[35] ”Northern sky Galactic Cosmic Ray anisotropy between 10 - 1000 TeV with the Tibet Air Shower
Array”, M. Amenomori et al., Proceedings of The
34th International Cosmic Ray Conference, The
Hague, The Netherlands, July 30 - August 6, 2015,
ID=0810.
[36] ”The Tibet AS+MD Project; status report 2017”,
M. Amenomori et al., Proceedings of The 35th International Cosmic Ray Conference, Busan, Korea,
July 12 - July 20, 2017, ID=831.
[37] ”Solar magnetic field strength and the Sun’s
Shadow observed by the Tibet Air Shower Array”,
M. Amenomori et al., Proceedings of The 35th International Cosmic Ray Conference, Busan, Korea,
July 12 - July 20, 2017, ID=028.
[38] ”Interplanetary Coronal Mass Ejection and theSun’s Shadow Observed by the Tibet Air Shower
Array”, M. Amenomori et al., Proceedings of The

35th International Cosmic Ray Conference, Busan,
Korea, July 12 - July 20, 2017, ID=031.
[39] ”Measurement of high energy cosmic rays by the
new Tibet hybrid experiment”, J. Huang et al.,Proceedings of The 35th International Cosmic Ray
Conference, Busan, Korea, July 12 - July 20, 2017,
ID=484.
Presentations in Internatinal Conference Proceedings[40] ”Modeling of the TeV galactic cosmic-ray
anisotropy”, T. K. Sako, COSPAR2012, Mysore,
India, July 14 - July 22, 2012.
[41] ” “Cosmic-ray shadow” of the Sun at 3 TeV observed by the Tibet Air Shower Array”, T. K. Sako,
The 33rd International Cosmic Ray Conference,
Rio de Janeiro, Brazil, July 2 - July 9, 2013.
[42] ”A Northern Sky Survey for TeV gamma-ray
steady point sources using the Tibet-III air shower
array”, A. Shiomi, The 33rd International Cosmic
Ray Conference, Rio de Janeiro, Brazil, July 2 July 9, 2013.
[43] ”Average mass of primary cosmic rays in the knee
energy region inferred from Tibet experiment”,
Y. Katayose, The 33rd International Cosmic Ray
Conference, Rio de Janeiro, Brazil, July 2 - July
9, 2013.
[44] ”Observation of Multi-TeV Gamma Rays from
MGRO J2019+37 and MGRO J2031+41 with
the Tibet Air Shower Array”, M. Ohnishi, The
33rd International Cosmic Ray Conference, Rio deJaneiro, Brazil, July 2 - July 9, 2013.
[45] ”Observation of thundercloud-related charged particles in Tibet”, K. Hibino, The 33rd International
Cosmic Ray Conference, Rio de Janeiro, Brazil,
July 2 - July 9, 2013.
[46] ”The TIBET AS+MD Project; progress report2013”, M. Takita, The 33rd International Cosmic
Ray Conference, Rio de Janeiro, Brazil, July 2 July 9, 2013.
[47] ”A Monte Carlo study to measure heavycomponent spectra of the primary cosmic-rays
at the knee by a new hybrid experiment (YACII+Tibet-III+MD)”, J. Huang, The 33rd International Cosmic Ray Conference, Rio de Janeiro,
Brazil, July 2 - July 9, 2013.
[48] ”Hadronic interaction and EAS muon investigatedwith the (YAC + Tibet-III + MD) hybrid experiment”, J. Huang, The 33rd International Cosmic
Ray Conference, Rio de Janeiro, Brazil, July 2 July 9, 2013.

82[49] ”Observation of the large-scale sidereal anisotropyof the galactic cosmic ray at 300 TeV with the Tibet Air shower Array”, Z. Y. Feng, The 33rd International Cosmic Ray Conference, Rio de Janeiro,
Brazil, July 2 - July 9, 2013.

[60] ”YAC sensitivity for measuring the lightcomponent spectrum of primary cosmic rays
at the “knee” energies”, L. M. Zai, The 34th
International Cosmic Ray Conference, The Hague,
The Netherlands, July 30 - August 6, 2015.

[50] ”Primary proton and helium spectra at energy
range from 50 TeV to 1015 eV observed with
(YAC+Tibet-III ) hybrid experiment”, J. Huang,
The 33rd International Cosmic Ray Conference,
Rio de Janeiro, Brazil, July 2 - July 9, 2013.

[61] ”Shower reconstruction performance of the new Tibet hybrid experiment consisting of YAC-II, TibetIII and MD arrays”, D. Chen, Proceedings of The
34th International Cosmic Ray Conference, The
Hague, The Netherlands, July 30 - August 6, 2015.

[51] ”Progress Report on the MD-A under TIBET III
array”, C. Liu, The 33rd International Cosmic Ray
Conference, Rio de Janeiro, Brazil, July 2 - July
9, 2013.

[62] ”Simulation Study On High Energy Electron andGamma-ray Detection With the Newly Upgraded
Tibet ASgamma Experiment”, Xu Chen, The 34th
International Cosmic Ray Conference, The Hague,
The Netherlands, July 30 - August 6, 2015.

[52] ”Study on the primary mass sensitivity of muon
multiplicity measured with (YAC-II +Tibet-III +MD) experiment”, D. Chen et al., The 33rd International Cosmic Ray Conference, Rio de Janeiro,
Brazil, July 2 - July 9, 2013.
[53] ”Test of the hadronic interaction model EPOSLHC and QGSJETII-04 with Tibet EAS coredata”, D. Chen, The 33rd International Cosmic
Ray Conference, Rio de Janeiro, Brazil, July 2 July 9, 2013.
[54] ”Progress report on the TIBET AS+MD Project”,
T. K. Sako, The 12th Asia Pacif, Makuhari Messe,
Chiba, Japan, July 14 - July 19, 2013.
[55] ”Cosmic-ray shadow of the Sun measured at 3 TeV
with the Tibet Air Shower Array” T. K. Sako,
COSPAR2014, Moscow, Russia, August 2 - August
10, 2014.
[56] ”Observation and modeling of cosmic-ray
anisotropy observed with the Tibet air shower
array”, M. Takita, Cosmic Ray Anisotropies 2015,
Bad Honnef, Germany, January 26 - January 30,
2015.
[57] ”Observation of intense fluxes of charged particles
in association with thundercloud in Tibet”, K. Hibino, The 34th International Cosmic Ray Conference, The Hague, The Netherlands, July 30 - August 6, 2015.
[58] ”Sidereal anisotropy of Galactic cosmic ray observed by the Tibet Air Shower experiment andthe IceCube experiment”, Y. Nakamura, The 34th
International Cosmic Ray Conference, The Hague,
The Netherlands, July 30 - August 6, 2015.
[59] ”Observation of primary cosmic rays with the new
Tibet hybrid experiment(YAC-II + Tibet-III +MD)”, J. Huang, The 34th International Cosmic
Ray Conference, The Hague, The Netherlands,
July 30 - August 6, 2015.

[63] ”On the primary model to explain the relation
between a rigidity-dependent spectral hardeningof proton and helium spectra and a sharp kneeof the all-particle spectrum”, Ying Zhang, The
34th International Cosmic Ray Conference, The
Hague, The Netherlands, July 30 - August 6, 2015,
ID=0546.
[64] ”Search for gamma rays above 100 TeV from the
Crab Nebula using the Tibet air shower array andthe 100 m2 muon detector”, T. K. Sako, The 34th
International Cosmic Ray Conference, The Hague,
The Netherlands, July 30 - August 6, 2015.
[65] ”Energy Determination and Gamma/Hadron Separation using the Lateral Distribution of EAS forthe 100 TeV Gamma-Ray Astronomy”, K. Kawata,
The 34th International Cosmic Ray Conference,
The Hague, The Netherlands, July 30 - August 6,
2015.
[66] ”The TIBET AS+MD Project; progress report2015”, M. Amenomori et al., The 34th International Cosmic Ray Conference, The Hague, The
Netherlands, July 30 - August 6, 2015.
[67] ”Long term stability analysis on the MD-A under
TIBET III array”, X. L. Qian, The 34th International Cosmic Ray Conference, The Hague, The
Netherlands, July 30 - August 6, 2015.
[68] ”Northern sky Galactic Cosmic Ray anisotropy between 10 - 1000 TeV with the Tibet Air Shower
Array”, Z. Y. Feng, The 34th International Cosmic Ray Conference, The Hague, The Netherlands,
July 30 - August 6, 2015.
[69] ”The Tibet AS+MD Project; status report 2017”,
M. Takita, The 35th International Cosmic Ray
Conference, Busan, Korea, July 12 - July 20, 2017.
[70] ”Solar magnetic field strength and the Sun’s
Shadow observed by the Tibet Air Shower Array”,

83

Y. Nakamura, The 35th International Cosmic Ray
Conference, Busan, Korea, July 12 - July 20, 2017.
[71] ”Interplanetary Coronal Mass Ejection and theSun’s Shadow Observed by the Tibet Air Shower
Array”, K. Kawata, The 35th International Cosmic Ray Conference, Busan, Korea, July 12 - July20, 2017.
[72] ”Measurement of high energy cosmic rays by the
new Tibet hybrid experiment”, J. Huang, The 35th
International Cosmic Ray Conference, Busan, Korea, July 12 - July 20, 2017.
[73] ”Study of the Magnetic Field Between the Sun
and the Earth by the Sun shadow in Cosmic Rays
with the Tibet Air Shower Array”, M. Tuang,
AOGS2017, Singapore, August 6 - August 10,
2017.
[74] ”Sub- and multi-TeV cosmic-ray anisotropies observed with Japanese underground muon detectors
and the Tibet Air shower experiment”, K. Munakata, Cosmic Ray Anisotropies 2017, Guadalajara, Mexico, October 10 - October 13, 2017.

84

CTA: CHERENKOV TELESCOPE ARRAY PROJECT
Cherenkov Telescope Array Project (CTA)[Spokesperson : Masahiro Teshima]
ICRR, Univ. of Tokyo, Kashiwa, Chiba 277-8582
In collaboration with the members of:
Aoyama Gakuin Univ., Kanagawa 252-5258, Japan;
Ibaraki Univ., Mito, Ibaraki 310-8512, Japan;
Osaka Univ., Toyonaka, Osaka 560-0043, Japan;
Kitasato Univ., Kanagawa 252-0373, Japan;
Kyoto Univ., Sakyoku, Kyoto 606-8502, Japan;
Kinki Univ., Osaka, Osaka 577-8502, Japan;
Kumamoto Univ., Chuoku, Kumamoto 860-8555,
Japan;
KEK, Tsukuba, Ibaraki 305-0801, Japan;
Konan Univ., Kobe, Hyogo 658-0072, Japan;
Saitama Univ., Saitama, Saitama 338-8570, Japan;
Tokai Univ., Hiratsuka, Kanagawa 259-1292, Japan;
The University of Tokyo, Bunkyo-ku, Tokyo 113-0033;
The University of Tokushima, Tokushima 770-8502;
Nagoya Univ., Nagoya, Aichi 464-8602, Japan;
Hiroshima Univ., Hiroshima 739-8526, Japan;
Miyazaki Univ., Miyazaki, Miyazaki 889-2192, Japan;
Yamagata Univ., Yamagata, Yamagata 990-0021,
Japan;
Yamanashi Gakuin Univ., Kofu, Yamamashi 400-8575;
Waseda Univ., Shinjyuku, Tokyo 169-0072, Japan;

Introduction

During the last decades, Very High Energy (VHE)
gamma ray astronomy has made spectacular progressand has established itself as a vital branch of astrophysics. To advance this field even further, we propose
the Cherenkov Telescope Array (CTA), the next generation VHE gamma ray observatory, in the frameworkof a worldwide, international collaboration. CTA is theultimate VHE gamma ray observatory, whose sensitivity and broad energy coverage will attain an order ofmagnitude improvement above those of current Imaging Atmospheric Cherenkov Telescopes (IACTs). By
observing the highest energy photons known, CTA will
clarify many aspects of the extreme Universe, including the origin of the highest energy cosmic rays in our
Galaxy and beyond, the physics of energetic particle
generation in neutron stars and black holes, as well as
the star formation history of the Universe. CTA will
also address critical issues in fundamental physics, suchas the identity of dark matter particles and the natureof quantum gravity.
VHE gamma rays from 100GeV to 10TeV can be observed with current ground-based IACTs. The historyof VHE gamma ray astronomy began with the discovery of VHE gamma rays from the Crab Nebula by the
Whipple Observatory in 1989. The current generation
IACTs featuring new technologies, such as H.E.S.S.,MAGIC, and VERITAS, have discovered more than 200
Galactic and extragalactic sources of various types todate.

CTA Project

CTA is designed to achieve superior sensitivity andperformance, utilising established technologies and experience gained from the current IACTs. The projectis presently in its pre-construction phase, with international efforts from Japan and the EU countries. It
will consist of several 10s of IACTs of three different
sizes (Large Size Telescopes, Mid Size Telescopes, andSmall Size Telescopes). With a factor of 10 increase insensitivity (1m Crab 10−14 erg s−1 cm−2 ), together
with much broader energy coverage from 20GeV up
to 300TeV, CTA will bring forth further dramatic advances for VHE gamma ray astronomy. The discovery
of more than 1000 Galactic and extragalactic sources isanticipated with CTA.
CTA will allow us to explore numerous, diverse topics in physics and astrophysics. The century-old question of the origin of cosmic rays is expected to be finallysettled through detailed observations of supernova remnants and other Galactic objects along with the diffuse
Galactic gamma ray emission, which will also shed lighton the physics of the interstellar medium. Observing
pulsars and associated pulsar wind nebulae will clarify physical processes in the vicinity of neutron stars
and extreme magnetic fields. The physics of accretion
onto supermassive black holes, the long-standing puzzle of the origin of ultrarelativistic jets emanating fromthem, as well as their cosmological evolution will be
addressed by extensive studies of active galactic nuclei (AGN). Through dedicated observing strategies,
CTA will also elucidate many aspects of the mysterious nature of gamma ray bursts (GRBs), the mostenergetic explosions in the Universe. Detailed studies of both AGNs and GRBs can also reveal the origin of the highest energy cosmic rays in the Universe,
probe the cosmic history of star formation including
the very first stars, as well as provide high precision
tests of theories of quantum gravity. For example, themulti-messenger observation with IceCube, Fermi and
MAGIC on TXS0506+056 has demonstrated the scientific potential of the next generation telescopes CTA
and IceCube Gen2 to identify the sources of Ultra HighEnergy Cosmic Rays. Finally, CTA will search for

85Fig. 1. Very High Energy Gamma Ray Sky (> 100GeV) in the galactic coordinate. More than 200 Galactic and extragalactic sources havebeen discovered by H.E.S.S., MAGIC and VERITAS. The low latitude sources on the galactic plane are pulsar wind nebulae, super nova
remnants, gamma ray binaries and un-known sources, and high latitude sources are mainly Blazars and AGNs.

Fig. 2. Computer graphics of the CTA North observatory. CTA consists of three types of telescopes, Large Size Telescopes (23m diameter),Mid Size Telescopes (12m) and Small Size Telescopes (4m, only in CTA south), and covers the broad energy band from 20GeV to300TeV.

signatures from elementary particles constituting dark
matter with the highest sensitivity so far. The realisation of its rich scientific potential of CTA is very muchfeasible, thanks to the positive experience gained fromthe current IACTs.

Structure of the large size telescope (LST)The structure of the large size telescope (LST) as
shown in figure 3 was designed by the MPI Munich
group together with MERO-TSK. The major part of
the telescope consists of the space frame structure withcarbon fiber reinforced plastic (CFRP) tubes. The total
weight of the telescope is designed to be about 100 tons

and allows the fast rotation of the telescope, 180 degrees
in 20 seconds, for fast follow-up observations of gamma
ray bursts using the location determined by gamma ray
satellites.
The telescope geometry is optimized to maximize
the cost performance by Monte Carlo simulations andtoy models. The baseline parameters are defined withthe dish size of 23m, the focal length of 28m and thenF/D = 1.2, and the camera FoV of 4.5 degrees with apixel size of 0.1 degrees.
The first Large Size Telescope (prototype/LST1)
has been successfully built at ORM. The construction
was done very smoothly, and we did not find any major
problem in the telescope structure and construction.

86

Fig. 3. Large Size Telescope (23m diameter). The telescope mechanical design was done by Max Planck Institute for Physics.
CTA Japan has made the contribution to the design and production of the imaging camera at the focal plane, ultrafast readout
electronics, and high precision segmented mirrors.

Activities of CTA JapanThe CTA-Japan consortium is aiming to contributeparticularly to the construction of the Large Size Telescopes (LSTs) and has been involved in their development. The LST covers the low energy domain from20 GeV to 1000 GeV and is especially important forstudies of high redshift AGNs and GRBs. The diameter and area of the mirror is respectively 23 m and
400 m2 to achieve the lowest possible energy thresholdof 20 GeV. All optical elements/detectors require highspecifications, such as, high reflectivity, high collection
efficiency, high quantum efficiency and ultra fast digitization of signals, and etc. For this purpose, CTA-Japan
has developed high quantum efficiency photomultipliers, ultrafast readout electronics and high precision segmented mirrors.
Segmented Mirrors for LST
The reflector with its diameter of 23m consists of
198 units of hexagonal shape 1.5m flat to flat segmented mirrors of 2m2 . The total area of the reflector
is about 400m2 . Each segmented mirror is attached to
the knots of the space frame structure through the interface plates with a universal joint and two actuators.
The segmented mirrors are of a sandwich structure,
consisting of a glass sheet of 2.7mm thickness - alu-

Fig. 4. The first Large Size Telescope LST1 which has a 23m diameter dish erected in the Observatory Roque de los Muchachos.
The mirror supporting structure is made of carbon fiber reinforced plastic (CFRP) to make the structure stiff and as light as
possible. The camera supporting arch and reinforcing ropes arealso made of CFRP.

minum honey-comb of 60mm thickness - and anotherglass sheet. The weight of a segmented mirror is 45kg.
The reflective layer of the mirror is coated with Cr andAl on the surface of the glass sheet with a protective
multi-coat layer of SiO2 , HfO2 , and SiO2 . By adjusting
the thickness of individual layers with SiO2 and HfO2 ,we can optimize the reflectivity to about 92% due tothe interference effect of multi-layers.
Permanent Active Mirror Control
We will define the optical axis (OA) of the LST
optics with two infra-red lazers at the center of thedish constantly shining two targets top and bottom of
the imaging camera. The individual segmented mirrors
have a CMOS Camera at the bottom corner of the HEX
mirror which observe the OA lazers at targets near theimaging camera to confirm the direction of the mirror
facet relative to the OA lasers (optical axis). The directional offset of the mirror facets will be estimated bytaking pictures of OA-laser spots on the targets witha CMOS camera viewing from the individual mirrors.
If any significant offsets are found, the direction of the

87Fig. 5. Hexagonal shape mirrors of 1510mm size delivered by the University of Tokyo and Sanko LTD. The area is about 2m2 and the
focal length is tuned between 28m and 29.2m depending on the location of mirror from optical axis. The PSF of individual mirrors ischaracterised by the parameter D80 within whose circle diameter the light is contained more than 80% of total reflected light, which isless than 16.7mm. Mirrors are produced with the cold slump technique. The mirror surface is protected with the multi-layer coatingwith SiO2 and HfO2 are produced by spattering method. The right picture shows the dish mounted with 198 segmented mirrors during
the construction period. They are covered by the light protection covers to avoid accidents with the sun light.

Fig. 6. The average spectral reflectivity and its dispersion of mirrors used in the LST1. The peak reflectivity of 92% is achievedat around 400nm, there is more than 85% reflectivity between
300nm and 600nm. D80 (the diameter containing 80% of totalreflected light at 1f position) of all mounted segment mirrors areless than 16.7mm, the averaged D80 is 13.6 mm, which is wellsmaller than the LST specification of 16.7mm.

corresponding mirror facets will be corrected by actuators. The 198 mirrors are divided into 16 groups. The
directions of 198 individual mirrors are monitored andcontrolled by 16 CPUs. The monitoring and control of
12-13 mirrors in each group will be done sequentiallyand performed typically in 10 seconds. This will bedone continuously during the observation. After the
first rotation for the GRB follow-up observation, or at
the beginning of the observation of any source, we will
use the look-up table corresponding to the zenith angle of the target source as the initial value of actuatorsand then move to the mode of permanent active mirror
control loop.

Fig. 7. The radius of curvature of individual segmented mirrors as afunction of distance from the optical axis. Red points represent
installed mirrors on LST1 and blue ones are ideal values with
the isochronous mirror configuration (similar with the parabolic
shape).

Imaging Camera

The imaging camera has a FOV of 4.5 degrees anda pixel size of 0.1 degrees. The actual size of the image plane will be about 2.3m in diameter. The signals
from the photomultipliers will be read with 1G samples/sec speed and be stored in the ring capacitors of4096 depth, which corresponds to 4 micro-seconds.
The camera will be sealed to resist humidity anddust in the field. The front side (entrance window) of
the imaging camera will be covered with uv-transparent
plexiglass. The water cooling plate is used to keep

88Fig. 10. The averaged Quantum Efficiency of 1000 PMTs produced
for LST camera. Q.E. Peak value of 42% is obtained at around
380nm. The best PMTs show the peak value of 45%. We haveasked Hamamatsu HPK to measure 10% tubes randomly (1000among 10,000 produced PMTs) to see the quality of the seriesof production.

Fig. 8. The precision of the permanent Active Mirror Control (AMC)
system examined with the test telescope structure in the MPI
backyard. The top picture shows the mirror facet direction after
the first adjustment and the bottom one is after the second trial,
after reading the position again with CMOS camera. These iteration can be done permanently every 10 seconds even during theobservation time. The green radius corresponds to the 14 arcseconds. We can monitor and control the mirror facet direction
with the precision better than 5 arc-seconds.

Fig. 9. R11920-100-20 PMT module. This module consists ofHamamatsu Photomultiplier R11920, Cockcroft-Walton HV, andUltra fast Preamplifier. CTA-Japan has developed this PMT
module together with Hamamatsu photonics.

the temperature of the camera and the electronics constant. As a part of the Camera mechanical structure
this cooling plate will also serve as a support for the
PMT/electronics clusters. The readout electronics andthe auxiliary electronics (HV, and amplifiers) will dissipate a heat of 2W/ch. 7-PMTs and readout electronics
are mechanically bundled as a PMT/electronics cluster.
Since the total number of pixels and clusters are 1855
and 265, the total heat dissipation inside the camera
will amount to 4-5kW.

Fig. 11. PMT Cluster, consisting of seven photomultipliers, CWHVs, Preamplifiers, readout electronics (Dragon card), and trigger system. Signals are digitized in this module and sent to DAQ
camera server system via G-bit Ethernet.

DAQ and Data Analysis

The computer cluster with 2000 cores and 3PB disk
is installed at the LST1 site. It is designed for the dataacquisition of 4 LSTs and 5 MSTs with high speed internet connection with telescopes (40Gbps/LSTs and
20Gbps/MSTs) and for the control of array / telescopes. We will develop the online / on-fly data analysis
pipe with this computer cluster and LST1 in the next
years.

Time Schedule and Responsibilities

As mentioned above, CTA-Japan has important responsibilities in the production and delivery of many
telescope elements, such as segmented mirrors, the active mirror control system, focal plane instruments and89

online calibration and online data analysis with LST1
and this computing system. This computing system
will mitigate the problem of huge data rates from CTA
telescopes, and relatively poor internet connections between La Palma and European countries and very poorconnections between Europe and Japan.
LST2-4 will be constructed in the next three years in
2019-2022 and already tendering of the foundation andtelescopes mechanics are ongoing in Spain. CTA-Japanhas produced all responsible elements for LST2-4 andquality control of products and assembly of elements
are ongoing. LST1-4 will be operated as a stereoscopic
system in 2022. CTA Japan has already produced mirrors, photo-sensors and readout electronics for LST2-4.
For the full construction (four LSTs in north and another four in south), we need more efforts to secure thefinance and manpower.

Fig. 12. LST1 Camera under assembling process in August 2018.
The camera consists of 265 PMT clusters and 1855 PMTs. PMT
clusters with 7 PMTs are inserted through the inclined slots andfixed from the frontal side by three pins and from the rear side by
two screws. The apparent green color of the camera is due to the
mixture of reflected blue light on the surface of the light guidewith UV-blue enhanced coating (95% reflectivity at 350nm) andyellow color direct light from PMT head.

readout electronics of the four Large Size Telescopes
installed in CTA North. CTA-Japan delivers these elements after production and procurements with budgets, the JSPS grant in aid research and budget fromthe Japanese government. The construction and realisation of LSTs in CTA south may be discussed in the
coming years.
In 2016 the CTA sites were decided to be Paranal,
Chile in the South and La Palma, Spain in the North,after detailed investigation and discussions at several
candidate sites. The design study and development of
the first Large Size Telescope had already been wellmatured even before this decision in 2016.
The construction of LST1 has started at ORM on
La Palma Spain after receiving the construction licensefrom the local government in the summer of 2017. After two year construction period, we will complete theconstruction in the fall of 2018. The local computing system of CTA North to take data from 4 LSTs
and 15 MSTs was installed in 2018. We can develop
the end to end system of data taking, data reduction,

90
Table 1. Timeline for the LST construction. LST1 related events have been almost done / performed. We learned all necessary procedureswith this construction. LST2-4 schedule is estimated from the experience of LST1 construction. Their financial resources are almost
secured. The possible schedule of LST5-8 in CTA South is also shown at the bottom, highly depending on the future funding.

Time

2014 Q3
2015 Q2
2015 Q4
2016 Q1
2016 Q3
2017 Q3
2018 Q42019 Q2
2018 Q2
2019 Q3
2020 Q2
2021 —
2022 Q42023 Q1
2023 Q3
2022 —
2025 —

Decision making
Organization
CTAC, LST
LST
LST
CTAO
Cabildo, LST
Cablido, LST
LST
CTAO
CTAO, LST
Cabildo, CTAO
Cabildo, LST
LST
LST
CTAO, LST
CTAO, LST
LST
LST

Telescopes

Events / Actions / Schedule

LST1
LST1
LST1
LST, MST
LST1
LST1
LST1
LST1
LST2-4
LST2-4
LST2-4
LST2-4
LST1-4
LST1-4
LST1-4
LST5-8
LST5-8Agreement for the construction of LST1 on the CTA site
Tendering Telescope mechanics
Decision on the LST1 construction at ORM, La Palma, Spain
Decision of La Palma as the CTA-North site
Start the construction of concrete foundation
License for the construction of Telescope mechanics
First Light, Commissioning and engineering run of LST1
Review of LST1
Tendering foundations and mechanics
License for the construction of infrastructure
Start construction of telescopes
Continue construction
First light, Commissioning and Engineering run as LST-array
Acceptance Review of LST array
In-kind Contribution, Operate CTA North as an observatory
Start the installation of CTA South and Tendering ?
Commissioning and Engineering run of LST5-8 ?

Summary from 2012 to 2018
• The optimisation of CTA array telescope configuration and the parameters of large size telescopes
was done with Monte Carlo simulation, and therequirements and specifications for the large size
telescopes were defined.
• The Baseline Design Document (BDD) and Technical Design Report (TDR) for the Large Size
Telescope (LST) were documented and includedin the TDR of CTA.
• The first stone ceremony for LST1-4 was held in
October 2015.
• After successful prototyping of segmented mirrors,
900 mirrors were produced and qualified. The
high reflectivity of 92%, small point spread function( ≤ 16mm), high durability, and light weight (≤ 50kg) were achieved.
• The 1.5 inch size hemispherical shape photomultiplier for CTA LSTs had been designed and developed with specification of the peak quantum
efficiency of 42% and low after pulse rate of 10−4 .
With this specification, 10,000 photomultipliers
are produced by Hamamatsu.
• The 10,000 preamplifier boards with specificationof low noise (∼ 104 e), a few GHz bandwidth andhigh/low dual gain were produced using PACTA
Chip, the ASIC developed in the IEEC, the University of Barcelona.

• 1,100 units of the GHz sampling fast 7ch readout
system Dragon boards were produced. The wide
dynamic range is achieved with the high and low
gain dual gain waveform readout system to measure high energy gamma rays from 20GeV to 3
TeV.
• The first telescope LST1 was inaugurated in October 2018 at CTA North site on La Palma.
• The very fast large flare from radio galaxy IC310
was observed with the MAGIC telescope. The
variability of minutes scale is observed from theblackhole with the light crossing time of 50 mins.
• The extension of the energy spectrum up to 1TeVin the pulsed component from Crab pulsar wasobserved with the MAGIC telescope.
• The observation of 100GeV gamma rays from thepossible PeV neutrino source TXS 0506+056.

91

CTA
(∗ )

Institute
Country
ICRR, Univ. of Tokyo
Japan
38
Aoyama Univ.
Japan
4
Ibaraki Univ.
Japan
5
Osaka Univ.
Japan
2
Kitasato Univ.
Japan
1
Kyoto Univ.
Japan
10
Yukawa InstituteJapan
1Kinki Univ.
Japan
3
Kumamoto Univ.
Japan
1
KEK
Japan
3
Konan Univ.
Japan
4
Saitama Univ.
Japan
5
Tokai Univ.
Japan
7
Univ. of TokyoJapan
4
Tohoku Univ.
Japan
1
Tokushima Univ.
Japan
1
Nagoya Univ.
Japan
9
ISEE, Nagoya Univ.
Japan
8
Hiroshima Univ.
Japan
3
Miyazaki Univ.
Japan
1
Yamagata Univ.
Japan
4
Yamamashi Gakuin Univ.
Japan
2
RIKEN
Japan
7
Rikkyo Univ.
Japan
1
Waseda Univ.
Japan
1
Total
126
(∗ ) Number of participants as of August 2018.

Members in ICRR

There are 126 collaborators in CTA-Japan consortium. Here we only list members in ICRR, the University of Tokyo.
StaffsMasahiro Teshima, Professor, 2010 to the present
Takanori Yoshikoshi, Assoc. Professor, 2010 to thepresent
Koji Noda, Assoc. Professor, 2018 to the present
Daniel Mazin, Research Assoc. Professor, 2014 to thepresent
Michiko Ohishi, Assist. Professor, 2010 to the present
Takayuki Saito, Research Assist. Professor, 2017 to
the present
Daniela Hadasch, Research Assist. Professor, 2014 to
the present
Postdoctoral Fellows
Mitsunari Takahashi, 2018 to the present
Masaaki Hayashida, 2011 to 2016
Daisuke Nakajima, 2011 to 2016

Technical Staff and Secretary
Hideyuki Ohoka, 2010 to the present
Kyosuke Awai, 2018 to the present
Nao Okazaki, 2016 to the present
Yusuke Inome, 2018 to the present
Midori Sugahara, 2011 to the present
Graduate Students
Satoshi Fukami (D3)
Yuki Iwamura (D2)Tomohiro Inada (D2)
Shunsuke Sakurai (D1)
Taku Kumon (M2)
Yukiho Kobayashi (M1)
Yoshiki Otani (M1)
One student was awarded doctor degrees and 9 students
earned master degrees during 2013–2018, supervised by
ICRR staff members.

List of Publications[1] Multimessenger observations of a flaring
blazar coincident with high-energy neutrino IceCube-170922A, IceCube, Fermi and
MAGIC Collaborations, M. M. Aartsen, H.Kubo,
D. Mazin, D. Hadasch, M. Hayashida, K. Noda,
T. Saito, M.Takahashi, M. Teshima, et al., Science
361 (2018) no.6398, eaat1378.
[2] A technique for estimating the absolute gainof a photomultiplier tube, M. Takahashi, et al.,Nucl.Instrum.Meth. A894 (2018) 1-7.
[3] Science with the Cherenkov Telescope Array, arXiv:1709.07997 (56 Cited)[4] Prospects for Cherenkov Telescope Array
Observations of the Young Supernova Remnant RX J1713.7−3946, CTA Consortium, Astrophys.J. 840 (2017) no.2, 74.
[5] Evaluation of novel PMTs of worldwide bestparameters for the CTA project R. Mirzoyan,
D. Nakajima, M. Takahashi, M. Teshima, T. Yamamoto it et al., Nucl.Instrum.Meth. A845 (2017)603-606[6] Large size telescope report, D. Mazin, J.
Cortina and M. Teshima, AIP Conf.Proc. 1792(2017) no.1, 080001[7] Status of the Cherenkov Telescope Array’s
Large Size Telescopes, J. Cortina and M.
Teshima, Proc. of ICRC2015 at Hague, Netherlands. arXiv:1508.06438.
[8] The Optical System for the Large Size Telescope of the Cherenkov Telescope Array, M.
Hayashida et al., PoS ICRC2015 (2016) 927.

92[9] Development of the photomultiplier tube
readout system for the first Large-Sized
Telescope of the Cherenkov Telescope Array, S. Masuda et al., PoS ICRC2015 (2016) 1003.
[10] Development of Slow Control Boards for
the Large Size Telescopes of the Cherenkov
Telescope Array, D. Hadasch et al., PoS
ICRC2015 (2016) 1024.
[11] Limits to dark matter annihilation crosssection from a combined analysis of MAGIC
and Fermi-LAT observations of dwarf satellite galaxies, MAGIC Collaboration, M.L. Ahnen
et al., JCAP 1602 (2016) no.02, 039.
[12] Black hole lightning due to particle acceleration at subhorizon scales, J. Aleksic, D.
Hadasch, M. Hayashida, H. Kubo, D.Mazin, K.
Noda, K. Saito, T. Saito, M. Teshima et al., Science 346 (2014) 1080-1084. (74 cited)[13] Introducing the CTA concept, B.S. Acharya etal., Astropart. Phys. 43 (2013) 3-18. (396 cited)[14] Design concepts for the Cherenkov Telescope Array CTA: An advanced facility forground-based high-energy gamma-ray astronomy, CTA Consortium (M. Actis, K. Ioka,
H.Kubo, R. Orito, H. Tajima, M. Teshima, T.
Totani, T. Yoshida, T. Yoshikoshi, 60 other CTAJapan members et al.). Exper.Astron. 32, 193(2011).
[15] Design Study of a CTA Large Size Telescope, M. Teshima et al., Proc. of ICRC2012 at
Beijing China, arXiv:1111.2183[16] Development of PMT Clusters for CTALST Camera, R. Orito et al., Proc. of ICRC2012
at Beijing China, arXiv:1111.2183[17] Development of the Readout System for
CTA Using the DRS4 Waveform Digitizing
Chip, H. Kubo et al., Proc. of ICRC2012 at Beijing China, arXiv:1111.2183

93

HIGH ENERGY ASTROPHYSICS GROUP
High-Energy Astrophysics Group
Introduction

The high energy astrophysics group aims at making
theoretical studies of violent astrophysical phenomena,in which cosmic-rays (nonthermal particles) are being
accelerated. Targets of the group’s study include highenergy astrophysical objects such as supernova explosions/shocks, neutron stars (NSs) and their nebulae,
merger of binary stars, and relativistic jets launchedfrom active galactic nuclei (AGNs) and gamma-ray
bursts (GRBs). The standard theory of cosmic ray
acceleration process invokes effective diffusive-shockacceleration (DSA) mechanism working around collisionless astrophysical shocks and their turbulent environment. The magnetic reconnection mechanism is another candidate for effective particle acceleration processes. Understanding of transport processes involvedin the DSA and magnetic reconnection mechanisms requires deep consideration based on microscopic plasma
physics.
The open problems in high-energy astrophysics are
not only the acceleration mechanism of cosmic rays.
How are relativistic jets from AGNs accelerated? Howare very bright gamma-ray emissions emitted in GRBs?
How is the magnetic energy of the pulsar winds converted into the kinetic energy to form ultra-relativistic
outflows? How are magnetic fields amplified in shocks
in supernova remnants (SNRs) or afterglows of GRBs?
How do relativistic particles propagate in interstellar
space and escape from the galactic plane? Those problems remain unsolved, and are our research themes [5].
In addition, studies have been made also for nonthermal phenomena within the heliosphere, such as interplanetary shocks and the earth’s bow shock, magnetic reconnection, the interaction processes between
the solar wind and the lunar surface.
We also have carried out observational studies.
Analysing a large set of the radio data taken withJapanese telescopes, we have studied giant radio pulses(GRPs) of pulsars to obtain the hint of the emission
mechanism. Our group has joined CALET, CALorimetric Electron Telescope, which is a mission for theJapanese Experiment Module-Exposed Facility (JEMEF) on the International Space Station. The CALET
mission carries out accurate measurements of high energy spectra of electrons, gammarays and nuclei.

Summary from 2012 to 2018

Our representative results are as follows:• Based on the turbulence acceleration model,
we have reproduced photon spectra of blazars,

gamma-ray bursts, the Fermi bubble, and pulsar
wind nebulae.
• We have discussed production of ultra highenergy cosmic-rays and high-energy neutrinos
from gamma-ray bursts.
• We estimated the detection rate of GW events for
the BH–BH or BH–NS binary mergers especiallyoriginated from the Pop III stars. We also proposed the binary merger progenitor model to be
the long gamma-ray burst progenitor. The GRB
rate we obtained is consistent with the observed
GRB rate evolution.
• We carried out a detailed radiative transfer simulation in ejecta at the NS–NS binary merger. Ourmodel succeeded in reproducing the complex evolution of the kilonova spectrum in GW170817.
• We operated simultaneous multi-frequency observations of the Crab giant radio pulses with four radio telescopes. Most of spectra can be fitted witha signle power-law between 0.3–2.2 GHz. Our results suggest an anti-correlation between the totalenergy release and spectral hardness.
• The CALET mission succeeded in obtaining theelectron+positron spectrum from 10 GeV to 5
TeV.

Non-thermal
Phenomena
messenger Astronomy

andMulti-

Our main research interest is the emission mechanism of the non-thermal photons in SNRs [41], GRBs
[7, 9, 10, 14, 22, 34, 36, 43, 51, 57], blazars (jets from
AGNs) [12, 19, 21, 27, 53], pulsars [13, 16, 17, 20, 29,
32, 35], pulsar wind nebulae (PWNe) [8, 11, 40, 42],
merger of binary stars [26, 30, 49], and huge gammaray bubbles above our galactic plane (Fermi bubble)[25].
A unique point in our study is a proposal of an alternative particle acceleration mechanism, the turbulence
acceleration model. Differently from DSA, particle acceleration via scattering by electromagnetic waves induced by turbulence can produce curved energy spectrum as shown in Figure 1, which agrees with the observed blazar spectra [12, 19, 53] (see Figure 2). Our
results suggest that the acceleration timescale is independent of the particle energy, which is different assumption from previous studies. We have proposed amodel, in which particles are accelerated via interaction
with fast mode waves so called transit time damping.

94

HHQ HH >HUJFP@















55  











 HH>H9@

pions or secondary muons/electrons/positrons are responsible for gamma-ray emission in hadronic models.
In most of high-energy objects, there is no consensus on the gamma-ray emission mechanism. Possible
way to determine the emission mechanism is detection of neutrinos from secondary pions in the hadronicmodel. We have discussed production of ultra highenergy cosmic-rays (UHECRs) and high-energy neutrinos from GRBs [9, 15, 22, 28, 51]. Our model of
UHECR production in GRBs [28] is unique one, which
is based on the turbulence acceleration model. We expect that the predicted UHECR and neutrino spectra(Figure 3) will be confirmed in near future.
H- H >HUJFPVVU@

Fig. 1. The evolution of the electron energy spectrum in our turbulence acceleration model for a blazar Mrk 421[53]. While apower-law energy distribution is predicted in the DSA model,
our spectra shows curved feature.

This naturally leads to a energy-independent acceleration timescale. With this idea, our model is the first one
to succeed in reproducing broadband spectra of blazars
with the turbulence acceleration model [53]. We havetested the turbulence acceleration model for GRB [22],the Fermi bubble [25], and PWN [42] as well.


,FH&XEH

'





&
%



1HXWULQR

$




$1,7$,,

7$

$0
.$2
'
&
%
$

 


3$2







HI H >HUJFPV@





 












 H>H9@

Fig. 2. Photon spectra for Mrk 421[53]. Our turbulence acceleration
models (red and blue lines, slightly diffenrent parameter sets)
reproduce the curved photon spectra.

Especially in the gamma-ray band, the possible
emission mechanisms are divided into two models: leptonic and hadronic models. In the leptonic model,
gamma-rays are emitted via inverse Compton (IC) scattering of electrons interacting with low-energy photons.
The seed photons for IC scattering are synchrotron photons emitted by the same electron population or external photons such as thermal radiation from hot plasma.
The DSA predicts the acceleration of not only electrons
but also protons. High-energy protons collide with lowenergy photons or background protons, and produce
pions. The secondary photons from decay of neutral H>H9@

Fig. 3. The diffuse UHECR spectra from GRBs in our model [28](thick solid lines, different parameter sets for A–D). The thickdashed lines are spectra neglecting the effects of photomeson
production and Bethe-Heitler pair production during propagation. The observed data for the UHECR by the Pierre Auger
Observatory (open circles) and the Telescope Array (green filled
circles) are shown. The thin lines show the all-flavor cosmogenic neutrino intensities, which are below the upper limits (grayshaded area) by IceCube and ANITA-II. For comparison, we also
plot the model spectra of the cosmogenic neutrinos by Kotera etal. (JCAP 10, 013 2010, thin dotted line, denoted as KAO10)
and prompt plus cosmogenic neutrinos by Asano and Mészáros[15] (thin dashed line, denoted as AM14).

Recently, IceCube reported a detection of a highenergy neutrino from the direction of the blazar TXS
0506+0561 .
Although this success in the multimessenger astronomy seems to support the hadronicmodel, most of interpretations based on the gamma-ray
spectrum disagree the hadronic origin for the gammaray emission itself2 . Actually, the hadronic model requires a huge jet luminosity, and the spectral shape inblazars favors the leptonic model [53]. The leptonic
model implies that the magnetic field is subdominantcomponent in the jet energy density, while the Poynting flux dominated jet seems consistent with the radio
observations of the M87 jet3 . This discrepancy strongly*1 Aartsen et al., Science, 361, 1378 (2018).
*2 see e.g., Murase et al., arXiv:1807.04748 (2018).
*3 Asada & Nakamura, ApJ, 745, L28 (2012).

95

Merger of Binary Stars

Advanced-LIGO, a ground-based detector in USA,achieved the first detection of a gravitational-wave(GW) event from a merger of binary black holes (BHs)
on 14th of September 20154 . This ignited the astronomy based on the GW observations. The star formation
rate (SFR) peaks at a redshift ∼ 2, but the SFR in theuniverse above z = 2 is still uncertain. In the very early
universe, gasses were not yet polluted by metals such
as C, O, Fe, etc., which act as coolant. In such environment, very massive stars so called Pop III stars tend
to be produced. A direct observation of Pop III stars
at very high redshift is almost impossible. GW observations of BH binaries, which are remnants of massive
stars, provide clue to revealing the SFR in early universe. The BH formation also relates to high-energy
phenomena such as GRBs. Following the history of the
SFR, we estimated the detection rate of GW events for
the BH–BH or BH–NS binary mergers especially originated from the Pop III stars [31, 33, 37, 39, 45] (seeFigure 4).
We also considered the binary merger progenitor
model to be the long GRB progenitor. A Wolf–Rayetstar and a giant star system or two giant stars merge
during the common-envelope phase. When a star in aclose binary system becomes a red giant, the companion star spirals into the core of the giant owing to theorbital energy loss by the friction. The envelope of the
giant is evaporated and the binary separation becomesso close that they merge. After the merger, a rapidly
rotating naked helium star remains. Such a compact
and highly rotating star is ideal to induce a GRB. Following this scenario, we estimated the GRB rate, andwe found the rate is consistent with the observed GRB
rate evolution. Future projects to detect high-redshict
GRBs such as High-Z GUNDAM may cerify the progenitors of GRBs.
*4 Abbott et al., PRL, 116, 061102 (2016).

104

54 3

2

1

0.5

0.1

0

Pop I, II sum
Pop III

3

10

2

10
R(t) [yr-1 Gpc-3]

motivates us to study blazars. Rich and complex structure may be hidden in relativistic jets, which may relate
to the jet launching mechanism.
We have studied mechanisms of the energy transfer
to non-thermal particles and photon and neutrino emissions, which are main targets for the next generation
astronomy, the multi-messenger astronomy. To develop
such a new field in astronomy, deep understanding ofplasma physics could be a key. For example, the X-ray
PWN profiles disagree with the classical model basedon fluid dynamics [40]. We need to incorporate particle
diffusion [58], which is an effect beyond the fluid picture. There are non-trivial mechanisms of interaction
between particles, waves, photons, and magnetic field[2, 20, 24, 32], which cannot be described by only fluid
dynamics. Such fundamental studies are also in our
research field.

z

1

10

0

10

10-1
10-2
10-3
10-4
0

2000

4000

6000
8000
t [Myr]

10000

12000

Fig. 4. NS-BH merger rate densities of summation of Pop I and II,
and Pop III origins [37]. A significant fraction of Pop III NS-BH
merger events is expected to be detected.

The detections of GW from a NS-NS merger by Advanced LIGO and Advanced Virgo, an accompanying
short GRB, and its optical/radio counterparts [46, 49]
are the most exciting news in 2017. Heavy nuclei areproduced in the neutron-rich ejecta, and the succeeding
decay of the unstable nuclei heats the material. The optical/IR counter part from this heated material showeda different evolution from the expected one. We carriedout a detailed radiative transfer simulation taking into
account the stuructured ejecta based on hydrodynamical simulations [56]. Our advanced model succeeded in
reproducing the complex evolution of the photon spectrum.

Radio Observation of Crab Pulsar

Crab pulsar, the remnant of the supernova explosion in 1054 A.D., is one of the well-known neutronstars. While its physical properties have been studied
for more than 40 years since its discovery, there remains an enigma about the origin of giant radio pulses(GRPs). Radio observation of Crab GRPs and its data
analysis have been a major project in our group. Huge
data sets sampled with high temporal resolution areaffected by the dispersion effect due to inter stellar
plasma. We have developed technique to carry out
dedispersion of radio data.
We operated simultaneous multi-frequency observations of the Crab GRPs at 0.3, 1.6, 2.2, 6.7, and 8.4
GHz with the four radio telescopes (Iitate Planetary
Radio Telescope, the 34m telescope at the Kashima
Space Technology Center, the 64m telescope at the
Usuda Deep Space Center of ISAS, and the Takahagi
32m antenna of Mizusawa VLBI observatory of NAOJ)[35], which is the widest frequency range ever observed
simultaneously. In the frequency range in 0.3–2.2 GHz,
70% or more of the spectra in our GRPs are consistent with single power-laws (see Figure 5). We do not96find strong evidence that the majority of GRPs disagreewith single power-law spectra, whose spectral index is
widely distributed from approximately −4 to −1. Our
results suggest an anti-correlation between the total energy release and spectral hardness. Those new discoveries provide constraint on not only the GRP models,
but also models of another sporadic phenomenon, fast
radio bursts (FRBs). Huge GRPs from young pulsars
are possible sources of FRBs. Future FRB observations
may find common or different properties with our data.

D

Fig. 5. Some examples of Crab GRP spectra observed by us [35].

dark matter signatures by carrying out accurate measurements of cosmic ray electrons in 1 GeV - 20 TeV andgamma-rays in 4 GeV - 10 TeV. We succeeded in obtaining the electron–positron spectrum from 10 GeV to
5 TeV [47, 52], which agrees with the result obatainedwith AMS-02, but does not agree with the DAMPE
result, in which a spectral cutoff is claimed.
The recent confirmation of the largely extended
gamma-ray halo around the Geminga pulsar by
HAWC5 suggests the suppresion of the particle diffusion around the pulsar. The CALET and HAWC results provide constraint on the electron–positron diffusion in the interstellar medium. Starting from ourstudy of PWNe [40, 42, 58], we are aiming at establishing a consistent model of production and propagationof electron–positron cosmic-rays.
Using the instruments onboard CALET, we set
upper-limits of the gamma-ray fluxes for the GW events(see Figure 6) [30, 55]. The upper-limits correspond toa luminosity of 1049 –1053 erg s−1 in the GeV energy
band, which is approximately the order of luminosityof typical short GRBs. This implies there will be a favorable opportunity to detect high-energy gamma-ray
emission in further observations if events with favorable
geometry will occur within our field-of-view.

R/D Studies on UHECRs and Extraterrestrial
GrainsAs the next generation technique to detect UHECRs, a space mission EUSO [6, 23, 54] and radio techniques have been proposed. EUSO will detect the UV
light emitted by cosmic-ray generated air-showers in
the earth’s atmosphere. On the other hand, a radioarray will measure the coherent radio signal emitted
by the electromagnetic air shower component.
CollabD
orating with the TA group of the high energy cosmic
ray division of the ICRR, we have made a R/D studyD Fig. 6. 90% C.L. upper limit on GW170104 energy flux in the energy
region 10–100 GeV shown in the equatorial coordinates [55]. Alsoof the Oactive
Omethod, namely, the detection of radarD  DDshown by green contours is the localization significance map ofechoes from extensive air showers of UHECRs. Using
the GW170104 signal reported by advanced LIGO.
the same radar echo technique, we have also joined aradar research project for extraterrestrial grains (meteors) [1, 4], which is also a research target for EUSO
[44].
Collaboration with other Projects
Our research fields closely relates to research targets
in the other groups in the ICRR, TA, Tibet (cosmicCALETThe CALET mission aims at revealing unsolved ray physics), CTA (gamma-ray emission mechanism),problems in high energy phenomena of the Universe Super Kamiokande (extragalactic neutrinos), Observaby carrying out accurate measurements of high energy tional Cosmology (GRB and star formation rate), andspectra of electrons, gamma-rays and nuclei [18, 38, KAGRA (binary mergers). Actually our group has col50]. HTV5 equipped with CALET was successfully laborated with the TA group (R/D study radar echoe
launched by the H-IIB at 8:50:49 p.m. on August 19 technique of the UHECR detection) and the CTA group2015 (JST) from the Tanegashima Space Center, and (several collaborating papers [21, 27, 53] have been pubCALET is observeing cosmic rays without apparent lished). Some of the CTA members join MAGIC team,which operates imaging atmospheric Cherenkov teleproblems.
Major scientific objectives are to search nearby scopes situated at La Palma, where the first large sizeDB

cosmic-ray sources, such as NSs [3] or PWNe [40], and*5 Abeysekara et al. 2017, Science, 358, 911 (2017).

97

telescope of CTA is now constructed. Now one of ourmembers has contributes to writing MAGIC papers byinterpreting the observation data. Those will be published in one year.
We also frequently teach and advise students in theother groups, especially the CTA and TA group, forpromoting their study and writing papers or thesis.
Gamma-ray detections from GRB 160509A with the
Fermi Large Area Telescope were analysed by a student
in the CTA group, and the results were summarized as adoctoral thesis. The results will be published with ourinterpretation based on our paper [43], which is also
based on a master thesis of a student in Osaka university. Thus, we contribute to education for students evenin other groups or universities. Now we are preparing
a paper on the high redshift GRB rate collaboratingwith a student of the Observational Cosmology group.
A new assistant professor joined 2018 concurrently belongs to the KAGRA group to analyse GW date withgeneral relativistic numerical simulations. When the
KAGRA starts its operation, he will greatly contributeto interpretation of the observation data.

Workshops

As a group in one of Joint Usage/Research Centers,
we have held workshops on high-energy astrophysics
every year. The list of the workshops is as follows:• “Space Particle Acceleration: Elementary Processes in Relativistic Plasma and Magnetopshereof Pulsar/Magnetar”, Jun. 2012
• “Physics for the Electron Energy Distribution”,
Nov. 2012• “Particle Acceleration and Radiation in Pulsar
Magnetosphere”, Mar. 2013
• “Sturucture of Strongly Magnetized Neutron Star
and Physical Processes of Particle Acceleration
and Photon Emission”, Oct. 2013
• “Particle Acceleration in the Universe and Transient Phenomena in Radio Band”, Mar. 2014
• “Cosmic-ray Research in the next 10 years”, Nov.
2014
• “Root of Relativistic Jet and Particle Acceleration”, Feb. 2016
• “High Energy Phenomena in Compact Objects”,
Oct. 2016• “Next Step with Gamma-ray Bursts”, Nov. 2017
In most of the workshops, we have assured one or
two hours for each lecturer. So the audience could
hear details in latest topics. Our workshops have provided valuable opportunity to learn studies in unfamiliar fields for astronomical community.

Members
Staff
Toshio Terasawa, Professor, Dec. 2009–Mar. 2016
Katsuaki Asano, Assoc. Professor, Apr. 2013–present
Kyohei Kawaguchi, Assis. Professor, Mar. 2018–present
Postdoctoral Fellows
Shota Kisaka, ICRR researcher, Apr. 2012–Mar. 2014
Yosui Akaike, JSPS fellow, Apr. 2012–Mar. 2015
Shuta J. Tanaka, JSPS fellow, Apr. 2013–Mar. 2016
Ryo Mikami, JSPS fellow, Apr. 2016–Sep. 2016
Tomoya Kinugawa, ICRR researcher, Apr. 2016–Mar.
2018
Collaboration Researcher
Hideaki Miyamoto, Apr. 2012–Mar. 2014
Toshio Terasawa, Apr. 2016–Mar. 2018
Graduate Students
Ryo Mikami, Apr. 2011–Mar. 2016
Ryuji Takeishi, Apr. 2011–Mar. 2013
Kento Sasaki, Apr. 2013–Mar. 2015
Sho To, Apr. 2013–present
Nagisa Hiroshima, Apr. 2014–Mar. 2016
Wataru Ishizaki, Apr. 2014–present

List of Publications
Papers in Refereed Journals[1] J. Kero, C. Szasz, T. Nakamura, T. Terasawa, H.
Miyamoto, & K. Nishimura “A Meteor Head Echo
Analysis Algorithm for the Lower VHF Band”,
Ann. Geophys. 30, 639-659 (2012)[2] Yasufumi Kojima, & Shota Kisaka “Magnetic
Field Decay with Hall Drift in Neutron Star
Crusts”, Mon. Not. R. Astron. Soc. 421, 2722-2730(2012)[3] Shota Kisaka, & Norita Kawanaka, “TeV Cosmicray Electrons from Millisecond Pulsars”, Mon.
Not. R. Astron. Soc. 421, 3543-3549 (2012)[4] J. Kero, C. Szasz, T. Nakamura, D. D. Meisel, M.
Ueda, Y. Fujiwara, T. Terasawa, K. Nishimura,& J. Watanabe, “The 2009–2010 MU Radar Head
Echo Observation Programme for Sporadic andShower Meteors: Radiant Densities and Diurnal
Rates”, Mon. Not. R. Astron. Soc. 425, 135-146(2012)[5] Toshio Terasawa, & Shuichi Matsukiyo, “Cyclotron Resonant Interactions in Cosmic Particle
Accelerators”, Space Sci. Rev. 173, 623-640 (2012)

98[6] J.H. Adams Jr., S. Ahmad, J.-N. Albert, et al.,
“An Evaluation of the Exposure in Nadir Observation of the JEM-EUSO Mission”, Astro. Part.
Phys. 44, 76-90 (2013)[7] Katsuaki Asano, “Wide-band Spectra of Prompt
Emission” EAS Publications Series 61, 115-122(2013)[8] Yoichi Yatsu, Katsuaki Asano, Nobuyuki Kawai,
Yuki Yano, & Takeshi Nakamori, “Spatially Resolved Spectroscopy of a Pulsar Wind Nebula in
MSH 15–56”, Astrophys. J. 773, 25(14pp) (2013)[9] Katsuaki Asano, & Peter Mészáros, “Photon and
Neutrino Spectra of Time-dependent Photospheric
Models of Gamma-ray Bursts”, J. Cos. Astropart.
Phys. 09, 008 (2013)[10] M. Ackermann, M. Ajello, K. Asano et al., “TheFirst Fermi LAT Gamma-ray Burst Catalog”, Astrophys. J. Supp. 209, 11(90pp) (2013)[11] Shuta J. Tanaka, & Fumio Takahara, “Constraint
on Pulsar Wind Properties from Induced Compton
Scattering off Radio Pulses”, Prog. Theor. Exp.
Phys. 2013, 123E01(24pp) (2013)[12] Katsuaki Asano, Fumio Takahara, Masaaki
Kusunose, Kenji Toma, & Jun Kakuwa, “Timedependent Models for Blazar Emissions with theSecond-order Fermi Acceleration”, Planet. Space
Sci. 59, 378-386 Astrophys. J. 780, 64(12pp)(2014)[13] R. Mikami, T. Terasawa, S. Kisaka, H. Miyamoto,
K. Asano, N. Kawai, Y. Yamakoshi, K. Nagata,
R. Kataoka, K. Takefuji, M. Sekido, H. Takeuchi,
H. Odaka, T. Sato, & Y. T. Tanaka, “Search for a
Correlation between Giant Radio Pulses and Hard
X-ray Emissions in the Crab Pulsar”, JPS Conf.
Proc. 1, 015106(4pp) (2014)[14] M. Ackermann, M. Ajello, Katsuaki Asano etal., “Fermi-LAT Observations of the Gamma-ray
Burst GRB 130427A”, Scinece 343, 42-47 (2014)[15] Katsuaki Asano, & Peter Mészáros, “Neutrino
and Cosmic-ray Release from Gamma-ray Bursts:
Time-dependent Simulations”, Astrophys. J. 785,54(5pp) (2014)[16] Makoto Takamoto, Shota Kisaka, Takeru Suzuki,& Toshio Terasawa, “The Evolution of High Temperature Plasma in Magnetar Magnetospheres and
its Implications for Giant Flares”, Astrophys. J.
787, 84(13pp) (2014)[17] Shota Kisaka, & Shuta J. Tanaka, “Synchrotron
X-ray Emission from Old Pulsars”, Mon. Not. R.
Astron. Soc. 443, 2063-2076 (2014)[18] Tae Niita, Shoji Torii, Katsuaki Kasahara, Hiroyuki Murakami, Shunsuke Ozawa, Yoshitaka
Ueyama, Yosui Akaike, Tadahisa Tamura, Kenji
Yoshida, Yusaku Katayose, Yuki Shimizu, &Hideyuki Fuke, “A Balloon Experiment Using
CALET Prototype (bCALET-2)”, Adv. SpaceRes. 55, 753-760 (2015)[19] Jun Kakuwa, Kenji Toma, Katsuaki Asano,
Masaaki Kusunose, & Fumio Takahara, “Synchrotron Self-Compton Emission by Relativistic
Electrons under Stochastic Acceleration: Application to Mrk 421 and Mrk 501”, Mon. Not. R.
Astron. Soc. 449, 551-558 (2015)[20] Shuta J. Tanaka, Katsuaki Asano, & Toshio
Terasawa, “Avalanche Photon Cooling by Induced Compton Scattering: Higher-order Kompaneets Equation”, Prog. Theor. Exp. Phys. 2015,
073E01(14pp) (2015)[21] Katsuaki Asano, & Masaaki Hayashida, “The
Most Intensive Gamma-ray Flare of Quasar 3C 279
with the Second-order Fermi Acceleration”, Astrophys. J. Lett. 808, L18(5pp) (2015)[22] Katsuaki Asano, & Kohta Murase, “Gamma-ray
Bursts as Multienergy Neutrino Sources”, Adv.
Astron. 2015, 568516(10pp) (2015)[23] J.H. Adams Jr., S. Ahmad, J.-N. Albert, D. Allard, L. Anchordoqui, V. Andreev, A. Anzalone, Y.
Arai, K. Asano et al., “Special Issue on the JEMEUSO Mission”, Exp. Astron. 40, 3-326 (2015)[24] Katsuaki Asano, & Toshio Terasawa, “Stochastic
Acceleration Model of Gamma-ray Burst with Decaying Turbulence”, Mon. Not. R. Astron. Soc.
454, 2242-2248 (2015)[25] Kento Sasaki, Katsuaki Asano, & Toshio Terasawa, “Time-dependent Stochastic Acceleration
Model for the Fermi Bubbles”, Astrophys. J. 814,93(9pp) (2015)[26] Ryo Yamazaki, Katsuaki Asano, & Yutaka
Ohira, “Electromagnetic Afterglows Associatedwith Gamma-ray Emission Coincident with Binary Black Hole Merger Event GW150914”, Prog.
Theor. Exp. Phys. 2016, 051E01(7pp) (2016)[27] M. Ackermann, R. Anantua, K. Asano, et al.,“Minute-timescale > 100 MeV γ -ray Variability
during the Giant Outburst of Quasar 3C 279 Observed by Fermi-LAT in 2015 June”, Astrophys. J.
Lett. 824, L20(8pp) (2016)[28] Katsuaki Asano, & Peter Mészáros, “Ultrahighenergy Cosmic Ray Production by Turbulence inGamma-ray Burst Jets and Cosmogenic Neutrinos”, Phys. Rev. D 94, 023005(10pp) (2016)

99[29] Kazuhiro Takefuji, Toshio Terasawa, Tetsuro
Kondo, Ryo Mikami, Hiroshi Takeuchi, Hiroaki
Misawa, Fuminori Tsuchiya, Hajime Kita, &Mamoru Sekido, “Very Long Baseline Interferometry Experiment on Giant Radio Pulses of CrabPulsar toward Fast Radio Burst Detection”, Pub.
Astron. Soc. Pac. 128, 084502(6pp) (2016)[30] O. Adriani, Y. Akaike, K. Asano, et al., “CALET
Upper Limits on X-ray and Gamma-ray Counterparts of GW 151226”, Astrophys. J. Lett. 829,
L20(5pp) (2016)[31] Takashi Nakamura, Masaki Ando, Tomoya
Kinugawa, Hiroyuki Nakano, Kazunari Eda,
Shuichi Sato, Mitsuru Musha, Tomotada Akutsu,
Takahiro Tanaka, Naoki Seto, Nobuyuki Kanda, &Yousuke Itoh, “Pre-DECIGO can Get the Smoking Gun to Decide the Astrophysical or Cosmological Origin of GW150914-like Binary Black Holes”,
Prog. Theor. Exp. Phys. 2016, 093E01(16pp)(2016)[32] Shota Kisaka, Katsuaki Asano, & Toshio Terasawa, “Electric Field Screening with Backflow at
Pulsar Polar Cap”, Astrophys. J. 829, 12(14pp)(2016)[33] Tomoya Kinugawa, Hiroyuki Nakano, & Takashi
Nakamura, “Gravitational Wave Quasinormal
Mode from Population III Massive Black Hole Binaries in Various Models of Population Synthesis”, Prog. Theor. Exp. Phys. 2016, 103E01(21pp)(2016)[34] S. Guiriec, C. Kouveliotou, D. H. Hartmann,
J. Granot, K. Asano, P. Meszaros, R. Gill, N.
Gehrels, & J. McEnery, “A Unified Model for GRB
Prompt Emission from Optical to γ -rays; Exploring GRBs as Standard Candles”, Astrophys. J.
Lett. 831, L8(6pp) (2016)[35] Ryo Mikami, Katsuaki Asano, Shuta J. Tanaka,
Shota Kisaka, Mamoru Sekido, Kazuhiro Takefuji, Hiroshi Takeuchi, Hiroaki Misawa, Fuminori
Tsuchiya, Hajime Kita, Yoshinori Yonekura, &Toshio Terasawa, “Wide-band Spectra of GiantRadio Pulses from the Crab Pulsar”, Astrophys.
J. 832, 212(25pp) (2016)[36] Makoto Arimoto, Katsuaki Asano, Masanori
Ohno, Peter Veres, Magnus Axelsson, Elisabetta
Bissaldi, Yutaro Tachibana, & Nobuyuki Kawai,“High-energy Non-thermal and Thermal Emission
from GRB 141207A Detected by Fermi”, Astrophys. J. 833, 139(13pp) (2016)[37] Tomoya Kinugawa, Takashi Nakamura, & Hiroyuki Nakano, “The Possible Existence of Pop
III NS–BH Binary and its Detectability”, Prog.
Theor. Exp. Phys. 2017, 021E01(9pp) (2017)[38] Y. Asaoka, Y. Akaike, Y. Komiya, R. Miyata,
S. Torii, O. Adriani, K. Asano, et al., “Energy
Calibration of CALET Onboard the International
Space Station”, Astro. Part. Phys. 91, 1-10 (2017)[39] Kohei Inayoshi, Ryosuke Hirai, Tomoya Kinugawa,& Kenta Hotokezaka, “Formation Pathway of Population III Coalescing Binary Black Holes throughStable Mass Transfer”, Mon. Not. R. Astron. Soc.
468, 5020-5032 (2017)[40] Wataru Ishizaki, Shuta J. Tanaka, Katsuaki
Asano, & Toshio Terasawa, “Broadband Photon
Spectrum and its Radial Profile of Pulsar Wind
Nebulae”, Astrophys. J. 838, 142(14pp) (2017)[41] F. Acero, R. Aloisio, J. Amans, et al., “Prospects
for CTA Observations of the Young SNR RX
J1713.7–3946”, Astrophys. J. 840, 74(14pp)(2017)[42] Shuta J. Tanaka, & Katsuaki Asano, “On theRadio-emitting Particles of the Crab Nebula:
Stochastic Acceleration Model”, Astrophys. J.
841, 78(11pp) (2017)[43] Takuma Fukushima, Sho To, Katsuaki Asano,& Yutaka Fujita, “Temporal Evolution of theGamma-ray Burst Afterglow Spectrum for an Observer: GeV–TeV Synchrotron Self-Compton Light
Curve”, Astrophys. J. 844, 92(11pp) (2017)[44] G. Abdellaoui, S. Abe, A. Acheli, et al., “Meteor
Studies in the Framework of the JEM-EUSO Program”, Planet. Space Sci. 143, 245-255 (2017)[45] Akinobu Miyamoto, Tomoya Kinugawa, Takashi
Nakamura, & Nobuyuki Kanda, “How to Confirm
the Existence of Population III Stars by Observations of Gravitational Waves”, Phys. Rev. D 96,
064025(10pp) (2017)[46] B. P. Abbott, et al., “Multi-messenger Observations of a Binary Neutron Star Merger”, Astrophys. J. Lett. 848, L12(59pp) (2017)[47] O. Adriani, Y. Akaike, K. Asano, et al., “Energy
Spectrum of Cosmic-ray Electron and Positronfrom 10 GeV to 3 TeV Observed with the Calorimetric Electron Telescope on the International
Space Station”, Phys. Rev. Lett. 119, 181101(6pp)(2017)[48] Tomoya Kinugawa, & Katsuaki Asano, “Long
Gamma-ray Burst Rate in the Binary Merger Progenitor Model”, Astrophys. J. Lett. 849, L29(5pp)(2017)[49] Katsuaki Asano, & Sho To, “Subsequent Nonthermal Emission due to the Kilonova Ejecta in
GW170817”, Astrophys. J. 852, 105(5pp) (2018)100[50] Y. Asaoka, S. Ozawa, S. Torii, O. Adriani, Y.
Akaike, K. Asano, et al., “On-orbit Operations
and Offline Data Processing of CALET onboard
the ISS”, Astro. Part. Phys. 100, 29-37 (2018)[51] Kai Wang, Ruo-Yu Liu, Zi-Gao Dai, & Katsuaki
Asano, “Hadronic Origin of Prompt High-energy
Emission of Gamma-ray Bursts Revisited: In theCase of a Limited Maximum Proton Energy”, Astrophys. J. 857, 24(12pp) (2018)[52] O. Adriani, Y. Akaike, K. Asano, et al., “Extended Measurement of the Cosmic-Ray Electron
and Positron Spectrum from 11 GeV to 4.8 TeV
with the Calorimetric Electron Telescope on theInternational Space Station”, Phys. Rev. Lett.
120, 261102(7pp) (2018)[53] Katsuaki Asano, & Masaaki Hayashida, “Blazar
Spectra with Hard-sphere-like Acceleration ofElectrons”, Astrophys. J. 861, 31(7pp) (2018)[54] G. Abdellaoui, et al., “EUSO–TA — First Resultsfrom a Ground-based EUSO Telescope”, Astro.
Part. Phys. 102, 98-111 (2018)[55] O. Adriani, Y. Akaike, K. Asano, et al., “Search
for GeV Gamma-ray Counterparts of Gravitational
Wave Events by CALET”, Astrophys. J. 863,
160(9pp) (2018)[56] Kyohei Kawaguchi, Masaru Shibata, & Masaomi
Tanaka, “Radiative Transfer Simulation for the
Optical and Near-infrared Electromagnetic Counterparts to GW170817”, Astrophys. J. Lett. 865,
L21(6pp) (2018)[57] Yutaro Tachibana, Makoto Arimoto, Katsuaki
Asano, et al., “Late Engine Activity of GRB
161017A Revealed by Early Optical Observations”,
Pub. Astron. Soc. Pac. 70, 92(9pp) (2018)[58] Wataru Ishizaki, Katsuaki Asano, & Kyohei
Kawaguchi, “Self-Consistent 1–D Model of Pulsar
Wind Nebulae with Particle Diffusion”, Accepted
for ApJ, arXiv:1809.09054

101

ASHRA (R&D)
In 2002, Sasaki et al. presented the distinctive detecting potential of the Earth-skimming tau neutrino(ES-ντ ) technique with air-shower (AS) telescope array detector [2], which can enjoy a large target massby imaging ASs produced by τ decays in the air. The
method has come to be used as a powerful tool to promote multi-astroparticle astrophysics including veryhigh energy (VHE) neutrino search [1, 7, 10, 11, 31,
32] 1 2 . Along with that, Sasaki et al. advocated thedesign of high resolution (1 arcmin) wide field of view(FOV) (50◦ ) AS telescope based on the modified BakerNunn optics [3], which can watch such a large air volume keeping the astrometrical advantage of cosmic ν sin a cost effective way. The camera mounted on such ahigh resolution and wide telescope is expected to efficiently trigger and read out images with pixels of ordersof million at least at the video rate. The only readoutthat meets the requirement seemed a CMOS sensor but
not a conventional PMT array camera. In 2002, Sasaki
et al. developed a prototype demonstrating the principle with the light and electron optics transporting
images to a fine CMOS sensor to realize a high resolution over a wide FOV [8, 12]. In 2003, Ashra-1 (All-sky
Survey High Resolution Air-shower detector Phase-1)
was funded to start the collaboration. 3
Ashra-1 4 [7, 14, 23] is a project to build an unconventional optical telescope complex on Mauna Loa(3,300 m asl.) on Hawaii Island as shown in Fig.1 (top),that images very wide FOV, covering most of the wholehalf sky, yet with the angle resolution of a few arcmin.
The project aims to observe Cherenkov (CE) and fluorescence (FL) light from the AS developments inducedby the primaries of ES-ντ ’s as well as gamma-rays (γ s)and cosmic-ray (CR) nucleons.
The key technical feature of the Ashra-1 detector
rests on the use of electrostatic lenses to generate convergent beams rather than optical lens system. This
electron optics requires: photoelectric lens imagingtube [18, 30]; the world’s largest imaging-tube uses electrostatic lens in addition to an optical system to generate convergent beams from photocathode of 20-in. diameter to output phospher window of 1-in. diameter,
enabling a very low cost and high performance image
sensor providing a high resolution over a wide FOV,
and image pipeline [8]; the image transportation fromimaging-tube (image intensifier) to a trigger device andimage sensors of fine pixels (CMOS), with high gain and*1 M. Ahnen et al.(MAGIC Collab.), Astropart. Phys, 102, 77–8
(2018).
*2 J. Alvarez-Muñiz et al., Phys. Rev. D, 97(2), 1–1 (2018).
*3 supported by Coordination Fund for Promoting Science and Technology (157-20004100) from MEXT of Japan.
*4 http://www.icrr.u-tokyo.ac.jp/∼ashra

Fig. 1. Ashra-1 Mauna Loa Observation Site (top), photo-electriclens imaging tube (bottom left) photoelectric image pipeline(bottom middle) and local exposure/readout fine sensor withtrigger (FST) (bottom right)

resolution, enabling very fine images with parallel selftrigger systems that trigger separately for optical flash,atmospheric CE and FL lights.
We succeeded in realizing the modified Baker-Nunn
optics light collector (LC) cost-effectively by combining three 1 m diameter acrylic aspherical lenses andseven 0.8 m diameter thin (∼1 cm) spherical float glass
segment mirrors. In 2004-2006, we made test observations with a prototype of the Ashra-1 LC at Haleakala
Observatory on Maui Island, which is owned by University of Hawaii. We searched for optical flash in thefield covering the WXM error box of GRB041211 withthe prototype [13]. The Ashra-1 wide FOV allows us
search for light or/and particle emission both before
and after rapid transients such as GRBs covering thestarting time [15, 16, 17, 26, 29, 31, 39].
The Ashra-1 has been partly operated since 2008
at the Mauna Loa observation site at 3300 m asl. on
Hawaii Island. The LC optics and the readout system
as the detector unit (Fig. 1) were confirmed to achievea total resolution of ∼ 3 arcminutes covering 42◦ FOVwith star light images 2 (left). Following the alert for
GRB081203A given by the SWIFT satellite, Ashra-1
succeeded in the first search for PeV-EeV ντ ’s originating from a GRB with the ES-ντ technique setting
stringent fluence limits [31]. Furthermore, with detailedMonte Carlo simulation, we performed systematic studies on the detection performance for ES-ντ ASs by the
Ashra-1 CE mode, confirming the good detection sensitivity due to the large angle method for quasi-horizontal
ASs with almost free CR background and very accurate pointing accuracy of less than 0.2◦ even in thecase of the monostatic observation [32]. In the third

102

Fig. 2. A real star image taken by Ashra-1 with 1 sec exposure time
without trigger (left), where big constellations are indicated on
it as guiding eyes. A real air-shower image triggered and readout by Ashra-1 with 200 ns exposure time (right).

Fig. 3. CR spectrum observed with Ashra-1 LC CE mode.

observation period (Obs3) in 2012-2013, we arranged
four sets of trigger pixels toward the 2.6◦ × 2.6◦ areaaround the center of 14.6◦ in altitude and 22.0◦ in azumuth to check the sensitivity for the background CR
CE ASs. From the analysis of the triggered events likeFig. 2 (right), we measured CR flux spectrum whichis consistent with conventional measurements [41], asshown in Fig. 3 (left) where the MC predictions arenormalized by the fluxes from conventional CR observations assuming the primary proton or iron.
Since 2013, we have proposed and prepared for thestage of Obs4, whereas the observation is still not
funded. The remaining element to be demonstrated
is a fast fine sensor with trigger (FST) to be served
as readout of the Ashra-1 FL mode. The observed FL
AS trajectories could have various time structure up

to their own energies and geometrical conditions. The
readout FST is supposed to have the functions of local
exposure and local readout corresponding to fast addressing signals indicating when and which pixel groups
on the sensor to be exposed by the signal light. We developed FST and performed careful tests so that we
confirmed such functions well as the readout of the FL
signals. Fig. 1 shows a picture of FST under testing
it. Adding that, automatic operation of the detector
system is useful and important improvement for thestage of more systematic operation. We have successfully developed a short pulse uniform LED flasher tobe equipped at the center of the main mirror of LC
to check the total gain and the non-uniformity of thetrigger readout system composed of PLI and PIP bycounting photons flash by flash. It provides us periodically high-quality gain calibration easily merged into
the available data flow without any specific data taking system. Adding to demonstrating the FL AS trigger readout as described above, we have planned toobserve at every night the ridge of our Galaxy within
the wide FOV combined with six Ashra-1 LCs aligned
along the tragectory of the Galactic center. The wide
FOV composed of six Ashra-1 LCs can efficiently cover
the Galactic ridge region simultaneously [36]. HESS
observation of the hard γ -ray spectra from the Galactic Center suggesting a Pevatron candidate is one offascinating physics motivation. 5
Based on Ashra-1 performance, we have planneda new extention, i.g. Ashra Neutrino Telescope Array (NTA) 6 , which is an AS imaging ν and γ observation system for the aim or scientific goal [33]: Clear
Discovery and Identification of Nonthermal Hadronic
Processes in the Universe, be it Galactic, Extragalatic,or Cosmogenic. By optimizing the layout of the
NTA stations to enhance the detection sensitivity forES-ντ ’s around 1 PeV from the detailed simulation
studies [32, 33], four NTA stations are to be deployedon Mauna Loa at 3000 - 3500 m asl. (NTA Summit Array), watching the air volume surrounding Mauna Loa
including the surface of Mauna Loa, the largest volcano, to efficiently detect CE and FL light generated
from τ ASs with both short and long decay lengths andγ ASs. The reconstructed AS images with fine resolution is powerful not only in the determination of point
sources of PeV ντ ’s but also FL observation for γ ASs
above PeV with the large effective area. Fig. 4 shows
NTA achieves the effective area similar with that of
IceCube at 3 PeV and larger than it above 30 PeV by
10-100 times and larger than either of IceCube Gen2 or
ARA-100 in the 30 PeV-30 EeV region. The sensitivity
for ES-ντ ’s with observing CE light can enhance more
the discovery potential around PeV or lower energy region. Fig. 4 shows the integral flux sensitivity limits for
γ ’s with NTA for one year observation time comparing*5 HESS Collaboration, Nature 531, 476 (2016).
*6 http://asrws300.icrr.u-tokyo.ac.jp/nta/

103(a) ICRR, Univ. Tokyo; (b) Toho Univ.; (c) Univ.
Hawaii Manoa; (d) Univ. Hawaii Hilo; (e) Nagoya
Univ; (f) Chiba Univ.; (g) Ibaraki Univ.; (h) Kanagawa Univ. (i) KEK (j) Univ. Torino (k) Academia
Sinica (l) National Taiwan Univ. (m) National Chiao
Tung Univ. (n) Univ.Immersion Program (o) Okayama
Univ.

List of Publications
2000–2012
Fig. 4. Effective area for ES-ντ ’s and DP-νe,τ ’s as changing theenergies with NTA in comparison with those for all flavor ν swith IceCube, IceCube Gen2, and ARA-100 (left). Integral flux
sensitivity limits (5σ or more than 10 events) for γ ’s versus theenergies with NTA (1yr observation) and other detectors (right).

with those of other detectors. The unique combination
among CE and FL observations for both ES-ντ ’s and γwith NTA will truly identify Pevatron(s) and open up
new types of search for ν ’s and γ ’s in the wide energy
range.
A Letter of Intent for NTA has been publishedsince 2013 [33, 34]. In 2014, a preliminary workshop(VHEPA2014) [37, 36, 38] was held at Kashiwa campusof the University of Tokyo to discuss the design of the
project and plans with the colleagues who share common interests. After a workshop in Taipei and an informal meeting to discuss the next post-IceCube detector
project at the 34th International Cosmic Ray Conference in The Hague in 2015 [40, 41], we held a workshop as VHEPA2016 at the University of Hawaii Manoa
in January, 2016 [42] to discuss more detailed physics
and NTA potential performance, as well as funding requests in each country. We have set up a Promotion
Working Group and our proto-collaboration to reconfirm the basic design of NTA, discuss strategies in themulti-messenger era [43], and publish White Paper andTechnical Design Report.

Members[Spokesperson: Makoto Sasaki]
ICRR, Univ. of Tokyo, Kashiwa, Chiba 277-8582
In collaboration with the members of: T. Aokia ,P. Binderd ,T. Browderc ,P. Doetinchemc ,bdY. Fukasaku , J. Goldman , J. Hamiltond , A. Hirosea ,S. Hainok , Y.B. Hsiungl , H.A. HuangN , T. Kimurag ,K. Kohrii , H. Kuze f , J. Learnedc , G.-L. Linm ,H. Mashiyamaa , S. Matsunoc , R. Mussa j , H. Oshimab ,S. Ogawab , M. Sasakia , H. Shibuyab , N. Sugiyamae ,M. Tanakai ,M.-Z. Wangrl ,Y. WatanabehoK. Yoshimura[1] M. Sasaki, in Proc. ICRR2000 Satellite Symposium: Workshop of Comprehensive Study of theHigh Energy Universe, edited by T. Kifune, J.
Okada, T. Kajita, and M. Sasaki (ICRR, the University of Tokyo, Kashiwa, 2000), p. 110.
[2] Sasaki, M., Asaoka, Y., Jobashi, M. “Detecting
very high energy neutrinos by the telescopearray”. Astroparticle Phys., 19(1), 37–46 (2003).
https://doi.org/10.1016/S0927-6505(02)00191-3.
[41][3] M. Sasaki, A. Kusaka and Y. Asaoka, “Design of
UHECR telescope with 1 arcmin resolution and50◦ field of view”, Nucl. Instrum. Methods A 492,
49 (2002). [34][4] Aita, Y., et al., “The ASHRA Detector”, ibid., pp.
1061-1064 (2003).
[5] Kohri, K., et al., “Particle Physics in ASHRA”,
ibid., pp. 1747-1750 (2003).
[6] Aita, Y., et al., “High Energy Astrophysics by
ASHRA”, ibid., pp. 2991-2994 (2003).
[7] Sasaki, M., “Very High Energy Particle Astronomy
with All-sky Survey High Resolution Air-shower
Detector (Ashra)”, Progress of Theoretical Physics
Supplement, 151, 192 (2003). [13][8] M. Sasaki, Y. Asaoka and M. Jobashi, “Selftriggered image intensifier tube for high resolution
UHECR imaging detector”, Nucl. Instrum. Methods A 501, 359 (2003). [29][9] Kuze, H., Fukagawa, S., Takeuchi, N., Asaoka,
Y., and Sasaki, M., “Development of a widearea imaging lidar for atmospheric monitoring”,
29th SICE Remote Sensing Symposium (Tsukuba),
61-64 (2003).
[10] Sasaki, M., “Very High Energy Particle Astronomy
with All-Sky Survey High Resolution Air-shower
Detector (Ashra)”, Modern Physics Letters A 19,
1107-1115, 2004. [13]104[11] P. Yeh et al., “PeV cosmic neutrinos from themountains”, Modern Physics Letters A Vol. 19,
Nos. 13-16 (2004) 1117-1124. [14][12] Arai, Y., et al., “ASHRA Trigger and Readout
Pixel Sensors”, Proc. 28th Intl. Cosmic Ray Conf.
(Tsukuba), pp. 961–964, 2003.
[13] Sasaki, M., et al., “GRB041211: Ashra Prototype
optical observation”, GCN Circ. 2846, (2004).
[14] Sasaki, M., et al., “Ashra-1 Analysis”, Proc. 29th
Int. Cosmic Ray Conf. (Pune, India), Vol. 8,
197-200, (2005).
[15] Sasaki, M., et al., “Observation of Optical Transients with the Ashra prototype”, ibid., Vol. 5,319-322, 2005.
[16] Sasaki, M., et al., “GRB050504: Ashra-P2/3 monitor and Ashra-AFT response”, GCN Circ. 3499,
2005.
[17] Sasaki, M., Manago, N., Noda, K., Asaoka, Y.,“GRB050502b: Early Observation”, GCN Circ.
3421, 2005.
[18] Asaoka.Y., Aita.Y., Aoki.T., and Sasaki.M., “Development of a 16-inch UV-Ray Image Intensifier
Tube,” IEEE Transactions on Nuclear Science, 52,
num 5, 1773-1778 (2005).
[19] Fukagawa, S., Kouga, I., Kuze, H., Takeuchi,N., Sasaki, M., Asaoka, Y., Ogawa, S., “Simulation study for aerosol distribution retrieval frombistatic, imaging lidar data”, Conference on Laserand Electro Optics / Pacific Rim 2005, C15, July
14, 2005.
[20] Fukagawa, S., Kouga, I., Kuze, H., Takeuchi, N.,
Sasaki, M., Asaoka, Y. and Ogawa, S., “Simulation study for aerosol distribution retrieval
from bistatic, imaging lidar data”, Conferenceon Quantum Electronics 2005 and the Pacific
Rim Conference on Lasers and Electro-Optics
2005 (IQEC/CLEO-PR 2005), Tokyo, Japan (July
11-15, 2005).
[21] Fukagawa, S., Koga, I., Kuze, H., Takeuchi, N.,
Sasaki, M., Asaoka, Y., Ogawa, S., “Environmental application of the all-sky survey high-resolution
air-shower (Ashra) telescope - aerosol distribution
measurement using a bistatic, imaging lidar”, Proceedings of the CEReS international symposium onradiation budget and atmospheric parameters studied by satellite and ground observation data, P-3,
pp.196-199 (2005.2).
[22] Katsuta.T., Yokomizo.S., Sasaki.M., “Study on
Machining of Large Acrylic Lens,” Japan Society ofPrecision Engineering, 73, No.2, 215-219, (2007).

[23] M. Sasaki, “The Ashra Project”, 30th Intl. Cosmic
Ray Conf. (Merida), ID1232, 2007.
[24] Y. Aita et al., “Ashra Mauna Loa Observatory andSlow Control System”, ibid., ID684.
[25] Y. Aita et al., “Hybrid Photo Detector as the
Ashra Trigger Sensor”, ibid., ID1279.
[26] Y. Aita et al., “GRB081203A: Ashra-1 observationof early optical and VHE-neutrino emission”, GCN
Circ., 8632 (2008).
[27] Y. Aita et al., “VHE neutrino pilot observation
with the Ashra detector”, 31th Intl. Cosmic Ray
Conf. (Lodz), ID313, 2009.
[28] Y. Aita et al., “Ashra Optical Transient Observation”, ibid., ID1410.
[29] Y. Asaoka et al., “GRB100906A: Ashra-1 observation of early optical emission”, GCN Circ., 11291(2010).
[30] Y. Asaoka and M. Sasaki, “Performance of a 20-in.
photoelectric lens image intensifier tube”, Nucl. Instrum. Methods Phys. Res. A 647, 34 (2011). [18][31] Y. Aita et al., “Observational Search for PeV-EeV
Tau Neutrino from GRB081203A”, Ap. J. Lett.
736, L12 (2011). [25]2013–2018[32] Y. Asaoka, M. Sasaki, “Cherenkov τ shower earthskimming method for PeV-EeV ντ observation with
Ashra”, Astroparticle Phys. 41, 7 (2013). [19][33] Sasaki, M., Hou, G. W.-S. “Neutrino Telescope
Array Letter of Intent: A Large Array of High
Resolution Imaging Atmospheric Cherenkov andFluorescence Detectors for Survey of Air-showers
from Cosmic Tau Neutrinos in the PeV-EeV Energy Range”, arXiv:1408.6244 (2014). [10][34] George W.-S. Hou, Makoto Sasaki, “Prospect towards Survey of Astronomical ντ Sources”, THE
UNIVERS EVol. 2, No. 4 October-December 2014,
Pages 15-26, ISSN:2309-852X.
[35] S. Ogawa (Ashra-1 Collaboration), Proceedings ofthe 12th Asia Pacific Physics Conference, JPS
Conf. Proc. , 013111 (2014).
[36] Sasaki, M., Kifune, T., “Ashra Neutrino Telescope
Array (NTA): Combined Imaging Observation of
Astroparticles — For Clear Identification of Cosmic Accelerators and Fundamental Physics Using Cosmic Beams —”, Proc. of the 7th International Workshop on Very High Energy Particle Astronomy in 2014 (VHEPA2014), 2014,1–8 (2017).
https://doi.org/10.7566/JPSCP.15.011013

105[37] G. W.-S. Hou (Taiwan), S. Ogawa (Toho) (Chair),M. Sasaki (Tokyo), N. Sugiyama (Nagoya), ”Proceedings of the 7th International Workshop on
Very High Energy Particle Astronomy in 2014
(VHEPA2014)”, JPS Conference Proceedings Volume 15 (June 2, 2017), e-ISBN: 978-4-89027-120-7.
[38] Y. Aita et al. (Ashra NTA Collaboration),“Ashra
NTA: Towards Survey of Astronomical Tau Neutrino Sources” JPS Conference Proceedings, 2014,3095-3098 (2017). http://dx.doi.org/10.7566/JPSCP.1.013095,[39] M. Sasaki, “Search for PeV-EeV Tau Neutrinos
and Optical Transients from Violent Objects with
Ashra-1,” Astroparticle, Particle, Space Physics
and Detectors for Physics Applications - Proceedings of the 14th ICATPP Conference on Astroparticle,Particle,Space Physics and Detectors
for Physics Applications (Villa Olmo,Como 2014),
112-119, DOI:10.1142/9789814603164-0018.
[40] G. W.-S. Hou, “Towards survey of astronomical ντ
source”,. PoS ( ICRC2015 ) 1167 (2015).
[41] Sasaki, M. et al., “Status of Ashra project”, 34th
Int. Cosmic Ray Conf. (Hague, Netherlands),
2015.
[42] 9th International Workshop on Very HighEnergyParticle
AstronomyWorkshop(VHEPA 2016) 07-09 Jan 2016. Honolulu,
HI, USA CNUM: C16-01-07. https://indico.phys.hawaii.edu/event/896/[43] Sasaki, M., ”Neutrino Telescope Array (NTA):
Multi-Astroparticle Explorer for PeV-EeV Universe — For Clear Identification of Cosmic Accelerators and Cosmic Beam Physics ”,— PoS (ICRC2017 ) 941 (2017).

106

ALPACA (R&D)

Fig. 1. From Ref.[2]. Experimental site for the ALPACA experiment,
Cerro Estuqueria (4,740 m above see level, 16◦ 23’ S, 68◦ 08’ W),near Mount Chacaltaya, in Bolivia.

Introduction

We have started up the ALPACA (Andes Large area
PArticle detector for Cosmic ray physics and Astronomy) project. The ALPACA experiment is composedof an 83,000 m2 air shower array and a 5,400 m2 underground muon detector array to make wide field-ofview high-sensitivity observation of high-energy cosmic
rays/cosmic gamma rays on the Cerro Estuqueria highland, 4,740 m above sea level around Mount Chacaltaya, Bolivia.

Experiment

The ALPACA[2],[3] is a cosmic-ray experiment witha large surface air shower array with a large underground muon detector array. The experimental site
(approximately 500 m×500 m ∼250,000 m2 in totalarea) is located on a flat high land called Cerro Estuqueria (4,740 m above sea level, 16◦ 23’ S, 68◦ 08’ W), as shown in Fig. 1, around Mount Chacaltaya, near
La Paz, Bolivia. In some part in this area, our detectors
will be set up.
We plan to set up a 5,400 m2 underground (approximately one to a few meters) muon detector array(MD) and an 83,000 m2 air shower array (AS), shownin Fig. 2. MD of water Cherenkov type is composed ofeight pools with each pool (approximately 1 m deep)
containing twelve 56 m2 unit detectors. AS is madeup of 401 1-m2 plastic scintillation counters at 15 m
spacing.
The AS field of view is roughly 2 steradian. The
expected angular resolution of AS is approximately 1
degree at 5 TeV and 0.2 degrees around 100 TeV for
gamma rays. For 100 TeV gamma rays, the AS energy
resolution is estimated to be ∼20-25 %. The hadronFig. 2. From Ref.[2]. Schematic view of the ALPACA experiment.
The small black squares indicate 401 1 m2 plastic scintillarion
detectors, forming an air shower array with 83,000 m2 in area.The
grey rectangles indicate eight underground muon detector pools,
each of which contains twelve 56 m2 muon detector units. The
total area of the underground muon detector array is 5,400 m2 .

rejection power of MD is more than 99.9 % at 100 TeV,
while keeping most of gamma-ray events. Long-term
detector stability, angular resolution, pointing accuracy
and energy scale can be calibrated by the cosmic-ray
shadow in the Moon as well as by some of the brightstable TeV gamma ray sources in the southern sky.

Covered Physics

Our reserch target is divided into four in ALPACA:

1. Measurement of high-energy (5 TeV – 1 PeV) cosmic
gamma rays.
2. Measurement of cosmic ray energy spectra aroundthe Knee energy region (100 TeV – 100 PeV)
3. Measurement of cosmic ray anisotropy > 5 TeV atsidereal time frame.
4. Measurement of the Sun shadow in cosmic rays > 5
TeV.
We aim at low-background detection of celestial
gamma rays in the 100 TeV region with the world-best
sensitivity (an order of magnitude better than any pre-

107

vious/existing experiments) and at locating the origins
of cosmic rays accelerated up to the knee energy region
in the southern sky. Presuming a Crab-like gamma-ray
source extending up with power-law index -2.6 locatedin the southern sky, the ALPACA experiment is sensitive to the source with ∼15 % Crab intensity during
one calendar year, as is demonstrated in Fig. 3.
10

3
[GeV]

[TeV]

[PeV]

Integral Flux x E (eV cm−2 s −1 )

Past

10

2

Present
FutureCrab flux
Tibet AS

10

1

10

CASA−MIAMAGIC
VERITAS
H.E.S.S.
HAWC

EGRET

Milagro

0

FERMI

10

10

−1

−2

10

ALPACACTA

5 sigma or 10 events
50 hours or 1 year (Wide FoV instrument)
8

10

9

10

10

10

11

12

10
10
Energy (eV)

13

10

14

10

15

10

16

Fig. 3. Sensitivity of ALPACA to high-energy gamma-ray point
source. Experimental data points are from HEGRA. The ALPACA sensitivity is evaluated from Ref.[1].

The AS + MD in the southern hemisphere will be aunique/complementary experiment to on-going experiments (FERMI, HESS, VERITAS, MAGIC, CALET,
Tibet ASγ , HAWC) and future projects (LHAASO,
CTA) in this field, which are either located in thenorthern hemisphere or aiming at gamma-ray astronomy below 10 TeV region, or having narrow field-ofview. Thus, the new energy window in the 100 TeV region observing gamma rays with wide field-of-view will
be opened first in the southern sky by the ALPACA
experiment. We expect to detect more than a dozen of
established TeV gamma-ray sources, i.e., young SNRs
(SN1006, RX J1713.7-3946, RX J0852.0-4622), Pulsar
Wind Nebulae, the galactic center, etc) in the 100 TeV
region, some of which may be cosmic-ray PeVatron candidates. Furthermore, our wide field-of-view sensitivity
to diffuse gamma rays allows us to study extremely diffuse gamma-ray sources which are difficult to detect byimaging air Cherenkov telescopes. The diffuse gamma
rays from the Fermi bubbles recently reported by the
Fermi-LAT group may be clearly detected, if they extend up to the 100 TeV region. Similarly, detection ofdiffuse gamma rays above 100 TeV from the extended
region around the galactic center is promising, where
the gamma-ray energy spectrum strongly suggests existence of PeVatron. Detection and spectral measurement of gamma rays in the 100 TeV region from thesecelestial sources, together with multi-wavelength (radio, X-ray, gamma-ray) observations, are key points
enabling us to discriminate between the two processes(cosmic-ray/electron origins), and to locate the acceleration site of cosmic rays and to examine the standard
acceleration model of cosmic rays. From the astronom-

ical point of view, we pioneer the ultra-high energy(above 100 TeV) gamma-astronomy in the southern sky.
Besides, gamma-ray emission from near-by extragalactic sources, e.g. M87, Cen A, gamma rays of dark matter origin, those from the Sun disk recently obseved byFermi may be interesting subjects.
We also aim at measuring energy spectra of proton, helium and iron components separately aroundthe knee energy region with the new AS + MD. The
standard cosmic-ray acceleration model at SNR predicts the knee energy of each nucleus component being
proportional to Z (atomic number). We can discriminate proton and iron components by MD, as an ironnucleus produces approximately 2 times more muonsthan a proton with the same energy. Thus, the cosmicray acceleration scenario (SNR shock acceleration) will
be verified by observing the linearly Z(atomic number)-dependent knee(=bent) positions of proton, helium, iron components around the knee energy region.
Precise cosmic-ray anisotropy measurement at sidereal time frame in the TeV energy region in the southern
sky provides unique data for the community to understand the magnetic field structure in the heliosphere.
The ALPACA experiment gives complementary datain the TeV region to those from IceCube above a few
tens of TeV.
Furthermore, measurement of the Sun shadow incosmic rays above the TeV energy region in the southern hemisphere also helps understand the modeling of
the magnetic fields between the Sun and the Earth,complementary to the observations in the northernhemisphere.

ALPAQUITA

As a proto-type experiment, the ALPAQUITA[4] air
shower array without MD, which is ∼20 % of the ALPACA air shower array in area will be constructed at
the experimental site, Cerro Estuqueria, in 2018.

108

THE ALPACAγ COLLABORATION
Institute
Country (∗ )ICRR, Univ. of Tokyo
Japan
5
Universidad Mayor de San Bolivia
5
AndresYachay Tech.
Ecuador
1
Kanagawa Univ.
Japan
2
Utsunomiya Univ.
Japan
1
Yokohama National Univ.
Japan
7
Shinshu Univ.
Japan
3
Osaka City Univ.
Japan
3
Chubu Univ.
Japan
2
National Institute of InforJapan1
matics
Osaka
ElectroJapan
1
Communicatoin Univ.
Tokyo Metropolitan College
Japan
1of Industrial Technology
Japan Atomic Energy Agency
Japan
1
Nihon Univ.
Japan
1
Aichi Institute of Technology
Japan
1
RIKEN
Japan
1
Hiroshima City Univ.
Japan
1
Total
37
(∗ ) Number of participants as of April 2018.

Members
StaffsMasato Takita, Associate Professor (February 2001
to November 2016), Professor (November 2016 to thepresent)
Takashi Sako, Associate Professer (TA/Tibet), October 2017 to the present
Munehiro Ohnishi, Assistant Professor, April 1993 to
the present
Kazumasa Kawata, Specially Appointed AssistantProfessor (April 2013 to March 2018), AssistantProfessor (Tibet/TA, April 2018 to the present)
Takahide Kobayashi, Technical Staff, from 1967 to
March 2013
Postdoctoral Fellows
Takashi Sako, from April 2010 to March 2017 andfrom June 2018 to the present
Graduate Students
Two graduate students obtained the master degrees
during 2012–2018, supervised by ICRR staff members.

List of Publications
Basically after Year 2012
Papers in Refereed Journals[1] “Exploration of a 100 TeV gamma-ray northern
sky using the Tibet air-shower array combinedwith an underground water-Cherenkov muondetector array”, T.K. Sako et al., Astroparticle
Physics 32, (2009) 177.
Papers in International Conference Proceedings[2] “The ALPACA Project”, M. Takita for the ALPACA Collaboration, THE EUROPEAN PHYSICAL JOURNAL, 145, 01002-1-3, (2017).
[3] “The overview of the ALPACA Experiment”,
T. Asaba et al, Proc. of ICRC2017, ID=827, Busan, Korea, July 12-20, (2017).
[4] “ALPAQUITA Array in the ALPACA Project”,
T. Asaba et al, Proc. of ICRC2017, ID=437, Busan, Korea, July 12-20, (2017).
Presentations in Internatinal Conference[5] “The ALPACA Project”, M. Takita, ISVHECRI2016, Moscow, Russia, August 22 - August 27,
2016.
[6] “The overview of the ALPACA Experiment”,
M. Ohnishi for the ALPACA Collaboration, The
35th International Cosmic Ray Conference, Busan,
Korea, July 12 - July 20, 2017.
[7] “ALPAQUITA Array in the ALPACA Project”,
K. Kawata for the ALPACA Collaboration, The
35th International Cosmic Ray Conference, Busan,
Korea, July 12 - July 20, 2017.
[8] “Overview and current status of the ALPACA experiment”, T. Sako for the ALPACA Collaboration, TeVPA2017, Colombus, Ohio, U.S.A., August 7 - August 11, 2017.
[9] “ALPACA experiment”, T. Sako for the ALPACA
Collaboration, The VII School on Cosmic Rays
and Astrophysics, Escuela Politecnica Nacional,
Quito, Ecuador, August 21 - September 1, 2017.

109

ASTROPHYSICS AND GRAVITYDIVISION

110

KAGRA GROUP
Introduction

KAGRA, Large-scale Cryogenic Gravitational wave
Telescope, aims at detecting gravitational waves anddeveloping gravitational wave astronomy, which was
initiated by the first detection of gravitational waves by
LIGO. KAGRA employs a 3 km L-shaped laser interferometer. KAGRA uses an optical configuration which
is similar to advanced LIGO and advance Virgo. However, there are some major differences. KAGRA uses
cryogenic sapphire mirrors as main test masses of theinterferometers, while advanced LIGO and advanced
Virgo use room-temperature fused silica mirrors. An
another major difference is its site; KAGRA is locatedin the underground site of the Kamioka mine, while
LIGO and Virgo are located on the surface. In the
Kamioka mine, two prototype detectors, LISM (a 20-m
scale interferometer, ran from 1999 to 2002) and CLIO(a 100-m scale interferometer, started in 2002), wereconstructed and operated to show the advantages of anunderground site. With these achievements and experiences, KAGRA was funded in 2010 and the excavationof the tunnel was started in May 2012.
The construction of the KAGRA detector is divided
into two stages: the initial KAGRA (iKAGRA) andbaseline KAGRA (bKAGRA). The iKAGRA detector
is a simple Michelson interferometer with a 2-Wattlaser, room-temperature mirrors, and a simple seismic
isolation system. The bKAGRA detector with final
configuration will be a Power Recycling, Fabry-Perot
Michelson interferometer with a Resonant Sideband
Extraction. It uses 180-Watt laser, cryogenic Sapphire
mirrors, and an advanced Seismic Attenuation System(SAS).
Figure 2 shows the estimated ultimate sensitivity
limits of KAGRA. Observation range for an inspiraland merger of neutron-star binary with the ultimate
sensitivity of KAGRA is about 152 Mpc with the samedefinition of the observation range as LIGO and Virgo.
In October 2015, most of the installation activities
for the initial KAGRA interferometer had been completed, and after commissioning work, the interferometer was operated for the first time in March 2016.
Though the interferometer configuration at that time
was simplified from the final KAGRA design, KAGRA
was operated for the first time as a full 3-km-scale interferometer connected to the data acquisition, transfer, and storage system. With that configuration, we
carried out a three-week test operation (iKAGRA operation) to check the overall performance as an interferometric gravitational wave antenna system.
Immediately after the end of the iKAGRA operation, installation of bKAGRA configuration started. In

Fig. 1. KAGRA’s scheduleFig. 2. Ultimate sensitivity limit of KAGRA. ”SQL”, ”seismic+gravity”, ”mirror”, ”suspension”, ”quantum” and ”total” mean standard quantum limit, seismic noise and gravity gradient noise, mirror thermal noise, suspension thermal noise, quantum noise andsum of all noises respectively. Observation range for an in-spiraland merger of neutron-star binary with the ultimate sensitivity
limit of KAGRA is about 152 Mpc with the same definition of
the observation range as LIGO and Virgo.

the first stage of installation of bKAGRA (Phase 1), a
3 km Michelson interferometer with cryogenic mirrors
were constructed. A test operation with this configuration (bKAGRA phase 1 operation) was performed in
May 2018.
In this report, we review the progress of the construction of KAGRA during 2012 and 2018. We also
summarize the iKAGRA operation and the bKAGRA
phase 1 operation. Finally, we explain the plan of theinstallation toward the final configuration of KAGRA,
and the plan of observation which will be done jointlywith LIGO and Virgo.

111
ETMY

IMC
PMC
PR2
laser
ETMX

IFI
PR3

BS

Fig. 4. Interferometer configuration of iKAGRA.

Fig. 3. A map of the Kamioka mine. The blue thick lines arebaselines of the KAGRA interferometer.

Summary of construction from 2012 to 2018

The excavation of the tunnel of KAGRA was started
in May 2012 after one year delay due to the Great East
Japan Earthquake occurred on March 11, 2011. Figure 3 shows a map around the Kamioka mine. The
geographical coordinates of the beamsplitter are 36.41
degrees North and 137.31 degrees East. The Y-arm isin the direction 28.31 degrees west of north. Though
the central station and the end stations are rather close
to the foot of the 1300-m high mountain, they are stillat least 200 m below the ground surface. The X and Y
arms are 3-km long. Access tunnels lead to the central
station and to the Y-arm end station. The excavationof the tunnel was finished in May 2014. Installationof the facilities proceeded in parallel with the excavation. This included electricity, air conditioning, watersupply, anti-dust wall painting, floor treatment, cranesetup, anchors, spiral steps, networks, telecommunication, laser clean room, etc. Soon after the tunnel andfacility neared completion in March 2014, installationof the vacuum system and interferometer components(the vibration-isolation systems, the laser source, theoptics for the interferometer, the digital control system,and so on) were started.

iKAGRA operation

By February 2016, we had completed the installation of the vacuum systems, clean booths, seismic isolation systems, and digital control systems to control theinterferometer. In March 2016, we aligned the optics
to make a 3-km Michelson interferometer, and started
the test run operation. The test run was split into two
periods, from March 25 to March 31 and from April
11 to April 25. Between the first and the second halfof the test run, we had a little interval to improve thesensitivity and the stability of the interferometer.
The interferometer configuration of iKAGRA isshown in Fig. 4. We used a single-frequency laser sourcewith a wavelength of 1064 nm. The laser beam was first

injected into a triangular cavity called the pre-modecleaner (PMC) and then to the suspended input modecleaner (IMC), to suppress unwanted spatial higher order modes. The PMC was constructed from three mirrors rigidly attached to an Invar spacer. It has a roundtrip length of 0.4 m and a finesse of 197. The IMC wasconstructed from three mirrors suspended by a double
pendulum on a vacuum-compatible vibration isolation
stack table. It has a round-trip length of 53.3 m and finesse of 540. The frequency of the laser was stabilized
to the PMC length below 0.3 mHz and the IMC length
above 30 Hz. The beam at IMC and the main interferometer is s-polarized. The incident power before the
IMC was 270mW.
The beam transmitted by the IMC is then sent
to an input faraday isolator (IFI), after which thebeam radius is expanded by the mirrors called PR2
and PR3. PR2 and PR3 stand for power recycling 2
and 3, and will be used for the folding mirrors of thepower recycling cavity in the bKAGRA phase. The
main 3-km Michelson interferometer consists of thebeam splitter (BS) and two end test masses (ETMX
and ETMY). The designed distance between BS andETMX is 2991.6 m, and that between BS and ETMY
is 2988.3 m. The IFI was put on a vibration isolation
stack table, and PR2 was fixed on a non-isolated table.
PR3, BS, ETMX and ETMY are suspended by double pendula. Of all these suspended mirrors, PR3 was
the largest (250 mm diameter and 100 mm thick) mirror, and was the only one with a full-sized suspension,
which will also be used in the bKAGRA phase withminor modifications.
All of the optics downstream of the IMC, including the IMC, were placed inside vacuum chambers, but
most of the parts were left at atmospheric pressure toallow for easier maintenance of the optics during thetest run. Only the IMC and the two 3-km arm ducts
are evacuated, to a pressure of ∼ 102 Pa. The differential length signal of the Michelson interferometer was
obtained from the reflected beam picked off by the IFI
with a photo-diode in air.
The differential length signal was fed back to theETMs via 3-km reflective memory network to lock theinterferometer fringe. During the first half of the test
run, we used the DC signal from the photo-diode andlocked the interferometer at the mid-fringe. During the112second half, we used the pre-modulation technique tolock the interferometer at the dark fringe. The darkfringe locking resulted in a lower coupling of laser intensity noise. We also increased the bandwidth of theservo from 8 Hz to 94 Hz by avoiding parasitic resonances of the ETMs. In the first half of the test run,
we did not have enough balancing of the mirror actuators, which resulted in the excitation of the suspension pitch modes at around 15 Hz. Moreover, the servo
bandwidth was automatically adjusted by monitoring
the loop gain using calibration lines in the second test
run. These improvements contributed to the high stability of the interferometer. The duty factor during thefirst half was 85.2 %, whereas that during the second
half was 90.4 %. The longest lock stretch was 3.6 hours
for the first half, and that for the second half was 21.3
hours. The high duty factor compared to other gravitational wave telescopes is mainly due to the simple
configuration of the interferometer, but the state machine automaton Guardian also helped recovering lockquickly after lock losses.

noise in iKAGRA was high because main mirrors were
inside the vacuum chambers but at atmospheric pressure. Note that the acoustic noise curve in Fig. 5 onlyshows the acoustic noise coupled via the BS chamber.
We turned off all the fans in the clean booths to confirm
that the sensitivity of the frequency regions where themeasured spectrum and the sum of all the known noise
sources do not match is also limited by acoustic noise.
However, the detailed coupling mechanism still needs
further investigation. At 3-4 kHz, the sensitivity was
limited by sensor noise, which mainly comes from theanalog-to-digital conversion of the photo-diode output.
These noise contributions will be further reduced in thebKAGRA phase by using better vibration isolation systems, a high vacuum system, and better whitening filters.

10-9
MeasuredSum
Acoustic noise
Actuator noise
Frequency noise
Seismic noise
Sensor noisebKAGRA seismic noise

10-10
10-11

strain [/rtHz]

10-12
10-13
10-14

Fig. 6. Spectrogram of seismic noise in the 0.1 Hz∼2 Hz band during
the second run. One can see large spikes on April 14th and 16th
due to a series of large earthquakes in Kumamoto. Some data on
April 17th and 21st is missing due to an issue in our data taking
system.

10-15
10-16
10-17
10-18
10-1

100

101

102

103

frequency [Hz]Fig. 5. The strain sensitivity of iKAGRA during the second run.
An expected seismic noise level in bKAGRA is also plotted asa reference. (The noise level is about one order of magnitude
smaller than other sites such as LIGO and Virgo.) The peaks
at 80 Hz and 113 Hz for the measured spectrum are from thecalibration lines. The acoustic noise plotted here only shows
the acoustic noise coupled via the BS chamber, but it is likelythat the acoustic noise is the sensitivity limiting noise source
also at neighboring frequencies. Actuator noise is the sum of
the displacements of the mirrors from electronics noise for theactuation. Frequency noise is the estimated laser frequency noise
suppressed through laser frequency stabilization using PMC and
IMC. Seismic noise is the ground displacement attenuated by the
mirror suspensions. Sensor noise is the sum of the ADC noise,the dark noise of the photo-diode, and the shot noise.

A typical sensitivity curve for iKAGRA during thesecond run is shown in Fig. 5. Below 3 Hz, the sensitivity is limited by seismic noise. Over 100 Hz to 3 kHz,the sensitivity is likely to be limited by acoustic noise,mainly from the fans of the clean booths. The acoustic

Figure 6 shows a spectrogram of seismic noise in the
0.1 Hz∼2 Hz band during the second run. In this period, a series of large earthquakes hit Kumamoto Prefecture that is about 700 km from KAGRA. According
to the United States Geological Survey, the foreshockwith a magnitude 6.5 hit Kumamoto at 12:26 UTC on
April 14 (2016) and the mainshock with a magnitude
7.3 hit the same area at 16:25 UTC on April 16 (2016).
The peak ground acceleration (PGA) levels in the horizontal direction measured at various points in KAGRA
are as follows. For the foreshock, PGAs at BS, ETMX,and ETMY were 20 mGal, 30 mGal, and 25 mGal, respectively. For the mainshock, the PGA at BS was
not measured as the seismometer was saturated (i.e.
over 95 mGal) as the sensor gain had been set higher
than the other two. PGAs at ETMX and ETMY were
306 mGal and 298 mGal, respectively. After the mainshock, the suspension system of BS was in trouble andit took half a day to fix it. The earthquake shook the
BS suspension and a screw to support a mirror before releasing touched one of the suspension fibers of
the BS. The interferometer was not in operation during the time when several aftershock earthquakes hit
Kumamoto.
During the test run, 65 people in total participatedin shifts to monitor the interferometer conditions. The

113

interferometer was monitored by at least three peopleon eight-hour shifts. A cumulative total of 186 people
from 35 institutes contributed. The members of theshift monitored the status of the interferometer lock,mirror suspensions, data acquisition system and datatransfer system to ensure that the interferometer data
was properly transferred.
The data including the gravitational wave signal andenvironmental monitor signals were transferred to thedata center at ICRR in Kashiwa and Osaka City University with latency of about 3 seconds. The amountof data was 7.5 TB in total. We also did a hardware
injection test right after the test run. The data management system and the results of the data analysis
and the detector characterization will be published inseparate papers.

bKAGRA phase 1 operation

After the iKAGRA test operation, we immediately
started the installation process toward bKAGRA configuration. In the phase 1 period, a simple Michelsoninterferometer with cryogenic sapphire mirrors as main
test masses was constructed.
A key feature of KAGRA is to cool sapphire mirrors.
Sapphire mirrors of KAGRA is suspended by 9-stage
suspension. The bottom 4 stages which include a sapphire mirror is called cryogenic payload. The first cryogenic payload with a sapphire mirror was installed at
Kamioka in November, 2017. Figure 7 shows the cryogenic payload installed at KAGRA site. This payload
was cooled down after installation and reached about
18 K, which is below the target temperature of KAGRA, 20 K.
Performance evaluation of sensors and actuators was
performed at the site after cooling. It was then confirmed that all sensors and actuators on the cryogenic
payload worked even at cryogenic temperature. Damping feedback system for the suspension eigenmode wasalso implemented, which is significant to operate KAGRA as an interferometer.
Major development of input and output optics werealso done. All mirrors between the input laser to thebeam splitter were installed. They are hanged withvarious types of vibration isolation systems, and areplaced in the vacuum chambers.
bKAGRA phase 1 operation was done during April
28th and May 7th in 2018. In the operation, ETMY
mirror was 20 K, but ETMX mirror was room temperature. Duty cycle was about 80-90 % during April 28th
and May 2nd, but is was about 20-60 % during May 3rd- 7th. The low duty cycle during the last 4 days wascaused by several distant major earthquakes and badweather condition (This is a very rare case in the undergroun site). Total amount of data taken was 8.875
days, and amount of data when the interferometer wasFig. 7. Cryogenic payload installed into the cryostat at KAGRA site

operational was 6.731 days. The longest continuous
operation was about 11 hours. Figure 14 is the noise
spectrum of iKAGRA and bKAGRA-1 and it shows the
improvement by 1-2 orders of magnitude. The observable range for binary neutron star was around 15 − 20
pc.
In February 2017, KAGRA’s main data server was
introduced in the main ICRR building at Kashiwa. The
main role of this system is to store all data producedby KAGRA detector, and it has 2.5 PB storage. It also
has 12 computing nodes, and total number of cores is334. Now, data taken by the digital system of KAGRA
detector is continuously transferred to the main dataserver and stored. By using this data, during the phase
1 operation, an attempt of the low latency data analysis
targeting coalescing compact binaries was performed byusing low latency data analysis pipeline, gstlal-inspiral
which has been developed and used by LIGO collaboration.

Future plan

So far, 7 gravitational wave events (6 binary black
holes and one binary neutron star) have been observed.
Next scientific observation by LIGO and Virgo (whichis called ”O3”) is planned to start from February 2019,and is expected to continue for about one year. We
understand that the most important thing for KAGRA

114

Fig. 8. Optical configuration of bKAGRA phase 1. This is a Michelson interferometer. A laser source is placed in IO and laser
beam is input into vacuum systems from MC. Each mirror issuspended by a vibration isolation system. Solid lines indicate
laser beams. Both ETMY and ETMX mirrors are sapphire mirrors. Only ETMY was cooled down to cryogenic temperature.

Fig. 10. Type-B vibration isolation system for beam splitter.
Fig. 9. Vibration isolation systems of KAGRA. KAGRA equips fourkinds of vibration isolation systems such as Type-A, Type-B, andType-Bp. We have installed PRM, PR2, BS, ETMX, and ETMY
in 2017. PR3 was upgraded to Type-Bp.

is to operate the interferometer as soon as possible,
and to join the observation of LIGO and Virgo, andto contribute to the observation of gravitational wave
signals. Now we are planning to join O3 at around autumn 2019. When we perform coincident observation,
we jointly analyze data from LIGO, Virgo and KAGRA detectors with LIGO and Virgo Collaborations.
In order to realize that, we reformed the organizationof data analysis and related subgroups. Since the neworganization structure is quite similar to that of LIGO
Scientific Collaboration, it will become easy to perform
collaboration between LIGO and Virgo.

Summary

The construction of the KAGRA detector has been
progressed rapidly since the start of the tunnel excavation in May 2012. We have performed test operationof the detector twice: iKAGRA operation with a room
temperature Michelson interferometer, and bKAGRA

phase 1 operation with a cryogenic mirror. We are now
making great efforts to install various components necessary to achieve good sensitivity and to perform anobservation run during LIGO and Virgo’s third observation run which is planned during February 2019 andJanuary 2020. It is expected that when KAGRA joinsa world-wide network of gravitational wave detectors,
KAGRA can contribute greatly to gravitational wave
astronomy and astrophysics.

115

Fig. 11. Beam splitter suspended in a vacuum chamber.

Fig. 13. A part of Type-A vibration isolation system.

Fig. 12. PR2 mirror suspended in a vacuum chamber. There isbeam splitter behind the PR2 mirror.

KAGRA Collaboration
Institute
ICRR, Univ. of Tokyo
NAOJ
KEK
School of Sci., Univ. of Tokyo
School of Eng., Univ.
of
Tokyo
Tokyo Tech
Osaka City Univ.
Kyoto Univ.
Univ.
ofElectroCommunications
ERI, Univ. of Tokyo
Hosei Univ.
AIST
NICT
Osaka Univ.
Kyushu Univ.
Niigata Univ.

Country
Japan
Japan
Japan
Japan
Japan

(∗ )
34
29
10
17
3

Japan
Japan
Japan
Japan

8
13
12
5

Japan
Japan
Japan
Japan
Japan
Japan
Japan

2
8
3
1
1
1
6

Rikkyo Univ.
Hiroshima Univ.
Ryukyu Univ.
Waseda Univ.
Toyama Univ.
Yokohama City Univ.
Fukuoka Univ.
Aichi Univ. of Technology
Inst. for Molecular Science
Kavli IPMU, Univ. of Tokyo
National Defense Academy
Inst. of Statistical Mathematics
National Inst. for Fusion Science
Toho Univ.
Max-Planck Institutes
CaltechUniv. of Mississippi
Univ. of Western Australia
Louisiana State Univ.
Beijing Normal Univ.
IUCAA
Moscow Univ.
LATMOS, CNRS
Univ. of Science and Technology of China
Tsinghua Univ.

Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan
Japan

1
1
2
2
19
1
1
1
1
1
1
1

Japan

1

Japan
Germany
USA
USA
Australia
USA
China
India
RussiaFrance
China

2
1
4
4
5
1
8
2
1
1
1

China

1

116

Members

Fig. 14. Average amplitude spectrum density of noise during bKAGRA phase 1 and iKAGRA. Red line (May 6 2018) was measured
during phase 1 operation.

Korea Univ.
Korea
Inje Univ.
Korea
Seoul National Univ.
Korea
Korea Astronomy and Space
Korea
Science Inst.
Myongji University
Korea
Hanyang University
Korea
Pusan National University
Korea
Korea Inst. of Science andKorea
Technology Information
National Inst. for MathematKorea
ical Sciences
Kyungpook National Univ.
Korea
Kunsan National Univ.
Korea
Sejong Univ.
Korea
Sogang Univ.
Korea
Tongji Univ.
Korea
Chinese Academy of Science
China
Univ. of California Berkeley
USA
Princeton University
USA
IISER-TVM
India
NIKHEF
Netherlands
RIKEN
Japan
Osaka Inst. of Technology
Japan
Warsaw University of TechPoland
nologyUniv.
of WisconsinUSA
Milwaukee
Academia Sinica
TaiwanNational Applied ResearchTaiwanLab.
University of Perugia
Italy
Chinese Univ. of Hong Kong
China
Total
(∗ ) Number of participants as of 2018.

1
3
2
1
1
1
1
1
4
1
1
1
2
1
2
1
1
1
1
1
2
2
1
3
1
2
2
288

StaffsKazuaki Kuroda, Prof., to Mar.2015
Seiji Kawamura, Prof., to Dec.2017
Masatake Ohashi, Prof.
Hideyuki Tagoshi, Prof., May 2017 to present
Yoshio Saito, Proj. Prof., Apr.2014 to present
Norikatsu Mio, Proj. Prof., Apr.2018 to present
Yuzuru Yoshii, Proj. Prof., Dec.2017 to present
Shinji Miyoki, Assoc. Prof.
Takashi Uchiyama, Assoc. Prof.
Kazuhiro Yamamoto, Assist. Prof., to Feb.2017
Osamu Miyakawa, Assist. Prof., Apr.2013 to present
Keiko Kokeyama, Assist. Prof., Feb.2015 to present
Kyohei Kawaguchi, Assist. Prof., Mar.2018 to present
Ryutaro Takahashi, Proj. Assist. Prof., to Oct.2013
Kazuhiro Hayama, Proj. Assist. Prof., to Mar.2018
Naoko Ohishi, Proj. Assist. Prof., Sep.2012 toMar.2015
Eiichi Hirose, Proj. Assist. Prof., Sep.2013 to present
Takafumi Ushiba, Proj. Assist. Prof., May 2018 topresent
Senior Fellows
Toshikazu Suzuki, Senior Fellow, Apr.2017 to present
Postdoctoral Fellows
Eiichi Hirose, to Mar.2013
Kieran Craig, Aug.2015 to Jul. 2017
Takafumi Ushiba, Apr.2017 to Feb.2018
Takahiro Yamamoto, Jul.2016 to present
Tatsuya Narikawa, to Mar.2018
Nami Uchikata, to Mar.2018
Takaaki Yokozawa, Dec.2017 to present
Hirotaka Yuzurihara, Jan.2017 to Present
Fabian Pena Arellano, Jul.2018 to Present
Lucia Trozzo, Jul.2018 to Present
Shoichi Oshino, Sep.2018 to Present
Graduate Students
2 students were awarded doctor degrees and 20 students
earned master degrees during 2012–2017, supervised by
ICRR staff members.

117

List of Publications[1] “Reduction of thermal fluctuations in a cryogenic laser interferometric gravitational wave detectord”, T. Uchiyama, S. Miyoki, S. Telada, K.
Yamamoto, M. Ohashi, K. Agatsuma, K. Arai,
M.-K. Fujimoto, T. Haruyama, S. Kawamura, O.
Miyakawa, N. Ohishi, T. Saito, T. Shintomi, T.
Suzuki, R. Takahashi, D. Tatsumi, Physical Review Letters, Vol.108, No.14, (2012)[2] “Calculation of thermal radiation input via funneling through a duct shield with baffles for KAGRA”, Y. Sakakibara, N. Kimura, K. Yamamoto,T. Suzuki, T. Tomaru, S. Miyoki, T. Uchiyama, K.
Kuroda, Class. Quantum Grav., 29 (2012) 205019.
[3] “Sapphire mirror for the KAGRA gravitational
wave detector”, E. Hirose, D. Bajuk, G. Billingsley,
T. Kajita, B. Kestner, N. Mio, M. Ohashi, B. Reichman, H. Yamamoto, and L. Zhang, Phys. Rev.
D 89, 062003 (2014).
[4] “Mechanical loss of a multilayer tantala/silica
coating on a sapphire disk at cryogenic temperatures: Toward the KAGRA gravitational wave detector”, E. Hirose, K. Craig, H.i Ishitsuka, I. W.
Martin, N. Mio, S. Moriwaki, P. G. Murray, M.
Ohashi, S. Rowan, Y. Sakakibara, T. Suzuki, K.
Waseda, K. Watanabe, and K. Yamamoto, Phys.
Rev. D 90, 102004 (2014)
Y. Sakakibara, N. Kimura, T. Suzuki, K. Yamamoto, D. Chen, S. Koike, C. Tokoku, T.
Uchiyama, M. Ohashi, K. Kuroda, Advances in
Cryogenic Engineering, 59, 11761183 (2014),[5] “Progress on the Cryogenic System for the
KAGRA Cryogenic Interferometric Gravitational
Wave Telescope”, Sakakibara, T. Akutsu, D.
Chen, A. Khalaidovski, N. Kimura, S. Koike, T.
Kume, K. Kuroda, T. Suzuki, C. Tokoku and K.
Yamamoto, Classical and Quantum Gravity, 31,
224003, (2014)[6] “Excavation of an underground site for a km-scalelaser interferometric gravitational-wave detector”,
T. Uchiyama, K. Furuta, M. Ohashi, S. Miyoki, O.
Miyakawa and Y. Saito, Classical and Quantum
Gravity, 31, 224005, (2014)[7] “Sapphire screws and strength test on them at liquid nitrogen temperature”, Eiichi Hirose, Yusuke
Sakakibara, Yukihiko Igarashi and Takashi Ishii,
Review of Scientific Instruments, 85, 104503,(2014)[8] “Vibration measurement in the KAGRA cryostat”,
D Chen, L Naticchioni, A Khalaidovski, K Yamamoto, E Majorana, Y Sakakibara, C Tokoku,
T Suzuki, N Kimura, S Koike, T Uchiyama

and S Kawamura, CLASSICAL AND QUANTUMGRAVITY, 31, 224001, (2014)[9] “Update on development of cryogenic sapphire
mirrors and their seismic attenuation system
for KAGRA”, E Hirose,T Sekiguchi, R Kumar,
R Takahashi, CLASSICAL AND QUANTUMGRAVITY, 31, 224004, (2014)[10] “Evaluation of heat extraction through sapphirefibers for the GW observatory KAGRA”, A Khalaidovski, G Hofmann, D Chen, J Komma, C
Schwarz, C Tokoku, N Kimura, T Suzuki, AO
Scheie, E Majorana, R Nawrodt, K Yamamoto,
CLASSICAL AND QUANTUM GRAVITY, 31,
105004, (2014)[11] “Method to reduce excess noise of a detuned cavity
for application in KAGRA”, S. Ueda, N. Saito,
D. Friedrich, Y. Aso, K. Somiya for the KAGRA
Collaboration, Classical and Quantum Gravity, 31,095003, (2014)[12] “Design study of the KAGRA output modecleaner”, A. Kumeta, C.Bond, and K.Somiya, Optical Review, 22, 149-152, (2015)[13] “Gravitational waves:classification, methods ofdetection, sensitivities and sources”, Kazuaki
Kuroda, Wei-Tou Ni and Wei-Ping Pan, International Journal of Modern Physics D, 24, 1530031,(2015)[14] “Ground- based Gravitational-wave detectors”,
Kazuaki Kuroda, International Journal of Modern
Physics D, 24, 1530032, (2015)[15] “Realistic loss estimation due to mirror surfaces
in a 10 meters-long high finesse Fabry-Perot filtercavity”, Nicolas Straniero, Jérôme Degallaix, Raffaele Flaminio, Laurent Pinard, and Gianpietro
Cagnoli, Optics Express, 23, 21455-21476, (2015)[16] “Gravitational wave astronomy: the current status”, David Blair , Li Ju, ChunNong Zhao, LinQing Wen, Qi Chu, Qi Fang, RongGen Cai, JiangRui Gao, XueChun Lin, Dong Liu, Ling-An Wu,
ZongHong Zhu, David H. Reitze , Koji Arai, Fan
Zhang, Raffaele Flaminio, XingJiang Zhu , George
Hobbs, Richard N. Manchester, Ryan M. Shannon,
Carlo Baccigalupi, Wei Gao, Peng Xu, Xing Bian,
ZhouJian Cao, ZiJing Chang, Peng Dong, XueFei
Gong, ShuangLin Huang, Science China-Physics
Mechanics and Astronomy, 58, 120402, (2015)[17] “Mechanical loss in state-of-the-art amorphous
optical coatings”, Massimo Granata, Emeline
Saracco, Nazario Morgado, Alix Cajgfinger, Gianpietro Cagnoli, Jérôme Degallaix, Vincent Dolique,Danièle Forest, Janyce Franc, Christophe Michel,118

Laurent Pinard, and Raffaele Flaminio, PhysicalReview D, 93, 12007, (2016)

and Nobuyuki Kanda, Physical Review D, 93,
82005, (2016)[18] “Characterization of the room temperature payload prototype for the cryogenic interferometric gravitational wave detector KAGRA”, Fabián
Erasmo Peña Arellano, Takanori Sekiguchi, Yoshinori Fujii, Ryutaro Takahashi, Mark Barton, Naoatsu Hirata, Ayaka Shoda, Joris van
Heijningen, Raffaele Flaminio, Riccardo DeSalvo, Koki Okutumi, Tomotada Akutsu, Yoichi
Aso, Hideharu Ishizaki, Takashi Uchiyama, Osamu Miyakawa, Masahiro Kamiizumi, Akiteru
Takamori, Ettore Majorana, Jo van den Brand andAlessandro Bertolini, Review of Scientific Instruments, 87, 34501, (2016)[26] “Observation of reduction of radiation-pressureinduced rotational anti-spring effect on a 23 mg
mirror in a Fabry-Perot cavity”, Yutaro Enomoto,Koji Nagano, Masayuki Nakano, Akira Furusawa and Seiji Kawamura, Classical and Quantum
Gravity, 33, 145002, (2016)[19] “5mg Suspended Mirror Driven by MeasurementInduced Backaction”, N. Matsumoto, K. Komori,Y. Michimura, G. Hayase, Y. Aso and K. Tsubono,
Physical Review A, 9, 33825, (2016)[20] “Performance test of pipe-shaped radiation shields
for cryogenic interferometric gravitational-wave
detectors”, Y. Sakaibara, N. Kimura, A. Akutsu,
T. Suzuki and K. Kuroda, Classical and Quantum
Gravity, 32, 155011, (2015)[21] “An experiment to distinguish between diffusive
and specular surfaces for thermal radiation in cryogenic gravitational-wave detectors”, Y. Sakaibara,
N. Kimura, T. Suzuki, K. Yamamoto, C. Tokoku,
T. Uchiyama and K. Kuroda, Prog. Theor. Exp.
Phys., Volume 2015, 073F01,(2015)[22] “The effect of crystal orientation on the cryogenic
strength of hydroxide catalysis bonded sapphire”,
K. Haughian, R. Douglas, A. A. van Veggel, J.
Hough, A. Khalaidovski, S. Rowan, T. Suzuki andK. Yamamoto, Classical and Quantum Gravity, 32,
75013, (2015)[23] “Vacuum and cryogenic compatible black surface
for large optical baffles in advanced gravitationalwave telescopes”, Tomotada Akutsu, Yoshio Saito,
Yusuke Sakakibara, Yoshihiro Sato, Yoshito Niwa,
Nobuhiro Kimura, Toshikazu Suzuki, Kazuhiro
Yamamoto, Chihiro Tokoku, Shigeaki Koike, Dan
Chen, Simon Zeidler, Optical Materials Express,
6, 1613-1626, (2016)[24] “Calculation method for light scattering caused bymultilayer coated mirrors in gravitational wave detectors”, Simon Zeidler, Tomotada Akutsu, Yasuo Torii, Eiichi Hirose, Yoichi Aso, and Raffaele
Flaminio, Optics express, 25, 4741-4760, (2017)[25] “Characterization of non-Gaussianity in gravitational wave detector noise”, Takahiro Yamamoto,
Kazuhiro Hayama, Shuhei Mano, Yousuke Itoh,[27] “Standard quantum limit of angular motion ofa suspended mirror and homodyne detection ofa ponderomotively squeezed vacuum field”, Yutaro Enomoto, Koji Nagano, and Seiji Kawamura,Physical Review A, 97, 12115, (2016)[28] “Mitigation of radiation-pressure-induced angular instability of a Fabry-Perot cavity consisting of suspended mirrors”, Koji Nagano, Yutaro Enomoto, Masayuki Nakano, Akira Furusawa, Seiji Kawamura, Physics Letters A, 380,
3871-3875, (2016)[29] “Design study and prototype experiment of the
KAGRA output mode-cleaner”, Kazushiro Yano,
Ayaka Kumeta, Kentaro Somiya, Journal of
Physics: Conference Series, 716, 12032, (2016)[30] “Measurement of Schumann Resonance at
Kamioka”, S. Atsuta, T. Ogawa, S. Yamaguchi,
K. Hayama, A. Araya, N. Kanda, O. Miyakawa,
S. Miyoki, A. Nishizawa, K. Ono, Y. Saito, K.
Somiya, T. Uchiyama, M. Uyeshima and K. Yano,
Journal of Physics: Conference Series, 716, 12020,(2016)[31] “The cryogenic challenge: status of the KAGRA
project”, Raffaele Flaminio and KAGRA collaboration, Journal of Physics: Conference Series, 716,
012034, (2016)[32] “Active damping performance of the KAGRA
seismic attenuation system prototype”, Yoshinori Fujii, Takanori Sekiguchi, Ryutaro Takahashi, Yoichi Aso, Mark Barton, Fabián Erasmo
Peña Arellano, Ayaka Shoda, Tomotada Akutsu,
Osamu Miyakawa, Masahiro Kamiizumi, Hideharu Ishizaki, Daisuke Tatsumi, Naoatsu Hirata, Kazuhiro Hayama, Koki Okutomi, Takahiro
Miyamoto, Hideki Ishizuka, Riccardo DeSalvo andRaffaele Flaminio, Journal of Physics: Conference
Series, 716, 012022, (2016)[33] ”Design and operation of a 1500-m laser strainmeter installed at underground site in Kamioka”,
Akito Araya, Akiteru Takamori, Wataru Morii,
Kouseki Miyo, Masatake Ohashi, Kazuhiro
Hayama, Takashi Uchiyama, Shinji Miyoki and
Yoshio Saito, Earth, Planets and Space 69, 77(2017)

119[34] ”Mirror actuation design for the interferometer
control of the KAGRA gravitational wave telescope”, Yuta Michimura, Tomofumi Shimoda,
Takahiro Miyamoto, Ayaka Shoda, Koki Okutomi,
Yoshinori Fujii, Hiroki Tanaka, Mark A. Barton, Ryutaro Takahashi, Yoichi Aso, Tomotada
Akutsu, Masaki Ando, Yutaro Enomoto, Raffaele
Flaminio, Kazuhiro Hayama, Eiichi Hirose, Yuki
Inoue, Takaaki Kajita, Masahiro Kamiizumi, Seiji
Kawamura, Keiko Kokeyama, Kentaro Komori,
Rahul Kumar, Osamu Miyakawa, Koji Nagano,
Masayuki Nakano, Naoko Ohishi, Ching Pin Ooi,
Fabian Erasmo Pena Arellano, Yoshio Saito, Katsuhiko Shimode, Kentaro Somiya, Hiroki Takeda,
Takayuki Tomaru, Takashi Uchiyama, Takafumi
Ushiba, Kazuhiro Yamamoto, Takaaki Yokozawa,
Hirotaka Yuzurihara, Classical and Quantum
Gravity 34, 225001(2017)[35] ”Construction of KAGRA: an undergroundgravitational-wave observatory”, The KAGRA collaboration, Prog. Theor. Exp. Phys. 2018, 013F01(2018)[36] http://gwcenter.icrr.u-tokyo.ac.jp/en/

120

OBSERVATIONAL COSMOLOGY GROUP
Summary from 2012 to 2018

We study cosmic reionization and early galaxy formation that are missing pieces of our understanding of
cosmic history. The outstanding scientific questions are
the unknown reionization physical process, the missing
ionizing photon problem, the formation of first stars,
and the structural/dynamical/chemical evolutions ofgalaxies at the early epoch of cosmic history. Our basic strategy is to carry out deep surveys with largest
telescope facilities such as Subaru, Hubble, and ALMA,
that have a sensitivity good enough for probing the distant universe. The main results based on our research
activities from April 2012 to March 2018 are summarized as follows.
1. Based on the on-going Subaru Hyper SuprimeCam (HSC) survey, we have constructed unprecedentedly large samples of luminous galaxies athigh redshifts, which enable us to investigate their
physical properties with great statistical accuracy.
We have revealed a significant excess of their number densities at the bright end compared to the
Schechter functional form, suggestive of inefficient
AGN feedback for star formation in massive galaxies at redshift z ∼ 4 − 7. We have also found atight correlation between galaxy halo mass andthe ratio of star formation rate to the dark matteraccretion rate with no significant change (evolution) at z ∼ 4−7, suggesting that it is a fundamental relation in high-z galaxy formation whose starformation is regulated by the dark matter massassembly.
2. We have analyzed the deep Hubble Space Telescope (HST) legacy survey data and found thatthe galaxy half-light radius at a given luminosity significantly decreases toward high z, and theirsize distribution is well represented by log-normal
functions whose standard deviation does not significantly change at z ∼ 0 − 6. In addition, we
have analyzed the deep HST data targeting six
massive galaxy cluster fields and detected intrinsically very faint high-z galaxies thanks to the gravitational lensing effect. We have found that theslopes of the UV luminosity functions (LFs) atz ∼ 6 − 8 are very steep and are even steeper atz ∼ 9 − 10. We have also found that reionization
scenarios exist that consistently explain all the observational measurements.
The details are described in the following section.

Fig. 1. Rest-frame UV LFs (i.e., differential number densities as afunction of rest-frame UV magnitude) of galaxies that take into
account quasar contamination correction at z ∼ 4. The green
shaded region corresponds to the best-fit Schechter functions
that take into account the effect of gravitational lensing withthe two cases of the optical depth estimates. The magenta curveis the best-fit Schechter function without considering the lensing effect and the blue curve is the best-fit double power law(DPL) function. For comparison, we also show previous results
for galaxies taken from the literature.

Research Activities

Subaru Hyper Suprime-Cam Survey
We are working on a large-area survey project with
Subaru HSC. This survey was awarded 300 nights ofSubaru observing time as a Subaru Strategic Program(SSP) [102, 103]. The HSC SSP survey began in 2014
and will last until 2019. This survey will cover 1400
deg2 with five optical broadband filters of g, r, i, z
and y down to 26 mag in g, r, and i band images and24 − 25 mag in z and y band images. This wide fieldsurvey will enable us to cover an unprecedentedly large
cosmic volume at z & 4 and to investigate very raresources such as luminous galaxies and galaxy overdense
regions. From early survey data products that were internally released in the HSC collaboration team in August 2016, we have constructed enormous samples ofhigh-redshift galaxy candidates at z ∼ 4 − 7, i.e., morethan 0.5 million Lyman-break galaxies (LBGs) selectedby the standard color selection technique, 358 out of
which are spectroscopically confirmed by our follow-up
spectroscopy and other studies [104]. We are also carrying out a large-area narrowband (NB) survey in the
HSC SSP [107]. The survey area of the latest HSCNB data reaches ' 21 deg2 , which is more than fourtimes larger than previous Subaru Suprime-Cam NB
fields. Based on the HSC NB data, we have constructeda large sample of 2230 Lyman-α emitters (LAEs) atz ' 5.7 − 6.6 [108].
Based on the HSC SSP LBG samples, we have studied the UV LFs at z ∼ 4, 5, 6, and 7 [104]. We have obtained UV LFs at z ∼ 4−7 that span a very wide UV lu∗ (−26 < M
minosity range of ∼ 0.002 – 100 LUVUV < −14121∗mag), where LUVdenotes the typical luminosity ofhigh-z galaxies, by combining LFs from our program
and the ultra-deep HST legacy surveys. Because our
HSC SSP data bridge the LFs of galaxies and active
galactic nuclei (AGNs) with great statistical accuracy,
we have carefully investigated the bright end of thegalaxy UV LFs that are estimated by the subtraction of
the AGN contribution either aided with spectroscopy or
the best-fit AGN UV LFs (Figure 1). We have foundthat the bright end of the galaxy UV LFs cannot be
explained by the Schechter function forms, which can
explain the UV LFs of low-z galaxies, at > 2σ significance, and require either double power-law functions
or modified Schechter functions that consider a magnification bias due to gravitational lensing. Our results indicate that AGN feedback for star formation in
massive galaxies would be more inefficient compared tolower redshifts where galaxy LFs are consistent with
Schechter functions.
We have also presented clustering properties of
LBGs at z ∼ 4 − 6 over the 100 deg2 sky (corresponding
to a 1.4 Gpc3 volume) identified in the early data of
the HSC SSP [105]. We have derived angular correlation functions (ACFs) for the HSC LBGs with unprecedentedly high statistical accuracies at z ∼ 4 − 6, andcompared them with the halo occupation distribution(HOD) models. We have clearly identified significant
ACF excesses in 1000 < θ < 9000 , the transition scale between one- and two-halo terms, suggestive of the existence of the non-linear halo bias effect. Combining the
HOD models and previous clustering measurements offaint LBGs at z ∼ 4 − 7, we have investigated the dark
matter halo mass (Mh ) of the z ∼ 4 − 7 LBGs and its
correlation with various physical properties including
the star formation rate (SFR), the stellar-to-halo mass
ratio (SHMR), and the dark matter accretion rate (Ṁh )over a wide mass range of Mh /M = 4 × 1010 − 4 × 1012 .
We have found that the SHMR increases from z ∼ 4
to 7 by a factor of ∼ 4 at Mh ' 1 × 1011 M , while the
SHMR shows no strong evolution in the similar redshift range at Mh ' 1 × 1012 M . Interestingly, we haveidentified a tight relation of SFR/Ṁh − Mh showing no
significant evolution beyond 0.15 dex in this wide mass
range over z ∼ 4 − 7 (Figure 2). This weak evolution
suggests that the SFR/Ṁh − Mh relation is a fundamental relation in high-redshift galaxy formation whose starformation activities are regulated by the dark mattermass assembly. Assuming this fundamental relation, we
have calculated the cosmic star formation rate densities(SFRDs) over z = 0 − 10 (a.k.a. the Madau–Lilly plot).
The cosmic SFRD evolution based on the fundamental
relation agrees with the one obtained by observations,suggesting that the cosmic SFRD increase from z ∼ 10
to 4 − 2 (decrease from z ∼ 4 − 2 to 0) is mainly driven
by the increase of the halo abundance (the decrease of
the accretion rate).
In [107], we have presented our NB survey strategy
and clustering properties investigated with ∼ 2000 Lyα

Fig. 2. SFR/Ṁh as a function of the halo mass. The filled blue,
green, orange, and red diamonds (circles) denote the ratio of
the extinction-corrected SFR to the dark matter accretion rate
at z ∼ 4, 5, 6, and 7, respectively, in this work The statisticalerrors for our data are smaller than the symbols (diamonds). We
also plot SFR/Ṁh at z ∼ 0, 4, 5, 6, 7, and 8 in the literature
with the black, blue, green, orange, red, and magenta crosses forcomparison. The black solid curve is the fitting formula for the
SFR/Ṁh – Mh relation for z ∼ 4 − 7, and the gray shaded region
represents the typical scatter (0.15 dex) of the data points. For
reference, we also plot the results for the subsamples with themagnitude cut of mcut
UV = 20.0 mag with the open diamonds.

emitters at z = 5.7 and 6.6 found in the early data of
the HSC SSP survey exploiting the carefully designed
narrowband filters. One of the NB filters whose central wavelength is around 0.9 µ m, NB921, was createdwith a support of KAKENHI (23244025; PI: M. Ouchi)
Grant-in-Aid for Scientific Research (A) through JSPS.
We have derived angular correlation functions with theunprecedentedly large samples of LAEs at z = 6 − 7over the large total area of 14 − 21 deg2 corresponding to 0.3 − 0.5 comoving Gpc2 . We have compared theLAE clustering results with two independent theoretical models that suggest an increase of an LAE clustering signal by the patchy ionized bubbles at the epochof reionization (EoR), and estimate the neutral hydrogen fraction of the intergalactic medium (IGM) to be
xHI = 0.15+0.15−0.15 at z = 6.6. Based on the halo occupation
distribution models, we have found that the & L∗ LAEs
are hosted by the dark-matter halos with an average+0.3
mass of log(hMh i /M ) = 11.1+0.2−0.4 (10.8−0.5 ) at z = 5.7(6.6) with a Lyα duty cycle (an abundance ratio of
LAEs to dark-matter haloes) of 1 % or less, where theresults of z = 6.6 LAEs may be slightly biased, due tothe increase of the clustering signal at the EoR. Ourclustering analysis reveals the low-mass nature of & L∗
LAEs at z = 6−7, and that these LAEs probably evolve
into massive super-L∗ galaxies in the present-day universe.

122

SFGs, rOpte

10

SFGs, rUVe

LBGs, rUVe

LUV = 0.3-1 L*z=3
Hu13

re [kpc]

Programs with Hubble Space Telescope
We worked on the UDF12 project of ultra-deep
imaging with HST at the near-infrared wavelengths
with the US, UK, and French colleagues (PI: R. Ellis). In [9], we have presented the results of the deepest
search to date for star-forming galaxies beyond a redshift z ' 8.5 utilizing a new sequence of near-infrared
Wide Field Camera 3 (WFC3) images of the Hubble
Ultra Deep Field. This UDF12 campaign completed
in September 2012 doubles the earlier exposures with
WFC3 in this field and quadruples the exposure in the
key F105W filter at λ ' 1.05µ m used to locate such distant galaxies. Combined with additional imaging in the
F140W filter at ' 1.4µ m, the fidelity of high-z galaxy
candidates is greatly improved. Using spectral energy
distribution fitting techniques on objects selected froma deep multi-band near-infrared stack we have found 7
promising galaxy candidates at z > 8.5. As none of thepreviously claimed galaxy candidates with 8.5 < z <10
is confirmed by our deeper multi-band imaging, ourcampaign has transformed the measured abundance ofgalaxies in this redshift range. Although we have recovered the galaxy candidate UDFj-39546284 (previously
proposed at z = 10.3), it is undetected in the newly
added F140W image, implying it lies at z = 11.9 or isan intense emission line galaxy at z ' 2.4. Although no
physically-plausible model can explain the required line
intensity given the lack of Lyman α or broad-band UV
signal, without an infrared spectrum we cannot rule out
an exotic interloper. Regardless, our robust z ' 8.5 − 10
sample demonstrates a luminosity density that continues the smooth decline observed over 6 < z < 8. Suchcontinuity has important implications for models of cosmic reionization and future searches for z >10 galaxies with James Webb Space Telescope (JWST). Further details of the UDF12 survey are provided in [27]and catalogs of z ∼ 7 − 8 sources used to estimate theUV luminosity function are provided in [17] and [20].
The spectral properties and sizes of the high-z UDF12
sources and measured and analyzed in [19] and [28], respectively. A review of the overall implications of thesurvey in the context of cosmic reionization is presented
in [16].
By extending these studies, we have exploited the
deep HST data of extragalactic legacy surveys to systematically investigate the redshift evolution of thegalaxy half-light radius re obtained from the HST samples of ∼ 190000 galaxies at z = 0 − 10 [62]. Our HST
samples consist of 176152 photo-z galaxies at z = 0 − 6
from the 3D-HST+CANDELS catalog and 10454 LBGs
at z = 4 − 10 identified in the CANDELS, HUDF09,
HUDF12, and HFF parallel fields, providing the largest
data set to date for galaxy size evolution studies. We
have derived re with the same technique over the wide
redshift range of z = 0 − 10, evaluating the effects of theuse of images at different wavelengths and the selection
bias of photo-z galaxies+LBGs as well as the cosmolog-

1

Cu14
On12

Ho14
Oe10

Gr13
Bo04

Aver

age

Med
ian
Mod
e

0

2

4 6 8 10 12
Redshift

Fig. 3. Difference of the size evolution results based on the average(crosses), median (filled circles), and modal (open diamonds)∗ . The red, cyan, andvalues of re in the bin of LUV = 0.3 − 1Lz=3

blue filled symbols indicate re (re in the rest-frame optical)
and reUV (re in the rest-frame UV) for the star-forming galaxies(SFGs) and reUV for the LBGs, respectively. The error bars for our
re are not plotted for clarity. The solid, dashed, and dot-dashedcurves denote the best-fit size evolution for the average, median,and modal re values, respectively, in the linear space. Note thatthese differences of the statistical results are found in the linear
space of re , because the re -distributions follow the log-normal
functions. The re values for LBGs in the literature are plottedwith gray symbols.
Opt

ical surface brightness dimming effect. We have foundthat re values at a given luminosity significantly decrease toward high z (Figure 3), regardless of statistics
choices (e.g., re ∝ (1 + z)−1.10±0.06 for median). For starforming galaxies, there is no evolution of the power-law
slope of the size–luminosity relation and the median
Sérsic index (n ∼ 1.5). Moreover, the re distribution iswell represented by log-normal functions whose standard deviation σln re does not show significant evolution
within the range of σln re ∼ 0.45 − 0.75. We have calculated the stellar-to-halo size ratio from our re measurements and the dark-matter halo masses estimated fromthe abundance matching study, and obtain a nearly
constant value of re /rvir = 1.0 − 3.5% at z = 0 − 8, wherervir is the virial radius of their host dark matter halos.
The combination of the re -distribution shape+standard
deviation, the constant re /rvir , and n ∼ 1.5 suggests a
picture in which typical high-z star-forming galaxies
have disk-like stellar components in a sense of dynamics
and morphology over cosmic time of z ∼ 0 − 6. If high-z
star-forming galaxies are truly dominated by disks, there /rvir value and the disk formation model indicate thatthe specific angular momentum of the disk normalized
by the host halo is ' 0.5 − 1. These are statistical results for major stellar components of galaxies, and thedetailed study of clumpy sub-components is presented
in [76]. In addition, we have presented clustering anal-

123

Fig. 4. Left panel: ρUV calculated with Mtrunc = −11.0. The black circles present ρUV from the best-fit luminosity functions at z ∼ 7 − 10(this work) and those at z ∼ 4 − 6 in the literature. The blue line and the light blue shade denote the best-fit function of ρUV and the1σ error, respectively. The up-scattering ρUV (z) at z ∼ 10 due to the large uncertainty in α does not affect the fitting result, because
the uncertainty in ρUV (z) is also large. Middle panel: τe integrating from z = 0 to a redshift z. The red line and the magenta shade
represent τe (z) and the 1σ error, respectively, that are consistent with ρUV (z) shown with the blue line in the left panel. The black line
and the gray region show the values of τe and its 1σ error, respectively, obtained by Planck. Right panel: evolution of QHII . The green
line and the light green shade present QHII and the 1σ error, respectively, that agree with ρUV (z) shown with the blue line in the left
panel. The symbols denote constraints of QHII obtained in the literature.

ysis results from the LBGs at z ∼ 4 − 7 identified in the
Hubble legacy deep imaging data [74].
Another program with HST, the Hubble Frontier
Fields (HFF; PI: J. Lotz), observed six massive clusters down to ∼ 29 mag at the 5σ level, which allows
us to detect background high-z sources with intrinsic
magnitudes of & 30 mag by lensing magnification [52].
By exploiting the HFF data, we have presented UV luminosity functions of LBGs at z ∼ 6 − 10 with the complete Hubble Frontier Fields data [119, 73]. We obtaina catalog of ∼ 450 LBG candidates (350, 66, and 40 atz ∼ 6 − 7, 8, and 9, respectively) with UV absolute magnitudes that reach ∼ −14 mag, ∼ 2 mag deeper thanthe Hubble Ultra Deep Field detection limits. We carefully evaluate number densities of the LBGs by Monte
Carlo simulations, including all lensing effects such as
magnification, distortion, and multiplication of images
as well as detection completeness and contamination
effects in a self-consistent manner. We find that UV
luminosity functions at z ∼ 6 − 8 have steep faint-end
slopes, α ∼ −2, and likely steeper slopes, α . −2 atz ∼ 9 − 10. We also find that the evolution of UV luminosity densities shows a non-accelerated decline beyond z ∼ 8 in the case of Mtrunc = −15, while an accelerated in the case of Mtrunc = −17, where Mtrunc is thefaint limit of the UV luminosity function. We examine
whether our results are consistent with the Thomson
scattering optical depth from the Planck satellite andthe ionized hydrogen fraction QHII at z . 7 based on thestandard analytic reionization model (Figure 4). We
find that reionization scenarios exist that consistently
explain all the observational measurements with the allowed parameters of fesc = 0.17+0.07−0.03 and Mtrunc > −14.0
for log ξion /[erg−1 Hz] = 25.34, where fesc is the escapefraction, and ξion is the conversion factor of the UV

luminosity to the ionizing photon emission rate. The
length of the reionization period is estimated to be∆z = 3.9+2.0−1.6 (for 0.1 < QHII < 0.99), consistent with therecent estimate from Planck. In addition, we have presented the results of size measurements for the z ∼ 6 − 9
LBGs identified in the HFF data and investigated theirsize-luminosity relation [60, 120].
Programs with ALMA
We have reported deep ALMA observations complemented by associated HST imaging for a luminous(mUV = 25) galaxy, “Himiko”, at z = 6.595 [29]. The
galaxy is remarkable for its high star formation rate,
100M yr−1 , which has been securely estimated fromour deep HST and Spitzer photometry, and the absence of any evidence for strong AGN activity or gravitational lensing magnification. Our ALMA observations probe an order of magnitude deeper than previous IRAM observations, yet fail to detect a 1.2 mm
dust continuum, indicating a flux of < 52µ Jy, which iscomparable to or weaker than that of local dwarf irregulars with much lower star formation rates. We have
likewise provided a strong upper limit for the flux of[CII] 158µ m, L[CII] < 5.4 × 107 L , which is a diagnostic of the hot interstellar gas that is often described asa valuable probe for early galaxies. In fact, our observations indicate that Himiko lies off the local L[CII] –star formation rate scaling relation by a factor of morethan 30. Both aspects of our ALMA observations suggest Himiko is a unique object with a very low dust
content and perhaps nearly primordial interstellar gas.
Our HST images provide unique insight into the morphology of this remarkable source, highlighting an extremely blue core of activity and two less extreme associated clumps. Himiko is undergoing a triple major
merger event whose extensive ionized nebula of Lyα124emitting gas, discovered in our earlier work with Subaru, is powered by star formation and the dense circumgalactic gas. We are likely witnessing an early massive
galaxy during a key period of its mass assembly close
to the end of the reionization era.

Si15

Ot15

On14

H13

Ka13

Fig. 5. Differential number counts at 1.2 mm. The red filled circles
are our number counts derived from our faint ALMA sources.
The red curve and the gray region denote the best-fit Schechter
function and the associated 1σ error. In the Schechter function
fitting, we use our number counts and the previous measurements shown with the black filled stars and the magenta filled
diamonds. Note that the faintest data point of our study (redopen circle) is removed from the sample of our Schether function fitting, due to the possible large systematic uncertainties.
The pale-green region presents a flux range where no reliable
number-count data bin exists (see text). The dashed and dotted curves show the model predictions for number counts basedon cosmological hydrodynamic simulations with GADGET-3 andsemi-analytic models, respectively. These model predictions areroughly consistent with our results. For the previous measurements of the number counts at 1.1 mm and 1.3 mm different
from our 1.2 mm band, we scale the flux densities.

We have also made use of deep ALMA data including archival ones to investigate statistical properties offaint dust continuum sources such as number densities
and clustering strengths [49]. In [71], we have presented
statistics of 133 faint 1.2 mm continuum sources detected in about 120 deep ALMA pointing data that include all the archival deep data available by 2015 June.
We have derived number counts of 1.2 mm continuum
sources down to 0.02 mJy partly with the assistance ofgravitational lensing (Figure 5), and find that the total
integrated 1.2 mm flux of the securely identified sources−2 that corresponds to 104+31 % ofis 22.9+6.7−5.6 Jy deg−25
the extragalactic background light (EBL) measured by
COBE observations. These results suggest that the
major 1.2 mm EBL contributors are sources with 0.02
mJy, and that very faint 1.2 mm sources with .0.02
mJy contribute negligibly to the EBL with the possible
flattening and/or truncation of number counts in thisvery faint flux regime. To understand the physical origin of our faint ALMA sources, we have measured thegalaxy bias bg by the counts-in-cells technique, whichis the relation of clustering strengths between galaxies
and underlying dark matter, and place a stringent upper limit of bg < 3.5 that is not similar to bg values ofmassive distant red galaxies and submillimeter galaxies
but comparable to those of UV-bright star-forming BzK
galaxies (sBzKs) and LBGs. Moreover, in the opticaland near-infrared (NIR) deep fields, we have identified
optical-NIR counterparts for 59% of our faint ALMA
sources, the majority of which have luminosities, colors,
and the IRX-β relation the same as sBzKs and LBGs.
We have thus concluded that about a half of our faint
ALMA sources are dust-poor high-z galaxies, and thatthese faint ALMA sources are not miniature (ultra-)luminous infrared galaxies simply scaled down with theinfrared brightness.
Programs with Optical Spectroscopy
We have conducted a systematic study of galactic
outflows in star-forming galaxies at z ∼ 0–2 based on
the absorption lines of optical spectra taken from SDSS
DR7, DEEP2 DR4, and Keck [100]. We have carefully
made stacked spectra of homogeneous galaxy samples
with similar stellar mass distributions at z ∼ 0–2, andperform the multi-component fitting of model absorption lines and stellar continua to the stacked spectra.
We have obtained the maximum (vmax ) and central(vout ) outflow velocities, and estimate the mass loading factors (η ), a ratio of the mass outflow rate to thestar formation rate (SFR). Investigating the redshift
evolution of the outflow velocities measured with theabsorption lines whose depths and ionization energies
are similar (Na ID and Mg I at z ∼ 0–1; Mg II and C II
at z ∼ 1–2), we have identified, for the first time, thatthe average value of vmax (vout ) significantly increases
by 0.05–0.3 dex from z ∼ 0 to 2 at a given SFR (Figure 6). Moreover, we have found that the value of η
increases from z ∼ 0 to 2 by η ∝ (1 + z)1.2±0.3 at a given
halo circular velocity vcir , albeit with a potential systematics caused by model parameter choices. The redshift evolution of vmax (vout ) and η is consistent with thegalaxy-size evolution and the local velocity-SFR surface
density relation, and explained by high-gas fractions inhigh-redshift massive galaxies, which is supported byrecent radio observations. We have obtained a scaling
relation of η ∝ vacir for a = −0.2±1.1 in our z ∼ 0 galaxies
that agrees with the momentum-driven outflow model(a = −1) within the uncertainty.
On-Going Projects and Future Prospects
Although our previous work provides various implications about the early epoch of cosmic history, large
uncertainties raised by the small statistics and systematics do not allow us to obtain conclusive results. One
example is shown in Figure 7 that presents the current
measurements of UV LFs of galaxies at z = 5 and 7. It is125

Fig. 6. Maximum outflow velocities as a function of SFRs. Eachsymbol corresponds to the elements of the absorption lines: Na
ID (blue circle), Mg I (green circle), Mg II (green diamond), C
II (orange diamond), and C IV (orange triangle). The circles,
diamonds, and triangle indicate the velocities of elements, whichhave the low (5–7 eV), medium (15–24 eV), and high (48 eV)
ionization energy, respectively. Error bars denote the 1σ fitting
errors. The light blue and green shades describe the result of thepower-law fitting to Na ID and Mg I, respectively, with vertical
1σ fitting error range. The green and orange lines denote thebest-fitting power-law function of Mg II and C II, respectively.

unclear whether the LF follows a Schechter function or
has a bright-end excess at the > 5σ level that is perhaps
the result of a heightened AGN population that may beproviding considerable feedback effects [104]. Anotherexample is presented in Figure 6. Outflows are a key
ingredient in the modeling of galaxies to regulate theirstellar mass growth. An open question is how the properties of outflows may vary with SFR and AGN luminosity at a given redshift. Theoretical models predictthat the outflow velocity (or the mass loading factor)
increases towards high-z thus preventing the productionof too many stars at early epochs. Recent observational
studies do show indications of strong outflow activity
in galaxies up to z ∼ 2 − 3 through UV metal absorption lines and broad emission-line components [100].
Although, large statistical uncertainties preclude any
definitive conclusion. Moreover, no statistical measureof outflows have been obtained at z > 3 to date.
First of all, the HSC SSP survey is ongoing andpromising to extend our previous work with better
statistics. Recently, we are also carrying out a multiple HSC NB survey, Cosmic HydrOgen Reionization
Unveiled with Subaru (CHORUS), which has been approved as a Subaru intensive program under the collaboration of Osaka Sangyo University, NAOJ, Ehime
University, and The University of Tokyo etc. Our contribution to the CHORUS project is to supply one of
the NB filters, NB973, which is created for identifying

Fig. 7. Bright-end of UV LF at z = 5 and z = 7. The magenta andblue circles indicate the expected PFS measurements (plus bestfit functions - curves) and uncertainties for the two cases with LFs
having a functional shape of either a Schechter or double-powerlaw model. The green curve shows the Schechter function including the effect of gravitational lensing. The black filled circles
represent the current LF at z = 5 and 7 [104] based on HSC imaging and spectroscopic follow-up that cannot distinguish between
Schechter and DPL functions due to uncertainties, contamination, and errors on the AGN fraction. Black open symbols areobservational data from the literature. The inset panel shows theerror contours of the best-fit parameters of the Schechter function. Solid-line contours are the expected PFS measurements,
while the dotted-line contours are the current observational results.

high-z galaxies with a support of KAKENHI (23244025;
PI: M. Ouchi) Grant-in-Aid for Scientific Research (A)through JSPS. Although our approved intensive program is a 2-year observation project which will be completed in 2018, some of our observing time was lost
due to bad weather. To complete our observations, wehave applied for additional observing time for the next
semester until January 2019. In addition to the HSC
SSP survey, we will work on the CHORUS project andplan to publish our scientific results by 2020.
Moreover, to address the open questions describedabove, we have started to work on the next generation Subaru wide-field spectrograph, the Prime Focus
Spectrograph (PFS) project [36]. After PFS starts science operations planned in 2021, we will start the PFS
survey under the large collaboration of The Universityof Tokyo, NAOJ, Princeton University and Taiwaneseinstitutes etc. Our current contribution to the PFS
project is to prepare for an SSP proposal particularly
for the section related to high-z galaxy studies. For

126

sured systemic velocity, we can further determine themaximum outflow velocity at z ∼ 2 − 6 and relations togalactic properties including starburst and AGN events
through spectral emission-line diagnostics.

Subaru Hyper Suprime-Cam Survey

Fig. 8. Maximum outflow velocity vmax as a function of SFR. Yellow, red, and purple stars represent the average vmax values witherrors at z ∼ 2, 3.5, and 5, respectively, based on simulated coadded PFS spectra. The black, dark-gray, and light gray data
points (lines) are the average vmax values (best-fit linear function)
obtained from the SDSS at z ∼ 0, DEEP2 at z ∼ 1, and a Kecksurvey at z ∼ 2. To illustrate an average measurement at a given
redshift and SFR, the inset panel shows the Si II1260 absorption line in a stacked spectrum based on simulated PFS spectra(texp = 6 hr each) of 191 galaxies (23.4 < yAB < 24) at z ∼ 3.5.
The black and red line represent the stacked PFS spectrum andthe best-fit model, respectively. The blue and green lines are
the wind and systemic components of the best-fit model. The
vertical dotted lines indicates the rest wavelength of Si II1260.

example, we propose to provide a definitive answer tothe open question on the shape of the bright-end of thegalaxy UV LF at z ∼ 4 − 7 (Figure 7) by combining thespectral analysis of high-z galaxies (AGN vs. star formation and outflows) and the precise spectroscopic redshifts. With PFS spectra, we will identify and remove
foreground contamination to accurately measure thebright-end LF with lessened systematics. The brightend luminosity function will allow us to determine theSchechter function parameters of M ∗ and φ ∗ , criticalfor obtaining the faint-end slope α (Figure 7; see inset
panel) that is key to understanding the global evolutionof galaxies, i.e. UV luminosity density, for the cosmic
star-formation density and the ionizing photon production rate connected to the cosmic reionization photon
budget. We also propose to measure maximum outflow
velocities of high-z galaxies. With PFS, we will use Mg
II2796, 2804, C II1335, and Si II absorption lines as indicators of outflows for galaxies at 2.1 < z < 2.4 where[O II] emission detected in the IR arm of PFS provides
the systemic velocity. At higher redshift z ∼ 3 − 6, we
will use the strong UV emission lines (e.g. C III]) toindicate the systemic velocity. In Figure 8, we show
the expected PFS measurements of the maximum outflow velocity as estimated from stacked simulated spectra. While this analysis is limited to galaxies with mea-

Institute
Country(∗ )ICRR, Univ. of Tokyo
Japan
15
Kavli IPMU, U. of Tokyo
Japan
24
School of Science, U. of Tokyo
Japan
28
NAOJ
Japan
47
JAXA
Japan
6
Tohoku Univ.
Japan
10
Tsukuba Univ.
Japan
1
Kyoto Univ.
Japan
1Osaka Univ.
Japan
2
Nagoya Univ.
Japan
8
Hokkaido Univ.
Japan
1
Hiroshima Univ.
Japan
4
Ehime Univ.
Japan
12
Tokyo Univ. of ScienceJapan
3
Others
Japan
17
ASIAA
Taiwan23
NCU
Taiwan9
NTHU
Taiwan6
Princeton Univ.
USA
27
Stanford Univ.
USA
1
JHU
USA
1
Univ. of Penn.
USA
1
Univ. of Geneva
Switzerland
1
ETH Zurich
Switzerland
1
SJTU
China
1
KIAA
China
1
Saint Mary’s Univ.
Canada1
ESO
Germany1
SRON
Netherlands
1
Total
254
(∗ ) Number of participants as of April 2018.

Members
StaffsMasami Ouchi, Assoc. Professor, July 2010 to thepresent
Yoshiaki Ono, Assist. Professor, April 2012 to thepresent
Postdoctoral Fellows
Rieko Momose, April 2012 to March 2015
Suraphong Yuma, May 2012 to March 2015
Masao Hayashi, April 2013 to March 2014
Takatoshi Shibuya, April 2013 to February 2018

127

Tomoki Saito, April 2014 to March 2015
Mariko Kubo, April 2014 to March 2016
Florent Duval, April 2015 to March 2016
Yi-Kuan Chiang, June 2016 to September 2016
Jun Toshikawa, April 2017 to the present
Ken Mawatari, April 2018 to the present
Graduate Students
One student was awarded a doctor degree and ten students earned a master degree during 2014–2018, supervised by Masami Ouchi.

List of Publications
After Year 2012
Papers in Refereed Journals

K. Nobuta et al., The Astrophysical Journal, 761,
143 (2012). Cited 30 times.
[8] “Intrinsic Shape of Star-forming BzK Galaxies.
II. Rest-frame Ultraviolet and Optical Structuresin GOODS-South and SXDS”
S. Yuma et al., The Astrophysical Journal, 761,
19 (2012). Cited 13 times.
[9] “The Abundance of Star-forming Galaxies in the
Redshift Range 8.5 − 12: New Results from the2012 Hubble Ultra Deep Field Campaign”
R. S. Ellis et al., The Astrophysical Journal Letters, 763, L7 (2013). Cited 297 times.
[10] “A Water Maser and NH3 Survey of GLIMPSE
Extended Green Objects”
C. J. Cyganowski et al., The Astrophysical Journal, 764, 61 (2013). Cited 28 times.

[1] “The First Systematic Survey for Lyα Emittersat z = 7.3 with Red-sensitive Subaru/SuprimeCam”
T. Shibuya et al., The Astrophysical Journal,
752, 114 (2012). Cited 95 times.

[11] “Nebular Attenuation in Hα -selected Starforming Galaxies at z = 0.8 from the NewHα Survey”
I. G. Momcheva et al., The Astronomical Journal,
145, 47 (2013). Cited 27 times.

[2] “Evidence for Low Extinction in Actively Starforming Galaxies at z > 6.5”
F. Walter et al., The Astrophysical Journal, 752,
93 (2012). Cited 42 times.

[12] “Gas Motion Study of Lyα Emitters at z ∼ 2 Using FUV and Optical Spectral Lines”
T. Hashimoto et al., The Astrophysical Journal,
765, 70 (2013). Cited 84 times.

[3] “The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and
Goals”
J.-C. Mauduit et al., Publications of the Astronomical Society of the Pacific, 124, 714 (2012).
Cited 68 times.

[13] “Calibrating [O II] star formation rates at z < 1from dual Hα -[O II] imaging from HiZELS”
M. Hayashi et al., Monthly Notices of the Royal
Astronomical Society, 430, 1042-1050 (2013).
Cited 22 times.

[4] “A Dual-Narrowband Survey for Hα Emitters at
Redshift of 2.2: Demonstration of the Techniqueand Constraints on the Hα Luminosity Function”
J. C. Lee et al., Publications of the Astronomical
Society of the Pacific, 124, 782 (2012). Cited 36
times.
[5] “Diffuse Lyα haloes around Lyα emitters at z = 3:
do dark matter distributions determine the Lyαspatial extents?”
Y. Matsuda et al., Monthly Notices of theRoyal Astronomical Society, 425, 878-883 (2012).
Cited 75 times.
[6] “Physical Conditions in Molecular Clouds in the
Arm and Interarm Regions of M51”
J. Koda et al., The Astrophysical Journal, 761,
41 (2012). Cited 33 times.
[7] “Black Hole Mass and Eddington Ratio Distribution Functions of X-Ray-selected Broad-line
AGNs at z ∼ 1.4 in the Subaru XMM-Newton
Deep Field”[14] “SEDS: The Spitzer Extended Deep Survey. Survey Design, Photometry, and Deep IRAC Source
Counts”
M. L. N. Ashby et al., The Astrophysical Journal,
769, 80 (2013). Cited 130 times.
[15] “First Spectroscopic Evidence for High Ionization
State and Low Oxygen Abundance in Lyα Emitters”
K. Nakajima et al., The Astrophysical Journal,
769, 3 (2013). Cited 70 times.
[16] “New Constraints on Cosmic Reionization fromthe 2012 Hubble Ultra Deep Field Campaign”
B. E. Robertson et al., The Astrophysical Journal, 768, 71 (2013). Cited 290 times.
[17] “The UV Luminosity Function of Star-forming
Galaxies via Dropout Selection at Redshifts z ∼ 7
and 8 from the 2012 Ultra Deep Field Campaign”
M. A. Schenker et al., The Astrophysical Journal,
768, 196 (2013). Cited 163 times.

128[18] “Evolution of star formation in the UKIDSS Ultra Deep Survey field - I. Luminosity functions
and cosmic star formation rate out to z = 1.6”
A. B. Drake et al., Monthly Notices of theRoyal Astronomical Society, 433, 796-811 (2013).
Cited 29 times.
[19] “The UV continua and inferred stellar populations of galaxies at z ' 7 − 9 revealed by the Hubble Ultra-Deep Field 2012 campaign”
J. S. Dunlop et al., Monthly Notices of the Royal
Astronomical Society, 432, 3520-3533 (2013).
Cited 108 times.
[20] “A new multifield determination of the galaxy luminosity function at z = 7 − 9 incorporating the2012 Hubble Ultra-Deep Field imaging”
R. J. McLure et al., Monthly Notices of the Royal
Astronomical Society, 432, 2696-2716 (2013).
Cited 234 times.
[21] “Star Formation on Subkiloparsec Scale Triggered by Non-linear Processes in Nearby Spiral
Galaxies”
R. Momose et al., The Astrophysical Journal Letters, 772, L13 (2013). Cited 30 times.
[22] “The Galaxy Environment of a QSO at z ∼ 5.7”
E. Bañados et al., The Astrophysical Journal,
773, 178 (2013). Cited 27 times.
[23] “Resolved Giant Molecular Clouds in Nearby Spiral Galaxies: Insights from the CANON CO (1–0)
Survey”
J. Donovan Meyer et al., The Astrophysical Journal, 772, 107 (2013). Cited 44 times.
[24] “Physical Properties of Spectroscopically Confirmed Galaxies at z ≥ 6. I. Basic Characteristicsof the Rest-frame UV Continuum and Lyα Emission”
L. Jiang et al., The Astrophysical Journal, 772,
99 (2013). Cited 46 times.
[25] “Physical Properties of Spectroscopically Confirmed Galaxies at z ≥ 6. II. Morphology of theRest-frame UV Continuum and Lyα Emission”
L. Jiang et al., The Astrophysical Journal, 773,
153 (2013). Cited 42 times.
[26] “On the evolution and environmental dependenceof the star formation rate versus stellar mass relation since z ∼ 2”
Y. Koyama et al., Monthly Notices of theRoyal Astronomical Society, 434, 423-436 (2013).
Cited 90 times.
[27] “The 2012 Hubble Ultra Deep Field (UDF12):
Observational Overview”

A. M. Koekemoer et al., The Astrophysical Journal Supplement Series, 209, 3 (2013). Cited 104
times.
[28] “Evolution of the Sizes of Galaxies over 7 < z < 12
Revealed by the 2012 Hubble Ultra Deep Field
Campaign”
Y. Ono et al., The Astrophysical Journal, 777,
155 (2013). Cited 86 times.
[29] “An Intensely Star-forming Galaxy at z ∼ 7 with
Low Dust and Metal Content Revealed by Deep
ALMA and HST Observations”
M. Ouchi et al., The Astrophysical Journal, 778,
102 (2013). Cited 123 times.
[30] “A fundamental metallicity relation for galaxies
at z = 0.84 − 1.47 from HiZELS”
J. P. Stott et al., Monthly Notices of the Royal
Astronomical Society, 436, 1130-1141 (2013).
Cited 63 times.
[31] “Nature of Hα Selected Galaxies at z > 2. I. Mainsequence and Dusty Star-forming Galaxies”
K.-i. Tadaki et al., The Astrophysical Journal,
778, 114 (2013). Cited 25 times.
[32] “First Systematic Search for Oxygen-line Blobs
at High Redshift: Uncovering AGN Feedback andStar Formation Quenching”
S. Yuma et al., The Astrophysical Journal, 779,
53 (2013). Cited 13 times.
[33] ““Direct” Gas-phase Metallicities, Stellar Properties, and Local Environments of Emission-line
Galaxies at Redshifts below 0.90”
C. Ly et al., The Astrophysical Journal, 780, 122(2014). Cited 44 times.
[34] “The Nature of Hα -selected Galaxies at z > 2.
II. Clumpy Galaxies and Compact Star-forming
Galaxies”
K.-i. Tadaki et al., The Astrophysical Journal,
780, 77 (2014). Cited 25 times.
[35] “The mass-metallicity relation at z ∼ 1.4 revealedwith Subaru/FMOS”
K. Yabe et al., Monthly Notices of the Royal Astronomical Society, 437, 3647-3663 (2014). Cited
48 times.
[36] “Extragalactic science, cosmology, and Galacticarchaeology with the Subaru Prime Focus Spectrograph”
M. Takada et al., Publications of the Astronomical Society of Japan, 66, R1 (2014). Cited 183
times.
[37] “Mapping the large-scale structure around a z =1.46 galaxy cluster in 3D using two adjacentnarrow-band filters”

129

M. Hayashi et al., Monthly Notices of the Royal
Astronomical Society, 439, 2571-2583 (2014).
Cited 16 times.
[38] “AzTEC/ASTE 1.1-mm survey of SSA22: Counterpart identification and photometric redshift
survey of submillimetre galaxies”
H. Umehata et al., Monthly Notices of the Royal
Astronomical Society, 440, 3462-3478 (2014).
Cited 23 times.
[39] “What is the Physical Origin of Strong Lyα Emission? I. Demographics of Lyα Emitter Structures”
T. Shibuya et al., The Astrophysical Journal,
785, 64 (2014). Cited 26 times.
[40] “What is the Physical Origin of Strong Lyα Emission? II. Gas Kinematics and Distribution of Lyα
Emitters”
T. Shibuya et al., The Astrophysical Journal,
788, 74 (2014). Cited 67 times.
[41] “Identification of the progenitors of rich clusters
and member galaxies in rapid formation at z > 2”
R. Shimakawa et al., Monthly Notices of theRoyal Astronomical Society, 441, L1-L5 (2014).
Cited 28 times.
[42] “Evidence for a Gas-rich Major Merger in a
Proto-cluster at z = 2.5”
K.-i. Tadaki et al., The Astrophysical Journal
Letters, 788, L23 (2014). Cited 14 times.
[43] “Diffuse Lyα haloes around galaxies at z = 2.2 −6.6: implications for galaxy formation and cosmic
reionization”
R. Momose et al., Monthly Notices of theRoyal Astronomical Society, 442, 110-120 (2014).
Cited 70 times.
[44] “Ionization state of inter-stellar medium in galaxies: evolution, SFR-M∗ -Z dependence, and ionizing photon escape”
K. Nakajima et al., Monthly Notices of theRoyal Astronomical Society, 442, 900-916 (2014).
Cited 128 times.
[45] “Large-scale environment of z ∼ 5.7 C IV absorption systems - I. Projected distribution of galaxies”
C. G. Díaz et al., Monthly Notices of theRoyal Astronomical Society, 442, 946-978 (2014).
Cited 15 times.
[46] “Constraining dust formation in high-redshift
young galaxies”
H. Hirashita et al., Monthly Notices of the Royal
Astronomical Society, 443, 1704-1712 (2014).
Cited 17 times.

[47] “A First Site of Galaxy Cluster Formation: Complete Spectroscopy of a Protocluster at z = 6.01”
J. Toshikawa et al., The Astrophysical Journal,
792, 15 (2014). Cited 23 times.
[48] “MOSFIRE and LDSS3 Spectroscopy for an [OII] Blob at z = 1.18: Gas Outflow and Energy
Source”
Y. Harikane et al., The Astrophysical Journal,
794, 129 (2014). Cited 7 times.
[49] “Faint Submillimeter Galaxies Revealed by Multifield Deep ALMA Observations:
NumberCounts, Spatial Clustering, and a Dark Submillimeter Line Emitter”
Y. Ono et al., The Astrophysical Journal, 795, 5(2014). Cited 47 times.
[50] “Accelerated Evolution of the Lyα Luminosity
Function at z & 7 Revealed by the Subaru Ultradeep Survey for Lyα Emitters at z = 7.3”
A. Konno et al., The Astrophysical Journal, 797,
16 (2014). Cited 78 times.
[51] “NIR Spectroscopic Observation of Massive
Galaxies in the Protocluster at z = 3.09”
M. Kubo et al., The Astrophysical Journal, 799,
38 (2015). Cited 21 times.
[52] “Hubble Frontier Fields First Complete Cluster
Data: Faint Galaxies at z ∼ 5 − 10 for UV Luminosity Functions and Cosmic Reionization”
M. Ishigaki et al., The Astrophysical Journal,
799, 12 (2015). Cited 120 times.
[53] “The Subaru High-z Quasar Survey: Discovery of
Faint z ∼ 6 Quasars”
N. Kashikawa et al., The Astrophysical Journal,
798, 28 (2015). Cited 45 times.
[54] “The Gas Inflow and Outflow Rate in Starforming Galaxies at z ∼ 1.4”
K. Yabe et al., The Astrophysical Journal, 798,
45 (2015). Cited 13 times.
[55] “First Infrared-Based Implications for the Dust
Attenuation and Star Formation of Typical Lyα
Emitters”
H. Kusakabe et al., The Astrophysical Journal
Letters, 800, L29 (2015). Cited 17 times.
[56] “The Relationship between Stellar Mass, Gas
Metallicity, and Star Formation Rate for
Hα -Selected Galaxies at z ≈ 0.8 from the NewHαSurvey”
M. A. de los Reyes et al., The Astronomical Journal, 149, 79 (2015). Cited 31 times.
[57] “The spectral energy distribution of the redshift
7.1 quasar ULAS J1120+0641”
R. Barnett et al., Astronomy & Astrophysics,
575, A31 (2015). Cited 19 times.

130[58] “When did Round Disk Galaxies Form?”
T. M. Takeuchi et al., The Astrophysical Journal,
801, 2 (2015). Cited 1 times.
[59] “Large-scale environment of z ∼ 5.7 C IV absorption systems -II. Spectroscopy of Lyman α emitters”
C. G. Díaz et al., Monthly Notices of the Royal
Astronomical Society, 448, 1240-1270 (2015).
Cited 11 times.
[60] “The Sizes of z ∼ 6 − 8 Lensed Galaxies from the
Hubble Frontier Fields Abell 2744 Data”
R. Kawamata et al., The Astrophysical Journal,
804, 103 (2015). Cited 62 times.
[61] “On the Diffuse Lyα Halo around Lyα Emitting
Galaxies”
E. Lake et al., The Astrophysical Journal, 806,
46 (2015). Cited 22 times.
[62] “Morphologies of ∼ 190,000 Galaxies at z = 0 − 10
Revealed with HST Legacy Data. I. Size Evolution”
T. Shibuya et al., The Astrophysical Journal
Supplement Series, 219, 15 (2015). Cited 112
times.
[63] “Deep rest-frame far-UV spectroscopy of the giant Lyman α emitter ‘Himiko”’
J. Zabl et al., Monthly Notices of the Royal Astronomical Society, 451, 2050-2070 (2015). Cited
17 times.
[64] “The Subaru-XMM-Newton Deep Survey(SXDS). VIII. Multi-wavelength identification,
optical/NIR spectroscopic properties, and photometric redshifts of X-ray sources”
M. Akiyama et al., Publications of the Astronomical Society of Japan, 67, 82 (2015). Cited
11 times.
[65] “A Close Comparison between Observed andModeled Lyα Lines for z ∼ 2.2 Lyα Emitters”
T. Hashimoto et al., The Astrophysical Journal,
812, 157 (2015). Cited 42 times.
[66] “Physical conditions of the interstellar medium instar-forming galaxies at z ∼ 1.5”
M. Hayashi et al., Publications of the Astronomical Society of Japan, 67, 80 (2015). Cited 17
times.
[67] “Evolution of star formation in the UKIDSS Ultra
Deep Survey Field - II. Star formation as a function of stellar mass between z = 1.46 and 0.63”
A. B. Drake et al., Monthly Notices of the Royal
Astronomical Society, 454, 2015-2025 (2015).
Cited 6 times.

[68] “Thirty Meter Telescope Detailed Science Case:
2015”
W. Skidmore et al., Research in Astronomy andAstrophysics, 15, 1945 (2015). Cited 30 times.
[69] “ALMA Deep Field in SSA22: A Concentrationof Dusty Starbursts in a z = 3.09 Protocluster
Core”
H. Umehata et al., The Astrophysical Journal
Letters, 815, L8 (2015). Cited 34 times.
[70] “Physical Properties of Spectroscopically Confirmed Galaxies at z ≥ 6. III. Stellar Populations
from SED Modeling with Secure Lyα Emission
and Redshifts”
L. Jiang et al., The Astrophysical Journal, 816,
16 (2016). Cited 18 times.
[71] “ALMA Census of Faint 1.2 mm Sources Down
to ∼ 0.02 mJy: Extragalactic Background Lightand Dust-poor, High-z Galaxies”
S. Fujimoto et al., The Astrophysical Journal Letters, 222, 1 (2016). Cited 50 times.
[72] “An extremely dense group of massive galaxies atthe centre of the protocluster at z = 3.09 in the
SSA22 field”
M. Kubo et al., Monthly Notices of the Royal Astronomical Society, 455, 3333-3344 (2016). Cited
13 times.
[73] “Precise Strong Lensing Mass Modeling of Four
Hubble Frontier Field Clusters and a Sample ofMagnified High-redshift Galaxies”
R. Kawamata et al., The Astrophysical Journal,
819, 114 (2016). Cited 75 times.
[74] “Evolution of Stellar-to-Halo Mass Ratio at z =0 − 7 Identified by Clustering Analysis with the
Hubble Legacy Imaging and Early Subaru/Hyper
Suprime-Cam Survey Data”
Y. Harikane et al., The Astrophysical Journal,
821, 123 (2016). Cited 40 times.
[75] “Statistical properties of diffuse Lyα haloes
around star-forming galaxies at z ∼ 2”
R. Momose et al., Monthly Notices of the Royal
Astronomical Society, 457, 2318-2330 (2016).
Cited 26 times.
[76] “Morphologies of ∼ 190,000 Galaxies at z = 0 − 10
Revealed with HST Legacy Data. II. Evolution of
Clumpy Galaxies”
T. Shibuya et al., The Astrophysical Journal,
821, 72 (2016). Cited 32 times.
[77] “A New Constraint on the Lyα Fraction of UVVery Bright Galaxies at Redshift 7”
H. Furusawa et al., The Astrophysical Journal,
822, 46 (2016). Cited 23 times.

131[78] “A Very Compact Dense Galaxy Overdensitywith δ ' 130 Identified at z ∼ 8: Implications for
Early Protocluster and Cluster Core Formation”
M. Ishigaki et al., The Astrophysical Journal,
822, 5 (2016). Cited 10 times.

[88] “The redshift-selected sample of long gamma-ray
burst host galaxies: The overall metallicity distribution at z < 0.4”
Y. Niino et al., Publications of the Astronomical
Society of Japan, 69, 27 (2017). Cited 2 times.

[79] “Bright and Faint Ends of Lyα Luminosity Functions at z = 2 Determined by the Subaru Survey:
Implications for AGNs, Magnification Bias, andISM H I Evolution”
A. Konno et al., The Astrophysical Journal, 823,
20 (2016). Cited 38 times.

[89] “The Galaxy-Halo Connection in High-redshift
Universe: Details and Evolution of Stellar-tohalo Mass Ratios of Lyman Break Galaxies on
CFHTLS Deep Fields”
S. Ishikawa et al., The Astrophysical Journal,
841, 8 (2017). Cited 7 times.

[80] “Detection of an oxygen emission line from ahigh-redshift galaxy in the reionization epoch”
A. K. Inoue et al., Science, 352, 1559-1562(2016). Cited 41 times.

[90] “Small-scale Intensity Mapping: Extended Lyα ,Hα , and Continuum Emission as a Probe of HaloStar Formation in High-redshift Galaxies”
L. Mas-Ribas et al., The Astrophysical Journal,
841, 19 (2017). Cited 13 times.

[81] “A Spectroscopically Confirmed Double Source
Plane Lens System in the Hyper Suprime-Cam
Subaru Strategic Program”
M. Tanaka et al., The Astrophysical Journal Letters, 826, L19 (2016). Cited 4 times.
[82] “Subaru High-z Exploration of Low-luminosity
Quasars (SHELLQs). I. Discovery of 15 Quasars
and Bright Galaxies at 5.7 < z < 6.9”
Y. Matsuoka et al., The Astrophysical Journal,
828, 26 (2016). Cited 59 times.
[83] “A Hard Ionizing Spectrum in z = 3−4 Lyα Emitters with Intense [O III] Emission: Analogs ofGalaxies in the Reionization Era?”
K. Nakajima et al., The Astrophysical Journal
Letters, 831, L9 (2016). Cited 32 times.
[84] “ALMA Reveals Strong [C II] Emission in a
Galaxy Embedded in a Giant Lyα Blob at z =3.1”
H. Umehata et al., The Astrophysical Journal
Letters, 834, L16 (2017). Cited 5 times.
[85] “Lyα emitters with very large Lyα equivalent
widths, EW0 (Lyα ) ' 200 − 400Å, at z ∼ 2”
T. Hashimoto et al., Monthly Notices of the Royal
Astronomical Society, 465, 1543-1562 (2017).
Cited 12 times.
[86] “A new quadruple gravitational lens from theHyper Suprime-Cam Survey: the puzzle of HSC
J115252+004733”
A. More et al., Monthly Notices of the Royal Astronomical Society, 465, 2411-2419 (2017). Cited
10 times.
[87] “Cosmic Galaxy-IGM H I Relation at z ∼ 2 − 3Probed in the COSMOS/UltraVISTA 1.6 Deg2
Field”
S. Mukae et al., The Astrophysical Journal, 835,
281 (2017). Cited 6 times.

[91] “Evolution of N/O abundance ratios and ionization parameters from z ∼ 0 to 2 investigated by
the direct temperature method”
T. Kojima et al., Publications of the Astronomical Society of Japan, 69, 44 (2017). Cited 14
times.
[92] “Imaging of diffuse H I absorption structure in
the SSA22 protocluster region at z = 3.1”
K. Mawatari et al., Monthly Notices of the Royal
Astronomical Society, 467, 3951-3962 (2017).
Cited 2 times.
[93] “Direct evidence for Lyα depletion in the protocluster core”
R. Shimakawa et al., Monthly Notices of
the Royal Astronomical Society, 468, L21-L25(2017). Cited 4 times.
[94] “Similarities and uniqueness of Ly α emittersamong star-forming galaxies at z = 2.5”
R. Shimakawa et al., Monthly Notices of theRoyal Astronomical Society, 468, 1123-1141(2017). Cited 7 times.
[95] “Systematic Survey for [O II], [O III], and HαBlobs at z = 0.1 − 1.5: The Implication for Evolution of Galactic-scale Outflow”
S. Yuma et al., The Astrophysical Journal, 841,
93 (2017). Cited 5 times.
[96] “A New Constraint on Reionization from the Evolution of the Lyα Luminosity Function at z ∼ 6−7
Probed by a Deep Census of z = 7.0 Lyα Emitter
Candidates to 0.3L∗ ”
K. Ota et al., The Astrophysical Journal, 844, 85(2017). Cited 27 times.
[97] “Minor Contribution of Quasars to Ionizing Photon Budget at z ∼ 6: Update on Quasar Luminosity Function at the Faint End with Subaru/Suprime-Cam”

132

M. Onoue et al., The Astrophysical Journal Letters, 847, L15 (2017). Cited 12 times.
[98] “The infrared-dark dust content of high redshift
galaxies”
A. Ferrara et al., Monthly Notices of the Royal
Astronomical Society, 471, 5018-5024 (2017).
Cited 9 times.

[107] “Systematic Identification of LAEs for Visible Exploration and Reionization Research Using Subaru HSC (SILVERRUSH). I. Program strategy
and clustering properties of ∼ 2000 Lyα emittersat z = 6 − 7 over the 0.3 − 0.5 Gpc2 survey area”
M. Ouchi et al., Publications of the Astronomical Society of Japan, 70, S13 (2018). Cited 42
times.

[99] “Demonstrating a New Census of Infrared Galaxies with ALMA (DANCING-ALMA). I. FIR Sizeand Luminosity Relation at z = 0 − 6 Revealedwith 1034 ALMA Sources”
S. Fujimoto et al., The Astrophysical Journal,
850, 83 (2017). Cited 13 times.

[108] “SILVERRUSH. II. First catalogs and propertiesof ∼ 2000 Lyα emitters and blobs at z ∼ 6 − 7identified over the 14 − 21 deg2 sky”
T. Shibuya et al., Publications of the Astronomical Society of Japan, 70, S14 (2018). Cited 24
times.

[100] “Evolution of Galactic Outflows at z ∼ 0 − 2 Revealed with SDSS, DEEP2, and Keck Spectra”
Y. Sugahara et al., The Astrophysical Journal,
850, 51 (2017). Cited 4 times.

[109] “SILVERRUSH. III. Deep optical and nearinfrared spectroscopy for Lyα and UV-nebularlines of bright Lyα emitters at z = 6 − 7”
T. Shibuya et al., Publications of the Astronomical Society of Japan, 70, S15 (2018). Cited 35
times.

[101] “Deep Submillimeter and Radio Observations in
the SSA22 Field. I. Powering Sources and the Lyα
Escape Fraction of Lyα Blobs”
Y. Ao et al., The Astrophysical Journal, 850, 178(2017). Cited 3 times.
[102] “The Hyper Suprime-Cam SSP Survey: Overview
and survey design”
H. Aihara et al., Publications of the Astronomical Society of Japan, 70, S4 (2018). Cited 108
times.
[103] “First data release of the Hyper Suprime-Cam
Subaru Strategic Program”
H. Aihara et al., Publications of the Astronomical Society of Japan, 70, S8 (2018). Cited 117
times.
[104] “Great Optically Luminous Dropout Research
Using Subaru HSC (GOLDRUSH). I. UV luminosity functions at z ∼ 4 − 7 derived with the halfmillion dropouts on the 100 deg2 sky”
Y. Ono et al., Publications of the Astronomical Society of Japan, 70, S10 (2018). Cited 38
times.
[105] “GOLDRUSH. II. Clustering of galaxies at z ∼4 − 6 revealed with the half-million dropouts overthe 100 deg2 area corresponding to 1 Gpc3 ”
Y. Harikane et al., Publications of the Astronomical Society of Japan, 70, S11 (2018). Cited 32
times.
[106] “GOLDRUSH. III. A systematic search for protoclusters at z ∼ 4 based on the > 100 deg2 area”
J. Toshikawa et al., Publications of the Astronomical Society of Japan, 70, S12 (2018). Cited
12 times.

[110] “SILVERRUSH. IV. Lyα luminosity functions at
z = 5.7 and 6.6 studied with ∼ 1300 Lyα emitterson the 14 − 21 deg2 sky”
A. Konno et al., Publications of the Astronomical Society of Japan, 70, S16 (2018). Cited 32
times.
[111] “A 16 deg2 survey of emission-line galaxies atz < 1.5 in HSC-SSP Public Data Release 1”
M. Hayashi et al., Publications of the Astronomical Society of Japan, 70, S17 (2018). Cited 4times.
[112] “Enhancement of galaxy overdensity aroundquasar pairs at z < 3.6 based on the Hyper
Suprime-Cam Subaru Strategic Program Survey”
M. Onoue et al., Publications of the Astronomical Society of Japan, 70, S31 (2018). Cited 5
times.
[113] “Luminous quasars do not live in the most overdense regions of galaxies at z ∼ 4”
H. Uchiyama et al., Publications of the Astronomical Society of Japan, 70, S32 (2018). Cited
8 times.
[114] “Clustering of quasars in a wide luminosity range
at redshift 4 with Subaru Hyper Suprime-Cam
Wide-field imaging”
W. He et al., Publications of the Astronomical
Society of Japan, 70, S33 (2018). Cited 6 times.
[115] “The quasar luminosity function at redshift 4
with the Hyper Suprime-Cam Wide Survey”
M. Akiyama et al., Publications of the Astronomical Society of Japan, 70, S34 (2018). Cited 15
times.

133[116] “Subaru High-z Exploration of Low-Luminosity
Quasars (SHELLQs). II. Discovery of 32 quasars
and luminous galaxies at 5.7 < z < 6.8”
Y. Matsuoka et al., Publications of the Astronomical Society of Japan, 70, S35 (2018). Cited
21 times.
[117] “The stellar mass, star formation rate and dark
matter halo properties of LAEs at z ∼ 2”
H. Kusakabe et al., Publications of the Astronomical Society of Japan, 70, 4 (2018). Cited 4times.
[118] “ALMA 26 arcmin2 Survey of GOODS-S at Onemillimeter (ASAGAO): X-Ray AGN Propertiesof Millimeter-selected Galaxies”
Y. Ueda et al., The Astrophysical Journal, 853,
24 (2018). Cited 8 times.
[119] “Full-data Results of Hubble Frontier Fields: UVLuminosity Functions at z ∼ 6 − 10 and a Consistent Picture of Cosmic Reionization”
M. Ishigaki et al., The Astrophysical Journal,
854, 73 (2018). Cited 49 times.
[120] “Size-Luminosity Relations and UV Luminosity
Functions at z = 6 − 9 Simultaneously Derived
from the Complete Hubble Frontier Fields Data”
R. Kawamata et al., The Astrophysical Journal,
855, 4 (2018). Cited 18 times.
Papers in Conference Proceedings[121] “Spectroscopic confirmation of three z-dropout
galaxies at z = 6.844 − 7.213: Demographics of
Lyα emission in z ∼ 7 galaxies”
Y. Ono, AIP Conference Proceedings, 1480,
277-280 (2012).
[122] “The Resolved Kennicutt-Schmidt Law in Nearby
Galaxies”
R. Momose et al., Molecular Gas, Dust, and Star
Formation in Galaxies, 292, 335-335 (2013).
[123] “Early Galaxies and Reionization: Implications
from Recent Observations”
M. Ouchi, The First Billion Years of Galaxies andBlack Holes, 3 (2014).
[124] “TMT Synergies with Subaru HSC+PFS andOther Facilities”
M. Ouchi, Thirty Meter Telescope Science Forum,
62 (2014).
[125] “Evolution and Feedback Effects of High-z StarForming Galaxies”
M. Ouchi, IAU General Assembly, 29, 2254171(2015).
[126] “Statistical Study of Stellar/AGN Feedback Evolution at High Redshifts”

S. Yuma et al., IAU General Assembly, 29,2254547 (2015).
[127] “Resolving the Extragalactic Background Lightwith Multi-field Deep ALMA Data”
S. Fujimoto et al., Astronomical Society of the
Pacific Conference Series, 499, 21 (2015).
[128] “Evolution and Feedback Effects of High-z StarForming Galaxies”
M. Ouchi, IAU Focus Meeting, 29B, 228-228(2016).
[129] “Redshift Evolution of Galactic Outflows Revealed with Spectra of Large Surveys”
Y. Sugahara, The Galaxy Ecosystem: Flow of
Baryons through Galaxies, 23 (2017).

134

THEORY GROUP
Introduction

Research Highlights

The research activities of the theory group cover abroad range of particle physics and cosmology. Research highlights of the theory group after 2012 are;
• A full two-loop calculation of the mass splitting of
the neutral and the charged winos in [164].
• The dark matter detection rates taking into account the final state ionization/excitation of theatoms caused by the nucleus recoil by dark matter(the Migdal effect) [19].
• Spontaneous suppression of the tensor-to-scalarratio r of the minimal chaotic inflation in the presence of a scalar field which takes a large expectation value during inflation [74].
• Improved analysis of effects of long-lived massive
particles decaying during the BBN epoch on theprimordial abundances of light elements [16].
• Field-theoretic lattice simulations of the axion abundance produced by the string and thedomain-wall network [93].
• Primordial black hole formation in a double inflation model and axion curvaton models [42, 34, 31,
14, 10, 9, 198].

Research Activities: Particle Phenomenology

The Standard Model of particle physics has been
confirmed by the discovery of the Higgs boson at the
LHC experiments in 2012. Particle physics has entereda new era for discovering new physics beyond the Standard Model.

15

0.5
0.4
0.3
0.2

τ [ns]10c τ [cm]

The primary interests of the theory group are particle physics phenomenology and particle cosmology.
The collaborative effort in cosmology and particle
physics provides synergy effects in understanding thefundamental law of the Universe.
The theory group has been led by Kawasaki and
Ibe. After 2012, the group hosted twelve PDs. The
research led to the publications of 194 papers in refereed journals, and 5 papers under referee. The groupalso conducted the education in the graduate school
in collaboration with the Department of Physics. The
group accepted twenty-seven graduate-school students
after 2012, and ten of them have acquired Ph.D.

5
0.1
two-loop
one-loop

0
100

150
200
mchargino [GeV]

0250

Fig. 1. The lifetime of charged wino evaluated by using δ m at
the one-loop (green band) and two-loop (red band). The dark
shaded region is excluded by the ATLAS collaboration at 95%CL√( s = 7 TeV, L = 4.7 fb−1 ).

Supersymmetry
For decades, supersymmetry has been widely studied as one of the top candidates for physics beyond
the Standard Model which allows a large separation of
low energy scales from high energy scales such as the
Planck scale. The precise unification of the gauge coupling constants at the scale of the grand unified theory(GUT) also strongly supports the minimal supersymmetric standard model (MSSM). The supersymmetricstandard model is entirely consistent with the featureof the elementary scalar field of the discovered Higgs
boson.
The pure gravity mediation model proposed by
M. Ibe in a collaboration with T. Yanagida is oneof the most attractive models which explains the observed Higgs boson mass, i.e. 125 GeV. We study various aspects of the pure gravity mediation model (seee.g. [186, 155, 148, 145, 139, 130, 125, 97, 92, 76]).
In particular, we performed a full two-loop calculation of the mass splitting of the neutral and the charged
winos in [164]. The wino (the superpartner of the
weak gauge boson) is the lightest supersymmetric particle (LSP) in the pure gravity mediation model, andthe neutral wino is a good candidate for dark matter.
Due to a tiny mass difference, the charged wino has along lifetime and can be searched for the disappearing
charged tracks inside the detectors at the LHC experiments. Our two-loop calculation shows that the splitting is reduced by a few MeV compared to the one-loop
calculation, which leads to about a 30% longer lifetimeof the charged wino (Fig. 1). Our results are used inanalyses of the wino searches by the ATLAS/CMS experiments.
In the theory group, we also studied various models

135of the supersymmetric Standard Model which address
phenomenological and cosmological aspects such as thegravitino dark matter, the muon anomalous magnetic
moments [194, 193, 175, 160, 158, 152, 150, 149, 124,
65, 62, 39, 37].
Strong CP problem
The strong CP problem is longstanding and probably one of the most puzzling issues of the Standard
Model. Although the Peccei-Quinn (PQ) mechanism
provides a successful solution to the problem, it is
not very satisfactory from various theoretical pointsof view, as it relies on a global PQ U(1) symmetry.
An even tiny explicit breaking of the PQ symmetry
spoils the PQ mechanism, while it is conceived thatany global symmetries are broken by quantum gravitational effects.
In theory group, we are intensively pursuing theoretical framework which leads to the global PQ symmetry as an accidental symmetry of gauge symmetries [27, 5, 142, 77, 22]. As the gauge symmetries are
not broken by quantum gravity effects, such construction makes the PQ mechanism more plausible. For example, we propose a general prescription to achieve theglobal PQ symmetry which originates from a gauged
U(1) symmetry, U(1)gPQ [27]. This mechanism is a generalization of the mechanisms which achieve the PQ
symmetry as an accidental symmetry resulting from(discrete) gauge symmetries in the literature.
We also proposed a model of axion where the PQ
breaking scale is in the TeV range [82, 29]. With sucha low scale PQ breaking, the PQ breaking by quantum
gravitational effects do not spoil the PQ mechanism.
By introducing a mirror copied sector of the Standard
Model with the larger mass scales, the QCD axion ismade heavy so that the axion evades experimental constraints. The Peccei-Quinn mechanism still solves thestrong CP-problem due to spontaneously broken Z2 exchanging symmetry between the Standard Model andits mirror copy. Due to its rather low mass scale, thismodel can be tested in collider experiments.

depend on the initial condition. As the annihilation
cross section of dark matter cannot exceed the so-called
unitarity limit, the mass of thermal relic dark matteris constrained to be less than a few hundreds TeV. In
Ref. [43], we challenged the unitarity limit and proposed a model of thermal relic dark matter where theannihilation cross section effectively exceeds the unitarity limit. As a result, we showed that thermal relic dark
matter can be in the PeV range, which, for example, allows a new interpretation of the flux of extraterrestrial
neutrinos in the PeV range observed by the IceCube
experiment.
The direct detection experiment of dark matter isobviously the most important channel to identify its nature, where it is detected by searching for its scatteringwith the atomic nuclei. In Ref. [19], we provide a coherent formalism of the dark matter detection rates whichtakes into account the final state ionization/excitationof the atoms caused by the nucleus recoil by dark matter (the Migdal effect). We found that the final stateionization/excitation can enhance the detectability ofrather light dark matter in the GeV mass range via thenuclear scattering (Fig. 2).

Dark Matter
The existence of dark matter has been establishedby a wide range of cosmological and astrophysical observations. Nevertheless, its nature remains elusive.
Identification of the nature of DM is one of the mostimportant challenges of modern particle physics. In
theory group, we have proposed many new models ofdark matter which are interrelated to physics beyond
the Standard Model. We are also studying how to identify dark matter experimentally through direct/indirect
detection experiments.
Among various candidates for dark matter, thermal
relic dark matter is one of the most attractive candidates. The beauty of thermal relic dark matter is thatthe resultant density is determined by the size of theannihilation cross section of dark matter and does not

Collider Phenomenology
Collider phenomenology has been providing many
important data on the standard model (SM) andphysics beyond the SM. In ICRR, theory group is also
studying physics beyond the SM by utilizing new datafrom the collider experiments [71, 52, 50, 41, 33].
The search for high-mass diphoton resonance is ongoing at the LHC, as it provides a definite evidence ofphysics beyond the Standard Model. Along with thediphoton resonance, many theoretical models also predict resonances decaying not into a pair of photons but
into a pair of highly collimated photon-jets. In Ref. [41],we studied how well we can distinguish such diphoton
resonance imposters, i.e. diphoton-jet resonances, from
diphoton resonances by examining detector responses
to the photon-jets. We find that the sum of pT of theFig. 2. The event rates at the single-phase experiments with liquid
Xe target for 2 GeV dark matter. The green, blue, and pink lines
show the ionization rates from n = 3, 4, and 5, respectively. Here,
we do not take the energy resolution into account.

136

Fig. 3. Predictions on the spectral index ns and the tensor-to-scalarratio r of the minimal chaotic inflation model with spontaneous
suppression. The solid lines show the predictions when we change
the model parameters of the additional scalar field.

first e+ + e− pair from the photon conversion provides
strong discrimination power. For example, it is possible to discriminate the diphoton-jet resonance from thediphoton resonance at around the 5σ level with an integrated luminosity of L = 100 fb−1 for a signal crosssection after the cut of 1 fb and ms = 750 GeV.
Higgs Properties
The discovery of the Higgs boson and precision measurement of the top quark mass at the LHC and Tevatron now allow us to study the detailed shape of the
Higgs potential by extrapolating the Higgs coupling
constants according to the renormalization group running. In fact, it has been shown that the electroweak
vacuum is not absolutely stable if the Standard Modelis valid up to O(1010 ) GeV or higher. The lifetime ofthe electroweak vacuum has been one of the important
topics in particle physics and cosmology.
In Ref. [26, 25, 18, 4], we performed a precise calculation of the decay rate of the electroweak vacuum in
the Standard Model using a gauge invariant calculationof the decay rate at the one-loop level. The decay rateof the electroweak vacuum in the standard model is+184 +144 +2
estimated to be log γ ×Gyr Gpc3 = −582+40−45 −329 −218 −1 ,where the 1st, 2nd, 3rd, and 4th errors are due to theuncertainties of the Higgs mass, the top quark mass, thestrong coupling constant and the choice of the renormalization scale, respectively.
We also studied high energy behaviors of the Higgs
potential in conjunction to physics beyond the Standard Mode and implications on cosmology [129, 109,
100].
Inflation Model Building
Cosmic inflation provides a compelling explanation
for why our universe is so huge, which is now considered
as an important pillar of big bang cosmology. However,

it is not known how inflation occurred and how the
Universe was reheated after inflation. In the theory
group, we are extensively studying models of cosmic
inflation as well as the thermal history of the Universe
after inflation including the origin of the asymmetry of
the matter-anti-matter in the Universe [168, 132, 122,
119, 115, 105, 103, 99, 85, 74].
Among slow-roll inflation models, chaotic inflationis the most attractive model, since it is free from theinitial condition problem. The recent observations of
the cosmic microwave background, however, disfavors
the chaotic inflation model with a quadratic scalar potential. In Ref. [74], we pointed out that the predictionof the minimal chaotic inflation model can be altered if
a scalar field other than the inflaton field takes a large
field value close to the Planck scale during inflation.
With such effects, we find that the minimal chaotic inflation model, especially the model with a quadratic
potential, is still consistent with recent observations of
the cosmic microwave background fluctuation without
modifying the inflation model itself (Fig. 3).

Research Activities: Astroparticle Physics

The progress in particle physics beyond the Standard Model leads us to more insight into the fundamental aspects of the early universe. At the same time,cosmology and astrophysics are used to test new theories in particle physics. Such particle cosmology is oneof main subjects of the theory group.
Big Bang Nucleosynthesis
The predicted abundances of the light elements in
Big Bang Nucleosynthesis (BBN) are sensitive to thecosmological scenario of the early universe. Exotic cosmological scenarios based on physics beyond the standard model would modify the light-element abundances
too much to be consistent with the observations. Thus,
the BBN provides significant constraints on the newparticles which change the cosmological evolution at
the cosmic time t ∼ 10−2 − 1012 sec. In 2005 we derived
general constraints on the relic abundances of longlived particles with hadronic decay modes as well asradiative decay modes.1 Since no reliable BBN constraints on unstable particles which decay into hadron
had not been obtained, our work attracted much attention. The paper gets more than 700 citations as ofthis writing. Since our previous analysis, we have been
updating constrains and applying to decay and annihilation processes of exotic particles at the BBN epoch.
In Ref. [16] we investigated effects of long-lived massive particles, which decay during the BBN epoch, on
the primordial abundances of light elements by improving the previous studies. Improvement includes upadate of the reaction rates of the standard BBN reactions, use of the most recent observational data of light*1 Phys. Rev. D 71, 083502 (2005) by M. Kawasaki∗ , K. Kohri and
T. Moroi

137

Fig. 4. Constraints on the abundance mX YX and lifetime τX of longlived particles which mainly decay into bb̄. Here mX (= Mbb̄ ) is themass of the decaying particle and YX is the ratio of the decaying
particle number density to entropy density. The black, red, green,
blue and magenta solid lines denote the BBN constraints for
mX = 0.03, 0.1, 1, 10, 100 and 1000 TeV, respectively. Theorange shaded regions are excluded by the constraint from the
CMB spectral distortion.

element abundances, effects of the interconversion ofenergetic nucleons at the time of inelastic scatterings
with background nuclei, and effects of the hadronic
shower induced by high energy anti-nucleons. Using
the improved calculation we derived upper bounds onrelic abundance of the decaying particle as a functionof its lifetime (Fig. 4). We also applied our analysis tounstable gravitino, the superpartner of the graviton insupersymmetric theories, and obtained constraints on
the reheating temperature after inflation. It was foundthat the upper bound on the reheating temperature TR
is 105 − 106 GeV for relatively light gravitino mass i.e.,
for m3/2 . a few TeV. In Ref. [67] we studied the effects
of dark-matter annihilation during the BBN epoch on
the BBN and derived upper bounds on the dark-matterpair-annihilation cross section.
The BBN also gives constraints on small-scaledensity perturbations. We derived the constraints
on the adiabatic density perturbations with scale ∼104−5 Mpc−1 [46] and baryonic isocurvature perturbations with scale & 4 × 108 Mpc−1 [196].
Axion Cosmology
The axion is a pseudo Nambu-Goldstone boson of
the PQ symmetry, which is introduced to the standard
model as a solution to the Strong CP problem. Theaxion has rich implications for astrophysics and cosmology.
When the spontaneous breaking of the PQ U(1) occurs after inflation, a cosmological network of axionicstrings is formed. The produced axionic strings lose

their energy by emitting axions and follow the so-called
scaling solution. How much the emitted axions contribute to the present dark matter density crucially depends on their energy spectrum. We estimated the energy spectrum precisely by using field-theoretic latticesimulations in a box of 5123 grids and found that the
spectrum has a sharp peak at the horizon scale. From
our spectrum we obtained constraint on the axion decay
constant Fa < 2 × 1011 GeV [93]. Recently, in Ref. [1]we upgraded our simulation in our previous study interms of the number of grids (40963 ) as well as thesuite of analysis methods. The improved simulation revealed that global strings do not evolve according to
the scaling solution but its scaling parameter, or thenumber of long strings per horizon, increases logarithmically in time. We also computed the spectrum of theaxion radiated from the axion strings. The result wasin agreement with the previous studies. Our results indicates the abundance of axion can be enhanced by afactor of two or three.
Axion models also predict a formation of domain
walls, when the temperature of the universe becomescomparable to ∼ GeV. At that time, NDW domain walls
are attached to strings, where NDW is an integer number
whose value depends on axion models. The subsequent
history of the universe is different between models with
NDW = 1 and NDW > 1.
In Ref. [93],2 we studied the scenario with NDW = 1
where domain walls are bounded by strings and they
immediately collapse due to the tension of walls. We
investigated the formation and annihilation of stringwall networks by performing field-theoretic lattice simulations. We analyzed the spectrum of axions radiatedfrom collapse of domain walls and found that axions
produced by the decay of domain walls give a significant contribution to the cold dark matter abundance.
Together with contributions from coherent oscillationand emission from strings, axions can account for all
dark matter of the universe for Fa ' (4 − 7) × 1010 GeV(Fig. 5).
On the other hand, for the models with NDW > 1 thestring-wall networks become long-lived, which leads toa serious problem in the standard cosmology. The problem can be avoided by introducing the explicit PQ symmetry breaking term, which leads to the annihilation ofstring-wall networks at a late time. In Ref. [187, 93],
we investigated the evolution of such long-lived domain
walls and estimated their decay rate. We found that if
we allow the mild tuning on the model parameters theaxion can be dark matter for Fa = O(108 − 1010 ) GeV(Fig. 5).
When the PQ symmetry is broken during inflation,
the axionic strings and domain walls are diluted away
by inflation. However, in this case the axion field acquires fluctuations with amplitude given by the Hubble*2 See also Phys. Rev. D 85, 105020 (2012) by T. Hiramatsu,
M. Kawasaki∗ , K. Saikawa∗ and T. Sekiguchi.

138
8

9

10

10

10

-2

10

11

10

-3

10

10

-4

10

10

10

12

-5

Fig. 5. The schematics of the parameter ranges where the axion
becomes cold dark matter for the models with NDW = 1 (orange
interval) and NDW = 6 (light blue interval). The blue intervals
represent the allowed region for δ = 10−8 , 10−5 , and 10−3 (δ :phase of the explicit U(1)PQ breaking term). The vertical dottedline corresponds to the bound from the observation of SN1987A.
Red and green intervals represent the regions that will be coveredby the axion search experiments, IAXO and ADMX, respectively

parameter during inflation, which leads to isocurvaturedensity perturbations. Since the isocurvature perturbations are stringently constrained by observations of thecosmic microwave background (CMB), the Hubble parameter during inflation should be small. This problem
is ameliorated if the PQ field has a larger expectation
value during inflation. However, in this case the PQ
field starts to oscillate after inflation and induces large
field fluctuations through parametric resonance, which
restores the PQ symmetry and leads to formation ofstrings and domain walls. In Ref. [146] we performed
lattice simulations and obtained the condition for the
PQ symmetry not to be restored. Furthermore we proposed models where the parametric resonance is ineffective and hence domain walls are not formed [72, 13].
We pointed out that the allowed region of the decay
constant Fa is reduced to a rather narrow region for agiven tensor-to-scalar ration r. Thus, if r is determinedin the near future we can predict the axion decay constant Fa [7].
Baryogenesis and Q-ball
Baryogenesis is one of the main issues in the theories of the early universe. Among many baryogenesis
scenarios proposed so far, the mechanism proposed by
Affleck and Dine is a promising candidate since it canbe realized in the SUSY standard model. It is known
that the Affleck-Dine mechanism is complicated by theformation of Q-balls which is a non-topological solution
in the SUSY standard model.
In Ref. [116] we investigated the Affleck-Dine baryogenesis in high-scale inflation models. High-scale inflation is observationally favored because gravitational

waves produced during inflation can be detected in future. We found that successful baryogenesis requires
too low reheating temperature to produce dark matterparticles thermally. Using the non-thermal dark matter production mechanism [127] and Q-balls we constructed consistent scenarios to account for the observed baryon and dark matter densities. In Ref. [20]we studied accidental suppression of the baryon asymmetry due to oscillation of the Affleck-Dine field after
inflation.
In Ref. [179] we reexamined the decay rate of a
Q-ball. We applied the results to a realistic modelin the gauge-mediated supersymmetry breaking andfound the branching ratio of the Q-ball decay into thegravitino much smaller than the one estimated in theprevious analysis. Based on this analysis, we reinvestigate the cogenesis scenario that the abundances of thebaryons and the gravitino dark matter is naturally explained by the decay of the Q-balls, and found that the
scenario works well [170]. Furthermore we revisited thescenario of baryon and dark matter cogenesis from Qballs in gravity mediation in respect of the improved
Q-ball decay rates, and found that the successful cogenesis takes place [166, 110].
If a Q-ball is stable, it can account for dark matter ofthe universe. We studied Q-ball dark matter in gaugemediated supersymmetry breaking. It was found that
the Q-ball is a good dark matter candidate and dark
matter Q-balls are detectable by IceCube-like experiments in the future [101, 91].
It is possible that Q-alls are electrically charged.
Such Q-balls are called gauged Q-balls. In Refs. [79,47, 28, 23] we studied the evolution of Q-balls withboth baryon and lepton numbers and found that theycan evolve into gauged Q-balls by emitting charged leptons. Those Q-balls are stable and can be charged dark
matter of the universe.
Formation of Primordial Black HolesIn September 2015, for the first time in history,
gravitational waves from the coalescence of binary
black holes were detected by the Laser Interferometer
Gravitational-Wave Observatory (LIGO). This event,
GW150914, comes from the merger of black holes(BHs) with ∼ 30M . Afterward, more gravitational
wave events caused by the merger of BHs with similar masses were reported. One of the promising candidates for BHs causing the LIGO GW events is primordial black holes (PBHs). PBHs are considered asa good candidate for dark matter of the universe. Although various observations give stringent constraints
on the DM PBH mass, there remains a window between
10−13 M − 10−10 M . Therefore, we studied formationof PBHs which account for the LIGO events and/or
dark matter.
In Refs. [42, 34, 31, 14, 10] we studied PBH formation in a double inflation model which has two stages ofinflation, pre-inflation and new inflation. The new in-

139

Fig. 6. PBH mass spectra for appropriate sets of model parameters(blue solid and brown lines). The red shaded regions show theobservational constraints on the monochromatic mass function.
The orange shaded regions are observational constraints which
we neglect due to uncertainties.

flation can yield small scale perturbations large enoughfor PBH formation, while the pre-inflation accounts forthe scalar perturbations at the large scale observed by
Planck. We found that the large density perturbations
necessary for the LIGO PBHs produce secondary gravitational waves on which the pulsar timing array experiments put severe constraints. To avoid the stringent
constraints the power spectrum has a sharp peak at thescale k ∼ 106 Mpc−1 . We showed that such a peak canbe realized in the double inflation model[34]. We also
showed that the double inflation model produce PBHs
with mass function peak at ∼ 10−13 M which accountfor all dark matter without conflicting with observational constraints [31]. Furthermore, in Ref. [10] weshowed that the double inflation can realize a power
spectrum with two peaks and hence can simultaneously
produce PBHs for all DM in a broad mass spectrum
around O(10−13 )M and PBHs for the LIGO events ina sharp mass spectrum at O(10)M (Fig. 6). We also
showed that the energy density of the secondary gravitational waves is large enough to be detected by future
detectors.
In Ref. [9] we investigated the axion curvaton modelfor PBH formation. The model realizes the highly bluetilted power spectrum of primordial curvature perturbations, which leads to large perturbation amplitude atsmall scales. We showed that this model can produce
PBHs for the LIGO events and the stringent constraints
from pulsar timing array experiments can be avoided
owing to non-Gaussianity. We also built another axion
curvaton model [198].
In Refs. [11, 195] we proposed a novel PBH formation scenario based on the Affleck-Dine mechanism forbaryogenesis. In this scenario, O(10)M PBHs and Qballs are formed from high baryon-density bubbles produced by the Affleck-Dine mechanism. The former account for the LIGO events and the latter explain thedark matter of the universe.

Fig. 7. The induced gravitational wave spectra for the same parameter set as Fig. 6. Green shaded region is excluded by thepulsar timing array experiments. The black dotted lines show
the prospects of the future experiments; SKA, eLISA, LISA and
DECIGO.

Members
StaffsMasahiro Kawasaki, Professor, 2004–present
Masahiro Ibe, Assoc. Professor, 2011–present
Postdoctoral Fellows
Motohiko Kusakabe, 2009–2012
Shuichiro Yokoyama, 2012–2014
Shohei Sugiyama, 2011–2013
Daisuke Yamauchi, 2011–2013
Etsuko Kawakami, 2014-2015
Kunio Kaneta, 2014-2015
Keisuke Harigaya, 2015-2015
Ryo Saito, 2015-2015
Yoshihiko Oyama, 2015-2017
Yutaro Shoj, 2016-2018
Kohei Hayashi, 2018-present
Ippei Obata, 2018-present
Graduate Students
Ten students were awarded doctor degrees and fifteen
students earned master degrees during 2012–2018, supervised by ICRR staff members.

140[10] Double inflation as a single origin of primordial
black holes for all dark matter and LIGO observa(The symbol ∗ indicates the authors belonging to ICRR.)
tions
Seventeen papers, [52] [155], [164], [187] [190],
K. Inomata∗ , M. Kawasaki∗ , K. Mukaida and[186], are cited more than fifty times. One papers,
T. T. Yanagida[93], [161] are cited more than a hundred times.
Phys. Rev. D 97, no. 4, 043514 (2018)
Papers in Refereed Journals (after April 2012)[arXiv:1711.06129 [astro-ph.CO]].

List of Publications[1] Long-term dynamics of cosmological axion strings
M. Kawasaki, T. Sekiguchi, M. Yamaguchi and
J. Yokoyama, PTEP (2018) 2018, no. 9, 091E01(2018) [arXiv:1806.05566 [hep-ph]].
[2] B − L as a Gauged Peccei-Quinn Symmetry
M. Ibe∗ , M. Suzuki and T. T. Yanagida
JHEP 1808, 049 (2018) [arXiv:1805.10029 [hepph]].
[3] Exploring compensated isocurvature perturbations with CMB spectral distortion anisotropies
T. Haga, K. Inomata∗ , A. Ota and A. Ravenni
JCAP 1808, no. 08, 036 (2018) [arXiv:1805.08773[astro-ph.CO]].
[4] Decay Rate of Electroweak Vacuum in the Standard Model and Beyond
S. Chigusa, T. Moroi and Y. Shoji∗Phys. Rev. D 97, no. 11, 116012 (2018)[arXiv:1803.03902 [hep-ph]].
[5] Gauged Peccei-Quinn symmetry – A case of simultaneous breaking of SUSY and PQ symmetry
H. Fukuda, M. Ibe∗ , M. Suzuki∗ and
T. T. Yanagida
JHEP 1807, 128 (2018) [arXiv:1803.00759[hep-ph]].
[6] Primordial black holes and uncertainties in the
choice of the window function
K. Ando∗ , K. Inomata∗ and M. Kawasak∗ iPhys. Rev. D 97, no. 10, 103528 (2018)[arXiv:1802.06393 [astro-ph.CO]].
[7] Cosmologically allowed regions for the axion decay
constant Fa
M. Kawasaki∗ , E. Sonomoto∗ and T. T. Yanagida
Phys. Lett. B 782, 181 (2018) [arXiv:1801.07409[hep-ph]].
[8] Oscillons from Pure Natural Inflation
J. P. Hong∗ , M. Kawasaki∗ and M. Yamazaki
Phys. Rev. D 98, no. 4, 043531 (2018)[arXiv:1711.10496 [astro-ph.CO]].
[9] Primordial black holes for the LIGO events in theaxionlike curvaton model
K. Ando∗ , K. Inomata∗ , M. Kawasaki∗ ,K. Mukaida and T. T. Yanagida, Phys. Rev.
D 97, no. 12, 123512 (2018) [arXiv:1711.08956[astro-ph.CO]].

[11] Cogenesis of LIGO Primordial Black Holes andDark Matter
F. Hasegawa∗ and M. Kawasaki∗
Phys. Rev. D 98, no. 4, 043514 (2018)[arXiv:1711.00990 [astro-ph.CO]].
[12] Inflaton fragmentation in E-models of cosmological α -attractors’ F. Hasegawa∗ and J. P. Hong∗
Phys. Rev. D 97, no. 8, 083514 (2018)[arXiv:1710.07487 [astro-ph.CO]].
[13] Domain wall and isocurvature perturbation problems in a supersymmetric axion model
M. Kawasaki∗ and E. Sonomoto∗Phys. Rev. D 97, no. 8, 083507 (2018)[arXiv:1710.07269 [hep-ph]].
[14] O(10)M primordial black holes and string axion
dark matter
K. Inomata∗ , M. Kawasaki∗ , K. Mukaida,
Y. Tada∗ and T. T. Yanagida
Phys.
Rev.
D
96,
no.
12,
123527(2017)
doi:10.1103/PhysRevD.96.123527[arXiv:1709.07865 [astro-ph.CO]].
[15] Gravitino problem in inflation driven by inflatonpolonyi Kähler coupling
F. Hasegawa∗ , K. Nakayama, T. Terada andY. Yamada
Phys.
Lett.
B
777,
270
(2018)
doi:10.1016/j.physletb.2017.12.038[arXiv:1709.01246 [hep-ph]].
[16] Revisiting Big-Bang Nucleosynthesis Constraints
on Long-Lived Decaying Particles
M. Kawasaki∗ , K. Kohri, T. Moroi and Y. Takaesu
Phys. Rev. D 97, no. 2, 023502 (2018)[arXiv:1709.01211 [hep-ph]].
[17] Adiabatic suppression of the axion abundance andisocurvature due to coupling to hidden monopoles
M. Kawasaki∗ , F. Takahashi and M. Yamada
JHEP 1801, 053 (2018) [arXiv:1708.06047 [hepph]].
[18] State-of-the-Art Calculation of the Decay Rate ofElectroweak Vacuum in the Standard Model
S. Chigusa, T. Moroi and Y. Shoji
Phys. Rev. Lett. 119, no. 21, 211801 (2017)[arXiv:1707.09301 [hep-ph]].

141[19] Migdal Effect in Dark Matter Direct Detection Experiments
M. Ibe∗ , W. Nakano∗ , Y. Shoji∗ and K. Suzuki∗
JHEP 1803, 194 (2018) [arXiv:1707.07258 [hepph]].

H. Fukuda, M. Ibe∗ and T. T. Yanagida
Phys. Rev. D 95, no. 9, 095017[arXiv:1702.00227 [hep-ph]].

(2017)[20] Oscillating Affleck-Dine condensate and its cosmological implications
F. Hasegawa∗ and M. Kawasaki∗
Phys. Rev. D 96, no. 6, 063518 (2017)[arXiv:1706.08659 [hep-ph]].

[30] Gravitino Problem in Minimal Supergravity
Inflation
F. Hasegawa∗ , K. Mukaida, K. Nakayama, T. Terada and Y. Yamada
Phys.
Lett.
B
767,
392
(2017)
doi:10.1016/j.physletb.2017.02.030[arXiv:1701.03106 [hep-ph]].

[21] Foreground effect on the J-factor estimation ofultra-faint dwarf spheroidal galaxies
K. Ichikawa, S. i. Horigome, M. N. Ishigaki,
S. Matsumoto, M. Ibe∗ , H. Sugai and K. Hayashi
Mon. Not. Roy. Astron. Soc. 479, no. 1, 64 (2018)[arXiv:1706.05481 [astro-ph.GA]].

[31] Inflationary Primordial Black Holes as All Dark
MatterK. Inomata∗ , M. Kawasaki∗ , K. Mukaida, Y. Tada
and T. T. Yanagida
Phys. Rev. D 96, no. 4, 043504 (2017)[arXiv:1701.02544 [astro-ph.CO]].

[22] Dynamical Clockwork Axions
R. Coy, M. Frigerio and M. Ibe∗
JHEP 1710, 002 (2017) [arXiv:1706.04529 [hepph]].

[32] Relaxation leptogenesis, isocurvature perturbations, and the cosmic infrared backgroundM. Kawasaki∗ , L. Pearce, L. Yang and A. Kusenko
Phys. Rev. D 95, no. 10, 103006 (2017)[arXiv:1701.02175 [hep-ph]].

[23] Gauged Q-ball Decay Rates into FermionsJ. P. Hong∗ and M. Kawasaki∗
Phys. Rev. D 96, no. 10, 103526 (2017)[arXiv:1706.01651 [hep-ph]].
[24] Does the detection of primordial gravitational
waves exclude low energy inflation?
T. Fujita, R. Namba and Y. Tada∗
Phys. Lett. B 778, 17 (2018) [arXiv:1705.01533[astro-ph.CO]].
[25] False Vacuum Decay in Gauge Theory
M. Endo, T. Moroi, M. M. Nojiri and Y. Shoji∗
JHEP 1711, 074 (2017) [arXiv:1704.03492 [hepph]].
[26] On the Gauge Invariance of the Decay Rate ofFalse Vacuum
M. Endo, T. Moroi, M. M. Nojiri and Y. Shoji∗Phys. Lett. B 771, 281 (2017) [arXiv:1703.09304[hep-ph]].
[27] A ”gauged” U(1) Peccei-Quinn symmetry
H. Fukuda, M. Ibe∗ , M. Suzuki∗ and
T. T. Yanagida
Phys.
Lett.
B
771,
327
(2017)
doi:10.1016/j.physletb.2017.05.071[arXiv:1703.01112 [hep-ph]].
[28] New type of charged Q -ball dark matter in gauge
mediated SUSY breaking models
J. P. Hong∗ and M. Kawasaki∗
Phys. Rev. D 95, no. 12, 123532 (2017)[arXiv:1702.00889 [hep-ph]].
[29] Dark Matter Candidates in a Visible Heavy QCD
Axion Model[33] Constraints on Lµ −Lτ gauge interactions from rarekaon decay
M. Ibe∗ , W. Nakano∗ and M. Suzuki∗
Phys. Rev. D 95, no. 5, 055022 (2017)[arXiv:1611.08460 [hep-ph]].
[34] Inflationary primordial black holes for the LIGO
gravitational wave events and pulsar timing arrayexperiments
K. Inomata∗ , M. Kawasaki∗ , K. Mukaida,
Y. Tada∗ and T. T. Yanagida
Phys. Rev. D 95, no. 12, 123510 (2017)[arXiv:1611.06130 [astro-ph.CO]].
[35] A map of the non-thermal WIMP
H. Kim, J. P. Hong∗ and C. S. Shin
Phys.
Lett.
B
768,
292
doi:10.1016/j.physletb.2017.03.005[arXiv:1611.02287 [hep-ph]].

(2017)[36] Squeezed bispectrum in the δ N formalism: local
observer effect in field space
Y. Tada∗ and V. Vennin
JCAP 1702, no. 02, 021 (2017) [arXiv:1609.08876[astro-ph.CO]].
[37] Revisiting gravitino dark matter in thermal leptogenesis
M. Ibe∗ , M. Suzuki and T. T. Yanagida
JHEP 1702, 063 (2017) [arXiv:1609.06834 [hepph]].
[38] Foreground effect on the J-factor estimation ofclassical dwarf spheroidal galaxies
K. Ichikawa, M. N. Ishigaki, S. Matsumoto,142

M. Ibe∗ , H. Sugai, K. Hayashi and S. i. Horigome
Mon. Not. Roy. Astron. Soc. 468, no. 3, 2884
(2017) [arXiv:1608.01749 [astro-ph.GA]].
[39] Lower limit on the gravitino mass in low-scalegauge mediation with mH ' 125 GeV
M. Ibe∗ and T. T. Yanagida
Phys. Lett. B 764, 260 (2017) [arXiv:1608.01610[hep-ph]].
[40] Elucidating Dark Energy with Future 21 cm Observations at the Epoch of Reionization
K. Kohri, Y. Oyama∗ , T. Sekiguchi and T. Takahashi
JCAP 1702, no. 02, 024 (2017) [arXiv:1608.01601[astro-ph.CO]].
[41] Cracking down on fake photons: Cases of diphoton
resonance imposters
H. Fukuda, M. Ibe, O. Jinnouchi and M. Nojiri
PTEP
2017,
no.
3,
033B05(2017)
doi:10.1093/ptep/ptx019[arXiv:1607.01936[hep-ph]].
[42] Primordial black holes as dark matter in supergravity inflation models
M. Kawasaki∗ , A. Kusenko, Y. Tada∗ and
T. T. Yanagida
Phys. Rev. D 94, no. 8, 083523 (2016)[arXiv:1606.07631 [astro-ph.CO]].
[43] Thermal Relic Dark Matter Beyond the Unitarity
Limit
K. Harigaya, M. Ibe∗ , K. Kaneta, W. Nakano∗ and
M. Suzuki∗
JHEP 1608, 151 (2016) [arXiv:1606.00159 [hepph]].
[44] Simple cosmological solution to the Higgs field instability problem in chaotic inflation and the formation of primordial black holes
M. Kawasaki∗ , K. Mukaida and T. T. Yanagida
Phys. Rev. D 94, no. 6, 063509 (2016)[arXiv:1605.04974 [hep-ph]].
[45] Revisiting constraints on small scale perturbations
from big-bang nucleosynthesis
K. Inomata∗ , M. Kawasaki∗ and Y. Tada∗
Phys. Rev. D 94, no. 4, 043527 (2016)[arXiv:1605.04646 [astro-ph.CO]].
[46] Revisiting constraints on small scale perturbations
from big-bang nucleosynthesis
K. Inomata∗ , M. Kawasaki∗ and Y. Tada∗
Phys. Rev. D 94, no. 4, 043527 (2016)[arXiv:1605.04646 [astro-ph.CO]].
[47] Charged Q-ball Dark Matter from B and L direction
J. P. Hong∗ , M. Kawasaki∗ and M. Yamada∗

JCAP 1608, no. 08, 053 (2016) [arXiv:1604.04352[hep-ph]].
[48] Dark matter annihilation and decay from nonspherical dark halos in galactic dwarf satellites
K. Hayashi, K. Ichikawa, S. Matsumoto, M. Ibe∗ ,M. N. Ishigaki and H. Sugai
Mon. Not. Roy. Astron. Soc. 461, no. 3, 2914
(2016) [arXiv:1603.08046 [astro-ph.GA]].
[49] Cosmology with the Square Kilometre Array by
SKA-Japan
D. Yamauchi et al. including Y. Oyama∗ [SKAJapan Consortium Cosmology Science Working
Group]
Publ. Astron. Soc. Jap. 68, no. 6, R2 (2016)[arXiv:1603.01959 [astro-ph.CO]].
[50] 750 GeV diphoton resonance in a visible heavy
QCD axion model
C. W. Chiang, H. Fukuda, M. Ibe∗ and
T. T. Yanagida
Phys. Rev. D 93, no. 9, 095016 (2016)[arXiv:1602.07909 [hep-ph]].
[51] Why three generations?
M. Ibe∗ , A. Kusenko and T. T. Yanagida
Phys. Lett. B 758, 365 (2016) [arXiv:1602.03003[hep-ph]].
[52] Revisiting Scalar Quark Hidden Sector in Light of
750-GeV Diphoton Resonance
C. W. Chiang, M. Ibe∗ and T. T. Yanagida
JHEP 1605, 084 (2016) [arXiv:1512.08895 [hepph]], 61 citations counted in INSPIRE as of Nov
2018.
[53] CMB Constraint on Dark Matter Annihilation after Planck 2015
M. Kawasaki∗ , K. Nakayama and T. Sekiguchi
Phys. Lett. B 756, 212 (2016) [arXiv:1512.08015[astro-ph.CO]].
[54] High-scale SUSY from an R-invariant New Inflation in the Landscape
M. Kawasaki∗ , M. Yamada∗ , T. T. Yanagida andN. Yokozaki
Phys. Rev. D 93, no. 5, 055022 (2016)[arXiv:1512.04259 [hep-ph]].
[55] Can massive primordial black holes be producedin mild waterfall hybrid inflation?
M. Kawasaki∗ and Y. Tada∗
JCAP 1608, no. 08, 041 (2016) [arXiv:1512.03515[astro-ph.CO]].
[56] Affleck-Dine baryogenesis just after inflation
M. Yamada∗
Phys. Rev. D 93, no. 8, 083516 (2016)[arXiv:1511.05974 [hep-ph]].

143[57] Axino dark matter and baryon number asymmetry
production by the Q-ball decay in gauge mediation
S. Kasuya, E. Kawakami∗ and M. Kawasaki∗
JCAP 1603, no. 03, 011 (2016) [arXiv:1511.05655[hep-ph]].

[67] Revisiting Big-Bang Nucleosynthesis Constraints
on Dark-Matter Annihilation
M. Kawasaki∗ , K. Kohri, T. Moroi and Y. Takaesu
Phys. Lett. B 751, 246 (2015) [arXiv:1509.03665[hep-ph]].

[58] Suppressing the QCD Axion Abundance by Hidden Monopoles
M. Kawasaki∗ , F. Takahashi and M. Yamada∗
Phys. Lett. B 753, 677 (2016) [arXiv:1511.05030[hep-ph]].

[68] Can thermal inflation be consistent with baryogenesis in gauge-mediated SUSY breaking models?
T. Hayakawa∗ , M. Kawasaki∗ and M. Yamada∗
Phys. Rev. D 93, no. 6, 063529 (2016)[arXiv:1508.03409 [hep-ph]].

[59] The two-field regime of natural inflation
A. Achúcarro, V. Atal, M. Kawasaki∗ and F. Takahashi
JCAP 1512, no. 12, 044 (2015) [arXiv:1510.08775[astro-ph.CO]].

[69] Adiabatic Invariance of Oscillons/I-balls
M. Kawasaki∗ , F. Takahashi and N. Takeda∗ ,Phys. Rev. D 92, no. 10, 105024 (2015)[arXiv:1508.01028 [hep-th]].

[60] Affleck-Dine leptogenesis and its backreaction to
inflaton dynamics
M. Yamada∗
Phys. Lett. B 754, 208 (2016) [arXiv:1510.08514[hep-ph]].
[61] ,”Spontaneous Baryogenesis from Asymmetric Inflaton
F. Takahashi and M. Yamada∗
Phys. Lett. B 756, 216 (2016) [arXiv:1510.07822[hep-ph]].
[62] ATLAS on-Z Excess via gluino-Higgsino-singlino
decay chains in the NMSSM
K. Harigaya, M. Ibe and T. Kitahara
JHEP 1601, 030 (2016) [arXiv:1510.07691 [hepph]].
[63] Cosmologically safe QCD axion without finetuning
M. Yamada, T. T. Yanagida and K. Yonekura
Phys. Rev. Lett. 116, no. 5, 051801 (2016)[arXiv:1510.06504 [hep-ph]].
[64] Cosmological problems of the string axion alleviated by high scale SUSY of m3/2 '10-100TeV
M. Kawasaki∗ , T. T. Yanagida and N. Yokozaki
Phys. Lett. B 753, 389 (2016) [arXiv:1510.04171[hep-ph]].
[65] Revisiting R-invariant Direct Gauge Mediation
C. W. Chiang, K. Harigaya∗ , M. Ibe∗ and
T. T. Yanagida
JHEP 1603, 145 (2016) [arXiv:1510.04047 [hepph]].
[66] Constraints on the neutrino parameters by futurecosmological 21 cm line and precise CMB polarization observations
Y. Oyama∗ , K. Kohri and M. Hazumi
JCAP 1602, no. 02, 008 (2016) [arXiv:1510.03806[astro-ph.CO]].

[70] Strongly broken Peccei-Quinn symmetry in theearly Universe
F. Takahashi and M. Yamada∗
JCAP 1510, no. 10, 010 (2015) [arXiv:1507.06387[hep-ph]].
[71] ,”Diboson Resonance as a Portal to Hidden Strong
Dynamics
C. W. Chiang, H. Fukuda, K. Harigaya∗ , M. Ibe∗and T. T. Yanagida
JHEP 1511, 015 (2015) [arXiv:1507.02483 [hepph]].
[72] Dynamics of Peccei-Quinn Breaking Field after Inflation and Axion Isocurvature Perturbations
K. Harigaya∗ , M. Ibe∗ , M. Kawasaki∗ and
T. T. Yanagida
JCAP 1511, no. 11, 003 (2015) [arXiv:1507.00119[hep-ph]].
[73] Thermalization Process after Inflation and Effective Potential of Scalar Field
K. Mukaida and M. Yamada∗
JCAP 1602, no. 02, 003 (2016) [arXiv:1506.07661[hep-ph]].
[74] Revisiting the Minimal Chaotic Inflation Model
K. Harigaya∗ , M. Ibe∗ , M. Kawasaki∗ and
T. T. Yanagida
Phys. Lett. B 756, 113 (2016) [arXiv:1506.05250[hep-ph]].
[75] Cosmologically safe QCD axion as a present fromextra dimension
M. Kawasaki∗ , M. Yamada∗ and T. T. Yanagida,
Phys. Lett. B 750, 12 (2015) [arXiv:1506.05214[hep-ph]].
[76] Cosmological Selection of Multi-TeV Supersymmetry
K. Harigaya∗ , M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys. Lett. B 749, 298 (2015) [arXiv:1506.00426[hep-ph]].

144[77] Peccei-Quinn Symmetry from Dynamical Supersymmetry Breaking
K. Harigaya∗ , M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys. Rev. D 92, no. 7, 075003 (2015)[arXiv:1505.07388 [hep-ph]].
[78] K. Harigaya∗ , M. Ibe∗ and M. Suzuki∗
JHEP
1509,
155
(2015)
doi:10.1007/JHEP09(2015)155 [arXiv:1505.05024[hep-ph]].
[79] Charged Q-balls in gauge mediated SUSY breaking models
J. P. Hong∗ , M. Kawasak∗ , and M. Yamada∗
Phys. Rev. D 92, no. 6, 063521 (2015)[arXiv:1505.02594 [hep-ph]].
[80] Muon g-2 in focus point SUSY
K. Harigaya∗ , T. T. Yanagida and N. Yokozaki
Phys. Rev. D 92, no. 3, 035011 (2015)[arXiv:1505.01987 [hep-ph]].
[81] Gravitational wave signals from short-lived topological defects in the MSSM
A. Kamada and M. Yamada
JCAP 1510, no. 10, 021 (2015) [arXiv:1505.01167[hep-ph]].
[82] Model of visible QCD axion
H. Fukuda, K. Harigaya∗ ,T. T. Yanagida
Phys. Rev. D 92, no. 1,[arXiv:1504.06084 [hep-ph]].

M.

Ibe∗

015021and(2015)[83] k,”Wino Dark Matter in light of the AMS-02 2015
Data
M. Ibe∗ , S. Matsumoto, S. Shirai and
T. T. Yanagida
Phys. Rev. D 91, no. 11, 111701 (2015)[arXiv:1504.05554 [hep-ph]].
[84] Observable dark radiation from a cosmologicallysafe QCD axion
M. Kawasaki∗ , M. Yamada∗ , and T. T. Yanagida
Phys. Rev. D 91, no. 12, 125018 (2015)[arXiv:1504.04126 [hep-ph]].
[85] Spontaneous thermal Leptogenesis via Majoron oscillation
M. Ibe∗ and K. Kaneta∗
Phys. Rev. D 92, no. 3, 035019 (2015)[arXiv:1504.04125 [hep-ph]].
[86] Indirect Probe of Electroweak-Interacting Particles at Future Lepton Colliders
K. Harigaya∗ , K. Ichikawa, A. Kundu, S. Matsumoto and S. Shirai
JHEP 1509, 105 (2015) [arXiv:1504.03402 [hepph]].

[87] Seminatural SUSY from the E7 nonlinear sigma
model
K. Harigaya∗ , T. T. Yanagida and N. Yokozaki
PTEP
2015,
no.
8,
083B03(2015)[arXiv:1504.02266 [hep-ph]].
[88] Gravitational waves from domain walls in the nextto-minimal supersymmetric standard model
K. Kadota, M. Kawasaki∗ , and K. Saikawa∗
JCAP 1510, no. 10, 041 (2015) [arXiv:1503.06998[hep-ph]].
[89] IceCube potential for detecting Q-ball dark matterin gauge mediation
S. Kasuya, M. Kawasaki∗ , and T. T. Yanagida
PTEP
2015,
no.
5,
053B02(2015)
doi:10.1093/ptep/ptv056[arXiv:1502.00715[hep-ph]].
[90] Affleck-Dine baryogenesis after D-term inflationand solutions to the baryon-dark matter coincidence problem M. Kawasaki∗ , and M. Yamada∗
Phys. Rev. D 91, no. 8, 083512 (2015)[arXiv:1502.03550 [hep-ph]].
[91] IceCube potential for detecting Q-ball dark matterin gauge mediation
S. Kasuya, M. Kawasaki∗ , and T. T. Yanagida
PTEP
2015,
no.
5,
053B02(2015)
doi:10.1093/ptep/ptv056[arXiv:1502.00715[hep-ph]].
[92] Light Higgsinos in Pure Gravity Mediation
J. L. Evans, M. Ibe∗ , K. A. Olive andT. T. Yanagida
Phys. Rev. D 91, 055008 (2015) [arXiv:1412.3403[hep-ph]].
[93] Axion dark matter from topological defects
M. Kawasaki∗ , K. Saikawa and T. Sekiguchi
Phys. Rev. D 91, no. 6, 065014 (2015)[arXiv:1412.0789 [hep-ph]], 106 citations counted
in INSPIRE as of Nov 2018.
[94] Coupling Unification and Dark Matter in a Standard Model Extension with Adjoint Majorana
Fermions
T. Aizawa∗ , M. Ibe∗ and K. Kaneta∗
Phys. Rev. D 91, no. 7, 075012 (2015)[arXiv:1411.6044 [hep-ph]].
[95] New resonance scale and fingerprint identificationin minimal composite Higgs models
S. Kanemura, K. Kaneta∗ , N. Machida andT. Shindou
Phys. Rev. D 91, 115016 (2015) [arXiv:1410.8413[hep-ph]].
[96] Mass of Decaying Wino from AMS-02 2014
M. Ibe∗ , S. Matsumoto, S. Shirai

and

145

T. T. Yanagida
Phys. Lett. B 741, 134 (2015) [arXiv:1409.6920[hep-ph]].
[97] Anomaly Mediated Gaugino Mass and PathIntegral Measure
K. Harigaya and M. Ibe∗
Phys. Rev. D 90, no. 8, 085028 (2014)[arXiv:1409.5029 [hep-th]].
[98] CDM/baryon isocurvature perturbations in asneutrino curvaton model
K. Harigaya, T. Hayakawa∗ , M. Kawasaki∗ and
S. Yokoyama∗
JCAP 1410, no. 10, 068 (2014) [arXiv:1409.1669[hep-ph]].

[106] Gravitational waves as a probe of the SUSY scale
A. Kamada and M. Yamada∗
Phys. Rev. D 91, 063529 (2015) [arXiv:1407.2882[hep-ph]].
[107] Cosmic neutrino background absorption line in
the neutrino spectrum at IceCube
M. Ibe∗ and K. Kaneta∗
Phys. Rev. D 90, no. 5, 053011 (2014)[arXiv:1407.2848 [hep-ph]].
[108] Relaxing Isocurvature Bounds on String Axion
Dark Matter
M. Kawasaki∗ , N. Kitajima and F. Takahashi
Phys. Lett. B 737, 178 (2014) [arXiv:1406.0660[hep-ph]].

[99] R-symmetric Axion/Natural Inflation in Supergravity via Deformed Moduli Dynamics
K. Harigaya, M. Ibe∗ and T. T. Yanagida
Phys. Lett. B 739, 352 (2014) [arXiv:1409.0330[hep-ph]].

[109] Vanishing Higgs potential at the Planck scale ina singlet extension of the standard model
N. Haba, H. Ishida, K. Kaneta∗ and R. Takahashi
Phys. Rev. D 90, 036006 (2014) [arXiv:1406.0158[hep-ph]].

[100] Gravitational effects on vanishing Higgs potential
at the Planck scale
N. Haba, K. Kaneta∗ , R. Takahashi and Y. Yamaguchi
Phys. Rev. D 91, no. 1, 016004 (2015)[arXiv:1408.5548 [hep-ph]].

[110] Solution to the baryon-dark-matter coincidence
problem in the constrained minimal supersymmetric model with a 126-GeV Higgs boson A. Kamada,
M. Kawasaki∗ , and M. Yamada∗
Phys. Rev. D 91, no. 8, 081301 (2015)[arXiv:1405.6577 [hep-ph]].

[101] Q-ball dark matter and baryogenesis in high-scale
inflation
S. Kasuya and M. Kawasaki∗
Phys. Lett. B 739, 174 (2014) [arXiv:1408.1176[hep-ph]].

[111] Wino Dark Matter and Future dSph Observations
B. Bhattacherjee, M. Ibe∗ , K. Ichikawa, S. Matsumoto and K. Nishiyama
JHEP 1407, 080 (2014) [arXiv:1405.4914 [hepph]].

[102] Thermal Effects and Sudden Decay Approximation in the Curvaton Scenario
N. Kitajima∗ D. Langlois, T. Takahashi,
T. Takesako∗ and S. Yokoyama∗
JCAP 1410, no. 10, 032 (2014) [arXiv:1407.5148[astro-ph.CO]].

[112] No quasistable scalaron lump forms after R2 inflation
N. Takeda∗ and Y. Watanabe
Phys. Rev. D 90, no. 2, 023519 (2014)[arXiv:1405.3830 [astro-ph.CO]].

[103] Phase Locked Inflation – Effectively TransPlanckian Natural Inflation
K. Harigaya and M. Ibe∗
JHEP 1411, 147 (2014) [arXiv:1407.4893 [hepph]].
[104] Blue-tilted Tensor Spectrum and Thermal History of the Universe
S. Kuroyanagi, T. Takahashi and S. Yokoyama∗
JCAP
1502,
003
(2015)
doi:10.1088/1475-7516/2015/02/003[arXiv:1407.4785 [astro-ph.CO]].
[105] Dynamical fractional chaotic inflation
K. Harigaya, M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys. Rev. D 90, no. 12, 123524 (2014)[arXiv:1407.3084 [hep-ph]].

[113] Non-perturbative approach for curvature perturbations in stochastic δ N formalism
T. Fujita, M. Kawasaki and Y. Tada
JCAP 1410, no. 10, 030 (2014) [arXiv:1405.2187[astro-ph.CO]].
[114] “Inflatonic baryogenesis with large tensor mode
N. Takeda∗
Phys. Lett. B 746, 368 (2015) [arXiv:1405.1959[astro-ph.CO]].
[115] Simple realization of inflaton potential on a Riemann surface
K. Harigaya and M. Ibe∗
Phys. Lett. B 738, 301 (2014) [arXiv:1404.3511[hep-ph]].
[116] Affleck-Dine Baryogenesis and Dark Matter Production after High-scale Inflation

146

K. Harigaya, A. Kamada, M. Kawasaki∗ ,K. Mukaida and M. Yamada∗ , Phys. Rev. D
90, no. 4, 043510 (2014) doi:10.1103/PhysRevD.90.043510 [arXiv:1404.3138 [hep-ph]].
[117] Isocurvature perturbations and tensor mode inlight of Planck and BICEP2
M. Kawasaki∗ , T. Sekiguchi, T. Takahashi and
S. Yokoyama∗
JCAP 1408, 043 (2014) [arXiv:1404.2175 [astroph.CO]].
[118] Curvaton in large field inflation
T. Fujita, M. Kawasaki and S. Yokoyama∗
JCAP 1409, 015 (2014) [arXiv:1404.0951 [astroph.CO]].
[119] High-Scale SUSY Breaking Models in light of the
BICEP2 Result
K. Harigaya, M. Ibe∗ , K. Ichikawa, K. Kaneta and
S. Matsumoto
JHEP 1407, 093 (2014) [arXiv:1403.5880 [hepph]].
[120] Compensation for large tensor modes with isocurvature perturbations in CMB anisotropies
M. Kawasaki∗ and S. Yokoyama∗
JCAP 1405 (2014) 046 [arXiv:1403.5823 [astroph.CO]].
[121] Neutrino masses, leptogenesis, and sterile neutrino dark matter
T. Tsuyuki∗
Phys. Rev. D 90, 013007 (2014) [arXiv:1403.5053[hep-ph]].
[122] Dynamical Chaotic Inflation in the Light of
BICEP2
K. Harigaya, M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys.
Lett.
B
733,
283
(2014)
doi:10.1016/j.physletb.2014.04.057[arXiv:1403.4536 [hep-ph]].
[123] Probing small-scale cosmological fluctuations
with the 21 cm forest: Effects of neutrino mass,
running spectral index, and warm dark matter
H. Shimabukuro, K. Ichiki, S. Inoue andS. Yokoyama∗
Phys. Rev. D 90, no. 8, 083003 (2014)[arXiv:1403.1605 [astro-ph.CO]].
[124] Testing the Minimal Direct Gauge Mediation at
the LHCK. Hamaguchi, M. Ibe∗ , T. T. Yanagida andN. Yokozaki
Phys. Rev. D 90, no. 1, 015027 (2014)[arXiv:1403.1398 [hep-ph]].
[125] Peccei-Quinn Symmetric Pure Gravity Mediation
J. L. Evans, M. Ibe∗ , K. A. Olive andT. T. Yanagida
Eur. Phys. J. C 74, 2931 (2014) [arXiv:1402.5989[hep-ph]].
[126] Baryogenesis from the gauge-mediation type Qball and the new type of Q-ball as the dark matter
S. Kasuya and M. Kawasaki∗
Phys. Rev. D 89, no. 10, 103534 (2014)[arXiv:1402.4546 [hep-ph]].
[127] Dark Matter Production in Late Time Reheating
K. Harigaya, M. Kawasaki∗ , K. Mukaida andM. Yamada∗ , Phys. Rev. D 89, no. 8,083532 (2014) doi:10.1103/PhysRevD.89.083532[arXiv:1402.2846 [hep-ph]].
[128] Critical constraint on inflationary magnetogenesis
T. Fujita and S. Yokoyama∗
JCAP 1403, 013 (2014) Erratum: [JCAP 1405,
E02 (2014)] [arXiv:1402.0596 [astro-ph.CO]].
[129] Flat Higgs Potential from Planck Scale Supersymmetry Breaking
M. Ibe∗ , S. Matsumoto and T. T. Yanagida
Phys. Lett. B 732, 214 (2014) [arXiv:1312.7108[hep-ph]].
[130] One-loop anomaly mediated scalar masses and(g − 2)m u in pure gravity mediation
J. L. Evans, M. Ibe∗ , K. A. Olive andT. T. Yanagida
Eur. Phys. J. C 74, no. 2, 2775 (2014)[arXiv:1312.1984 [hep-ph]].
[131] Mixed (cold+warm) dark matter in the bino-winocoannihilation scenario
M. Ibe∗ , A. Kamada and S. Matsumoto
Phys. Rev. D 89, no. 12, 123506 (2014)[arXiv:1311.2162 [hep-ph]].
[132] Lower Bound on the Garvitino Mass m3/2 >O(100) TeV in R-Symmetry Breaking New Inflation
K. Harigaya, M. Ibe and T. T. Yanagida
Phys. Rev. D 89, no. 5, 055014 (2014)[arXiv:1311.1898 [hep-ph]].
[133] Decay rates of Gaussian-type I-balls and Boseenhancement effects in 3+1 dimensions
M. Kawasaki∗ , and M. Yamada∗
JCAP 1402, 001 (2014) [arXiv:1311.0985 [hepph]].
[134] Hypercharged Dark Matter and Direct Detection
as a Probe of Reheating
B. Feldstein, M. Ibe∗ and T. T. Yanagida
Phys. Rev. Lett. 112, no. 10, 101301 (2014)[arXiv:1310.7495 [hep-ph]].

147[135] Halo/galaxy bispectrum with primordial nonGaussianity from integrated perturbation theory
S. Yokoyama∗ , T. Matsubara and A. Taruya
Phys. Rev. D 89, no. 4, 043524 (2014)[arXiv:1310.4925 [astro-ph.CO]].
[136] I-ball formation with logarithmic potential
M. Kawasaki∗ and N. Takeda∗
JCAP 1407, 038 (2014) [arXiv:1310.4615 [astroph.CO]].
[137] CMB distortion anisotropies due to the decay ofprimordial magnetic fields
K. Miyamoto∗ , T. Sekiguchi, H. Tashiro andS. Yokoyama∗
Phys.
Rev.
D
89,
no.
6,
063508(2014)
doi:10.1103/PhysRevD.89.063508[arXiv:1310.3886 [astro-ph.CO]].
[138] Axions as Hot and Cold Dark Matter
K. S. Jeong∗ , M. Kawasaki∗ , and F. Takahashi
JCAP 1402, 046 (2014) [arXiv:1310.1774 [hepph]].
[139] A Closer Look at Gaugino Masses in Pure Gravity Mediation Model/Minimal Split SUSY Model
K. Harigaya, M. Ibe∗ and T. T. Yanagida
JHEP 1312, 016 (2013) [arXiv:1310.0643 [hepph]].
[140] On the estimation of gravitational wave spectrum
from cosmic domain walls
T. Hiramatsu, M. Kawasaki∗ , and K. Saikawa
JCAP 1402, 031 (2014) [arXiv:1309.5001 [astroph.CO]].
[141] ,”A new algorithm for calculating the curvature
perturbations in stochastic inflation
T. Fujita, M. Kawasaki∗ , Y. Tada and
T. Takesako∗
JCAP 1312, 036 (2013) [arXiv:1308.4754 [astroph.CO]].
[142] Peccei-Quinn symmetry from a gauged discrete
R symmetry
K. Harigaya, M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys.
Rev.
D
88,
no.
7,
075022(2013)
doi:10.1103/PhysRevD.88.075022[arXiv:1308.1227 [hep-ph]].
[143] Type-I cosmic string network
T. Hiramatsu, Y. Sendouda, K. Takahashi,
D. Yamauchi∗ and C. M. Yoo
Phys. Rev. D 88, no. 8, 085021 (2013)[arXiv:1307.0308 [astro-ph.CO]].
[144] Higher order statistics of curvature perturbations
in IFF model and its Planck constraints
T. Fujita∗ and S. Yokoyama∗

JCAP 1309, 009 (2013) [arXiv:1306.2992 [astroph.CO]].
[145] Non-Universalities in Pure Gravity Mediation
J. L. Evans, K. A. Olive, M. Ibe∗ and
T. T. Yanagida
Eur. Phys. J. C 73, no. 10, 2611 (2013)[arXiv:1305.7461 [hep-ph]].
[146] Domain wall and isocurvature perturbation problems in axion models
M. Kawasaki∗ , T. T. Yanagida and K. Yoshino∗
JCAP 1311, 030 (2013) [arXiv:1305.5338 [hepph]].
[147] Gravitational waves from a curvaton model withblue spectrum M. Kawasaki∗ , N. Kitajima∗ and
S. Yokoyama∗
JCAP
1308,
042
(2013)
doi:10.1088/1475-7516/2013/08/042[arXiv:1305.4464 [astro-ph.CO]].
[148] AMS-02 Positrons from Decaying Wino in thePure Gravity Mediation Model
M. Ibe∗ , S. Matsumoto, S. Shirai and
T. T. Yanagida
JHEP 1307, 063 (2013) [arXiv:1305.0084 [hepph]].
[149] Recent Result of the AMS-02 Experiment and
Decaying Gravitino Dark Matter in Gauge Mediation
M. Ibe∗ , S. Iwamoto, S. Matsumoto, T. Moroi and
N. Yokozaki
JHEP 1308, 029 (2013) [arXiv:1304.1483 [hepph]].
[150] Muon g-2 and 125 GeV Higgs in Split-Family Supersymmetry
M. Ibe∗ , T. T. Yanagida and N. Yokozaki
JHEP 1308, 067 (2013) [arXiv:1303.6995 [hepph]].
[151] Implications of Planck results for models with local type non-Gaussianity
T. Suyama, T. Takahashi, M. Yamaguchi and
S. Yokoyama∗
JCAP 1306, 012 (2013) [arXiv:1303.5374 [astroph.CO]].
[152] Focus point gauge mediation in product groupunification
F. Brümmer, M. Ibe and T. T. Yanagida
Phys. Lett. B 726, 364 (2013) [arXiv:1303.1622[hep-ph]].
[153] Statistics of general functions of a Gaussian field-application to non-Gaussianity from preheating
T. Suyama and S. Yokoyama∗
JCAP
1306,
018
(2013)

148

doi:10.1088/1475-7516/2013/06/018[arXiv:1303.1254 [astro-ph.CO]].
[154] T. Abe, R. Kitano, Y. Konishi, K. y. Oda, J. Satoand S. Sugiyama, EPJ Web Conf. 49, 15018 (2013)[arXiv:1303.0935 [hep-ph]].
[155] Universality in Pure Gravity Mediation
J. L. Evans, M. Ibe∗ , K. A. Olive andT. T. Yanagida
Eur.
Phys.
J.
C
73,
2468
(2013)
doi:10.1140/epjc/s10052-013-2468-9[arXiv:1302.5346 [hep-ph]], 51 citations counted
in INSPIRE as of Nov 2018.
[156] ”Testing general scalar-tensor gravity and massive gravity with cluster lensing
T. Narikawa, T. Kobayashi, D. Yamauchi∗ and
R. SaitoPhys. Rev. D 87, 124006 (2013) doi:10.1103/PhysRevD.87.124006 [arXiv:1302.2311 [astro-ph.CO]].
[157] Heavy gravitino in hybrid inflation
M. Kawasaki∗ , N. Kitajima∗ , K. Nakayama andT. T. Yanagida
JCAP 1306, 037 (2013) [arXiv:1301.6281 [hepph]].
[158] A Simple Solution to the Polonyi Problem in
Gravity Mediation
K. Harigaya, M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys. Lett. B 721, 86 (2013) [arXiv:1301.3685[hep-ph]].
[159] Scale-dependent bias due to primordial vector
field
M. Shiraishi, S. Yokoyama∗ , K. Ichiki and T. Matsubara
Mon. Not. Roy. Astron. Soc. 432, no. 3, 2331
(2013) doi:10.1093/mnras/stt594 [arXiv:1301.2778[astro-ph.CO]].
[160] Natural supersymmetry’s last hope: R-parity violation via UDD operators
B. Bhattacherjee, J. L. Evans, M. Ibe∗ , S. Matsumoto and T. T. Yanagida
Phys. Rev. D 87, no. 11, 115002 (2013)[arXiv:1301.2336 [hep-ph]].
[161] Axions: Theory and Cosmological Role
M. Kawasaki∗ , and K. Nakayama
Ann. Rev. Nucl. Part. Sci. 63, 69 (2013)[arXiv:1301.1123 [hep-ph]], 129 citations counted
in INSPIRE as of Nov 2018.
[162] Cosmological and astrophysical constraints on superconducting cosmic strings
K. Miyamoto∗ and K. Nakayama
JCAP 1307, 012 (2013) [arXiv:1212.6687 [astroph.CO]].

[163] Extended analysis of CMB constraints on nonGaussianity in isocurvature perturbations
C. Hikage, M. Kawasaki∗ , T. Sekiguchi
and T. Takahashi, JCAP 1303 (2013) 020[arXiv:1212.6001 [astro-ph.CO]].
[164] Mass Splitting between Charged and Neutral
Winos at Two-Loop Level
M. Ibe∗ , S. Matsumoto and R. SatoPhys. Lett. B 721, 252 (2013) [arXiv:1212.5989[hep-ph]], 96 citations counted in INSPIRE as of
Nov 2018.
[165] Effects of power law primordial magnetic field on
big bang nucleosynthesis
D. G. Yamazaki and M. Kusakabe∗Phys. Rev. D 86, 123006 (2012) [arXiv:1212.2968[astro-ph.CO]].
[166] Opening the window to the cogenesis with
Affleck-Dine mechanism in gravity mediation
A. Kamada, M. Kawasaki∗ , and M. Yamada∗
Phys. Lett. B 719, 9 (2013) [arXiv:1211.6813 [hepph]].
[167] Smooth hybrid inflation in a supersymmetric axion model
M. Kawasaki∗ , N. Kitajima∗ , and K. Nakayama
Phys. Rev. D 87, no. 3, 035010 (2013)[arXiv:1211.6516 [hep-ph]].
[168] Chaotic Inflation with a Fractional Power-Law
Potential in Strongly Coupled Gauge Theories
K. Harigaya, M. Ibe∗ , K. Schmitz and
T. T. Yanagida
Phys.
Lett.
B
720,
125
(2013)
doi:10.1016/j.physletb.2013.01.058[arXiv:1211.6241 [hep-ph]].
[169] Hubble-induced mass from MSSM plasma
M. Kawasaki∗ , F. Takahashi and T. Takesako∗
JCAP
1304,
008
(2013)
doi:10.1088/1475-7516/2013/04/008[arXiv:1211.4921 [hep-ph]].
[170] Revisiting the gravitino dark matter and baryon
asymmetry from Q-ball decay in gauge mediation
S. Kasuya, M. Kawasaki∗ , and M. Yamada∗
Phys. Lett. B 726, 1 (2013) [arXiv:1211.4743 [hepph]].
[171] Non-Gaussianity from Attractor Curvaton
K. Harigaya, M. Ibe∗ , M. Kawasaki∗ and
T. T. Yanagida
Phys. Rev. D 87, no. 6, 063514 (2013)[arXiv:1211.3535 [hep-ph]].
[172] CMB constraint on non-Gaussianity in isocurvature perturbations
C. Hikage, M. Kawasaki∗ , T. Sekiguchi and149

T. Takahashi
JCAP 1307, 007 (2013) [arXiv:1211.1095 [astroph.CO]].
[173] Evolution and thermalization of dark matter axions in the condensed regime
K. Saikawa∗ and M. Yamaguchi
Phys. Rev. D 87, no. 8, 085010 (2013)[arXiv:1210.7080 [hep-ph]].
[174] ”Non-Gaussianity from Axionic Curvaton
M. Kawasaki∗ , T. Kobayashi and F. Takahashi
JCAP 1303, 016 (2013) [arXiv:1210.6595 [astroph.CO]].
[175] Heavy Squarks and Light Sleptons in Gauge Mediation From the viewpoint of 125 GeV Higgs Boson and Muon g-2
M. Ibe∗ , S. Matsumoto, T. T. Yanagida andN. Yokozaki
JHEP 1303, 078 (2013) [arXiv:1210.3122 [hepph]].
[176] Forecast constraints on cosmic strings from future
CMB, pulsar timing and gravitational wave direct
detection experiments
S. Kuroyanagi, K. Miyamoto∗ , T. Sekiguchi,
K. Takahashi and J. Silk
Phys. Rev. D 87, no. 2, 023522 (2013) Erratum: [Phys. Rev. D 87, no. 6, 069903 (2013)][arXiv:1210.2829 [astro-ph.CO]].
[177] Scale-dependent bias with higher order primordial non-Gaussianity: Use of the Integrated Perturbation Theory
S. Yokoyama∗ and T. Matsubara
Phys. Rev. D 87, 023525 (2013) [arXiv:1210.2495[astro-ph.CO]].
[178] Imprints of nonthermal Wino dark matter on
small-scale structure
M. Ibe∗ , A. Kamada and S. Matsumoto
Phys. Rev. D 87, no. 6, 063511 (2013)[arXiv:1210.0191 [hep-ph]].
[179] Q ball Decay Rates into Gravitinos and Quarks
M. Kawasaki∗ and M. Yamada∗ Phys. Rev. D 87,
no. 2, 023517 (2013) [arXiv:1209.5781 [hep-ph]].
[180] Minimal Dilaton Model
T. Abe, R. Kitano, Y. Konishi, K. y. Oda, J. Satoand S. Sugiyama∗, Phys. Rev. D 86, 115016 (2012) [arXiv:1209.4544[hep-ph]].
[181] CMB power spectra induced by primordial crossbispectra between metric perturbations and vector
fields
M. Shiraishi, S. Saga and S. Yokoyama∗
JCAP 1211, 046 (2012) [arXiv:1209.3384 [astroph.CO]].

[182] Production of 9Be through alpha-fusion reactionof metal-poor cosmic ray and stellar flare’ M. Kusakabe∗ and M. Kawasaki∗
Astrophys. J. 767, 5 (2013) [arXiv:1208.4210[astro-ph.CO]].
[183] Gravitational waves from smooth hybrid new inflation
M. Kawasaki∗ , K. Saikawa∗ and N. Takeda∗
Phys. Rev. D 87, no. 10, 103521 (2013)[arXiv:1208.4160 [astro-ph.CO]].
[184] Non-Gaussian bubbles in the sky
K. Sugimura, D. Yamauchi∗ and M. Sasaki
EPL 100, no. 2, 29004 (2012) [arXiv:1208.3937[astro-ph.CO]].
[185] Remarks on Hubble Induced Mass from Fermion
Kinetic Term
M. Kawasaki∗ and T. Takesako∗
Phys. Lett. B 718, 522 (2012) [arXiv:1208.1323[hep-ph]].
[186] Pure gravity mediation of supersymmetry breaking at the Large Hadron Collider
B. Bhattacherjee, B. Feldstein, M. Ibe∗ , S. Matsumoto and T. T. Yanagida
Phys. Rev. D 87, no. 1, 015028 (2013)[arXiv:1207.5453 [hep-ph]], 73 citations counted in
INSPIRE as of Nov 2018.
[187] Axion cosmology with long-lived domain walls
T. Hiramatsu, M. Kawasaki∗ , K. Saikawa∗ and
T. Sekiguchi
JCAP 1301, 001 (2013) [arXiv:1207.3166 [hepph]], 62 citations counted in INSPIRE as of Nov
2018.
[188] Stochastic Approach to Flat Direction during Inflation
M. Kawasaki∗ and T. Takesako∗
JCAP 1208, 031 (2012) [arXiv:1207.1165 [hepph]].
[189] Full bispectra from primordial scalar and tensor
perturbations in the most general single-field inflation model
X. Gao, T. Kobayashi, M. Shiraishi, M. Yamaguchi, J. Yokoyama and S. Yokoyama∗
PTEP 2013, 053E03 (2013) [arXiv:1207.0588[astro-ph.CO]].
[190] Seesaw Mechanism with Occam’s Razor
K. Harigaya, M. Ibe∗ and T. T. Yanagida
Phys. Rev. D 86, 013002 (2012) [arXiv:1205.2198[hep-ph]], 56 citations counted in INSPIRE as of
Nov 2018.
[191] Weak lensing generated by vector perturbations
and detectability of cosmic strings

150

D. Yamauchi∗ , T. Namikawa and A. Taruya
JCAP 1210, 030 (2012) [arXiv:1205.2139 [astroph.CO]].
[192] Updated constraint on a primordial magnetic
field during big bang nucleosynthesis and a formulation of field effects
M. Kawasaki∗ and M. Kusakabe∗Phys. Rev. D 86, 063003 (2012) [arXiv:1204.6164[astro-ph.CO]].
[193] The Lightest Higgs Boson Mass in the MSSM
with Strongly Interacting Spectators
J. L. Evans, M. Ibe∗ and T. T. Yanagida
Phys. Rev. D 86, 015017 (2012) [arXiv:1204.6085[hep-ph]].
[194] A 125 GeV Higgs Boson Mass and Gravitino Dark
Matter in R-invariant Direct Gauge Mediation
M. Ibe∗ and R. SatoPhys. Lett. B 717, 197 (2012) [arXiv:1204.3499[hep-ph]].
Papers under referee[195] Primordial Black Holes from Affleck-Dine Mechanism
F. Hasegawa∗ and M. Kawasaki∗arXiv:1807.00463 [astro-ph.CO].
[196] Big Bang Nucleosynthesis Constraint on Baryonic
Isocurvature Perturbations
K. Inomata∗ , M. Kawasaki∗ , A. Kusenko and
L. YangarXiv:1806.00123 [astro-ph.CO].
[197] Optical Ring Cavity Search for Axion Dark Matter
I. Obata∗ , T. Fujita and Y. Michimura
arXiv:1805.11753 [astro-ph.CO].
[198] “Formation of primordial black holes as dark matter or LIGO black hole binaries in an axion-likecurvaton model
K. Ando∗ , M. Kawasaki∗ and H. Nakatsuka∗
arXiv:1805.07757 [astro-ph.CO].
[199] A Model of Composite B − L Asymmetric Dark
MatterM. Ibe∗ , A. Kamada, S. Kobayashi∗ and
W. Nakano∗
arXiv:1805.06876 [hep-ph].

151

FACILITIES152

KAMIOKA OBSERVATORY
Kamioka Observatory
1.

Introduction

There are four operating bodies for the undergroundsite in Kamioka: 1) Kamioka Observatory, Institutefor Cosmic Ray Research (ICRR), the University ofTokyo; 2) Tohoku University Research Center for Neutrino Science; 3) Kamioka Satellite, Kavli Institute for
the Physics and Mathematics of the Universe (Kavli
IPMU), the University of Tokyo; 4) KAGRA Observatory, ICRR, the University of Tokyo; (in order offoundation). They are cooperating each other in many
aspects. They work together for safety issues and someof the maintenance costs for the underground environments in common area are shared. But they are independent and operating different facilities and haveseparate budgets.
Kamioka Observatory is located at 1000 m underground (2,700 meter water equivalent) of the Kamioka
Mine, Gifu prefecture, about 200 km west of Tokyo.
Kamioka Observatory was established in 1995, initiallyto house and operate the Super-Kamiokande (SK) detector. Kamioka Observatory has expanded its role inorder to operate XMASS: the direct dark matter searchexperiment. Kamioka Observatory also accepts experiments to use underground spaces by external research
institutions based on the proposals: CANDLES is adouble beta decay experiment using 48 Ca led by Osaka
University and NewAGE is a dark matter experiment
led by Kobe University to detect directions of recoil
nucleus. There are also experiments to measure precise
value of the vertical gravity by a super-conductive gravity meter and to measure the earth’s horizontal crustal
strain to the level of 10−12 by a laser displacement meter.

2.

Brief History and Major Achievements

In 1991, the construction budget for SK was approved and the 5 year construction project had started.
SK is a 50, 000 ton water Cherenkov detector, and theinner 32,000 tons of water are optically separated fromthe outer-layer, anti-counters, of 2m thickness on anaverage. One year after the construction had started,
the US group joined and they bore a responsibility toconstruct and operate anti-counters. The SK detector was built to resolve the remaining problems from
Kamiokande, namely the solar neutrino problems andthe atmospheric neutrino anomaly. Detection of neutrino bursts from supernovae and proton decay werealso very interesting subjects.
In 1995, the Kamioka Observatory was officiallyfounded to operate the SK detector. One year before
that, in 1994, the computer building was constructed
and the computer system was installed.
In 1996, the detector construction was completed
and started data taking. Two years later, in 1998, SK
found the up-down asymmetry of the µ -like events inthe zenith angle distribution of the atmospheric neutrinos, which provided a clear evidence for neutrino oscillation independent of the flux calculation.
In 1999, the first long baseline neutrino oscillation
experiment of the earth scale, K2K, had started, injecting man-made neutrinos produced at KEK PS to
SK, 250 km away from the neutrino source. The experiment was completed in 2004 observing 112 µ -likeevents whereas 156 such events were expected without
oscillation. This confirmed the neutrino oscillation observed in atmospheric neutrinos.
In June 2001, SK published the 1256 days of data
for solar neutrino observations. On the same day, SNO
announced their first result on charged current interactions. They have concluded 3.6σ effect by comparing
the SK electron scattering data and the SNO charged
current data that there are non-electron type neutrinosin the solar neutrinos detected on the earth. This was
the first evidence for solar neutrino oscillation.
In 2009, the new long baseline oscillation experimentcalled T2K was started to look for as yet undetermined
mixing angle of θ13 . The intense neutrino beam from
JPARC was directed to the SK detector 295 km away.
In 2011, T2K presented the first evidence for non-zeroθ13 , namely observation of electron-like events in muonneutrino beam.

3.

Operation

In Kamioka Observatory we have two technicalstaffs who work for the daily maintenance of the underground laboratory. Another technical staff works
for safety issues such as management of chemicals. The
experiments and related equipments are basically operated by the scientists from each collaboration.
The number of full-time scientific staff of Kamioka
Observatory is 24 (3 professors, 5 associate professors
and 16 research associates) and there are three staffs
in administration office. These scientific personnel are
conducting researches using SK and XMASS and have
responsibilities to operate these detectors. In addition,
they have responsibilities on the matters related to
the facilities of the underground laboratory and on the
supports for about 5000 person×day visiting scientists
per year, especially for those taking SK shifts andanalyzing the data. The safety issues are one of the153

important subjects.

4.

Procedure of approving experiments in
Kamioka Observatory

Once every year, ICRR receive experimental proposals not only for underground experiments but also
for all the relevant experiments to ICRR. And the program assessment committee including some members
from outside of ICRR decides if the proposals to be approved or not. This judgement is mostly done basedon the scientific point of view. Following this approval,
the Kamioka steering committee discusses and decides
if the approved proposals would fit to the undergroundlaboratories. Basically, the Observatory provides thespace and facility like electricity, water and so on andthe user will bring all the necessary equipments. Practically before any decision is made, the experimental
collaboration would negotiate with the Observatory if
their experiment would fit in the underground or tojudge if the experiment needs a new cavity or not.
After the approval, the observatory manages theiractivities in a point of view for safety. For example, if
one uses liquid scintillator, one needs to get permission
to use liquid scintillator from the local fire department
and there are many rules and regulations to follow.
Then the Observatory sometimes ask the experimental group to change a design by taking into accountthose regulations.

5.

Facilities

5.1. Underground Facilities
The entrance of the underground facilities is locatedat the distance about 10 minutes drive from the groundoffice buildings. A horizontal access of a few minutes
drive of 1.7 km drift brings users to the area where
undergrounds facilities are located. The Kamioka mine
is no more operating and therefore users can access tothe facilities 24 hours a day by just driving a car. The
map of the facility area is shown in Fig. 5.1.
SK and KamLAND (Tohoku University) are shownin circle. CLIO and Laser strain meter are placed at the
L-shape laboratory and the length of each arm is 100
m. Labs A to G are under the supervision by Kamioka
Observatory and Labs 1 and 2 are the Kavli IPMU laboratories. Lab.A is a back yard of SK, where we havethe electron LINAC for the calibration for SK and low
background test equipments. Lab. B is a L-shape laboratory; the length of each arm is 20 m. NewAGE is
operating at the end of one arm and the other arm
was used by the Super-Conductive Gravity meter until
2016. The crossing area is used for various test experiments and preparations. Lab.C (15 m wide and 20
m long) is a hall for XMASS and Lab.D houses CANDLES, the double beta decay experiment using 48 Ca.
Lab.E is hosting 200 ton EGADS water tank, which wasbuilt for testing the feasibility of the SK-Gd project.
Lab.G area was newly excavated in 2015 and a Gd
dissolving system and a Gd-loaded water recirculation
system have been constructed since then. Lab.1 hasvarious devices to detect radio-active contaminations,
impurities and so on. Among them are two high purity
Germanium detectors, Gas analyzer, Particle counters,
Scanning Electron Microscope and so on. Lab.2 is used
for computer clusters. All the laboratories are within
400 m distances.
5.2. Environmental Backgrounds
The deep underground provides the low radioactive
and low seismic noise environment and gives good opportunities to perform physics experiments requiringsuch experimental conditions.
At the depth of 2,700 m water equivalence, thecosmic ray muon flux, about 250 muons/m2 /day, is∼1/100,000 of the surface flux.
The environmental gamma ray backgrounds fromthe wall depend upon locations and energy. It ranges
from 0.1∼0.2 /MeV/cm2 /sec at around 1 MeV and
0.01∼0.02 /MeV/cm2 /sec at around 2 MeV.
The thermal and non-thermal neutron fluxes in
Kamioka Observatory are measured to beφnthermal= 8.3±0.6×10−6 cm−2 s−1 andφnnon−thermal = 1.2±0.1×10−5 cm−2 s−1 ,respectively. In order to reduce those environmental
γ and neutron backgrounds, water shields are most effective and commonly used in Kamioka. There are 2
to 3 m water layers surrounding the inner volume of
SK which would effectively reduce those backgrounds.
The 800 tons water tank for the XMASS experiment
provides 10−7 reduction for neutrons.
The Rn concentration of the mine air varies ∼100
Bq/m3 to ∼2000 Bq/m3 depending on the season of
the year. To reduce the exposure of workers and experiments, fresh air from outside of the mine is brought into
the experimental areas. 100 m3 of fresh air per minutes
is brought through a 1.8 km pipe. Rn concentration of
the fresh air is ≤50 Bq/m3 and the Rn concentration of
the experimental areas are always kept < 100 Bq/m3 .
To further reduce the effects of Rn for experiments, so
called ”Rn-free air” generation system which is basedon charcoal filters is installed in the mine. The Rnconcentration of ”Rn-free air” is < 1 mBq/m3 and 18
m3 per hour of flow is available for purging detectors
and buffer tanks for the ultra pure water system. The
U/Th and K contamination in the rocks in the mineare ∼1.0 ppm for U, ∼3.2 ppm for Th and 1.6% for K
at a typical location.
To study and establish low background environment, we have ICP-MASS spectrometer, GAS MASS
spectrometer, and low background Ge detectors in theunderground laboratory. Those are available for theusers of the underground observatory.
An electron LINAC of which energy ranges 5 MeV to

154

G

Fig. 1. Layout of Kamioka Observatory

15 MeV is set up near the SK detector for the purposeof the low energy calibration. The energy of the LINAC
covers exactly the energy range of the detectable solar
neutrinos in SK. The DT neutron generator can also be
used to produce 16 N in the water as a calibration source.
Other radio-active calibration sources are available in
the underground laboratory.
5.3. Ground Facilities
The office building for Kamioka Observatory(ICRR) and Kamioka Satellite (Kavli IPMU) are built
close each other and actually connected by passage.
Kamioka Observatory operates dormitory where wehave 18 single rooms and 1 twin room and 1 Japanesestyle tatami room. Three meals are served at the dormitory, but need reservation. Kamioka Observatory
operates computer system, electronics workshop andchemical workshop in the office building. The computer system consists of 2380 cores with 2.40 GHz andthe memory size is 4 GB/core. Disk space available is
9 PB and the access speed is greater than 2.5 GB/s intotal. We also have a tape backup system with 4.2 PB.
The system is mostly used for the SK experiment. All
the data from SK are stored in this system and accessedalso from abroad.

155

AKENO OBSERVATORY
Introduction

The Akeno Observatory is situated in Akeno of
Hokuto-city situated 20 km northwest of Kofu and
130 km west of metropolitan Tokyo. The location isat the longitude of 138.5◦ E and the latitude of 35.8◦ N.
The altitude is ∼900 m above sea level. It was established in 1977 as a research center for air shower studies
in the very high energy region, and it has been administered by the ICRR as a facility of the joint researchof ICRR.
The Akeno Air Shower Experiment started in 1979
with an array covering 1 km2 area. The array was enlarged to 20 km2 in 1984 and was gradually expanded
to Akeno Giant Air Shower Array (AGASA) of approximately 100 km2 area by 1990. The AGASA was built
to detect Ultra-High Energy Cosmic Rays (UHECRs)
in the energy range of 1020 eV.

Fig. 1. The main campus of the Akeno Observatory. There are
the movable tent for a small atmospheric Cherenkov telescope,
the large experimental hall, the research building and the lodging
facility from the left to the right.

the TA extensions2 were assembled (Fig. 2). The assembly work of the surface detector electronics for the
TA extensions was also performed.

The 40th anniversary of the Akeno Observatory

The ceremony of the 40th anniversary of the Akeno
Observatory was held at a hotel in Kofu in Yamanashi
Prefecture on September 8th in 2017. Before the ceremony, a tour of the observatory was held. The lecturesand ceremony were performed with about 120 participants, followed by the celebration party.

Scientific Activities since 2012

The study of UHECRs by AGASA in Akeno was
succeeded by the Telescope Array (TA) experiment in
Utah, USA since 2008. After the shutdown of AGASA
in 2004, all the AGASA detectors were dismantled andthe Akeno Observatory has been used for small-scalecosmic-ray experiments, astrophysical observations andas a test/assembly/R&D facility of TA and other experiments by the ICRR and university researchers1 . As of
2018, there are the main campus of approximately ten
thousand square meters (Fig. 1), one small site of the
MITSuME telescope, three small sites each of a muonhouse, and one small site of an unused muon house.
One assistant technical staff works twice per week inorder to maintain the Akeno Observatory.
Research and development for the Telescope Array observation in Utah by the TA collaboration: In 2016 and 2017, plastic scintillator counters for*1 In Japanese Fiscal Year (JFY) 2017, 14 proposals that included theusage of the Akeno Observatory were accepted for the ICRR joint
research program.

Fig. 2. Assembly of a plastic scintillator counter at the large experimantal hall of the Akeno Observatory

Observation by the multi-color imager for transients, survey and monstrous explosions (MITSuME) by N. Kawai (Tokyo Institute of Technology), et al.: One of the three MITSuME robotic
telescopes was installed in the Akeno Observatory in2003 on the roof of the unused concrete muon house(Fig. 3). The telescope has an aperture of 50 cm, an
FOV of 28’ × 28’ and is equipped with a tricolor CCD
camera capable of g0 RC IC -bands photometry. It is operated remotely from the Tokyo Tech. Upon receiving a
GRB alert from Swift or Fermi satellite, it quickly directs the telescope (9◦ /s maneuverability) toward the
GRB direction, and makes a prompt observation of
the GRB and its afterglow. Since its commissioningin 2004, the MITSuME telescope in Akeno has been
*2 E. Kido et al., “The TAx4 experiment”, CRI199 and S. Udo et al.,“The Telescope Array Low-energy Extension”, CRI069, 35th ICRC
(Busan) 2017

156

making more than ∼10 GRB follow-up observations every year, and detected GRB afterglows. It has been
also joining the multi-wavelength campaign for observing AGNs and other transient objects. The MITSuME
telescope joined J-GEM and GROTH3 , and performed
follow-up observation of gravitational wave events for
GW 1701044 and GW 170817A with visible light. In
November of 2017, the two MITSuME telescopes at
the Akeno Observatory and the Okayama Astrophysical Observatory did cooperative follow-up observationof burst phenonmena.
Fig. 4. One of the three muon detector housings with concrete
shielding.

Fig. 3. The dome in which the MITSuME telescope was installed in
Akeno.

Observation of galactic cosmic rays by large area
muon telescope by A. Oshima (Chubu University), et al.: Four layers of proportional counter telescopes, each with 25 m2 area, were installed in eachof three muon houses in Akeno (Fig.4) and the cosmic ray muons have been measured since 2003. The
mode energy of the primary cosmic rays is approximately 25 GeV corresponding to 2-m thick concrete
ceiling of the muon house and the latitude of Akeno
Observatory. It is also aimed that the measurement in
Akeno is combined with a simultaneous measurementwith GRAPES-3 in Ooty, India, and the modulation effects of galactic cosmic rays by the solar activity such as
the Forbush decrease and its precursor are monitored.
The instruments are now under redevelopment in accordance with the upgrade for the GRAPES-3 muon
telescope. The analysis of the data with the Akeno
muon telescope was carried out and clear solar time
variation of muon flux was reported in the annual report of ICRR 2015.
Research and development for a small atmospheric Cherenkov telescope by T. Yoshikoshi(ICRR), et al.: A small alt-azimuth telescope with
an aperture of three meters was setup at the main
campus of the Akeno Observatory (Fig. 5) for prototype tests with atmospheric Cherenkov observations of
gamma rays 5 . This telescope is the only telescope to*3 Kawai et al., “MITSuME, Murikabushi and Subaru Telescopes”,
GROWTH Conference 2016, 2016/07/25-27, Caltech*4 LVC-GCN #20430*5 T. Yoshikoshi et al., “A 3-Meter Atmospheric Cherenkov Telescope
as a Test Bench for Very High Energy Gamma-Ray Astrophysics
Projects”, 34th ICRC (The Hague), 887 (2015)

observe atmospheric Cherenkov light emitted from air
showers induced by TeV gamma rays in Japan. Refurbishing of the telescope control system, the unit mirror
and optics were assembled in 2010 and 2011. In JFY
2016, an atmospheric Cherenkov light event was firstly
observed with this telescope. By the observation in
JFY 2017, the data of the Crab Nebula were taken.

Fig. 5. The Cherenkov telescope at the tour of the 40th anniversaryof the Akeno Observatory

Research of ultra-low frequency anti-vibration
system for KAGRA by R. Takahashi (NAOJ),
et al.: Since 2011, the large experimental hall of the
Akeno Observatory has been partially utilized for theevaluation of the performance of ultra-low frequency
anti-vibration system and advance check of fitting of
the system at the installation site of the KAGRA Observatory.

157

NORIKURA OBSERVATORY
Introduction

Norikura Observatory (36.10◦ N and 137.55◦ E) was
founded in 1953 and attached to ICRR in 1976. It is
located at 2770 m above sea level, and is the highest
altitude manned laboratory in Japan (Fig. 1). Experimental facilities of the laboratory are made available toall the qualified scientists in the field of cosmic ray research and associated subjects. The AC electric poweris generated by the dynamo and supplied throughout
the observatory. The observatory can be accessed easily by car and public bus in summer (July-September).
The 60th anniversary of Norikura Observatory was celebrated in 2013.
Fig. 2. A dynamo of 115KV.

thunderclouds, all of which require long-term observation. These researches have been carried out by thegroup of user universities, where ICRR provides themwith laboratory facility. A part of the facility has been
open for the environmental study at high altitude suchas aerosol-related mechanism in the atmosphere, observation of total ozone and UV solar radiation, forbotanical study in the high-altitude environment, etc..

Cosmic Ray Physics
Fig. 1. Norikura Observatory

Norikura Observatory gave manned operation to theobservations by the qualified scientists all the year until the year 2003. However, the feasibility of the automatic operation of Norikura Observatory during winter period has been tested since winter 2004 in orderto study the possibilities to reduce maintenance andlabor costs without causing serious inconveniences forthe researches. A long-distance (∼40km) wireless LAN
system (11M bps) was set up in 2003. Two new easyto-handle and easy-to-maintain dynamos of 115 KVA
each, as shown in Fig. 2 were installed in 2004 as well.
The unmanned operation of Norikura Observatory has
been mostly successful in winter, during which the battery backed-up solar panels and/or wind power generators kept supplying the electricity to the wireless LAN
and on-going cosmic-ray experiments.
Present major scientific interests of the laboratory is
focused on the modulation of high energy cosmic raysin the interplanetary space associated with the solar
activity, the generation of energetic particles by the solar flares, and the particle acceleration mechanism inSpace weather observation [represented by
Kazuoki Munakata, Shinshu University]
Space weather observation is actively made by a
25 m2 muon hodoscope at Norikura Observatory. Mt.
Norikura muon hodoscope has started operation in
May, 1998 and successfully observed a clear precursory signature of the interplanetary shock arrival at
Earth. With its improved angular resolution of muon
incident direction, the detector succeeded for the firsttime to observe a loss-cone signature which is an intensity deficit within a narrow cone around the interplanetary magnetic field (IMF). The observation of theloss-cone precursor gives us unique information for the
space weather forecast and for understanding the interplanetary disturbances in near Earth space.
Following this successful observation, we installeda small muon hodoscope in Kuwait City, Kuwait as
the fourth detector in our Global Muon Detector Network (GMDN) with other three multidirectional muon
detectors in Nagoya (Japan), Hobart (Australia), andSão Martinho (Brazil). The GMDN has started operation in March, 2006 monitoring the intensity of ∼ 50
GeV cosmic rays over an entire sky around Earth.
The cosmic ray observations using muon detector are158

complementary to observations with neutron monitors
monitoring a lower energy range below ∼ 10 GeV andthe observations with GMDN have a great advantage
particularly in precise measurement of the cosmic ray
anisotropy, i.e. the dependence of intensity on incident
direction in space, which gives us valuable informationof the spatial distribution of the cosmic ray density inthree dimensions. The Mt. Norikura muon hodoscopeand GMDN have revealed the dynamic variations of
the anisotropy which give us important informationof the space weather. It has been already confirmedthat the GMDN can measure the rapid variation of
the anisotropy in the “cosmic ray burst” observed inJune 2015 in 10 minute time resolution. The Kuwait
muon hodoscope was enlarged three times in March
2016 and one minute data are now available from all offour detectors in the GMDN enabling us to analyze theanisotropy in 1 minute time resolution.
Recently, we also developed the method of the correction of the atmospheric temperature effect on muoncount rate by using the GMDN data. This is a significant step, because it makes possible for the first time
the analysis of the long-term variation of ∼ 50 GeV
cosmic ray density (i.e. isotropic intensity) which waspossible so far only for cosmic ray below ∼ 10 GeV using
the neutron monitor data nearly free from the temperature effect. We have already published the long-term
variation of the anisotropy observed by Nagoya muon

detector.1 2 3 4 5 6 7 8 9 10
Solar neutron observation [represented by Yutaka Matsubara, Nagoya University]
Observation of solar neutrons in solar cycle 24 hascontinued at Norikura Observatory of ICRR since fiscal
2007 to understand the acceleration mechanism of highenergy (>100 MeV) ions associated with solar flares.
These neutrons are produced by the interaction between accelerated ions and the solar atmosphere. Neutrons are not reflected by the interplanetary magnetic
field, and thought to be more informative than accelerated ions themselves to study the acceleration mechanism at the solar surface. Solar neutron events detected*1 “ Solar neutron events in association with large solar flares in
November 2003”, Watanabe, K. et al., Adv. Space Res., 38, 425–
430, 2006.
*2 K. Munakata, M. Kozai, P. Evenson, T. Kuwabara, C. Kato, M.
Tokumaru, M. Rockenbach, A. Dal Lago, R. R. S. Mendonca, C.
R. Braga, N. J. Schuch, H. K. Al Jassar, M. M. Sharma, M. L.
Duldig, J. E. Humble, I. Sabbah, and J. Kota, “Cosmic Ray Short
Burst Observed with the Global Muon Detector Network (GMDN)
on June 22, 2015”, Astrophys. J., 862:170 (9pp), 2018 (August 1).
*3 R. R. S. Mendonca, C. R. Braga, E. Echer, A. Dal Lago, M. Rockenbach, N. J. Schuch„ K. Munakata, “Deriving the solar activity cycle modulation on cosmic ray intensity observed by Nagoyamuon detector from October 1970 until December 2012”, Proc.
IAU Sympo., 328, 1-4 (IAU-16-IAUS328-0453), 2016 (October 20).
*4 R. R. S. Mendonca, C. R. Braga, E. Echer, A. Dal Lago, K. Munakata, T. Kuwabara, M. Kozai, C. Kato, M. Rockenbach, N. J.
Schuch, H. K. Al Jassar, M. M. Sharma, M. Tokumaru, M. L.
Duldig, J. E. Humble, P. Evenson, I. Sabbah, “Temperature effectin secondary cosmic rays (muons) observed at ground: analysis of
the global muon detector network data”, Astrophys. J., 830:88(25pp), 2016 (October 20). Cited by 2
*5 M. Kozai, K. Munakata, C. Kato, T. Kuwabara, M. Rockenbach,
A. Dal Lago, N. J. Schuch, C. R. Braga, R. R. S. Mendon, H. K.
Al Jassar, M. M. Sharma, M. L. Duldig, J. E. Humble, P. Evenson,
I. Sabbah, and M. Tokumaru, “Average spatial distribution of cosmic rays behind the interplanetary shock– Global Muon Detector
Network observations”, Astrophys. J., 825:100 (19pp), 2016 (July10). Cited by 7
*6 D. Ruffolo1, A. Saiz, P.-S. Mangeard, N. Kamyan, P. Muangha, T.
Nutaro, S. Sumran, C. Chaiwattana, N. Gasiprong, C. Channok,
C. Wuttiya, M. Rujiwarodom, P. Tooprakai, B. Asavapibhop, J. W.
Bieber, J. Clem, P. Evenson, and K. Munakata, “Monitoring shortterm cosmic-ray spectral variations using neutron monitor timedelay measurements”, Astrophys. J., 817:38 (12pp), 2016 (January
20). Cited by 8
*7 M. Kozai, K. Munakata, C. Kato, T. Kuwabara, J. W. Bieber, P.
Evenson, M. Rockenbach, A. Dal Lago, N. J. Schuch, M. Tokumaru,
M. L. Duldig, J. E. Humble, I. Sabbah, H. K. Al Jassar, M. M.
Sharma, J. Kota, “The spatial density gradient of galactic cosmic
rays and its solar cycle variation observed with the Global Muon
Detector Network”, Earth, Planets and Space, 66, 151-158, 2014
(November 14). Cited by 12*8 K. Munakata, M. Kozai, C. Kato, J. Kota, “Long term variationof the solar diurnal anisotropy of galactic cosmic rays observedwith the Nagoya multi-directional muon detector”, Astrophys. J.,
791:22, 1-16, 2014 (August 10). Cited by 19*9 M. Rockenbach, A. Dal Lago, N. J. Schuch, K. Munakata, T.
Kuwabara, A. G. Oliveira, E. Echer, C. R. Braga, R. R. S. Mendonca, C. Kato, M. Kozai, M. Tokumaru, J. W. Bieber, P. Evenson, M. L. Duldig, J. E. Humble, H. K. Al Jassar, M. M. Sharma,
I. Sabbah, “Global muon detector network use d for space weather
applications”, Space Sci. Rev., 182, 1-18, 2014 (May 9). Cited by
18*10 K. Munakata“For space weather applications”, Space Sci. Rev.,
182, 1-18, 2014 (May 9).

159

on the ground are rare, and about 10 events were reported before solar cycle 24. The group led by Institute for Space-Earth Environmental Research, Nagoya
University has operated a world-wide network of 7 solar neutron telescopes in the world. The solar neutrontelescope operated at Norikura Observatory has an areaof 64 m2 , which is largest among the 7 stations. The
solar neutron telescope at Norikura consists of plastic
scintillation detector and proportional counters. The
neutron is detected when a recoil proton is produced in
the scintillator, and the energy of the recoil proton ismeasures. Proportional counters are used both to vetocharged particles and measure the direction of recoil
protons. The telescope is operated by solar power during the winter period when the Norikura observatory is
closed.
Solar cycle 24 was its maximum in February 2014and has decreased its activity since then. We searched
for solar neutron signals from the world-wide networkof the solar neutron telescopes between January 2010
and December 2014 when the large (≥X1.0 class) solar flare occurred. No solar neutron event was detectedby this search. We statistically studied the relation between upper limits of the neutron flux and the energy ofsoft X-rays during the solar flare. This comparison was
also made for the successful detections of solar neutrons
before solar cycle 24. The conclusion from this study isthat the total energy obtained by neutrons during solar
flare does not exceed 0.1 % of the total energy of soft
X-rays.11
Relativistic electron acceleration in thunderstorm electric field and high-energy atmosphericphenomena at lightning [represented by Teruaki
Enoto, Kyoto University, for GROWTH Collaboration]
The Gamma-Ray Observation of Winter THundercloud (GROWTH) collaboration is aiming at revealing
high-energy atmospheric phenomena occurring at lightning and in thunderstorms. The project started in 2006
and has detected bremsstrahlung gamma rays from relativistic electrons accelerated by strong electric fields
in winter thunderstorms. This gamma-ray radiation
events were named “long burst” by our group (as known
as gamma-ray glow), which lasts for a minute time-scalecorresponding with passage of a thundercloud above
our detectors. Winter thunderstorms observed along
the Japan sea are ideal targets for our observation campaigns thanks to its low altitude of the cloud base andfrequent energetic lightning, while observations of summer thunderclouds at mountain tops are also important to measure the phenomena very close to or eveninside thunderstorms. Our collaboration has used the*11 D. Lopez et al., Estimates of the neutron emission during solar flaresin the rising and maximum period of solar cycle 24, Astroparticle
Physics, 76 (2016) 19-28.

Mount Norikura cosmic ray observatory to study thelong bursts, and successfully recorded events.
The GROWTH collaboration newly launched multipoint mapping observation campaigns in 2015. The primary purpose is to study life cycle of the electron acceleration sites in thunderstorms comparing with weathermonitoring data. Another purpose of the project is
to reveal mysterious “short burst” events, which havebeen sometimes detected in our past observations, associated with lightning discharges with its duration
shorter than a second. Financially supported by the
ICRR joint research programme, academic crowdfunding “academist”, and JSPS/MEXT KAKENHI grant,
we have developed portable and high-performance radiation detectors. In FY2016 and FY2017, the collaboration deployed radiation detectors at the Mt. Norikura
cosmic ray observatory. So far, there is no detectionof “long burst” nor “short burst” events from summer
thunderclouds during the two years. However, we successfully used these summer campaigns as pilot observations toward the winter campaigns to check our operation and capability of the detectors. The Norikura observations are also educationally important as a training yard for Ph.D students in the team. In 2017 winter,our new mapping system at Kashiwazaki, Niigata, provided us a chance to solve the mystery of the shortburst. This phenomena is revealed to be photonuclear reaction triggered by gamma rays from a lightning discharge. This discovery was selected, by the
Physics World, as one of the top 10 breakthrough in thephysics field in 2017.12 We are now trying to develop
the “high-energy atmospheric physics” of lightning andthunderstorms, a new interdisciplinary field combining
the gamma-ray and radio observations.13 14 15 16*12 Enoto, T. and Wada, Y. and Furuta, Y. and Nakazawa, K. andYuasa, T. and Okuda, K. and Makishima, K. and Sato, M. andSato, Y. and Nakano, T. and Umemoto, D. and Tsuchiya, H, ”Photonuclear reactions triggered by lightning discharge”, Nature 551,
(2017) 481-484.
*13 Dwyer, J. R. and Smith, D. M. and Cummer, S. A., ”High-Energy
Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related
Phenomena”, 173 (2012) 133-196.
*14 Tsuchiya, H. and Enoto, T. and Torii, T. and Nakazawa, K. andYuasa, T. and Torii, S. and Fukuyama, T. and Yamaguchi, T.
and Kato, H. and Okano, M. and Takita, M. and Makishima, K.,”Observation of an Energetic Radiation Burst from Mountain-Top
Thunderclouds”, Physical Review Letters, 102, (2009) 255003.
*15 Wada, Y. and Bowers, G. S. and Enoto, T. and Kamogawa, M.
and Nakamura, Y. and Morimoto, T. and Smith, D. M. and Furuta, Y. and Nakazawa, K. and Yuasa, T. and Matsuki, A. andKubo, M. and Tamagawa, T. and Makishima, K. and Tsuchiya,
H., ”Termination of Electron Acceleration in Thundercloud by Intracloud/Intercloud Discharge”, Geophysical Research Journal 45,(2018) 5700-5707.
*16 Tsuchiya, H. and Enoto, T. and Yamada, S. and Yuasa, T. and
Kawaharada, M. and Kitaguchi, T. and Kokubun, M. and Kato,
H. and Okano, M. and Nakamura, S. and Makishima, K., ”Detection of High-Energy Gamma Rays from Winter Thunderclouds”,
Physical Review Letters, 99, (2007) 165002.

160

Study of Secondary Cosmic Rays from Thundercloud at Mt. Norikura [represented by Atsushi
Shiomi, Nihon University]
In order to study the relativistic electron acceleration mechanism by thundercloud-derived electricfield and the relation between thunder and cosmic-ray
air shower, we started an experiment in 2015, mainly
using gamma-ray detectors at Norikura Observatoryof Institute for Cosnic Ray Research, the Universityof Tokyo. This experiment consists of gamma-ray
detectors, a lightning sensor, an electric field meter, aweather monitor, and an air shower array. Gamma-ray
detectors using three crystals, NaI, CsI, and BGO
respectively, cover the energy range over 3 orders ofmagnitude from 70 keV to 120 MeV as a whole.
Observation period: 22 days from August 24 to
September 14, 2015
During this time, the thundercloud did not pass, andno gamma-ray burst derived from thundercloud was
detected. However, the gamma ray detector was able
to observe gamma rays derived from radon of less than
3 MeV as expected during rainfall. Also, gamma ray
detectors using CsI and BGO for detecting gamma
rays of 3 MeV or more did not observe an increasein gamma rays of 3 MeV or more. This means that
gamma rays from radon can be distinguished fromcontributions from other gamma rays not derived fromradon, and it can be expected that these detectors
can operate normally even when they are installedat an altitude about 4000 m for a certain future project.
Observation period: 30 days from July 19 to September
16, 2016 (excluding the summer season)
No direct lightning strike event occurred during theobservation period, but 6 atmospheric electric field
fluctuations exceeding 30 kV/m, which seemed to be
an influence of thunder cloud passage, were observed.
In one of the events, it seems there was a lightning
strike in the vicinity.
Observation period: 41 days from July 31 to September
9, 2017
Unfortunately, during this observation period there wasno lightning strike nearby, but it was observed thatthe thundercloud passed several times. Currently, thedata at the time of the thundercloud passing is being
analyzed in detail (Fig. 3).
Study of gamma ray bursts from mountain-top
thunderclouds [represented by Makoto Minowa,
The University of Tokyo]
We observed gamma ray bursts that arise in relation to thunderclouds at the Norikura Observatory of
ICRR(2,770 m above sea level). Measurement was carried out by placing PANDA64 detector outdoors of theobservatory. The detector is made of 64-module plas-

Fig. 3. Electric field (green), rainfall (blue), count value (purple) of
each detector at the time of thundercloud passing.

tic scintillators(total mass about 640 kg) developed forreactor operation monitoring. Our measurement has
unprecedented features including high statistics, goodenergy response, direction sensitivity and neutron identification.
Long-duration persistent bursts were observed 12
times in 54 days from July to September 2014 and theirenergy spectrum extended up to 25 MeV in the largest
burst. The duration of the bursts ranged from a few to
ten minutes. Since these bursts were found in the energy range higher than 3 MeV, they were not attributed
to the rain fallout of radon and its daughter nuclei.
According to the thundercloud information provided
by the Japan Meteorological Agency, the bursts wereobserved when there was thunder activity near the observatory. The observation is qualitatively in good
agreement with thundercloud radiation bursts previously observed in mountain areas or coastal areas of
the Sea of Japan.
Monte Carlo simulation showed that thebremsstrahlung γ -rays by source electrons withmonochromatic energy of 40–80 MeV falling downwards from altitude of 400–1000 m produced theobserved total energy spectra of the bursts well. It
is supposed that secondary cosmic ray electrons,
which act as seed, were accelerated in electric field ofthunderclouds and multiplied by relativistic runaway
electron avalanche.
The estimated energy of the source electrons washigher than that of the bursts we previously observed
at Ohi Power Station at sea level. Additionally, estimated electron flux at the estimated source height was
remarkably lower than that of the Ohi site. These results give new restrictions to the model of electron acceleration and multiplication process in electric field of161

thunderclouds.17 18 19 20
Development of high energy proton irradiation
technique for devices used in spaceship [represented by Kyo Kume, The Wakasa Wan Eenergy
Research Center]
Space exploration is presently interesting in businessfield. Ion beam irradiation verification for devices to be
mounted on spaceships is required to simulate cosmic
rays expected in the universe to estimate lifetime ofthese devices.
Flux estimation technique of primary ion beam inwide range from an accelerator is needed In this kindof cosmic ray simulation field. The desired flux ofthe ion beam for this kind of field is between 102 and106 protons·cm−2 ·s−1 in typical proton cases. Plastic scintillators can be used in lower intense region tocount direct primary ions, while ionization chambers
can be used in higher intense region to count ionization
caused by primary ions. But there have been no definite modalities available to measure throughout thiswhole intensity region.
One of the candidate techniques is to measure secondary γ -ray intensity emitted through a beam transport, which has nearly a maximum energy of the primary ion beam around 100 MeV for this kind of simulation field. This technique has a feature that detector components do not occupy the beam path and thepresence of the detector do not influence the main simulation field at all.
NaI(Tl) scintillator system for high energetic γ -raymeasurement which had been used in previous thunder lightning γ -ray measurement was used. This system has a 5-inch NaI(Tl) scintillator with NT100GPS
pulse hight analysis system of Laboratory Equipment.
Ion beam accelerator experiments using 100 MeV proton beam were carried out at the Wakasa Wan Energy
Research Center (WERC). Background measurements
were carried out at Norikura Observatory of ICRR insummer, where one can expect high energetic γ -ray,which has the similar energy region compared to theaccelerator field of this study. EFM100 atmosphericelectric field monitor system of Boltek was added at
Norikura to measure accidental high energetic γ -rays
caused by thunder lightning.
*17 Yo Kato, “Observational study of thundercloud radiation bursts
using a segmented organic scintillator installed at a mounaintop”,
Ph.D. thesis, The University of Tokyo, September, 2015.
*18 Y. Kato, “Thundercloud-related radiation bursts observed at acoastal area and a mountaintop using segmented organic scintillators”, Thunderstorms and Elementary Particle Acceleration(TEPA-2015), 5-9 October 2015, Yerevan, Armenia.
*19 Y. Kato, “Development of Plastic Anti-neutrino Detector Array(PANDA)”, Applied Antineutrino Physics(AAP) 2015, 7-8 December 2015, Virginia Tech Research Center, United States.
*20 Y. Kato “Observation of thundercloud radiation bursts using segmented plastic scintillators”, European Geosciences Union General
Assembly (EGU), 8-13 April 2018, Austria Center Vienna, Austria.

Fig. 4. Time structure of γ -ray above 3 MeV at WERC 100 MeV
proton beam delivery. Vertical axis shows γ intensity ( events/ms) while horizontal shows time after beginning of the operation (s ).

As shown in Fig.4 of a time structure of γ -ray during 100 MeV proton beam delivery duration obtained at
WERC, the result clearly shows that this measurement
system can distinguish the beam ON/Off, while thissystem is still in verification for a quantitative discussion. The dead time of the system should be defined.
In the meantime, thunder lightning events weresearched using data obtained at Norikura. No events
have been distinguished. The whole data at Norikurashow a stable condition of the whole system for a coupleof months.
With the help of this study, the trial to carry out
cosmic ray simulation at the ion beam accelerator facility ( WERC ) has been successfully carried out. One
will keep trying to estimate the quantitative propertyof the system in ion beam environment.21 22 23 24
Development of high energy proton irradiation
technique for devices used in spaceship [represented by Kazuaki Yajima, National Instituteof Radiological Sciences, National Institutes for
Quantum and Radiological Sciences and Technology]
Aircraft crew are exposed to elevated levels of cosmic rays at aviation since the dose rate of cosmic rays
increases with altitude. The occupational doses of air*21 T. Torii et al., ”Gradual increase of energetic radiation associatedwith thunderstorm activity at the top of Mt. Fuji”, Geophys. Res.
Lett 36(13) , 2009.
*22 T. Kuritai et al., ”The Status of the synchrotron of the Wakasa
Wan Energy Research Center”, Proc. 12th Annual Meeting of theParticle Accelerator Societ of Japan, 288, 2015.
*23 K. Kume, T. Torii, M. Takita and T. Hasegawa, ”Developmentof a beam fluence measurement technique at atmosphere”, Igaku
Butsuri, Vol.36 Suppl.3 178, 2016 ( in Japanese ).
*24 K. Kume, T. Hasegawa, S. Hatori, M. Takita and H. Tsuji, ”Space
engineering application of therapeutic broad beam for cosmic rai
simulation”, Igaku Butsuri, Vol.37 Suppl. 112, 2017 ( in Japanese
).

162

craft crew have generally been evaluated by model calculation. It is necessary to verify the calculation withmeasurements to maintain accuracy and credibility ofdose assessment. The purposes of this study were toconstruct a compact and inexpensive cosmic-ray neutron monitoring system which was based on a remcounter at Norikura Observatory (2770 m above sea
level), and to examine the feasibility of it. The monitoring system was installed in the Norikura Observatory
in 2014. It consisted of an extended-energy neutron
rem counter with wide energy range from 25 meV to 5
GeV, a custom-made data logger connected to LAN,and a battery power unit. The measured data wasreceived in National Institute of Radiological Sciencein Chiba-city via the ICRR network. This monitoring
system succeeded in continuous monitoring more thanten months twice during 2014 to 2016. The averaged
counting rate was about 1 count per minute, which was
equivalent to neutron ambient dose equivalent rate ofabout 15 nSv/h by the preliminary evaluation. It is a
future problem to reveal the cause of the reduction incounting rate seen over from November to April.
Evaluation of Response to the Gamma-ray of
the Emulsion Telescope (2007, 2013 [represented by Shigeki Aoki for GRAINE collaboration, Kobe University]
GRAINE project (Gamma-Ray Astro-Imager with
Nuclear Emulsion) has been developing the observation of cosmic γ -ray in the energy range 10 MeV–100
GeV with precise (0.08◦ at 1–2 GeV), polarizationsensitive, large-aperture-area (∼10 m2 ) balloon-borne
telescope using nuclear emulsion film. Under the development of the telescope, we performed test observation at Norikura Observatory (2770 m a.s.l.) in 2007
and 2013 using prototype emulsion telescope in order toconfirm detection performance using atmospheric γ -ray.
2007 test was the first trial of the detection to theγ -ray spread wide incoming angle. We established configuration of the telescope and its analysis scheme.
Based on this experience, we finalized the design of thefirst balloon-borne emulsion telescope and performed
1st balloon experiment (GRAINE 2011) at the Taiki
Aerospace Research Field of JAXA in June 2011.
In 2013 test, we introduced self-produced nuclear
emulsion gel film with higher volume occupancy of silver bromide crystals with respect to conventional ordinary gel in order to improve track finding efficiency aswell as signal-to-noise ratio. We obtained high (> 97%)
track finding efficiency in a single film and confirmed
γ -ray detection capability at 100 MeV energy region(Figure 5).
Based on this experience, we performed 2nd balloon
experiment (GRAINE 2015) in Japan-Australia JAXA
collaborative balloon experiment at the Alice Springs

Fig. 5. 3-D view of γ → e+ +e− detected in the chamber employed in
the obseration test at Norikura Observatory. The reconstructedenergy of this event was 160 MeV.

balloon-launching station in May 2015. 25 26 27 28 29
30 31

Environmental Study

Aerosol sampling at Mt. Norikura [represented
by Fuyuki Tokanai, Yamagata University]
Aerosol in the atmosphere has been sampled since2013 at the Norikura observing site using air-samplers
to investigate the production of cosmogenic nuclide 7 Be
in a free troposphere above 2 km in the altitude. The
aerosol size distribution of 7 Be was measured for theaerosols sampled by an Andersen sampler enable to separate aerosols to nine classes from 0.43 µ m to 11 µ m.
The 81.7% of 7 Be was covered with the aerosol sizes
less than 1.1 µ m and the 7 Be with the aerosol sizes
above 1.1 µ m decrease with an exponential function.
The 7 Be concentration at Mt. Norikura was approximately 9.4 times greater than that at the ground level*25 Satoru Takahashi,Ph.D Thesis (2011) Nagoya University,
http://hdl.handle.net/2237/14900.
*26 Hiroaki Kawahara, Master Thesis (2015), Nagoya University*27 “GRAINE project: gamma-ray observation with a balloon-borne
emulsion telescope”, Hiroki Rokujo on behalf of GRAINE Collaboration, Proceeding of Science KMI2013 (2014) 042.
*28 “GRAINE project: The first balloon-borne, emulsion gamma-ray
telescope experiment”, S. Takahashi, S. Aoki, K. Kamada, S. Mizutani, R. Nakagawa, K. Ozaki and H. Rokujo, PTEP 2015 (2015)
no.4, 043H01.
*29 “Gamma-Ray Astro-Imager with Nuclear Emulsion, GRAINE (inJapanese)”, Satoru Takahashi, Shigeki Aoki for GRAINE collaboration, Journal of the SPSTJ, Vol,78(2015) No.4, pp.228-234.
*30 “GRAINE 2015, a balloon-borne emulsion γ -ray telescope experiment in Australia”, Satoru Takahashi et al. (GRAINE collaboration), PTEP 2016 (2016) no.7, 073F01.
*31 “GRAINE project, prospects for scientific balloon-borne experiments”, Satoru Takahashi, Shigeki Aoki for GRAINE collaboration, Advances in Space Research, Articles in press.

163

in Yamagata-city (Fig.6.). Its ratio is almost consistent to a simulated ratio 8.8 of 7 Be productions due tosecondary cosmic rays in the atmosphere by EXPACS.
This experimental result is useful for an estimation ofaltitude distribution of cosmogenic nuclide.

conduits were not observed even in severely-embolized(complete loss of conductivity) shoots. Rather, the pits
(valves of partitions inter-conduits) closed before thesevere frost-drought in mid-winter, thereby resulting insevere-embolism (complete loss of conductivity). Thus,by pit (valve) cloure, shoots could maintain water in thexylem throughout the winter, which is thought to be an
adaptation against lethal filling of air in the conduits
during severe frost-drought.
3. Photosynthesis of Haimatsu (Pinus pumila)At wind-exposed sites on Mt. Norikura, photosynthesis in Haimatsu was suppressed by lower mesophyll
CO2 conductance, and not by stomatal closure. 32 33
34

Fig. 6. 7 Be concentration as a function of aerosol size

Adaptation of alpine plants to severe environmental conditions [represented by Emiko
Maruta, Kanagawa University]
Trees in the alpine regions experience harsh conditions including strong winds, low temperatures, desiccation, and heavy snow. Thus, plants growing in suchregions are predicted to have adaptations to these environmental stressors. Through the inter-university research of ICRR, we obtained an opportunity to intensively study plant responses to environmental factors.
We identified several characteristics unique to alpine
plants, some of which contradict conventional knowledges.
1. Adaptation of leaf cuticles in sub-alpine fir (Abies
mariessii) at the alpine tree-line
Leaf browning and death are frequently observed inevergreen conifers at the alpine tree-line. These arethought to due to increased transpiration caused by athinner cuticle and/or mechanical damage to the cuticle by wind-blown snow and ice particles. However, inthe sub-alpine fir (Abies mariessii) at Mt. Norikura,mechanical damage was not observed, and the cuticle
was rather thick, which may be an adaptation against
overwintering at the alpine regions.
2. Embolism of sub-alpine fir (Abies mariessii) at thealpine tree-line
Trees at high altitudes experience severe embolism(loss of xylem conductivity for water) during winters,
which is attributed to the entrapment of air in xylem
conduits during frost-drought. However, in the subalpine fir (Abies mariessii) at Mt. Norikura, air-filled

Investigation of alpine plants on Mt. Norikura[represented by Koichi Takahashi, Shinshu University]
We studied mainly the following three researches on
Mt. Norikura after 2012.
1) Long-term monitoring and community assembly of
alpine plants
We made 40 plots for long-term monitoring of alpine
vegetation because climate change possibly affects distributions of alpine plants. We also examined the community assembly process of alpine plants at the 40 plots
from the view points of habitat filtering and limiting
similarity. Habitat filtering and limiting similarity relate environmental conditions and interspecific competition, respectively. It is suggested that habitat filtering is more important than limiting similarity for thecommunity assembly of alpine plants.
2) Soil respiration rates along an altitudinal gradientThis study investigated seasonal changes of soil respiration rates from forest soil along an altitudinal gradient (1600 m to 2800 m above sea level). The soil
respiration rate positively correlated with soil temperatures and forest biomass. It is suggested that forest
productivity is an important factor for soil respiration
rates.
3) Genetic differentiation of Solidago virgaurea complex
Plant species distributed along wide altitudinal orlatitudinal gradients show phenotypic variation due totheir heterogeneous habitats. This study investigatedwhether phenotypic variation in populations of the Solidago virgaurea complex along an altitudinal gradientis caused by genetic differentiation. Population genetic*32 Nakamoto A., Ikeda T., Maruta E. (2013) Needle browning anddeath in the flagged crown of Abies mariesii in the timberline ecotone of the alpine region in central Japan. Trees 27:815-825.
*33 Maruta E., Yazaki K.(submitted) Mechanism of embolism as
induced by pit closure during winter in sub-alpine fir (Abies
mariesii)on Mt. Norikura.
*34 Nagano S., Nakano T., Hikosaka K., Maruta E. (2013) Pinus pumila
photosynthesis is suppressed by water stress in a wind-exposed
mountain site. Arctic, Antarctic, and Alpine Research 45:229-237.

164

analyses with microsatellite markers were used to infer
the genetic structure and levels of gene flow between
populations. However, the population genetic analysis
suggested an extremely low level of genetic differentiation of neutral genes among the nine populations.
This study suggests that genome regions responsible
for adaptive traits may differ among the populations
despite the existence of gene flow and that phenotypicvariation of the S. virgaurea complex along the altitudinal gradient is maintained by strong selection pressure.
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49*35 Takahashi, K., Hirosawa, T. and Morishima, R. (2012) How thetimberline formed: altitudinal changes in stand structure and dynamics around the timberline in central Japan. Annals of Botany
109: 1165-1174.
*36 Takahashi, K. and Okuhara, I. (2013) Forecasting the effects ofglobal warming on radial growth of subalpine trees at the upperand lower distribution limits in central Japan. Climatic Change
117: 278-287.
*37 Takahashi, K. and Obata, Y. (2014) Growth, allometry and shadetolerance of understory saplings of four subalpine conifers in central
Japan. Journal of Plant Research127: 329-338.
*38 Takahashi, K. (2014) Effects of wind and thermal conditions ontimberline formation in central Japan: a lattice model. EcologicalResearch 29:121-131.
*39 Takahashi, K. and Koike, S. (2014) Altitudinal differences in bud
burst and onset and cessation of cambial activity of four subalpine
tree species. Landscape and Ecological Engineering 10:349-354.
*40 Takahashi, K. and Murayama, Y. (2014) Effects of topographic andedaphic conditions on alpine plant species distribution along a slope
gradient on Mount Norikura, central Japan. Ecological Research
29: 823-833.
*41 Singh, D., Takahashi, K., Park, J. and Adams, J. M. (2016) Similarities and contrasts in the archaeal community of two Japanesemountains: Mt Norikura compared to Mt Fuji. Microbial Ecology
71: 428-441.
*42 Takahashi, K. and Furuhata, K. (2016) Shoot growth and seasonalchanges of non-structural carbohydrate concentrations at the upper and lower distribution limits of three conifers. Landscape andEcological Engineering 12: 239-245.
*43 Takahashi, K. and Tanaka, S. (2016) Relative importance of habitat
filtering and limiting similarity on species assemblages of alpine
and subalpine plant communities. Journal of Plant Research 129:
1041-1049.
*44 Takahashi, K. and Matsuki, S. (2017) Morphological variations of
the Solidago virgaurea L. complex along an elevational gradient on
Mt. Norikura, central Japan. Plant Species Biology 32: 238-246.
*45 Kerfahi, D., Tateno, R., Takahashi, K., Cho, H., Kim, H. andAdams, J. M. (2017) Development of soil bacterial communities on
volcanic ash microcosms in a range of climates. Microbial Ecology
73: 775-790.
*46 Sakurai, A. and Takahashi, K. (2017) Flowering phenology andreproduction of the Solidago virgaurea L. complex along an elevational gradient on Mt. Norikura, central Japan. Plant Species
Biology 32: 270-278.
*47 Dong, K., Moroenyane, I., Tripathi, B., Kerfahi, D., Takahashi,K., Yamamoto, N., An, C., Cho, H., and Adams, J. (2017) Soil nematodes show a mid-elevation diversity maximum and elevational
zonation on Mt. Norikura, Japan. Scientific Reports 7: 3028.
*48 Hirano, M., Sakaguchi, S. and Takahashi, K. (2017) Phenotypic
differentiation of the Solidago virgaurea complex along an elevational gradient: Insights from a common garden experiment andpopulation genetics. Ecology and Evolution 7: 6949-6962.
*49 Takahashi, K., Otsubo, S. and Kobayashi, H. (2017) Comparisonof photosynthetic traits of codominating subalpine conifers Abies
veitchii and A. mariesii in central Japan. Landscape and EcologicalEngineering 14: 91-97.

165

KAGRA Observatory
Introduction

KAGRA observatory is located in the Ikenoyamamountain on the border between Gifu and Toyama prefecture, about 35 km south of Toyama city in Japan.
The observatory was established in 2016 in order to operate Large-scale Cryogenic Gravitational Wave Telescope (nicknamed “KAGRA”). KAGRA itself has a Lshape tunnel facility, and it is located more than 200 m
under Ikenoyama-mountain. The corner station of the
L-shape tunnel is accessible through a 500 m horizontal
access tunnel from Atotsu area. The observatory has
its own surface research buildings and rental space inthe community center of Hida city located about 5 km
away from the Atotsu entrance of KAGRA.

March 2019 and start joint observation with LIGO and
Virgo within 2019.

Scientific Goal

KAGRA aims to observe several gravitational waves(GWs) per year with its designed sensitivity as one ofobservatories of the world GW detection network including Advanced-LIGO, Advanced-Virgo and planned
LIGO-India. KAGRA project (formerly named LCGT)
was partially approved in 2010 as one of Leadingedge Research Infrastructure Program, and also supported by Program for Promoting Large-scale Science
Projects, Subsidy for Facilities Expense and Grants-inAid for Scientific Research from Ministry of Education,
Culture, Sports, Science and Technology (MEXT).

Fig. 1. Surface Research Building.

KAGRA Collaboration

In KAGRA project, Institute for Cosmic Ray Research plays a role of a host promoting institute, andNational Astronomical Observatory in Japan (NAOJ)
and High Energy Accelerator Research Organization(KEK) are the main support organizations, then morethan 280 researchers in 87 institutes and universities in
the world are collaborating for construction and data
analysis of KAGRA.

Construction Status

The tunnel excavation started in May 2012, andfinished in March 2014. After that, the basic laboratory environment was prepared until September 2015.
A Michelson interferometer with 3km arm (iKAGRA)
was demonstrated in March 2016, and the first engineering run was performed until May 2016. At present(June 2018) all interferometer components are being installed to complete KAGRA observatory that adopts a
power recycled Fabry-Perot Michelson type interferometer with resonant sideband extraction technique. We
hope to start KAGRA observatory operation aroundFig. 2. Atotsu Entrance of KAGRA.

166

RESEARCH CENTER FOR COSMIC NEUTRINOS
Research Center for Cosmic Neutrinos (RCCN) wasestablished in 1999 in order to promote the activities of neutrino researches. The center belongs to the
Cosmic Neutrino Research Division, and the scientificstaffs of the center have been working mainly for several research projects of the division such as SuperKamiokande, T2K, Hyper-Kamiokande. The research
activities, that these staffs have been made, are already included in the corresponding sections of theseprojects. This section describes other RCCN activities
which have been made as a role of the inter-university
joint-use research institution.
RCCN is in charge of the management of the ICRR
main computing system, which is also provided as aservice of the joint-use research program. Every researchers, who are accepted for the research program,are able to have the computing account and utilize
the computing resources. The system is managed under the ICRR computer committee, which is composedof ICRR staffs. The main part of the system consists of the computing clusters containing about 2000
CPU cores, and the file system of 4 Petabyte (PB) disk
space. A high-speed network is connected in-between
them, which enables users to read/write the large sizeof the files speedy and analyze the data efficiently. The
system also provides several servers of system control,
networking, internet services such as web, mail, online
storage, etc. The system has been upgraded in every
5∼6 years in order to have the benefits of the evolution of the latest computer performance. The current
system was renewed in January 2014 (Fig. 1). It has
been running stably without any serious trouble. The
overall efficiency of providing the computing resources
is greater than 95% at present (as of year 2018). The
next upgrade of the system is planned in 2020.
RCCN has been organizing the series of the domestic workshop since 2000, in order to stimulate the activities of the neutrino research. The workshop havebeen accepted as one of ICRR joint-use research program, with the external members of other universities.
The workshop has been held every year, and focusedon the several topics related to neutrino physics, which
are listed in Table 1. The photo of the latest workshop
held in 2018 is shown in Fig. 2.
RCCN is also contributing to the outreach of the scientific research results. RCCN and the Public Relation
Office of ICRR have been performing the public lectures every year, which are co-sponsored by ICRR and
the Kavli IPMU (Institute for the Physics and Mathematics of the Universe). The list of the public lectures,
held between 2013 and 2018, is shown in Table 2.
RCCN has been accepting several applications ofFig. 1. ICRR computering system managed by RCCN and ICRR
computer committee.

Fig. 2. Domestic neutrino workshop held in February 2018.

the joint-use research programs of ICRR, especially forthe low background radioisotope measurement in theunderground laboratory of the Kashiwa campus, andthe simulation researches using the ICRR computing
system. The underground laboratory is equipped withfour Germanium detectors (Fig. 3), mainly utilized forthe measurements of cosmic radioactive isotopes. The
scientific activities which have been made by using thislaboratory are described in other section.

167
Table 1. Series of the domestic neutrino workshop organized by RCCN between 2013 and 2018.

1
2
3
4
5
6

Date
March 15, 2013
January 20, 2014
February 21, 2015
February 20, 2016
February 4, 2017
February 24 2018

Main topic(s)θ13 and high energy cosmic neutrino
Absolute mass of neutrino measured from cosmology and ground experimentNon-standard framework of neutrino mixing – sterile neutrino and nonstandard interaction
Atmospheric neutrino
Physics of neutrino interaction
High energy neutrino and CP

Table 2. List of public lectures organized by RCCN and ICRR Public Relation Office. These lectures are co-hosted by ICRR and Kavli
IPMU.

1

Date
April 13, 2013

2

April 14, 20143

April 18, 2015

4

April 16, 2016

5

April 15, 2017

6

April 14, 2018

Lecturers
Ouchi Masami (ICRR)
Nojiri Mihoko (KEK, Kavli IPMU)
Terasawa Toshio (ICRR)
Kouno Toshitake (Kavli IPMU)
Nomura Yasuki (UC Berkley, Kavli IPMU)
Miyoki Shinji (ICRR)
Hayato Yoshinari (ICRR)
Nishimichi Akihiro (Kavli IPMU)
Sagawa Hiroyuki (ICRR)
Toda Yukinobu (Kavli IPMU)
Nakahata Masayuki (ICRR)
Shrai Satoshi (Kavli IPMU)

Fig. 3. Low-background underground laboratory in Kashiwa campus.

168

LOW-LEVEL RADIOISOTOPE MEASUREMENT
FACILITY
Low-level radioisotope measurement facility wasbuilt at Kashiwa campus in 2000 with the relocation of
ICRR from Tanashi campus in Tokyo. Historically, thefacilities originated in Nokogiri-yama laboratory set up
under Mt. Nokogiri-yama approximately 45 years ago,
and the initial objectives were low-level counting to investigate cosmogenic nuclides in extraterrestrial materials such as cosmic dust falling down to the earth surface and/or the ocean. The new laboratory at Kashiwa
is located in a basement 23 m from the surface. The
floor space is approximately 47 m2 and all of the walls
are painted by ”MINEGUARD” which is a special paintto block radon gas from the reinforced concrete.
In this laboratory, four High-Purity Germanium(HPGe) detectors are installed to measure natural radioactivity of various kinds of samples by gamma ray
spectroscopy (Fig. 1).

Fig. 1. High-Purity Germanium (HPGe) detectors installed in
Kashiwa Underground Laboratory

All of HPGe detectors are well type with U-style
cryostats. Each HPGe detector is surrounded by an ultra low-background shield with 15 cm thick low background lead. The specifications of these detectors areshown in Tab.1. The data acquisition for the HPGe
detector are computer controlled, and the identification of the radioisotopes is automated. In addition,
environmental data of the laboratory such as temperature, humidity, atmospheric pressure, radon concentration, weights of liquid nitrogen vessel for cooling HPGe
detectors, are monitored and logged by the network
data logging system as shown in Fig.2. Oxygen-density-

Det. No.
1
2
3
4

Table 1. Specifications of HPGe detectors.

Well Dia.(mm)
10
16
16
16

Vol.(cc)
190
170
116
160

Manu. Year
1997
1997
1986
2005

meter is continuously monitored by a TV camera, andis displayed at the entrance level floor. Before entering
the underground laboratory, we are obliged to checkif (1) Ventilator is working, and (2) Oxygen density is
not lower than 20.9%. This facility has been utilized for
low-level counting of the cosmogenic and environmental
radioisotopes in a variety of samples.
Theme 1: Detection of time variation of cosmogenic 7 Be
by Fuyuki Tokanai (Yamagata Univ.) et al.: The temporal variation of the 7 Be concentrations in atmosphere
has been measured during 18 years since 2000 at Yamagata 1 . This corresponds to the period from the solar
maximum of the 23th solar cycle to almost the end of
the 24th cycle. The yearly profile of the 7 Be concentration indicates the variation in galactic cosmic rays
owing to solar modulation because 7 Be is produced byspallation of the air elements due to cosmic rays. The
yearly profile of the 7 Be concentration showed a clear
anticorrelation with that of the number of sunspots.
Corresponding to the 41% reduction in the number ofsunspots from the 23th cycle to the 24th cycle, the 7 Be
concentration increased by 39.5%, indicating that thevariation in the 7 Be concentration can quantitatively
reflect the variability of solar activity, even in different
solar cycles. The daily variation of the concentrationsof 7 Be has been measured since June 2014 at Bangkok
in Thailand (13.45◦ N, 100.19◦ E) to observe the variations in the concentration of cosmogenic nuclides at low
latitudes. The 7 Be concentration shows a clear seasonalvariation with high and low concentrations from January to May and from June to December, respectively,in each year, with the highest 7 Be concentration 2.3
times larger than the lowest 7 Be concentration. Froman analysis of air-mass trajectories, it was found thatthe high and low 7 Be concentrations approximately correspond to ”Continental and Pacific Ocean trajectories”
and ”Indian Ocean trajectories”, respectively.
Theme 2: Evaluation and monitoring of radioactive Cs
released from the TEPCO Nuclear Power Plant since the2011 Tohoku earthquake by Kotaro Shirai (Univ. Tokyo)
et al: The 2011 earthquake and associated tsunami
disrupted the operation of the Fukushima Daiichi Nuclear Power Plant (FDNPP), which resulted in thewidespread release of large amounts of radio- cesium(134 Cs and 137 Cs) into the local environment during theperiod 12-23 March 2011. Understanding of the dy*1 S. Suzuki, H. Sakurai, F. Tokanai et al., “Observation of cosmogenic
nuclide Be-7 concentrations in the air at Bangkok and trajectory
analysis of global air-mass motion”, 35th International Cosmic Ray
Conference (ICRC2017): Bexco, Busan, Korea, July 12-20, 2017,
Proceedings of Science (ICRC2017) 070 (2017) 1-8.

169
Kashiwa Underground Laboratory①

Internet

157.82.231.248⑫

192.168.0.254
Fast SWIII-8P

⑮

⑦

④

Data
Radon Scout⑥
BUFFALOUSB

960
X.X.X.10⑩⑪⑭

TCS-715X

TCS-715X

TCS-715X

TCS-715X

X.X.X.6

X.X.X.4

X.X.X.3

X.X.X.2

7142

7142

7142

7142

A/D

/Centronix⑬

X.X.X.15

X.X.X.20

OPTIPLEX⑯

LHD-NAS250W

Logger18○

LSW4-GT-8NS/BK

BUFFALO
WHR-300H

⑤

P2

③

②
AIM1

B1F EV HallCRTRS232C

AIM2
Atomic

Det01

Det02

Det04

Det03

W1

W2

W3

W4

pressure
ITV

Radon

Temp & Humid

20.9

OxygenTCS-715X
X.X.X.5
7142

7143
TCS-714X
X.X.X.1

7144

7142

192.168.0.X
714X

xX
○

Private IP adress
Port
Cable number

Fig. 2. Data acquisition and monitoring of Kashiwa Underground Laboratory

namics of radioactive Cs is scientifically important aswell as socially important. Two topics have been studied to solve the query, (1) the behavior of radioactive
Cs in a coastal marine environment and (2) the coastal
monitoring of radioactive Cs through the observationof mussels.
(1) Vertical profiles of the 137 Cs activity in coastal
sandy sediment from Funakoshi bay, Iwate, were determined. The depth of 137 Cs intrusion was estimatedto be 10-15 cm, where 137 Cs was initially deposited on
the surface layer of the sandy bottom. In combination
with the observation of bioturbation structures basedon a view of burrows using a CT scanner, the main
driver of 137 Cs redistribution was identified as bioturbation by Echinocardium cordatum2 . (2) Since sessile
bivalves such as mussels and oysters are filter feeders
and accumulate heavy metals and organic pollutantsin their bodies, monitoring of the contaminant concentration in their soft tissue is one of the most common
methods of environmental monitoring and is used in theMussel Watch Program. Radioactive Cs has also beenmeasured at various locations for monitoring purposes.
Thus, mussels were collected along the Pacific coast ofJapan from Iwate to Fukushima from 2011 to 2015, andthe radioactive Cs in the soft tissue was measured toclarify the temporal and spatial changes in radioactive
Cs released from FDNPP. For the 2011 samples, 137 Cs
was above the detection limit at all the sampling points
with the coast to the south of FDNPP showing higher
concentrations than the northern coast. In addition,
the 137 Cs level has been decreasing since 2011. Since*2 Koji Seike, Kotaro Shirai, Naoko Murakami-Sugihara, ”Usingtsunami deposits to determine the maximum depth of benthic burrowing”, PLOS ONE. 12 (8) e0182753, DOI:10.1371/journal.pone.
(2017) 0182753

mussels feed particulate organic materials, quantitativevalue of radioactivity obtained from these mussels canbe used as baseline data for ecosystems.
Theme 3: Evaluation of the erupted radioactivities into
the environment by Hisayuki Arakawa (Tokyo Univ. of
Marine Sci. and Tech. et al.: As a result of theGreat East Japan Earthquake and associated tsunami
in March 2011, the Fukushima Daiichi Nuclear Power
Plant (FDNPP) released a large amount of radioactive
materials into the environment, resulting in the contamination of many marine organisms. Marine plantsof various species were collected from the Iwaki coast of
Fukushima Prefecture from May 2012 to June 2015 andanalyzed to determine variations in 110m Ag, 134 Cs, and
137 Cs with time. The results indicated that (1)110m Ag
was present in all seaweed and seagrass collected in
May 2012. (2) The concentration of 110m Ag in thesea grass Phyllospadix iwatensis decreased significantlywith time, while the ecological half-life of110m Ag in P.
iwatensis was longer at locations nearer to FDNPP.

170

APPENDIX

171

A.1: ORGANIZATION

Fig. A.1-1 Organization of the Institute for Cosmic Ray Research as of April 2018.

172

A.2: ICRR STAFFS (2018)
The number of tenured researchers (Professors, Associate Professors, and Research Associates) in ICRR
was slightly increased; it is 42 in 2018. Essentially,
there is the balance between newly allocated posts by
the University of Tokyo, and the un-filled posts (after
retirement or departure of the staff) under a generalrule in the University of Tokyo, which is based on alaw to restrict the total number of employees in thegovernmental agency.
The University of Tokyo established the post re-allocation system to assign new posts to departments
and institutes. Under this system, each department
or institute submits a post re-allocation proposal to
the University of Tokyo. Then the posts might be reallocated to them after the evaluation of the proposal.
In 6 years, ICRR got some re-allocations.
The Table below summarizes staffs of ICRR in 2018.
In FY2018, ICRR accepted one visiting professors andtwo visiting associate professors, which are not shownin this table.

Table A.2-1 Numbers of ICRR staffs as of May 2018.

Professors
9

Associate
Professors
15

Research
Associates
18

Technical
Staffs13

Project
Professors1
1*1 Project Associate Professors and Project Research Associates have fixed terms.
*2 3 JSPS Post-Doctoral Fellows studying in ICRR.

Project
Associate
Professors1
1

Project
Research
Associates1
13

Post-Doctoral
Fellows
9 + 42

Total
79

173

A.3: BUDGET (FY2012–2017)
After the restructuring of national universities in
2004, the same level of government revenue funding as
of FY2004 has been maintained for each fiscal year witha limited amount of yearly cut due to the government’s
general saving policy (Table A.3-1). Funding for specific research projects, designated as “Promotion of Sci-ence” in Table A.3-2, changes depending on the projectstatus. KAGRA construction started in FY2010 andfinished in FY2016. An increase of the total annual
budget in FY 2016 is due to CTA construction. Other
sources of research fund are listed in Table A.3-3.

Table A.3-1 Government Revenue Funding (in thousand yen).

2012
658,000
1,172,000
1,830,000

Personnel Expenses
Supplies and Facilities
Total

2013
687,000
1,095,000
1,782,000

2014
706,000
1,282,000
1,988,000

2015
684,000
1,595,000
2,279,000

2016
683,000
1,288,000
1,971,000

Table A.3-2 Research Grant for Promotion of Sciences (in thousand yen).

KAKENHI
KAGRA
Construction
BudgetCTA Construction Budget
Total

2012
305,100
5,956,579

2013
282,600
725,6702014
328,400
468,400

2015
499,787
674,000

2016
626,100
576,000

2017
475,800
0

0

0

0

0

1,346,000

06,261,679

1,008,270

796,800

1,173,787

2,548,100

475,800

Table A.3-3 Other research funds (in thousand yen).

Trust FundPrivate Scholarship
OthersFoundation
Total

2017
779,000
1,514,000
2,293,000

2012
0
85
0
–
85

2013
36,321364
3,000–
39,685

2014
38,250181
300–
38,731

2015
13,5000201–
13,701

2016
0
0
300
18,182
18,4822017
0
17,765130
62,42180,316

174

B.1: AGREEMENTS OF INTERNATIONAL ACADEMIC
COOPERATION
Table B.1-1 Agreements of academic cooperation with the ICRR, or the University of Tokyo, and overseas universities and research

Country
Bolivia
China
USA
USA
USA
RussiaAustralia
Korea
USA
Italy
China
UK
Taiwan
China
USA
Italy
China
Spain
TaiwanGermany
Korea
PolandSpain
Canadaorganizations.

University/Institute
Universidad Mayor de ‘San Andres’The institute of High Energy Physics, Chinese Academy of Sciences
College of Science, University of Utah
School of Physical Science, the University of California, Irvine
Graduate School of Arts and Sciences, Boston University
Institute for Nuclear Research, Russian Academy of Science
Faculty of Life and Physical Sciences, University of Western Australia
College of Natural Science, Seoul National University
CIT LIGO Laboratory, California Institute of Technology
European Gravitational Observatory/ Virgo Collaboration
Shanghai United Center for Astrophysics, Shanghai Normal University
Institute for Gravitational Research, University of Glasgow
College of Science, National Tsing Hua University
SICCAS-GCL Research & Development Center,
Shanghai Institute of Ceramics, Chinese Academy of Sciences
College of Science, Louisiana State University
University of Sannio at Benevento Department of Engineering
National Astronomical Observatories, Chinese Academy of Sciences
The Faculty Sciences, Autonomous University of Madrid
Institute of Physics Academia Sinia
Max Plank Institute for Physics
Sungkyunkwan University, College of Science, Center for Cosmic Ray Research
National Centre for Nuclear ResearchInstituto de Astrofisica de Canarias
TRIUMF

Year of Agreement
1981
1995
1995
1995
1995
2001
2001
2009
2009
2011
2011
2011
2011
2012
2012
2012
2012
2013
2017
2017
2017
2018
2018
2018

175

B.2: INTERNATIONAL MEETINGS
Date
May 28–29, 2012
Oct. 3–4, 2012
Dec. 21–22, 2012
Jun. 10–11, 2013
Nov. 29–30, 2013
Dec. 4–5, 2013
Jan. 14–17, 2014
Jun. 20–21, 2014
Dec. 19–20, 2014
Jan. 31, 2015

Table B.2-1 List of international meetings hosted or co-hosted by ICRR.

Place
Kashiwa
Tokyo
Korea
Osaka
Korea
Tokyo
KashiwaTokyo
ToyamaKashiwa

Feb. 9–10, 2015
Jun. 17–20, 2015

Tokyo
Osaka

Oct. 26–30, 2015
Nov. 16–21, 2015

Kashiwa
Osaka

Dec. 2–3, 2015
Jan. 13–14, 2016

Tokyo
Kashiwa

May 30–Jun. 1, 2016

Ibaraki

May 30–Jun. 1, 2016

Kashiwa

Jun. 16–20, 2016
Nov. 11–14, 2016

Kashiwa
Kyoto

Sep. 6–9, 2017

KashiwaMar. 5–7, 2018

KashiwaMar. 26–30, 2018

Tokyo

Symposium/Workshop Title
2nd Japan Korea workshop on KAGRA
1st EliTES General Meeting
3rd Japan Korea workshop on KAGRA
4th Japan Korea workshop on KAGRA
5th Japan Korea workshop on KAGRA
2nd EliTES General Meeting
CTA LST General Meeting
6th Japan Korea workshop on KAGRA
7th Japan Korea workshop on KAGRA
The Inaugural Symposium of the HyperKamiokande Proto-Collaboration and Signing
Ceremony
3rd EliTES General Meeting
Gravitational Wave Physics and Astronomy Workshop (GWPAW) 2015
Tev Particle Astrophysics (TeVPA) 2015
10th International Workshop on Neutrino-Nucleus
Interactions in the Few-GeV Region
4th EliTES General Meeting
The extreme Universe viewed in very-high-energy
gamma rays
Third International Meeting for Large Neutrino Infrastructures
PhyStat-ν Workshop on Statistical Issues in Experimental Neutrino Physics
CTA Consortium Meeting Kashiwa
International Conference on Ultra-Hgh Energy
Cosmic Rays (UHECR2016)
5th Hyper-Kamiokande Proto-Collaboration Meeting
The first annual symposium innovative area ”Gravitational Wave Physics and Astronomy: Genesis”
Tokyo Spring Cosmic Lyman Alpha Workshop

Participants
26
80
20
34
36
80
83
49
34
108
42
133
169
109
38
68
80
90
170
100
82
99
120

176

B.3: INTER-UNIVERSITY COLLABORATIVERESEARCHES
As one of the “Joint Usage/Research Centers” inJapan, ICRR has been supporting inter-university collaborative researches. Annually ICRR issues calls forapplications of collaborative researches. The following
tables summarize the annual numbers of the applications after selection.

Table B.3-1 Annual numbers of applications for collaborative research programs.

Facilities
Kamioka Observatory
Akeno Observatory
Norikura Observatory
Low-Level Radioisotope Measurement Facility
KAGRA Observatory1
Laboratorial Facility in Kashiwa
Computer Facility in Kashiwa
Conference Facility in Kashiwa
Overseas Facilities2
Annual Sums

2012
33
4
7
6
15
10
12
7
11
105

2013
33
4
9
5
21
9
17
7
12
117

2014
38
5
10
4
22
5
14
10
11
119

2015
39
7
11
3
19
9
12
10
13
123

2016
39
4
12
4
17
3
12
11
23
125

2017
42
4
10
4
14
3
12
13
26
128

Subtotal
224
28
59
26
108
39
79
58
96
717

*1 The first laser interferometer using cryogenic mirrors developed in Kashiwa. KAGRA Observatory was established in 2016.
*2 The Tibet ASγ Observatory, Observatory for Highest Energy Cosmic Rays in Utah, Chacaltaya Observatory of Cosmic Physics, and The
Cherenkov Telescope Array in La Palma. Applications for domestic activities corresponding to these overseas works are included here.

Research Project Titles
1. Study of simulation for atmospheric neutrino
2. Study of atmospheric neutrino flux and neutrino oscillations
3. Studying the Neutrino Mass Hierarchy With Atmospheric Neutrinos
4. Study of flavor identification of atmospheric and beam neutrinos
5. Study of solar neutrino energy spectrum
6. Precise measurement of Day/Night effect for B8 solar neutrinos
7. Study for Supernova monitor
8. Improved measurement of solar neutrino at Super-Kamiokande and SK-Gd
9. Study of Supernova Relic Neutrinos
10. Search for proton decay via e+ π 0 mode
11. Study of proton decay p→ν K +12. Study in upward-going muons and high energy neutrinos
13. Sidereal daily variation of ~10TeV galactic cosmic ray intensity observed by the Super-Kamiokande
14. Tokai to Kamioka Long Baseline Experiment T2K
15. Neutrino interaction study using accelerator data

177

16. Study of the electron neutrino appearance measurement in the T2K experiment
17. Study to improve sensitivity of neutrino oscillation measurement in T2K experiment
18. Joint Oscillation Analysis With the T2K and Super-Kamiokande Experiments
19. Energy cailbration for Super-Kamiokande
20. Research and development of computer simulation of Super-Kamiokande detector.
21. Development of low concentration radon detection system
22. R&D of Megaton scale water Cherenkov Detector Hyper-Kamiokande
23. Development of the Large Aperture Photodetector for a next-generation neutrino detector
24. Development of software for the next generation neutrino detector
25. A Search for Dark Matter using Liquid Xenon Detector
26. Study of annual modulation for dark matter search with XMASS
27. Study for future XMASS detector
28. Development of calibration system for XMASS detector
29. Micro-analysis of gaserous cantamination in Xe
30. Study on neutrino physics using liquid xenon
31. Radon emanation measurement from material using dark matter search experiment.(2)32. A study on scattering processes of scintillation photons in liquid xenon
33. Study on surface background removal in the dark matter search
34. Study of Double beta decay of 48Ca
35. Direction-sensitive dark matter search
36. Study for lowering backgrounds of radioisotopes in large volume detectors
37. Studies on the background evaluation using laser spectroscopy analysis
38. Development of a radioactivity assay system for underground experiments
39. Dark Matter Search with double-phase Argon detector
40. Integration of crustal activity observation around the Atotsugawa fault
41. Strain, tilt, seismic measurement in Kamioka-mine
42. Keeping neuclear emulsion plates in a box made of lead blacks at Kamioka Underground Lab
43. R&D for a Small Atmospheric Cherenkov Telescope in Akeno Observatory
44. Development of new surface detector for observation of ultra high energy cosmic ray at Telescope Arraysite
45. Development of the national control station for the fully remote operation of TA-FD
46. Multi-Color Imager for Transients, Survey and Monstrous Explosions
47. Observation of Galactic Cosmic Ray Intensities using Large Area Muon Telescopes
48. Study for cosmic ray detector with capability of indivisual particle identification and particle tracking
49. The development for the platform of advanced field measurement

178

50. Observation of solar neutrons in solar cycle 24
51. Space weather observation using muon hodoscope at Mt. Norikura
52. Observation of cosmogenic nuclides concentrations at Mt. Norikura
53. Study of secondary cosmic rays from Thundercloud at Mt. Norikura
54. Relativistic Electron Acceleraton in Thunderstorm Eelectric Field
55. Observational study of electron acceleration mechanism in thunderclouds
56. High-energy phenomena via interaction between cosmic-rays and thunderstorms
57. Development of high energy proton irradiation technique for devices used in spaceship
58. Investigation of alpine plants on Mt. Norikura
59. Effect of forest fragmentation on the belowground microorganisms
60. Symbiosis betweem Pinus pumila and Nucifraga caryocatactes on Mt. Norikura
61. The CTA Project
62. CTA-Japan Physics Research
63. Development of Focal Plane Instruments for the CTA Large Sized Telescope
64. Development of the readout system for the CTA large sized telescopes
65. Installation and commissioning of the first Large Size Telescope of CTA in La Palma, Canary Islands, Spain
66. Integration & Commissioning of the Slow Control Program for the Camera of the first Large Size Telescopeof CTA in La Palma, Spain
67. Development of the optical system for CTA Large size telescopes
68. Development of a calibration system of the CTA-LST PMT modules
69. Development of camera for CTA small-sized telescopes
70. CTA Monte Carlo Simulation
71. Development of advanced photon counter for the future IACT
72. Study of High Energy Gamma-ray Objects with the MAGIC telescope
73. The study on simultaneous observations of gamma ray bursts by CTA and LEAP
74. Study of Extremely-high Energy Cosmic Rays by Telescope Array
75. Observation of Ultra High Energy Cosmic-Ray with New Fluorescence Detector at the Telescope Array Site
76. Timing and position calibration of surface detectors of TA×4 and TALE experiment
77. Research and development of the surface detectors for the TALE experiment
78. Study of radio detection of highest energy cosmic rays
79. Research and development of a Fresnel lens air fluorescence telescope for the next generation UHECR
observation
80. The observation of abnormal shower event with lightning by TA surface particle detector
81. Development of solar power system and detector protection system for the new-type fluorescent detector

179

82. Calibration of fluorescence detector responce and optical system with standard light source mounted on
UAV
83. Development and analysis of night cloud observation by CCD camera for automatic observation of air
fluorescence detector
84. Study of absolute energy calibration of air shower by a compact electron linac
85. Observation of airshower fluorescence light at the TA FD site by using an Imaging UV telescope
86. Experimental Study of High-energy Cosmic Rays in the Tibet ASγ experiment
87. Study of High Enegitic Radiation from Thundercloud in Tibet
88. Sidereal daily variation of ~10TeV galactic cosmic ray intensity observed by the Tibet air shower array
89. Study of the composition of cosmic-rays at the Knee
90. A study on variation of interplanetary magnetic field with the cosmic-ray shadow by the sun.
91. Air shower observation for high-energy gamma ray and cosmic ray detections at the Chacaltaya Cosmic
Ray Observatory
92. Study on High Energy Cosmic Ray Sources by Observation in Spac with CALET
93. Design study of a Compton camera for study of cosmic rays
94. Observation with Ahsra
95. Integration of the optical fiber trigger system for Ashra
96. Comparative study of astrophysical particle acceleration processes
97. The extreme Universe viewed in very-high-energy gamma rays 2017
98. Development of a new code for cosmic-ray air shower simulation
99. Cosmic ray interactions in the knee and the highest energy regions
100. YMAP symposium 2017 (Basic Part)
101. Research of Large-scale Gravitational wave Telescope (VII)102. Research on ultra-low frequency anti-vibration system for KAGRA
103. Development of High Performance Cryogenic Mirror Control System
104. Research on cryogenic payload for KAGRA
105. Study for improving a curing time of silicate bonding by controlling gas environment.
106. Development of Very Low Vibration CryoCooler System
107. Development of ultra-low loss coating for the KAGRA sapphire mirror - 2
108. Numerical Simulation of Electro-Magnetic Wave Propagation in Gravitational wave Detector V
109. Control and automatic operation for KAGRA
110. Construction of KAGRA data transfer and storage system (3)111. Data analysis of KAGRA detector (III)112. R&D for the intensity stabilization of the laser system in KAGRA
113. Technical development for effects on high optical power for bKAGRA mode cleaner

180114. Precise geophysical observation at the Kamioka underground site and modeling of crustal activities
115. Study of Gravitational-wave by cryogenic laser interferometer CLIO in KAMIOKA Mine
116. Development of precision profiler for mirrors of LCGT interferometer 7
117. Development of optical cavity for ultranarrow stable lasers
118. Research and development on advanced gravitational wave observatory network for KAGRA
119. Cosmic Reionization and Galaxy Formation Probed with Large Optical Near-Infrared Telescope120. Evolution of the universe and particle physics
121. Detection of time variations for cosmogenic nucleid Be-7
122. Evaluation of the erupted radioactivities into the environment
123. Frontier of the planetary material science
124. Time profile of radioactive Cs concentration and its aerosol size distribution in local area
125. Continuous Measurement of Underground Laboratory Environment
126. Precise calculation of the atmospheric neutrino flux
127. Neutrino Workshop
128. CRC workshop for future plans in cosmic ray research

181

B.4: EDUCATIONAL ACTIVITIESOne of the important functions of ICRR is to educate graduate students directly at ICRR as a part of the
Graduate School of Science, the University of Tokyo,

and indirectly at graduate schools of domestic/foreign
universities through collaborative research programs.

Table B.4-1 Annual numbers of accepted master/doctor theses.

Master theses at ICRR
Master theses at domestic universities
Total numbers of master theses in Japan1
Doctor theses at ICRR
Doctor theses at domestic universities
Doctor theses at foreign universities
Total numbers of doctor theses

2012
9
15
24
5
4
19
28

2013
9
9
18
3
2
14
19

2014
18
8
26
3
5
10
18

2015
12
10
22
8
5
9
22

2016
16
14
30
4
10
1
15

2017
13
10
23
3
7
17
27

Subtotal
77
66
143
26
33
70
129

*1 Since the numbers of master theses are not available for all the collaborating foreign universities, only the domestic numbers are tabulated.

182

B.5: PUBLIC RELATIONS
6.

Overview

Four staffs take charge of public relations; two ofthem are assigned in Kashiwa ICRR headquarters andeach one in both Kamioka Observatory and KAGRA
Observatory. Under the supervision of ICRR director,
Public Relations Committee, organized by the 4 members, create and manage following public relations activities.

3.3 The 360◦ Panoramic Video for Virtual Reality(VR)A goggle with a smartphone display on its glass
top provides a chance to explore Super-Kamiokande,
XMASS and KAGRA observatory as a VR experience. [Find more on http://www.icrr.u-tokyo.ac.jp/
panorama/index.html]

9.

Exhibitions

1. Publications
2. Multimedia Contents
3. Exhibitions
4. Original Goods
5. Interactive Events

7.

Publications

2.1 News Letter
ICRR News is published quarterly and provides
summary of the seasonal news topics and photos.
2.2 Brochure
ICRR Brochure, published annually, offers a basic
overview of the entire ICRR research activities.
2.3 Annual Report
ICRR Annual Report provides detailed information onscientific outcomes of every research group in ICRR.

8.

ICRR collaborates with other organizations to create
new type of exhibitions. Main collaborators are as follows:• National Museum of Nature and Science, Tokyo• National Museum of Emerging Science and Innovation (Mirailan)• Local government and universities located near
ICRR experimental facilities

Multimedia Contents

3.1 Websites
Not only PR office in Kashiwa ICRR headquarters, butalso every research group such as Super Kamiokande(SK) and KAGRA, create their original websites tointroduce a series of topics. In the summer of 2018,
SK detector was opened for the first time in 12 years
because of the refurbishment. ICRR posted a featureof 9 questions and answers on SK refurbishment on
the ICRR website. [Find more on http://www.icrr.utokyo.ac.jp/sk_open/]
3.2 Videos
An 8-minutes-video which is edited in English withJapanese subtitles, introduces ICRR experimental facilities all over the world. [Find more on http://
www.icrr.u-tokyo.ac.jp/beta/movie.html]

10.

Original Goods

ICRR creates several kinds of original goods for sale:jigsaw puzzle featuring SK, note, mug, calendar and soon. The purpose is to promote public relations and to183

support research activities in ICRR: for example, employing young researchers who already have promising
results.

11.

6.3 Spring School

Interactive Events

6.1 Open Campus

Several thousand people visit ICRR headquarters every
year on the Kashiwa Open Campus days. They are
attracted by science cafe, workshops, original goods saleand other entertainments the PR Committee createdwith many students and researchers.
6.2 ICRR x Kavli IPMU Public Lecture

ICRR collaborates with Kavli Institute for the Physics
and Mathematics of the Universe (Kavli IPMU) to organize public lectures in both spring and fall every year.
The number of people wishing to attend the lecture
largely exceeds the quota. In the spring of 2018, four
hundred people who won the lottery enjoyed the lecture.

ICRR offers 30 university students from all over Japan,
sometimes from overseas, to spend in ICRR for 5 days
and 4 nights in March every year from 2012. This program, called “Spring School,” provides lectures, experiments, data analysis, and presentation to researchersand colleagues. Students, tutors and also the ICRR director who never met before get along with each othersoon and have a profitable experience under one roof.

184

C: REPORT FROM THE FUTURE PLANNING
COMMITTEE (2013 and 2017)

Report from the University of Tokyo, Institute for Cosmic Ray Research (ICRR),
Future Planning Committee
September 26, 2013
History
andObjective
BehindEstablishment of the Committee

the Town Hall Meeting examined mainly thescientific significance of the proposed plans,
including future plans from outside the ICRR, theCommittee evaluates projects that the ICRR has
assumed to implement, which sets it aside from thefuture plan discussions with the CRC and its aims.

the

The University of Tokyo, Institute for Cosmic Ray
Research (ICRR), has organized Future Planning(Sub-) Committee meetings on three occasions since1987. At the subcommittee, future plans for the
institute, especially large-scale projects that could
serve as a pillar for the institute’s future research,were evaluated and recommendations were made.
The institute has been striving to realize these plans.
The Kamioka Gravitational Wave Detector (KAGRA)
project that was recommended at the 2nd (1993）andthe 3rd（2007）committee meeting, has moved onto
the construction phase after being chosen as a stateof-the art research project, and has been allocated abudget for the cavity excavation needed for detector
construction. The ICRR decided that the time hascome for considering a new future project that canfollow Super-Kamiokande and KAGRA, and thiscommittee was established in January 2012 toprovide recommendations for a way forward for the
ICRR and its in-house projects.
Also, the ICRR has restructured its organization
by terminating projects such as the CANGAROO
experiment and SDSS experiment, and establishing
new research groups such as the Cherenkov Cosmic
Gamma-ray Group, the Observational Cosmology
Group, and the High Energy Astrophysics Group. It
also conducts relatively small scale research on suchareas as primary cosmic rays (studying topics
including changes in cosmic-ray intensity and therelation with the climate) and Ashra (All-sky Survey
High Resolution Air-shower detector). The role ofevaluating and providing recommendations on the
directionality, scientific significance, and humanresource allocation for these research projects was
assigned to the Committee.
The Committee invited advocates from each future
plan to the meeting to conduct hearings on the detailsof each plans. In 2012, the Cosmic Ray Conference(CRC) held a town hall meeting discussing future
research plans in the field of cosmic rays. To
efficiently obtain information on each experimentplan, the Committee has co-hosted events, beginningwith the 3rd CRC Town Hall Meeting (2012, June 30).
(Members of the Committee coordinated andattended the 2nd CRC Town Hall Meeting (2012,
January 22) as well, but did not take the form of cosponsorship.) However, it should be noted that while

Committee Members
K. Saito (Chairperson) Director, National Institutesof Natural Sciences
H. Aihara, Professor, Graduate School of Science, theUniversity of Tokyo
Y. Ito, Professor, Institute for Space-EarthEnvironmental Research, Nagoya University
K. Inoue Professor, Research Center for Neutrino
Science, Tohoku University
N. Kanda, Professor, Osaka City University
Graduate School of Science
T. Kishimoto, Professor, Osaka University Graduate
School of Science
N. Sugiyama, Professor, Nagoya University
Graduate School of Science
Y. Mizumoto, Professor, National Astronomical
Observatory of Japan
T. Terasawa, (Secretary) Professor, The University ofTokyo, Institute for Cosmic Ray Research
K. Okumura, (Vice secretary, clerk) Associate
professor, The University of Tokyo, Institute for
Cosmic Ray Research
T. Kajita, (Observer) Director, The University ofTokyo, Institute for Cosmic Ray ResearchCommittee History･ 1st Meeting (January 25, 2012)－ On the establishment of the Committee anditems requested for evaluation－ Discussion on future policies･ 2nd Meeting (February 21, 2012)－ Hearing on Cherenkov Telescope Array (CTA)
from two project members－ Discussion within the Committee on the
hearings[Follow-up questions from the Committee weresent to the group from the Secretariat on a later
date.]

1decade is attributed to the development ofobservations in the energy regions of several dozen
GeV to several TeV by HESS, MAGIC, and VERITAS,
which stemmed from the pioneering ground-based
high energy gamma-ray observations, such as
Whipple, HEGRA, and CANGAROO. The next
generation, ground-based high energy gamma-ray
experiment, Cherenkov Telescope Array (CTA), aims
to maintain this momentum of research—andadvance it further. In fact, CTA plans to increase thesensitivity of the conventional ground-level gammaray experiment by approximately 10 times, andexpand the target energy region in both ends of thespectrum by nearly an order of magnitude.
Astronomical and astrophysical themes in which
progress can be expected by the experiment are as
follows:
1. The origin, acceleration, and propagation ofcosmic rays
2. Galactic high energy astronomical objects(Pulsar, Magnetar, Pulsar wind nebula, Supernova
remnant (SNR), X-ray Binaries, Globular clusters).
3. The origin, acceleration, and propagation of
extra-galactic high-energy cosmic rays
4. Extra-galactic high energy astronomical objects(active galactic nucleus (AGN), gamma-ray bursts(GRB), nearby galaxy clusters, nearby starburst
galaxies, etc).
5. Cosmological studies using gamma-rays from
active galactic nuclei (AGN) and gamma-ray bursts(GRB)
6．Dark matter annihilation in a Galactic Center,dwarf galaxies, galactic halos, galaxy clusters, andsearch for gamma-ray decay
７ ． Tests for Lorentz invariance at the Planckscale
In this way, the scientific perspective of CTA
extends not only to each field of high energy
astrophysics, but also cosmology and particle
physics; that the list of participants of this projectencompasses nearly all researchers in the world
involved with high energy gamma-ray astronomy. In
response to these trends internationally, Japan’shigh energy gamma-ray astronomy community hasorganized the CTA-Japan Consortium, which has
been examining future plans. The following is thesummary of the hearings on the plans.

･ 3rd Meeting (April 10, 2012)－ Continued the previous hearings on the CTA
from two project members－ Discussion within the Committee on the
hearings･ 4th Meeting (May 29, 2012)－ About cohosting the town hall meeting withCRC
－ Discussion based on the response from the
CTA group･ 5th Meeting (July 17, 2012)－ Hearing on Telescope Array (TA) from one
project member－ Discussion on personnel matters of the HighEnergy Astrophysics Group[Follow-up questions from the Committee weresent to the TA group from the Secretariat on alater date.]･ 6th Meeting (August 7, 2012)－ Hearing on the Tibet Experiment from one
project member－ Discussion on the Tibet Experiment within
the Committee－ Discussion based on the response from the TAgroup･ 7th Meeting (November 1, 2012)－ Hearing on the Hyper-Kamiokande from one
project member－ Hearing on the XMASS Experiment from one
project member－ Hearing on the Ashra Experiment from one
project member･ 8th Meeting (December 21, 2012)－ Discussion on the Hyper-Kamiokande,
XMASS, and Ashra－ Discussion on the interim report･ 9th Meeting (February 26, 2013)－ Discussion on the interim report･ 10th Meeting (April 19, 2013)－ Hearing on the joining of TA group to EUSO,
and details on TA×4 Plan from one project
member－ Discussion on the interim report

About Cherenkov Telescope Array (CTA)
Experiment

There are three kinds of telescope: LST (Large Size
Telescope), MST, and SST, having a 23-meter, 12meter, and 6-meter diameter, respectively. The
northern and southern hemispheres are being
considered for the site, and at each site, 4 LSTs, 23
MSTs, and 32 SSTs are planned to be set up. (SSTs

Project Outline
Much of the rapid advancement in theunderstanding of the origin of cosmic rays in the past2

start in 2015, and partial observation is scheduled tocommence in 2017. The overall budget that includes
both the northern and the southern hemisphere sites
is 190 million euros (totalling about 20 billion yen, asof 2012), and Japan is aiming for about 20% for thecost allocation, which is 4 billion yen. The breakdown
is about 1.3 billion yen for the mirror, 0.9 billion yen
for the photomultiplier, 1.4 billion yen for theelectronics, 0.3 billion for the camera box, and 0.1
billion yen for the personnel expenses. As the hostinstitution of CTA-Japan, the ICRR needs to secure
observation time, operate the experiment, andbecome the point-of-contact for providing observation
data and analysis tools. In addition to theapproximately 4 billion yen for construction, about
0.2 billion yen must be secured annually for
maintenance, so the ICRR needs to submit a budget
request.

are not planned for the northern hemisphere.)Japan’s core responsibilities are the LST, the mirror
and the imaging camera, and the camera module (thephotomultipier and the electronics part), andGermany (MPI, Munich) is to be responsible for the
structuring, such as the gantry. Mr. Teshima from
ICRR is managing the LST Project. LST is an
important telescope in filling the gap in the energy
range between Fermi and currently working IACT.
The low energy threshold target is at 10 GeV. Also,
the requirements specifications for LST is the moststringent out of the three telescopes, and Japan’s
technical contribution is expected by the world’s
experiment teams in this regard. If Japan is able
make a significant contribution to LST, we can expect
to obtain a drastic increase in observation time in theCore Programme. Regarding the development of themirror, which is Japan’s main contribution to LST,
the aim is to extend the equipment life by multilayer
coating (sputtering) and to achieve high reflectanceof 95% or more. In addition, in order for fast GRB
follow-up observation to be possible, it is necessary todrive the telescope at high speed, so measures arebeing taken to reduce the weight. As for the imaging
camera module, the photomultiplier tubes are madeby Hamamatsu Photonics. The electronics is able tosecure 3-digit dynamic range with 2 gains, achieving
pulse height sampling and high speed readings at 2
gigahertz. There are some issues to be solved
including the trigger system between modules andthe cooling system that uses a camera box to cool theheat generated from the electronics. Going forward,
the plan is to build a camera prototype with the
Kakenhi scientific research grants, and test the MST
prototype in Germany. There is no major technicaldevelopment factor in producing the mirror, so it isthought that the remaining developmental factors
can be sufficiently accomplished by Japanesetechnology. The overall production of the mirror isplanned at 1,600 mirrors, the photomultiplier at
16,000 tubes, and the electronics at 16,000 channels.
In addition to the ICRR-led group of researchers
responsible for LST, members of CTA-Japan from
Nagoya University Solar-Terrestrial Environment(STE) Laboratory are to be in charge of MST and
SST. In order to secure financial resources for these,
separately from the funding for the LST plan,measures to obtain competitive funding have alreadybeen put in place.

The total number of CTA-Japan members is 84(as of October 2012). Besides those responsible for
equipment development, there is a science groupcomposed of about 35 individuals, whose main
research topics are AGN, GRB, and SNR. (Thebreakdown for the 35 members is: about 20
theoretical researchers and 15 experimental
researchers. To organize headcount by each research
topic, with some duplications, there are 10 members
for AGN, 12 members for GRB, and 20 members for
SNR). At the same time, by using the author list fromthe Task-B paper (a report on the study to quantify
scientific progress under each scientific theme) CTAJapan’s contribution to the world—for GRB, AGN,
and SNR—is found to be 11/20 (out of 20 authors, 11
people are from CTA-Japan), 5/28, and 1/ 14,
respectively. As a strategy to lead the course of future
research topics, the group plans to compile
observation proposal papers, give presentations at
general meetings, and target new themes like Fermi
Bubbles. Also, the CTA group made a request forcollaboration with X-rays and radio observationgroups, wishing to have a connection especially with
the X-ray observation group, so a joint meeting is to
be organized with the X-ray group in 2013.
As preparation for the CTA-Japan data analysis,
the group acquired Monte Carlo (MC) simulation
code, analyzed MC summary data, and based on thesummary data, reproduced the CTA sensitivity
curve. Currently, there are 13 members in CTAJapan MC team, which is too small of a team tothoroughly investigate all physics, calling for
expansion of the scale of activities. As the future
course of action for the analysis group, the group

Based on the CTA experiment time schedule, by2010 the design (LOI and documents on the designconcept) has been completed, and from 2010 to 2014,preliminary studies are to begin. Construction is to3

only calibrated and already reduced summary data issent from Europe, it becomes impossible to carry out
detailed analyses on the whole observation data.
Taking into consideration also the problem of datatransfer, there was an opinion that it may benecessary to establish a system within the ICRR
where analytic processes such as calibration can be
carried out. As for graduate students who can takean active part in observation shifts and analyzing
data, cooperation with university laboratories that
conduct CTA experiments, such as those at Kyoto
University, Nagoya University and Ibaraki
University, is necessary. Moreover, the Committee
expects the team to investigate measures to be taken
in order to jointly acquire budgets with theselaboratories and universities.

cites, in preparation for the launch of CTA operation,aims to set up the environment necessary for
analysis, improve the MC team’s activity scope, andcooperate with the science group in the future. Also,
the group will investigate the possibilities such as
using the ICRR as the data center to secure sufficientstorage and participating in the European group’s
grid system. The development items for the analysis
method include: an expansion of the field-of-view for
GRB follow-up observations by improving themethodology of the stereo observation, andincreasing the rate of GRB detection. In addition,
there is an issue with the network speed fortransferring data from the Europe region to the
ICRR,（3 -terabyte data requires over 1 month to besent）that going forward, measures need to be taken
to reinforce the network and/or to secure a different
method for transferring data.

In particular, building the workforce structure in
the ICRR is believed to be an urgent task. The CTA
experiment is planned to be conducted over the longterm, over a 10-year span. Thus, a system must be inplace to stably provide experimental operation anddata distribution and observation shifts for theduration of the term. Furthermore, to be able toannounce the latest results from the ICRR at any
time, talented people who are knowledgeable of theexperiment and well-versed in relevant theories need
to be posted at the institute. In particular, in order topromptly release scientific achievements and otherexperimental results immediately after the onset ofthe research, a plan that shows scientific strategies
and expected achievements at each experimental
phase needs to be prepared in advance.

Committee Evaluation
CTA’s plan was evaluated highly by the Committee
for its scientific achievement goals, breadth ofresearch topics, high-level technology and experiencein the production of the CTA-Japan telescope anddetectors, that it was concluded that it could be a
candidate for the next large-scale project for the
ICRR. The CTA plan is anticipated to make asignificant contribution to the elucidation of theorigin of cosmic rays, which is an important subject
in cosmic ray research. In Japan, the ICRR shouldtake the lead in promotion of the plan. A discussion
was made on the issue of having a project, unlike
Super Kamiokande and KAGRA, that is not hosted
by Japan, become the pillar of ICRR researches.
However, the consensus was that considering thecurrent globalization trend where countries arecollaborating without borders, the plan was only
following those norms.

Telescope Array (TA) Project
Research Results and Future Project Outline
The TA experiment is an experiment aiming toelucidate extreme phenomena of the universe bymeasuring the energy spectrum, arrival directions,mass composition of ultra-high-energy cosmic rays
(UHECR) with energies greater than 1018 eV. The
experimental site is located in Utah, United States(at an altitude of 1,400 meters); 507 surface detectors(SD) and 3 stations of atmospheric fluorescencetelescope (FD) are installed in the area ofapproximately 700 square kilometers.

At the same time, the shortage of manpower inconstruction of telescopes and detectors, mass
production of detector parts, and in the data analysis
in general, is a concern. There is a shortage of
personnel who can concentratesolely on theconstruction. Therefore, it is necessary to ensure
such talent is arranged, by leveraging posts such as
project assistant professors who have longer officeterms than the regular postdoctoral researchers.
Particularly, in order to become one of the first toachieve results in international projects, it is vital toallocate more human resources and expand the scaleof activities in data and simulation analyses. Also, if

The breaks in the cosmic energy spectrum wereobserved at 1018.7 eV and 1019.7 eV. The significance ofthe cutoff above 1019.7 eV is 3.9 sigma. Taking into
account
thesystematicerrorofenergy4

20 years of the current TA’s data within the first 5
years—that is, a 2-year construction period followed
by a 3-year observation period. It is hoped that thequestion of anisotropy of the highest energy cosmic
rays will be settled with sufficient statisticalaccuracy.

determination, it was confirmed that the shape of thespectrum obtained from TA and all otherexperiments agreed well below 1019.5 eV. The analysisof the mass composition of the cosmic rays
investigated from the distribution of maximum of airshower profiles (Xmax) provides the result that the
TA data are consistent with protons in the energy
region above 1018.2 eV. On the other hand, the results
at Pierre Auger Observatory (PAO) showed thetransition to iron towards high energy side, so that
there is inconsistency in the two experimental data.
An analysis is underway on the autocorrelation ofarrival directions of cosmic rays and on thecorrelations with active galactic nuclei (AGN),
astronomical objects, and large scale structures(LSS). There is no contradiction with the assumptionthat these are ultra-high-energy protons generated
in LSS. However, the probability is 2% that thiscorrelation occurs in the isotropic distribution. The
probability is low but it is not sufficient to exclude
isotropic assumption completely. Also, the clusters
detected by AGASA were not reproduced in the
autocorrelation analysis.

Also, the central figures of the TA experiment inJapan, such as the TA team from the ICRR, isparticipating in the RIKEN-led, The groups from theTA experiment in Japan including the TA team in
ICRR participate in the highest-energy cosmic-ray
observation project, JEM-EUSO, which is led by
RIKEN and will be mounted on the International
Space Station (ISS). And the TA group supports thetest observations of JEM-EUSO prototype at a TA
site.
TA2 is the next generation air shower
experimental plan, aiming to elucidate the origin of
UHECR and to limit cosmic ray source density, andto identify chemical compositions, by expanding thedetection area by 60 times compared to TA.
Approximately 10 thousand SDs are placed at 2 kmintervals, and the total area is 39,200 square
kilometers (60 times the TA). The estimated budget
is about 100 million dollars (approx. 10,000 dollars
per unit, for 10,000 SDs) but the specifics of the plan
will be finalized in the future based on thecomparison results between PAO and TA, as well as
results from TA×4 and JEM-EUSO.

As the next plan for the TA experiment, the TA Low
Energy Extension (TALE) is currently in progress. In
this experiment, the plan is to conduct precise
measurements on the mass composition, which be anindicator of the transition from galactic source cosmic
rays to extragalactic cosmic rays in the energy region
from 1016.5 eV up to 1020.5 eV with the TALE and TA.
For this project, we will add the high-density SD
array and relocate and enhance the FD that was usedin the HiRes experiment. The FD, under the
responsibility of the United States, is already underconstruction, and as for the SD, which Japan is incharge of, the scale of the budget is approximately
100 million yen. In the fiscal year 2013, we plan toconstruct the SD and start its observations. In orderto verify the differences in results such as cosmic-ray
energy scales and mass composition obtained by PAO
and TA, a number of steps are being planned, such as
exchanging light intensity calibration data andatmospheric data between the experimental groups,
making analysis programs public, organizing
collaborative analyses, and exchanging FDs and SDs.
The exchange of light intensity calibration data has
already begun, in 2012, and it can continue until
2018.

Committee Evaluation
Regarding the TA experiment, the researchachievements such as the construction and operationof the experimental equipment, observations, energy
spectrum measurements, chemical composition ofcosmic rays, and correlation analysis of cosmic ray
arrival directions were highly commended within theCommittee. However, in terms of the discrepancies inthe energy scale and the chemical compositions withthe data from PAO, it is not clear whether these areascribed to systematic errors such as from thecalibration, or from simulations, and the Committee
recognized it was important to identify the cause andresolve the issue before proceeding to the next plan.
Therefore, it is essential to deepen the understandingof the observation data, by performing energy
calibration with LINAC and carry out comparative
verification with PAO.
With regards to the TALE experiment, it isexpected that it will contribute to a greater

TA×4 is a plan to deploy 500 SDs at 3 different sites
with 2-kim spacing. The plan is to quadruple thetotal area including the current TA, and to install an
additional FD at one site. It is possible to acquire over5

structure in the vicinity of the solar system.

understanding of the cosmic ray spectrum and thechemical composition in a wider energy range, and by
expanding to the low energy end to fill the gap
between the observed energy region of the Tibet
experiment, a more comprehensive understanding
can be expected about cosmic ray generation andpropagation.
TA×4 plan aims to accumulate data equivalent of
20 years of the TA data in total, in the short period of
the next five years. The Committee appreciates that
it is a realistic plan that saves resources. However,
the TA×4 plan was submitted during the Committee
discussion, giving the impression that the concrete
planning of this experiment started later comparedto other projects. The Committee expects theplanning be expedited and the project be
implemented as soon as possible.
With regard to TA group members such as thosefrom the ICRR participating in JEM-EUSO, and eachaiming to make contributions via
their own
capacities, the Committee believes that the diverse
experiences of the members related to the highest
energy cosmic rays will be advantageous in JEMEUSO, and concludes that this situation isappropriate.
With the TA2 Project, in accordance with thecurrent policy of the TA group, it is desired that thediscussions based on PAO and TA comparison
results, and results from TA×4 and JEM-EUSO
should be continued.

The ongoing expansion work and the next term
plan for the Tibet experiment are described below.
With the conventionally used AS array (having theeffective surface area of approximately 37,000 square
meters by the counter array consisting of plastic
scintillators and photomultiplier tubes), it wasdifficult to distinguish between gamma rays andnucleons. For that reason, currently there is a planunderway to expand the facility by building muon
detectors (MD) under the AS arrays. MD is a watertank installed 2.5 meters underground, and isdesigned to detect muon Cherenkov light generated
by cosmic ray particle events, and with anticoincidence, it is aimed to significantly improve theS/N of gamma-ray observation. As a result,meaningful gamma-ray observation can be expectedin the energy region over 10 TeV up to regions
exceeding 100 TeV. The prototype for MD wasconstructed in 2007, and for the most part confirmed
the performance expected from the simulations.
Currently, there are 5 MDs being constructed (thefinal target is 12). Moreover, 100 counters (with 1.5
meter-intervals) consisting of lead, iron and plastic
scintillators are currently being built in the 160square meter area, at the center of the AS array(YAC-II). With these in place, the energy spectrum
and composition ratio of protons and helium in the“knee” region are expected to be determined.
The construction of the remaining seven MDs, theexpansion of the AS arrays (37,000square meters →83,000 square meters), and the construction of YACIII are being considered, as the next term plan of the
Tibet experiment. YAC-III plans to build 400
detectors at the center of the AS array, in an area of
5,000 square meters, aiming to determine the energy
spectrum and the composition ratio of the ironcomponent in the “knee” region. The budget is 400
million yen for the MD, 150 million yen for the AS
expansion, and 250 million yen for the YAC-III,
totalling 800 million yen. It is a 6-year plan, which
breaks down to 3 years of construction and 3 years ofobservation. Kakenhi scientific research grant isbeing considered for the budget acquisition, and infor the fiscal year 2012, and Grant-In-Aid for
Scientific Research (A) was adopted. The budget
allocation is planned to be 3:1 for Japan and China,
respectively.

The Next Term Tibet Experiment
Hitherto Research Results and Future Project
Outline
The Tibet experiment, conducted on a highland at
an altitude of 4,300 meters, aims to study the energy
spectrum, the chemical composition and anisotropy
observation of cosmic ray particles (nucleons) fromseveral TeV to the “knee” region (1015－1016 eV), andto conduct a high energy celestial gamma-ray
observation in the regions from several TeV to over
100 TeV. As for the achievements of the Tibet
experiment, there are a few events detected for the
first time. These include: the deficit of the cosmic rayintensity due to the shielding effect of the sun andthe moon, the Compton-Getting effect associatedwith the revolution of the earth, change in the
structure of magnetic field near the sun that isdependent on solar activities, and the cosmic ray
anisotropy in the regions of a few TeV that provide
information on the interstellar magnetic field

Committee Evaluation
Knowing the cosmic ray chemical composition in6the “knee” region is important in investigating theorigin, acceleration, and propagation issues ofgalactic cosmic rays. At present, the composition
model that is considered standard is based on the
KASCADE observation, but muons are the main
component of the air shower at KASCADE, (1 atm
altitude) and its development heavily depends on
hadronic interaction models. On the other hand, thealtitude at the maximum development of air showers
generated by cosmic ray particles in the “knee” region
almost equals that of the Tibetan altitude (X=600
grams). The main component of the air showers
detected at this altitude is electromagnetic, and theirmodel dependence is low, which is advantageous fordetermining the chemical composition.
Meanwhile, HESS, MAGIC, and VERITAS, which
are international gamma-ray observation projects
that employ Cherenkov light detectors, are fiercelycompeting for results. Also, as the next term plan,
CTA is starting to take form. However, as it waspointed out at the 2007 Future Planning Committee,
CTA has a narrow field of view, and as the scale of its
budget is large, it may take years to materialize.
Complementarily, the Tibet observation has a wide
field -of -view and requires a small-scale budget, anda quick implementation of the plan is expected toachieve reasonable scientific results. Considering theconstruction for the North American experiment
HAWC—that has similar scientific objectives and isalready in progress—a prompt implementation of theplan is extremely important if they are to achieve
pioneering results with the Tibet MD plan.

Project Outline
Hyper-Kamiokande (Hyper-K) is a thirdgeneration proton decay/neutrino experimental planusing a water Cherenkov detector, following theprevious Kamiokande and Super-K projects. A wide
range of physics is envisaged from this plan,
including measurements of charged parity (CP)asymmetry in lepton by accelerator neutrinos,
determination of neutrino mass hierarchy inatmospheric neutrinos, nucleon decay search, studyof solar neutrinos, supernova burst neutrinos,
supernova relic neutrinos, and solar flare neutrinos.
The ability of the water Cherenkov detector has
already been demonstrated with Kamiokande andSuper-Kamiokande, and its excellent scalability is
notable. Hyper-K detector consists of two sets oftanks 54 meters high and 250 meters long, with a
total mass of 1 million tons (of which the effective
volume is 560,000 tons, resulting in a 25-foldincrease from Super-K), 20％ photo-sensitive area,
and about 100,000 photosensors in total. Tochibora
mine (650m-deep underground located about 10
kilometers away from Super-K) in Kamioka Town,
Hida City, Gifu Prefecture is being considered as a
candidate site for the detector. Cavern analysis anddesign for the detector are nearly complete, and thedetector design is almost finalized. Either 20-inch
photomultiplier tubes (PMT) or the hybrid
photodetector (HPD), a new model that is designed to
improve performance and costs, is considered as thephotosensor. A prototype of the 20-inch HPD is to becompleted in 2013, and its feasibility test isscheduled to be conducted in a water tank with 200
tons of gadolinium water in Kamioka. In addition,
the designing of the plastic liner shielding, which isused as the outer wall of the water tank and thedevelopment of DAQ are ongoing.
In order to form a collaborative body throughinternational cooperation, in August 2012, an Open
Meeting was held at the Kavli IPMU of the
University of Tokyo at Kashiwa Campus, and about
100 people from 35 different research facilities from
9 different countries came together to participate in.
At the meeting, the participants reviewed projects,
formed working groups, and assigned conveners toaccelerate the detector R&D.
The estimated cost of the construction is about 80
billion (with an error of 25％) of which 30 billion isfor the cavern excavation, 30 billion for the watertank construction, and 20 billion for the photosensors (assuming highly photoelectric efficient HPD
is to be deployed), however, further cost reduction isrecognized as an issue to be solved. Although it isFinally, the relationship between the Tibet
experiment group and the TA group is described here.
Although the observation altitude is different, as the
TALE experiment starts, having both the Tibet andthe TALE experiments will enable a wide andcontinuous measurement of energy in higher regions
than the “knee” region. Both are groups within the
ICRR, and as such:○ Unified research on cosmic-ray energy
spectrum/chemical composition,○ Efficient use of human resources drawing on thesimilarities of technologies used (detectors,
simulations etc.), training students and postdoctoral
fellows, facilitating acquisition of external funds, andother possibilities that can be utilized to strengthen
the mutual collaboration in advancing the researchmust be considered.

Comments on Hyper-Kamiokande (Hyper-K)
7detector, and thus, it is necessary to demonstrate its
performance and capability.

contingent upon the budget acquisition period, theconstruction is scheduled to start in 2016, and theoperation is to start in 2023.
One of the most anticipated physics topics withaccelerator neutrinos is the search for the CP
asymmetry in the lepton sector. If the CP symmetryis violated, the probability of occurrence of electron
neutrinos from muon neutrinos for neutrino andantineutrino beams are different and thus, it ispossible to check whether there are differences in theprobability of occurrence for each or not. Assuming
the beam power of neutrino is 750kW and theexperiment period is 10 years, about 4000 electron
neutrino events and about 2000 anti-electron
neutrino events are expected to be detected, andassuming the systematic error to be 5% or less, it will
be possible to detect CP violation with the confidence
level of over 3 sigma. The systematic errors are
expected to be further reduced with the advancementof the currently ongoing T2K experiment. With
atmospheric neutrinos, it is anticipated that themass hierarchy of neutrinos can be determined bymeasuring the matter effect of neutrino oscillationsin the Earth’s interior. The oscillation between muon
neutrino and electron neutrino is amplified byneutrinos in the several GeV region passing throughthe Earth. However, the amplitude differs dependingon the mass hierarchy, so with the Hyper-K detector
and the observation period of 10 years, it will bepossible to determine the mass hierarchy with over 3
sigma of confidence even taking the uncertainty ofthe oscillation parameters into account. Regarding
the supernova burst neutrinos, it is expected that theverification of the theoretical model of supernova
burst mechanism and the measurement of the mass
hierarchy of neutrinos will be possible, by measuring
the time dependencies of flux, energy and neutrino
types. For proton decays, in p→eπ0 decay mode, thesearch is made possible up to 1.3×1035 years oflifetime (90％ reliability), and in many other modes,
the sensitivity will be improved by an order ofmagnitude from Super-K that the discovery of
nucleon decay is also anticipated.
Other competing experiments include LBNE in the
United States, and LBNO in Europe, and theseexperimental projects are planning to employ liquidargon detectors. LBNE is scheduled to start its
experiment in 2022 and is mainly focused on themeasurement of CP asymmetry by accelerator based
long-baseline neutrino oscillations. LBNO has longbaseline of 2,300km and it has significant sensitivity
in identifying mass hierarchy. However, there are
only limited precedents of the large-scale liquidargon detectors, unlike the water Cherenkov

Committee Evaluation
Hyper-K is an experimental plan that uses a largescale water Cherenkov detector following theprevious Kamiokande and Super-Kamiokande. As
mentioned above, its technology and analyticalmethods have been well demonstrated andconfirmed. Other than the scale of the budget, there
are no other significant concerns with the project.
The required person-power has been estimated andit goes beyond that of the Super-Kamiokande project,
and thus asking the ICRR alone for contribution isclearly not enough. More human resources must be
secured by other means such as leveraging
international cooperation.
In terms of the budget, 80 billion yen stands atsuch a large scale that it has never been requested
for before from the ICRR, and therefore, thepossibility of a joint budget request with KEK and JPARC must be examined, also. For the budget
request as well as its acquisition to be successful, allneutrino researchers in Japan need to unite toestablish a system that can promote Hyper-K plan.
In that respect, it is important that the directionalityof the future plan is shared and aligned with the
KEK group. (Written as of November, 2012. After theabove note was made, an agreement was made with
KEK group which affirmed Hyper-K project to be
Japan’s future neutrino plan).
There is no issue with the scientific significance or
the employed technology with accelerator neutrinos
and atmospheric neutrinos, however, the cosmic ray
muon-sourced background level is high in the solar
neutrino observation indicating a
possible
impediment in the research development in the
future, so it may be necessary to examine the courseof the experimental plan more closely.
In addition, there is a concern about the timing of
the acquisition of the budget, as it may be too pressed
for time for the construction scheduled to start in2016 and for the operation scheduled to start in 2023.
Therefore, it is necessary to review the budget
acquisition plan in detail, and work towards
actualizing it. Moreover, reportedly there is still a
25% error in the 80 billion construction cost estimate
at present. The Committee urges the group to
improve the accuracy of the budget estimate byclarifying each country’s cost allocation, and to
examine the estimate in depth for a successfulacquisition of the budget.
8reasons, the background rate is approximately 2orders of magnitude higher than the target value.
Nonetheless, using the 6.7-day observation data fromXMASS-I, DM and axions were searched. For DM
search, by using the fiducial volume cut analysis, it
is expected to be possible to set limits from 2.2×10-41
to 10-43 cm2 in scattering cross-sections for the range
and from 4.7×10-43 to 1.1×10-44 cm2 in cut analyses.
Currently, the detector is being refurbished, andthe plan is to cover the aluminum sealing on the PMTwith a copper plate, and to further remove the GoreTex in order to reduce the background rate down to
1/100th of the current rate. XMASS-1.5 is also in theplanning, for fundamental amelioration andsensitivity improvement, as well as to compete with
the new project emerging abroad called Xenon-1ton.
In order to improve the surface background
identification, PMTs with a round shape lightreceiving surface will be developed, Gore-Tex will be
removed, and the background due to the lead on thesurface of the detector will be reduced. For the DM
search, the expected physics results include thecross-section 10-46 cm2 and less (at an energy range
over 5 keV ） and for the lower energy range, thesensitivity improvement for the scattering crosssection up to a few10-42 cm2, and an improvement by
2 orders of magnitude for the axion search comparedto the current capability. The construction isscheduled to start in 2014, and the experiment is tostart in 2015.
The main foreign competitor is Xenon-1ton (Gran
Sasso, scheduled to start operation in 2015). Similar
to XMASS-1.5, they are expected to achievesensitivity lower than 10-46 cm2, however, there aresome technical developmental factors such as highelectric pressure, that still remain. Other foreign
projects include LUX (Xe 300kg, Homestake), LZ (Xe7ton), and Super-CDMS. Among these project
designs, what characterizes XMASS is its sensitivity
to both nuclear recoil and electron/gamma ray.
In terms of manpower, except for the contribution
from Korea for the development of calibration
sources, it is comprized of personnel from domestic
research institutions, and the total number ofcollaborators is 41. The total budget for XMASS-1.5
is planned to be 1.3 billion yen.

XMASS Experiment
Hitherto Research Results and Future Project
Outline
The XMASS project is based on a single-phasedetector that uses liquid xenon as target material,and is a multi-purpose cosmic elementary particle
experiment that aims for not only the direct search ofdark matter (DM) but also observation of solar
neutrinos and double-beta decay. At present, as
phase 1 of the experiment, XMASS-I (effective
volume 100kg, total mass 835kg), is already underoperation. For the next phase, XMASS-1.5 (effective
volume 1 tons, total mass 5 tons) is in the planning,and XMASS-II (effective volume 10 tons, total mass
25 tons) will be the final phase. With the XMASS
detector, it is possible to eliminate the gamma-ray
and neutron-induced external background by the
self-shielding effect of xenon. Also, as the detector
extension is comparatively easy, this project is moreadvantageous than its competitors, such as Xenon100, which employ a gas-liquid dual phase detector.
XMASS-I detector is comprized of 630 2-inch
photomultiplier tubes (PMT) and 62% of the innerwall is made of a photoelectric surface. As a measureagainst ambient background, the detector is set upcompletely submerged in the 800 ton water tank, inorder to shield gamma rays and neutrons. The teamalso developed PMTs with low radioactivity
background. In addition, for the xenon internal
background, krypton (Kr) and radon (Rn) were
eliminated almost to the required target value. The
construction was completed in September 2010, andby 2012 May, the commissioning was completed. The
photoelectron yield is about 14.7 p.e. per 1 keV, whichis 6 times higher than Xenon 100. With the 122 keVgamma-ray calibration using Cobalt 57, the energy
resolution is evaluated at 4%. The position resolutionof the detector core is 1.4 cm in RMS.
More than expected amount of background wasobserved from the commissioning data, and theidentification of its source was carried out. One of the
backgrounds was gamma rays from uranium 238 andlead 210 contained in the aluminum used tocompress the quartz window and the metal pipes of
the PMT. The radiation measurement using agermanium detector and simulations explain theamount of background energy over 5 keV. The energy
below 5 keV is assumed to be background from thecarbon contained in Gore-Tex used to fill in the gap
between the PMT and the holders. For theses

Committee Evaluation
XMASS is an experiment led by a Japanese groupthat is capable of competing with other experiments
overseas, in the area of dark matter search. The
expected scientific achievements range widely from9

terms of the timeline, it is planned that the R&D andbudget application are to be completed between 2013
and 2014, the construction to start from 2015, andthe observation to start from 2018.

the field of axion dark matter to solar neutrinos. The
experiment demonstrates excellent backgroundremoval by the detector and the energy thresholdetc., and it shows high potential. The members of theexperiment group have advanced technicalcapabilities in areas such as removal of impurities inxenon, and thus, considering the background of thegroup, there seems to be no issue technologically interms of detector manufacturing. Also, in terms of
the project structure and scale, presently there seemto be no issues. It is considered to be a reasonable
experiment to execute at the Kamioka Observatory
in the next 10 years, which is when the HyperKamiokande project is planned to start.
The remaining issues are the background and thebudget, but the competitor, Xenon-1ton, despite
uncertainties in its feasibility, has been approved andbudgeted. The Committee urges to resolve thesurface background of XMASS-I by upgrading thedetector as soon as possible, and to seek various
possibilities to acquire a budget for XMASS-1.5, inorder to implement the plan.

Committee Evaluation
The Ashra experiment has been working on the
research and development of cosmic ray observation
in a very ambitious way, such as wide filed-of-view
imaging observation using the image intensifier, andthe observation of very-high-energy, earth-skimming
tau neutrinos of astronomical origin. Some of theresults obtained from this detector development
research are quite unique. Also, through a series ofdevelopment studies, the project attained somephysics results such as cosmic ray spectrum
measurement in the PeV region, neutrino search,and GRB optical flash search.
However, the Committee must conclude that Ashra
is drastically behind its initial plan. The projects
abroad that were expected to become competitors
while Ashra was still in the planning stage are now
delivering some results, so the Committee concludes
there is little significance in continuing theverification tests for Ashra for another period of timein the future. Moreover, NTA was suggested as a
future plan following Ashra. However, at present, thescale of the joint research organization, especially the
manpower within the ICRR, is insufficient to fulfil
the proposed plan, and therefore the Committee
concludes that it cannot endorse implementing the
NTA plan in such a way that the institute acts as themain body of the program.
Finally, the Committee agrees that the results
obtained from the Ashra’s detector development canbe efficiently utilized in some form in the future, andtherefore recommends to compile the technicalresults of the research in detail in papers and soforth.

Ashra Project
Hitherto Research Results and Future Project
OutlineFor the last decade, the Ashra group has beendeveloping a detector that captures images of air
shower by using imaging intensifier, and conducting
observations in Mt. Mauna Loa. Ashra has been
aiming at very-high energy astronomy, observation of
gamma-ray burst (GRB)-origin tau neutrinos, andobservation of optical flash from GRB in the visible
light region.
In the interview with this Committee, in addition
to the details of Ashra, the Neutrino Telescope Array(NTA) project was also reported. The NTA plan is to
succeed from the Ashra experiment the air shower
imaging technology, and to perform observation on
the very-high-energy astronomical-origin neutrinos
ranging from 106 to 109 GeV. The experiment islocated on the island of Hawaii, United States, in anarea surrounded by three mountains. Very-highenergy events due to air shower of tau neutrinos
generated by passing through the mountains will beobserved.
The
observable
solid
angle
is
approximately π steradian, and the angularresolution is evaluated at about 0.2°. Currently, there
are about 30 collaborators from Japan, Taiwan, andthe United States, and the target is to involve over
100 collaborators from 10 different countries. In

Gadzooks!
Project Outline
Gadzooks! is a plan that aims to improve physics
in supernova background neutrinos and supernova
burst neutrinos and other events, by dissolving
gadolinium sulphate at 0.1% in pure water in SuperKamiokande equipment and thereby improving
neutron capture efficiency of anti-electron neutrino
reactions.
The primary background of the supernova
10background neutrino is electron events originatingfrom atmospheric neutrinos that are generated by
the decay of muons with energy levels lower than the
Cherenkov threshold. By detecting the delayed
signal with energy approximately 8 MeV that isreleased when neutrons are captured by gadolinium,
it is possible to reduce this background by about onefifth, thereby improving the detector sensitivity forthe supernova background neutrinos. Also, in the
supernova burst neutrinos, electron events are
sensitive with respect to the direction of arrival ofneutrinos, however, by neutron tagging it is possible
to use the sample that excludes anti-neutron events
to calculate the direction, which then makes it
possible to improve the resolution for the arrival
direction by nearly 2-fold. In addition, further
development of physics can be expected in areas
including verification of supernova burst models
such as for neutronization burst and Si burning, andin supernova burst sensitivity in nearby galaxies.
Currently, in the Kamioka mine shaft, a 200-ton testtank is constructed and set up, and demonstration
tests (EGADS) are being conducted for Gadzooks!,
including tests for the Cherenkov radiationtransmission, equipment corrosion, and water
circulation equipment.

Hitherto Research Results and Future Prospects
The Committee asked the Theory Group, theObservational Cosmology Group, and the HighEnergy Astrophysics Group to each submit
documents of a few pages reporting researchachievements and future prospects. The following is
the record of the discussions and evaluations made at
the Committee based on these reports.
The Theory Group has been conducting theoreticalresearch on various subjects related to particle
physics and cosmology. In the field of particle physics,
theories such as the grand unified theory andsupersymmetry standard model, which is consideredhopeful due to the presence of a candidate for dark
matter, are the targets of study. In the field ofcosmology, focusing on the particular area called
particle cosmology, subjects such as cosmic inflation,
baryogenesis, cosmological nucleosynthesis based on
the supersymmetry theory, dark matter, and thegeneration and the evolution of cosmological density
perturbation are studied. Over the past 5 years, thegroup has published 20 to 40 articles per year in peerreviewed
journals.
Recentmajor
researchachievements include limitations to nucleosysnthesis
and gravitino in the early universe, the Axion
cosmology,
limitationson
equal
curvature
perturbation in inflation, Higgs particles in LHC
experiments, and research on the supersymmetry
standard model. Going forward, while paying close
attention to the latest results from LHC and CMB
observations, the group aims to propose a strong
physics candidate that surpasses the standard modeland to explore cosmological reasoning. Also, thegroup will accept students and researchers, andmaintain activities to contribute to the field ofparticle physics/cosmology.

Committee Evaluation
At the Committee, no hearing on the Gadzooks!
experiment was conducted with the parties involved,
so the discussion on this topic was based on theinformation obtained at the town hall meeting hosted
by the Cosmic Ray Conference. At the Committee,
the Gadzooks! project is recognized as a researchplan that is a natural extension in the realm ofSuper-Kamiokande
research
activities.
The
Committee highly appreciates its significance inphysics, and concludes that it demonstrates highfeasibility due to R&D that have been steadily
executed. Regarding the transition of SuperKamiokande detector to Gadzooks!, the Committee
shares the opinion that the final decision isdependent of the Super-Kamiokande group,
nonetheless, the experiment is considered feasible
also in terms of obtaining a large-scale budget.
Therefore, the Committee concludes an early
realization of the plan is desired.

The observational cosmology group has gone
through an organizational change from the SDSS
group, and since 2010 the group has been embarking
on preparatory research for deep space exploration
using Subaru Telescope Super Wide Camera (Hyper
Suprime-Cam: HSC). Using HSC, the deep spacevisible light imaging survey will be conducted overthe next five years from 2013 to 2017, to detect about
10 thousand Lyα spectral lines at z=5~7, and byanalyzing the data, the group aims for a unified
understanding of the reionization of the universe.
After 2018, the group will consider participating in
projects such as the infrared spectrum survey using
Subaru Prime Focus Spectrograph (PFS) and the

Other research groups (the Theory Group,
the Observational Cosmology Group, andHigh-Energy Astrophysics Group)11with investigators from the KAVLI Institute for the
Physics and Mathematics of the Universe (Kavli
IPMU), whose building is adjacent to the ICRR
building.

super large-scale, next-generation telescope, theThirty Meter Telescope (TMT). As a collaboration,
the group plans to provide data analysis tools andcatalog a database inside and outside of Japan, as
well as to host lectures and research workshops on
deep imaging data analysis.

Overall Conclusion

The High-Energy Astronomy Group was firstestablished within High-Energy Cosmic Ray Divisionof ICRR in December 2009, and has been conducting
research with an aim to study various high-energy
astrophysical events related to the origin of cosmic
rays, by employing theoretical and analyticalmethods. Thus far, the group has achieved results inthe area of high-energy astrophysics, such as studies
on explosion energy release phenomena and its
related research on particle acceleration phenomena.
The group has numerous research targets in common
with other groups within the ICRR. The subjects thatare expected to grow in the future are researches on
extragalactic super-high energy cosmic rays, theacceleration mechanism of galactic cosmic rays,
anisotropy, propagation, electromagnetic field
detection methodologies on ultra-high energy cosmic
rays, high energy γ-ray astronomy, and binary
neutron start system merger events.

Before this Future Planning Committee
commenced, the ICRR organized Future Planning
Subcommittee meeting on three occasions since the1980s to examine future plans for the institute. At
the 1st committee meeting, Super-Kamiokande plan
was recommended with the highest ranking. This
plan has since been implemented, has madesignificant scientific achievements, and continues to
produce new results. At the 2nd meeting, theresearches on the highest-energy cosmic rays andgravitational waves were strongly recommended.
The Telescope Array plan that promotes the studies
on the highest energy cosmic rays has since been
implemented. Its achievements are materializing atpresent, and the results have already received highevaluation by the international cosmic-ray
community. At the 3rd committee meeting, the subjectof gravitational waves was again strongly
recommended. The promotion of the plan called
KAGRA, which aims to detect gravitational waves,
was approved by the Committee, and it is now underconstruction. In this way, recommendations made by
the ICRR Future Plan Discussion (Sub) Committee
have greatly influenced the execution of the researchplans of the institute, and these researches havemade many scientific achievements. Looking back onsuch history, the conclusion of this Committee,
therefore, will have significant weight.

Committee Evaluation
It is noteworthy that two members of thetheoretical group comprehensively share and cover abroad field of research, from particle physics tocosmology, and have been delivering numerous
research results while maintaining a high level ofactivities. In addition, many excellent researchers
currently active in the field of theoretical researchoriginally resided in this group, that it seems thegroup has also contributed greatly to educating
researchers.
At the ICRR, in addition to the Theoretical Groupand Observational Cosmology Group, the HighEnergy Astronomy Group was established, and these3 research groups together resulted in the formationof a researcher’s group that covers a wide range ofstudies including particle physics, cosmic rays, andcosmology. Taking advantage of these characteristics,
it is recommended that the group investigate newcollaborative research possibilities. Moreover, theCommittee expects the 3 groups will not become
siloed but, instead, expand their network andinteract with other experimental and observationalgroups. In fact, the Committee encourages thecontinuation of already active collaborative research

The Committee convened a total of 10 meetings
over the period of about one year and a half since thebeginning of 2012, and various possibilities for the
future plan were discussed. The details of theCommittee’s evaluation on each future plan havebeen described in the previous chapters of this report.
Although it is extremely difficult to rank these futureplans, based on the comprehensive evaluation whichtakes into account the aspects of its expectedscientific achievement, scientific ripple effects, andtechnical and budgetary feasibility of implementing
the plan, the Committee’s conclusions are reported
below.
For the next plan the ICRR must promptly work
towards realizing, the Committee recommends a12

expedited and the project be implemented as soon as
possible.
Moreover, since XMASS-1.5 and TA×4 are not
final plans, but have their next stages already being
planned in detail, and hence also from theperspective of setting these future experiments inmotion, early realization of these projects isdesirable.
As the TALE experiment is to start, having boththe Tibet and the TALE experiments will enable awide and continuous measurement of energy inhigher regions than the “knee” region. Suchcomprehensive implementation of air shower
observation in such a wide energy range by one
research facility is internationally unique. In order totake advantage of this, it is important for TA andTibet, as well as for the theoretical groups, to work
together and strengthen their cooperation. In termsof joining JEM-EUSO, the Committee expects active
contributions from Japan’s groups, even whileleveraging the TA group’s experience.

focus on CTA experiments that aim at dramaticadvancements of gamma-ray astronomy. The CTA
plan must be executed through an international
collaborative effort. Hence, in order for the Japanesecommunity to make a significant contribution and toactively implement the research, it is important tofurther reinforce Japan’s research team organization
comprised of the ICRR and other research groups, forthe development of both equipment and software.
Regarding the advancement of neutrino physics
such as neutrino oscillation, Japan has been leading
the world for more than 10 years. HyperKamiokande is a vital project that the Committee
believes will be able to continue such trends, andeven further develop it. In particular, themeasurement of CP asymmetry and elucidation ofmass hierarchy are one of the top-priority issues inparticle physics, and their feasibility is consideredsufficiently high based on the experiences from thecurrent experiments. The Committee hopes to see agreat progress in the research and development ofHyper-Kamiokande, to prepare for its swift
implementation when the opportunity arises. At thesame time, the budget scale is estimated at
approximately 80 billion yen, and therefore it isrecommended to those involved to clarify details—
such as the cost ratio of each country to improve theaccuracy of the budget estimate—in order to go
forward with attaining the budget.
Many other future plans of the ICRR that were
examined by the Committee can also be expected todemonstrate high scientific significance andsufficient scientific results. As for the XMASS
experiment, if the background issue is resolved andthe superiority of its methodology can bedemonstrated, it can be expected—considering thescalability of the XMAS-1.5 detector—to drastically
improve detection sensitivity for dark matter.
Considering their experimental scale, not only
budgetary requests but also every other funding
possibilities (including competitive funds) must besought for XMASS-1.5: the next Tibet plan (which
conducts gamma-ray observation in the 100TeV
region), Gadzooks! (which searches for neutrinos
originated from supernovae from the past, and TA×4
(which expands the observational area for thehighest energy cosmic-rays by 4 times). The ICRR is
also to support these plans and advocate for their
speedy implementation. While its scientificsignificance is recongnized, the impression has been
given that the TA×4 experiment’s logistics andplanning had a slower start compared to otherprojects. The Committee expects the planning to be

For the 3 groups of theory, observationalcosmology, and high energy astrophysics, theCommittee expects the groups to achieve results
through implementing each other’s collaborative
studies, and to promote research exchange with otherexperiment observational groups.
Still, in order to realize all the large- and mediumsize future plans examined this time by theCommittee, there is not only the difficulty offinancing, but also the difficulty of securing humanresources. Therefore, it must be noted that, even forplans that were evaluated highly scientifically by theCommittee, research teams must be flexible in theiractions—by changing the order of plans,reevaluating research organizational structures, andso on—though the project’s prompt implementation
has been deemed difficult.
The ICRR has been providing various resources
including experiment facilities, computers, andresearch groups, to support small-scale cosmic-ray
research projects at different universities to date,fulfilling the role of Joint Usage/ Research Center(the former Joint Research Laboratories). The
Committee hopes to see the institute continue therole of being the base for cosmic-ray studies. Finally,
the Committee anticipates KAGRA to make steadyprogress in its ongoing construction, and to produce
scientific results as soon as possible.

13

The Institute for Cosmic Ray Research of the University of Tokyo
Future Planning Committee Final Report
October 26th, 2017
Background and Objectives of Committee
Establishment

The ICRR steering committee has decided to appointa new Future Plan Committee, which focuses its
efforts around the Hyper-K plan. This committee
examines the work following the 2013 Future Plan
Committee Report [1] and evaluate the purpose andthe feasibility of the project to determine whether
this project is appropriate as the next main project ornot. This committee also has decided to discuss new
projects which are proposed after the previous
Future Plan Discussion Committee.

The University of Tokyo, Institute for Cosmic Ray
Research (ICRR), has coordinated Future Plan
Discussion Committees four times since 1987 under
the supervision of the organizing Committee. In
these committees, discussions and nominations of
the research projects of the next generations,especially on the ones, which could be the flagship(projects) of the institute. Based on therecommendations, the institute has strived to realize
the plans. At the first committee in 1987, the SuperK was proposed and realized, thereby contributing toimmense research progress. The second committee,
in 1993, proposed highest-energy cosmic rays
research, and this was realized in the form of theTelescope Array (TA) experiments. Furthermore, thegravitational wave telescope KAGRA, proposed in
the second and third Committee (2007), has its
detector under construction. To succeed Super-K and
KAGRA in a timely manner, it was recommended—at the fourth Committee (2013)—that Cherenkov
Telescope Array (CTA) experiments were to beprioritized, therefore permitting the construction of
the CTA following the 2016 budget request. The
Institute is at a point where future plans to follow up
KAGRA and CTA must be debated.

At the time this Committee commenced, the science
council of Japan had been developing master plans
for the 23rd large-scale research facility plan. The
Ministry of Education, Culture, Sports, Science andTechnology (MEXT) had also decided on discussing
the reassessment of the roadmap in early 2017. It
was projected that Hyper-K would be one of the main
topics in this discussions, so this Future Plan
Committee held a discussion on Hyper-K prior to theother projects to accommodate the developments thatwere to happen in the MEXT meetings and,published an interim report on March 24th, 2017.
Regarding the new research plans, proposals were
solicited from faculty members of ICRR, from
October 8th to November 1st 2016, which resulted in
3 proposals: “ALPACA Experiment Plan,” “Proposal
for extension of the high-energy cosmic rayobservation on the ground,” and ““Composite
Imaging Observations of Cosmic Elementary
Particles.”In the 2012-2013 Future Plan Review Committee, alarge-scale plan, Hyper-Kamiokande (Hyper-K), wasalso discussed. It was recommended that, “We havehigh expectations for progress on the development
research of Hyper-K so that we are ready to realize
the plan whenever the opportunity arises.”

In addition, on May 1st, 2017, the research
representative of the XMASS collaboration requested
a discussion on changes to the current plan and it
was decided at the 8th committee meeting (May 10th,2017) that this was to be regarded as a new proposal.
This final report is comprised of the discussions on
the four new proposals, the summary of the plans
and the evaluation of the Committee.

Since the budget is estimated to be around 80 billion
yen, it is recommended that outstanding details are
finalized, such as responsibilities and budget ratios
for each participant country, so that the budget
request can be moved forward [1].
Subsequently, ICRR and Institute of Particle andNuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK) have agreed tocollaborate regarding Hyper-K. Also, these two
institutes have set up an external advisorycommittee for the further discussion to realize theproject.

Members
Y. Okada (Chairperson)
Director, High-Energy
Accelerator Research Organization

1

H. Aihara
Professor,
Graduate
School of Science, The University of Tokyo
Y. ItoProfessor, Institute forSpace-Earth Environmental Research, Nagoya
University
K. Inoue
Professor,
ResearchCenter for Neutrino Science, Tohoku University
S. Ogio
Professor, Osaka City
University Graduate School of Science
T. Kishimoto
Professor,
Osaka
University Graduate School of Science
J. Hisano
Professor, Center for
Theoretical Studies, Nagoya University
M. Mori
Professor, Ritsumeikan
University College of Science and Engineering
M. Nakahata (Secretary) Professor, Institute for
Cosmic Ray Research, The University of Tokyo
M. Ibe (Vice secretary, clerk) Associate
professor,
Institute for Cosmic Ray Research, The
University of Tokyo
T. Kajita (Observer)
Director, Institute for
Cosmic Ray Research, The University of Tokyo

•

•

•

•

Background
•

•

•
•

Discussions on interim report
5th Meeting (February 21st, 2017)- Reports on the submission of the draft
interim report to the faculty and the steeringcommittee- Hearings on new proposals
March 6th - March 24th, 2017
- Discussed revisions via email based on thecomments received from the faculty meeting(commenced on Feb. 23rd, Mar. 23rd) and thesteering committee (commenced on Mar. 6th)
March 24th 2017
- The Institute for Cosmic Ray Research of the
University of Tokyo Future Plan Discussion
Committee finalized the interim report
6th Meeting (April 10th, 2017)- Discussions on the new proposal the“ALPACA Project”- Discussions on the new proposal “Future
Plans on the Ground-level observations of
Ultra-High Energy Cosmic Rays”- Discussions on the new proposal “Composite
Imaging Observations of Cosmic Elementary
Particles”- Discussions on the final report
7th Meeting (April 27th, 2017）- Discussions on the draft final report
8th Meeting (May 10th, 2017)- Discussions on the draft final report- Procedures for handling new/revised
proposals
9th Meeting (June 12th, 2017)- Review on the direct search of dark matter- A hearing and discussions on the new
proposal “A Proposal for the New
Development of the Direct Dark Matter
Search Experiment with Liquid Xenon”
10th Meeting (July 3rd, 2017)- Additional hearing and discussions on the
new proposal “A Proposal for the New
Development of the Direct Dark Matter
Search Experiment With Liquid Xenon”
11th Meeting (August,18th)- Discussions on the new proposal “A Proposal
for the New Development of the Direct Dark
Matter Search Experiment With Liquid
Xenon”- Discussions on the final report
September 19th - 22nd, 2017
- Based on the comments received from thesteering
committee(commencedon
September 19), revisions were discussed via
email.

-

1st Meeting (October 4th, 2016)- Introduction- Report from Hyper-K Advisory Committee- About Hyper-K Plan
- On the status of the evaluations by thecommunity- Future course of action- Discussion procedures for Hyper-K- Inviting other project proposals andthe review timeframe
2nd Meeting (November 14th, 2016)- Hearing on Hyper-K Plan
- On the set-up for promoting Hyper-K- Physics of Hyper-K I (solar, supernova
neutrino bursts)- Physics of Hyper-K II (CPV, Proton
decay)- Discussions on Hyper-K Plan
- New proposals- Soliciting proposals from faculty
members- Proposals- Discussion procedures for new
proposals
3rd Meeting (December 13th, 2016)- Discussions on interim report- Hearings on new projects
4th Meeting (February 2nd, 2017)

•
•

•

•

•

•

2

•

•

Furthermore, while in the former design the detector
was instrumented with photodetectors of regular
photon-sensitivity at photocathode coverage of 20%,the new design installs photodetectors with doubled
photon-sensitivity and increases the photocathodecoverage to 40%. As a result, it is possible tosuppress the background of nucleon decay events byneutron tag (simultaneous measurement of 2.2 MeV
gamma rays associated with neutron capture byprotons), thus maintaining the experimental
sensitivity (the discovery capabilities). The Hyper-K
group is also considering the possibility of having thesecond detector in the future.

12th Meeting (October 16th, 2017)- Discussion on the draft final report based oncomments received from the faculty meeting(commenced on September 26).
October 26th, 2017
- The Institute for Cosmic Ray Research of the
University of Tokyo Future Plan Discussion
Committee concluded the final report

Hyper-Kamiokande Plan
Overview of Hyper-K plan and Updates from the
Future Planning Committee

The total construction cost for one tank is estimatedto be 67.5 billion yen, which consists of 16.3 billion
yen for excavation, 9.2 billion yen for the disposal of
the sludge and the wastewater treatment, 2.4 billion
yen for the geological surveys and the design work forconstruction, 12.5 billion yen for the liner and thestructure, 19.6 billion yen and 1.8 billion yen for the
optical sensors for the internal and the externaltanks respectively, 1.2 billion yen for the electronic
circuit, 3.3 billion yen for the pure water system, 100
million yen for the measurement computer, 100
million yen for the calibration equipment, and 100
million yen for the construction management. Out of
the total, 12.4 billion yen, which covers a half of theinner tank optical sensors, the whole outer tank
optical sensors, the electric circuit, the measurement
computer and the calibration equipment, is expected
to be contributed from the international
collaborators.

Hyper-Kisathird-generation
nucleondecay/neutrino experimental plan using a water
Cherenkov detector, following the previous
Kamiokande and Super-K projects. A wide range ofphysics is envisaged from this plan, including
measurements of charged parity (CP) asymmetry in
lepton sector by accelerator neutrinos, determinationof neutrino mass hierarchy in atmospheric neutrinos,
nucleon decay search, studies of solar neutrinos,
supernova burst neutrinos, and supernova relicneutrinos. A candidate site for the detector is
Tochibora mine (underground, 650m-deep, locatedabout 10 km away from Super-K) in Kamioka Town,
Hida City, Gifu Prefecture.
The detector design examined at the previous Future
Planning Discussion Committee comprised two setsof cylindrical tanks, 54-meters high and 250-meters
long with total mass of 1 million tons (of which theeffective volume is 560,000 tons, resulting in a 25fold increase from Super-K), 20% photo-sensitive
coverage with about 100,000 photosensors in total.
The detector size was then optimized, and the revised
design submitted to this committee consisted of acylindrical tank of 60 meters in height and 74 meters
in diameter (260,000 tons in total mass with aneffective volume of 190,000 tons,) with 40,000 ultrasensitive photosensors (twice the conventional
sensitivity) mounted in the inner tank.
The
production cost is scaled down in the new design byreducing the total mass. Although the effective
volume is smaller compared to the former design, thesensitivity in CP asymmetry measurements will not
be affected owing to the revised plan of the J-PARC
beam power upgrade from the previously expected
0.75 MW to 1.3 MW.

The international collaborative research group that
officially launched in January 2015 has now growninto an international joint experiment with about 300
members, representing 15 different countries. The
research organization is comprised of 10 working
groups operating under the project leader with eachoverseeing a specific experimental component, theInternational Steering Committee (ISC) that makes
decisions on the major policies of the project, and theInternational Board of Representative (IBR) thatconsists of the representatives from the member
countries.
The construction is expected to be completed in 2025
following 2 years for the access tunnel excavation, 2
years for the experimental cave excavation, 2 years
for the tank construction, and 1 year for the photosensor and the supporting structure installation, if
the construction is budgeted. Then, the detector isfilled with pure water and starts its operation from
2026.

3

astronomy can provide information that cannot be
obtained from the other observational methods.
The Committee highly commends Hyper-K, themultipurpose detector, as it is able to contribute to
the developments of diverse fields of science,
including the precision measurements of neutrino
oscillation parameters, the contribution to theneutrino astronomy, and the direct verification of theGrand Unified Theory. Hyper-K is an excellent
detector that not only provides a solution to theurgent issue of neutrino CP symmetry violation, butalso can serve as a long-term neutrino astronomy
observation site. The detector evaluated by thiscommittee is the 260 thousand-ton design that was
included in the SCJ Master Plan 2017.

Progress in the Research Environment
Until now, the following developments were made in
relation to the promotion of Hyper-K:(1) In the KEK's project implementation plan (KEKPIP), improving J-PARC performance for HyperK was recognized as the most crucial among theother four top-priority research plans that shouldbe implemented by new budgetary requests [2].
(2) Japan Association of High Energy Physicists(JAHEP) made a recommendation in its futureplans that the massive neutrino detector be madeone of the core large-scale future projects [3].
(3) Cosmic Ray Researchers Congress (CRC) placed
Hyper-K as one of the projects of utmost
importance [4]. Furthermore, the CRC steeringcommittee submitted a letter “A Request from
CRC for the Early Realization of HyperKamiokande Project” to the Chairperson ofParticle Physics and Nuclear Physics Subcommittee of the Science Council of Japan(March 31, 2016). The steering committee
recognized the project as a super-large scale
future plan of utmost importance in the field ofcosmic ray research since the completion of
KAGRA.
(4) Based on the discussions evolved in the researchcommunity, the Hyper-K project was submitted
to the Science Council of Japan (SCJ) with the
project title “Nucleon decay and neutrino
oscillation experiment using a large-scaleadvanced detector,” which was then selected in
the Master Plan 2017. The council also
acknowledged it as one of the key large-scale
research plans.
(5) A memorandum for cooperation was signed by
the host institutions (ICRR and KEK-IPNS). The
International Advisory Committee was set up,
and discussions by the committee members aremaking progress. The International Advisory
Committee concluded Hyper-K is highly
competitive internationally, and therefore must
be promoted going forward.

In recent years, it has been known that the neutrino
mixing angle θ13 has a sufficiently large value that it
may lead to the early discovery of the violation of theCP symmetry. Leptogenesis (lepton number
generation by the universe) by the seesaw
mechanism that incorporates right-handed neutrino
is a strong candidate for the origin of the baryon
number of the universe. Finding the presence of theCP violation in the lepton sector can mark an
important step towards the elucidation of
leptogenesis.
Hyper-K is at an advantage internationally in termsof becoming the first to discover the CP violation with
the neutrino oscillation, having the most reliable andthe matured technology. Hyper-K is capable ofsearching a wide range of the CP violation
parameters, and as the result of the T2K experimentsuggests, in cases where the CP violation parameteris at its maximum, its sensitivity exceeds 5 sigma. A
way to steadily reduce the systematic errors isoutlined—and it indicates a high degree ofachievability. Hyper-K's ability to determine themass hierarchy by using the high statisticalobservation of atmospheric neutrinos is alsocommendable.
Discovering nucleon decay is the direct means toverify the Grand Unified Theory (GUT). Hyper-K is
the only future plan that is capable of extending thesensitivity of e+π0 mode lifetime to 1035 years.
e+π0mode is caused by a gauge boson (X Boson) newly
introduced in GUT. Based on the measurements ofthree coupling constants using the Standard Model,
it is expected that supersymmetric Grand Unified
Theory is being realized at 1016GeV, that if the massof X Boson is in the vicinity of 1016GeV, there is ahigh probability Hyper-K is able to detect nucleon
decay. Also, νK+ mode that greatly restricts theCommittee Evaluation
The precision measurements of the neutrino
oscillation parameters and the search for the nucleon
decay lead to elucidation of physics beyond theStandard Model of particle physics, and these play a
crucial role in shedding light on the early stages of
the universe. In addition, through its unique means
to study the internal structure of stars, neutrino4has already started and it is essential Hyper-K is put
into motion as soon as possible.

model of supersymmetric GUT will be improved in
the new design, having 2 to 3 times the current
sensitivity. The Committee recognizes Hyper-K as
an appropriate project for the next-generation
nucleon decay search.

With regard to proton decay search, the sensitivity ofthe e+π0 mode is essentially determined by the size ofthe effective volume. With νK+mode, Hyper-K
demonstrates sensitivity close to that of DUNE’s. As
such, and also for the field of proton decay search, asteady progress can be anticipated by utilizing theexisting technology and enlarging the detectors.

The supernova explosion mechanism and the precise
measurements of solar neutrinos are important
subjects in neutrino astronomy. With Hyper-K, we
can expect about 100,000 neutrino induced events
observed from supernova explosions within ourgalaxy. These events allows us to study theneutralization burst in the initial stage of explosion
and the short scale time structure from the shock
induced instability during the explosion process andthe core rotations. It is anticipated that these new
information will be of help in understanding the
explosion mechanism. The Committee recognizes
Hyper-K as a vital project in promoting neutrino
astronomy.

Regarding neutrino astronomy/atmospheric neutrino
observation, there is the IceCube/DeepCore projectthat utilizes Antarctic ice sheets, and ANTARES
project that uses the water at the MediterraneanSea, where observations are being conducted forneutrinos over 20GeV. In addition, the PINGU
project is in the planning stage, to extend thesensitivity of IceCube to low energy, equal to 10GeV
and lower. Hyper-K is able to observe solar neutrinos
and supernova neutrinos, which cannot be detectedby these projects, and therefore these projects arecomplementary.

The Committee recognizes the significance inincreasing the observational accuracy of solar
neutrinos, especially in improving the measurement
precision for neutrino mass difference. Currently,
there is a discrepancy in 2 sigma level between Δm221
obtained from solar neutrino observations and theΔm221 obtained from KamLAND’s nuclear reactor
neutrino observations, and thus it is important todetermine whether this is due to the statistical orsystematic errors or due to the unknown newphysics. Although the estimation of the cosmic
background is in progress, more detailed study basedon various simulations is required. In addition, theinfluence of radon from bedrocks and the reduction ofradon due to water flow should also be considered.

As such, Hyper-K demonstrates uniqueness not seen
in its competitors, and due to the urgency in studying
the CP violation, which is now a global race, theCommittee concludes that an immediate start ishighly desired.
It is commendable that, compared to the previous
draft plans, cost-saving measures have been taken
without compromising the detector sensitivity to CP
violation and the nucleon decay search byreexamining the design. Moreover, the Committee
acknowledges that the cost uncertainties pointed out
at the previous Future Plan Discussion Committee
have been reduced. For example, the expenses for thesludge disposal and the wastewater treatment,
which were not estimated previously are now
included in the cost estimates. Also, for the cost ofcavity excavation, which accounts for a major portionof the expenses, a conservative estimate is madebased on the boring survey. Progress is also seen in
the specific designing of the tank structure (including
the surface treatment such as the waterproof sheets
and the polyethylene liner), and the designing andthe testing of the shockwave prevention cover. For
these reasons, it can be concluded that the details of
the plan have been improved and the technicalfeasibility, including expenses is confirmed.
After the last Future Plan Discussion Committee, theInternational Collaborative Research Group was
officially launched in January 2015, paving the way
to establish a collaborative system, and the

As a competing project, there is the LBNF/DUNE
project in the United States, measuring CP violationand searching for nucleon decay using a 40,000-ton
liquid argon TPC detector, whose construction isalready underway and aiming to commence
operation from 2026. This project has equivalentsensitivity to Hyper-K in measuring CP violation andit has become an international competition. Against
the liquid argon TPC detector, which requires
technological development, Hyper-K is in anadvantageous position in terms of feasibility because
similar type of detector, including Super-K, havebeen in realized with existing technology and has
already proved its capability. Hyper-K is able toaccumulate useful data from the onset of theresearch scheduled in 2026, and deemed likely to
discover CP violation ahead of DUNE. It should be
noted that the construction of LBNF/DUNE project5

sharing between ICRR and KEK. Based on the
points made above and its scientific significance, theCommittee recognizes Hyper-K as an appropriate
plan for ICRR’s next major project, and concludes
that its prompt realization is desirable.

Committee commends this development. Notably, it
is highly evaluated that an agreement on theconstruction and operational expense allocations is tobe signed by foreign delegates in March 2017. The
required next step is the further substantiation of theplan.
The Committee applauds that the signing of thememorandum for cooperation was completed by thehost institutions (ICRR and KEK-IPNS), and that
the International Advisory Committee wassubsequently established where discussions by themembers are already making progress. The divisionof responsibility is more clearly defined, where ICRR
is to focus on the construction and the operation ofHyper-K, and KEK to focus on the contraction andthe operation of the J-PARC beam and the neardetector, showing a significant improvement since
the last committee meeting. ICRR having the
responsibility over the construction and theoperation of Hyper-K sends a strong message that
this project is a future plan in the field of the cosmic
ray research. In fact, it is noteworthy that Hyper-K
not only acts as a far detector for long baseline
experiments, but also serves as a comprehensive
cosmic ray research facility.

The ALPACA Project
Project Outline
ALPACA Project is a collaborative experiment planby 14 Japanese and Bolivian research institutions,led by the Institute of Cosmic Ray Research (ICRR)
of the University of Tokyo and San Andres
University. The plan is composed of an air shower
array with an effective area of 83,000m2 and theworld’s biggest (5,400m2) muon detector array, set up
on a highland in Mt. Chacaltaya in Bolivia (at analtitude of 4,740m). By coupling the two detectors, itaims to make a wide field-of-view imaging in the
100TeV region （ 10TeV-1000TeV ） of ultra-highenergy gamma rays, employing the world’s highest
sensitivity. The two major research objectives for theexperiment are as follows:(1) To become the world’s pioneer in probing theacceleration point sources of cosmic rays in the over
100TeV region by detecting the point sources of ultrahigh energy gamma rays in the southern sky.
(2)To aim for discovering Galactic PeVatrons byinvestigating the extended region around theGalactic Center and the diffuse gamma-ray emission
in the 100TeV region from Fermi bubbles’ diffuse
gamma-ray sources.

However, it is estimated that about 20 personnel,
twice the number of staff at Super-K, are required forthe internal system of ICRR, indicating that
additional six personnel are needed. In cases wheresecuring full-time researchers is deemed to bedifficult, the Committee recommends to hire project
researchers in order to propel the project, and at thesame time to take measures to develop more young
researchers. It is commendable that there was a
proposal for establishing an organizational structurethat accommodates a large-scale budget. Further
improvement of the organization is required,however. This includes the setting up of a promotion
team lead by the project managers who have
responsibility over the implementation of theconstruction. The Committee urges that ICRR and
the international collaboration team cooperate todevelop a management system appropriate for theconstruction and the execution. The Committee
views that it is also important to seek advices, as
deemed necessary, from a third-party committee ofexperts who have experience in executing large-scale
projects.

Furthermore, this experiment will cover various
other research themes such as the elucidation of thechemical composition of the primary cosmic-ray
nuclei in the cosmic ray “knee” region, precise
measurements of the cosmic ray anisotropy above theTeV energy region that is still unobserved in thesouthern hemisphere, and the elucidation of thegeneral structure of the solar terrestrial magnetic
fields by precise measurements of the Sun’s shadow
against the cosmic ray flux.
The ALPACA experiment consists of an air shower
array with 401 1m2 scintillation detectors deployed
at a 15m-apart grid covering 83,000m2 , and of 96
sets of 56m2 underground water Cherenkov muon
detectors. Regarding the performance of thisdetector, it is estimated that the primary gamma-ray
modal energy detected is 5TeV, the angular

The Committee recognizes significant improvementsin the budget estimation accuracy that was an issue
identified at the last meeting, in the international
collaboration formation, and in defining the role-6Tibet ASγ and BASJE members are participating inthis project, so Tibet ASγ’s technology and thealready-establishedinternational
cooperation
system with BASJE can be leveraged. Considering
the results from the Tibet ASγ experiment in thenorthern hemisphere,10 to 20 high-energy gammaray point sources are expected to be discovered with
the ALPACA experiment. Furthermore, as ALPACA
has the angular resolution of 0.2°, which is excellent
for an air shower array, a detailed analysis of diffuse
gamma-ray sources is possible. The Committee
considers that with this experiment it may bepossible to also search for electrons and gamma rays
that originated from dark matter in the southernhemisphere.

resolution is 0.2° at 100TeV, and the energy
resolution is 25% at 100TeV.
The key to this experiment lies in the successful
selection of gamma rays by the number of muons in
air shower. With the planned detector, by the
optimized selection process using muon numbers
within 100m of the core, it is estimated that, for the
primary energy at 100TeV, the removal rate ofprimary cosmic-ray nuclei to be 99.9%, while having
the gamma-ray survival rate at 90%. For that reason,
it is estimated that the world’s highest sensitivity
can be achieved by this experiment, going beyond the
CTA experiment, for gamma-ray sources over 40TeV.
Moreover, for more diffused sources, the sensitivity
is expected to surpass that of the CTA experimenteven at lower energy levels, for example, over 30 TeV
for sources extending over 0.1°and over 10 TeV forsources extending over 1.5°.

A reasonable cost estimate is made from years ofexperienceandprototypeproductionthat
implementation of the plan seems feasible with theacquisition of the large-scale science grants.

The necessary cost for the construction is estimatedto be approximately 500 million yen, and hence alarge-scale Kakenhi (Grants-in-Aid for Scientific
Research) is planned to be utilized. After the
preparation, the construction is expected to becompleted in four years, and the stationary
continuous observation to start in the fifth year andto continue for the next 10 years.

The South-HAWC plan that aims to carry out a widefield-view continuous survey of gamma rays in theenergy region from 100GeV to 100TeV and ALPACA
experiment overlap and compete in the 10TeV region,
so it is desired to implement the ALPACA
experiment in advance of the South-HAWC plan.
Regarding the HAWC experiment and the LHAASO
experiment in the north, ALPACA is complementary
in its field of observation where observation is madein the south. CTA, an experiment that has a highangular resolution and limited observation time, and
ALPACA that employs wide field-of-view and longterm continuous observation, are mutuallycomplementary. ALPACA is to widely survey thesouthern hemisphere, HAWC/LHAASO to widely
survey in the north, and CTA to survey specific areas
in detail, which subsequently creates a synergistic
effect and utilizes CTA’s observation time more
efficiently. For this reason, ALPACA shouldcommence its survey in the southern hemisphere intime for the launch of CTA observation in 2021,
emphasizing the point that the speedy realization of
the plan is essential.

The advantage of its implementation in Bolivia isthat an easily accessible flat land can be secured, atan altitude of over 4000m where air shower before its
attenuation is detectable, and it is in the southernhemisphere where the Galactic Center, the mostpromising candidate of the galactic cosmic-ray
source, can be observed. It is also because the country
shares strong rapport with—and a history of—cosmic
ray observation research with Japan since the 1960s,
and that adequate infrastructures (such as
electricity, water supply, roads etc.) are available.
Committee Evaluation
In the southern hemisphere, since there are anumber of interesting celestial objects—from the
Galactic Center and Fermi bubble to supernova
remnants and pulsar wind nebulae—that thediscovery of Galactic PeVatrons can be anticipated.
As such, the scientific significance of this plan thataims to survey gamma rays in the 100TeV region isvery high. The plan demonstrates high feasibility inboth technology and budget, and hence theCommittee evaluates it as a research that should bepromoted.

Tibet ASγ experiment has completed theconstruction of the muon detector, and has entered astationary observation mode that it can be said thatthe time is ripe to embark on the ultra-high energy
gamma-ray detector in the southern hemisphere.
However, there is a concern that human resources
may be insufficient to execute two experiments
simultaneously. The Committee asks to consider
personnel allocation for the steady execution of the
project.

7

to elucidate the origin of the highest energy cosmic
rays based on the results expected from the TAx4
experiment and the increased data with 16-foldincrease from the TAx4, which is to be obtained fromthe 10-year observation.

Future Plans of Observations of Ultra-High
Energy Cosmic Rays on the groundAs for other future plans concerning the observationof the highest energy cosmic rays, there is the Auger
Prime experiment, which is an upgrade from theAuger experiment, as well as the JEM-EUSO
experiment, which is a plan to detect cosmic-ray
shower from the satellite orbit. Both have theirstructures for collaboration in place. The joint
discussion with the Auger experiment group is at thestage where a WG has been set up. Regarding the
JEM-EUSO experiment, it will be succeeded by the
Russia-led experiment K-EUSO Project, but thedetection aperture remains twice the size of TAx4.
TA group will continue to maintain the cooperative
framework with other projects, such as TA-EUSO.

Project Outline
TA2 project is an air shower detection plan that has
detection area 60 times larger than the current TA
experiment, having 10,000 ground detectors in anarea of 40,000km2. About 1000 highest energy cosmic
rays above 57EeV will be observed annually. The
field-of-view is divided into 60 regions, and energy
spectrum is measured in each region. The aim is to
examine with high statistical precision the source ofanisotropy of the highest energy cosmic rays, which
is becoming apparent by the TA experiment. For the
ground detector, the scintillator is mainly used, butwater tanks and lead-plate-type detectors are also
being considered as the particle discrimination
capability is required. It is also under consideration
to carry out chemical composition measurement bydeploying the atmospheric fluorescencetelescope,
which has been drastically reduced in cost. Total
construction cost is estimated to be in the scale of 10
billion yen. It is planned to be realized through a big
experimental project under the international
collaboration led by researchers from TA and Auger
projects. It is estimated that Japan will bear
approximately 20% of the total construction cost. The
construction is planned to start in 2025, aiming for
full-scale operation from 2028.

Committee Evaluation
Observation of the high-energy cosmic rays is an
important research subject with respect to theelucidation of cosmic ray acceleration mechanism
over 1020 eV and its celestial accelerator, and the
propagation mechanism. In the last decade,understanding of the highest-energy cosmic rays has
been dramatically advanced by the progress made by
the TA and Auger projects. In particular, thediscovery of HotSpot by the TA experiment in thenorthern hemisphere is commendable as it gives animportant clue to the origin of high-energy cosmic
rays. In addition, the discrepancies between the TAexperiment and the Auger experiment in theirobservation
results—suchaschemicalcompositions—are
being
reconciled
throughcollaborative efforts by the two groups. Attempts toadvance the understanding of chemical composition,such as the Auger Prime experiment, are making
headway. The Committee commends the results fromthe current TA experiment and its development into
TAx4
regardinghighest-energy
cosmic-ray
observation. At the same time, it also acknowledges
that the TA2 plan that can further advance theseaccomplishments is now being studied. The strategyof the current TA2 plan is mainly to increase the areaof ground detectors, and after statisticallyestablishing the HotSpot in the northern hemisphereby the TAx4 experiment, the main focus is on
accomplishing even greater statistical accuracy todetermine the disparities in the energy spectrum
according to the arrival directions. The optimizationof these TA2 experiments strongly depends on the
results of ongoing TAx4 experiments and Auger

In proposing the TA2 experiment, regarding the
progress of the highest energy cosmic ray researchusing air shower observation, expected results andthe timeline of the currently under construction
TAx4 experiment as well as the TALE SD experimentwere explained—based on the results from thepresently ongoing TA experiment. A holistic roadmapof the highest energy cosmic-ray observations waspresented. According to the roadmap, the TAx4
experiment that has the detection surface area fourtimes larger than the current TA experiment, is to becontinued with some other funds after its current
budget ends in 2020. Within 10 years of observation,
it is aimed to obtain six times more data than fromthe current TA experiment, to affirm the arrival
direction of the highest energy cosmic rays claimedby the TA experiment with the statisticalsignificance of 3.4σ (HotSpot) at the 5σ confidence, as
well as to probe Hotspot structure and search forother Hotspots. The proposed TA2 experiment aims

8

Prime experiments. With other experiments such as
GZK neutrino search by the IceCube experiment, theintegrity as a multi-messenger observation isquestioned such that further evaluation thatincorporates these results is necessary.

The Future Plan Discussion Committee of 2012-2013
reported that it could not endorse executing the NTA
plan in such a way that the ICRR is positioned as themain body. Taking into consideration the change inthe environment since high-energy neutrinos derived
from astronomical sources were detected by IceCube,
the discussion at this committee was focused onwhether there has been a new development thatwould greatly alter the previous committee’s
conclusion.

On the other hand, international cooperation isindispensable to advance a large-scale future plan of
this level. The Committee commends that acooperative framework is being established between
the TA and Auger experiments. The Committee
recommends continuing further studies towardrealization of the high statistical accuracy of highestenergy cosmic-ray observations in future groundbased experiments. At the same time, the Committee
also recommends to take specific measures toadvance building the structures needed for this large
experimental
project
underinternational
collaboration.

The detection technology for implementing NTA canbe described as ambitious, based on technologicaldevelopments aimed at Ashra-1. There is also
progress seen in the forming of the research
structure, as a Letter of Intent was issued in 2014
and a subcommittee which promotes international
collaborative experiments has been set up.
However, the development of elemental technology
has resulted in considerably longer time than theoriginal estimation. There is insufficient material
that points to significant progress since the last
Future Plan Discussion Committee, in developing
the detection technology that can provide basis forrealizing the NTA plan. In addition, there are no
feasibility studies of the Earth-skimming neutrino
observation by means of atmospheric fluorescencethat have been published.

Composite Imaging Observation of Cosmic
Elementary Particles
Project Outline
Neutrino Telescope Array (NTA) plan is to install awide-field observation station equipped with an
optical imaging device at four locations in themountains of Mauna Loa, Hawaii Island at analtitude between 3000 and 3500m. The plan is tocarry out observations on the Earth-skimming highenergy tau neutrinos, large zenith-angle high-energy
electron neutrinos/tau neutrinos, and high-energy
cosmic rays/gamma rays, by utilizing Cherenkov
light and atmospheric fluorescence, as aninternational,
collaborative
experimentwithresearchers from the United States and Taiwan. The
optical imaging device utilizes the technology
developed by Ashra-1, including a condenser, animage intensifier, a trigger sensor circuit, and a
CMOS sensor circuit, to achieve high sensitivity,
high resolution of 0.1 degrees or less, and a wide
field-of-view. If the neutrino observation in the PeV
region can be realized, data complementary to otherexperiments such as the IceCube experiment, can beobtained. It is possible to observe gamma rays inwide field-of-view up to the PeV region. Within fiscal
2017, the Technical Design Report is to be published,
and if the plan goes smoothly, the operation is tocommence from fiscal 2022.

The Future Plan Discussion Committee 2012-2013
reported that “The scale of the joint researchorganization, especially the manpower within the
ICRR, is insufficient to fulfil the proposed plan, andtherefore the Committee concludes that it cannot
endorse implementing the NTA plan in such a waythat the institute acts as the main body of the
program.” The Committee made the discussion on
the promotion organization for the plan again,
however, the Committee concludes that a project
team required to lead the experiments has not been
formed domestically yet and the promotion
organization remains inadequate.
Based on the above, the Committee has not been able
to acknowledge any significant progress being madethat would overturn the last Committee’s decision.
Therefore, the Committee concludes that it cannot
endorse the NTA plan as the ICRR’s future plan.

A Proposal for the New Development of
Direct Dark Matter Search Experiment with
Liquid Xenon

Committee Evaluation9

It is assumed that target size will become the decisive
factor in the future, creating a situation where alarge-scale detector, something that can only be
produced singularly in the world, becomes essential,
which then leads to the formation of an international
collaborative experiments that will have to integrate
such initiatives as XMASS. In view of this situation,
there was a proposal from the XMASS experimentgroup to shift the policy from the current one thataims at realizing XMASS-1.5 to forming a researchgroup that participates in overseas G2 experiments
and promotes research and development for the
future G3 experiment. There are two G2
experiments, LZ and XENONnT, but the researchgroup is already having discussions based on thepremise to join XENONnT. Both projects plan to
conduct searches on the scattering cross section of 1048cm2, and their targets are similar. However,XENON1T, which forms the basis for XENONnT,
has already yielded superior results compared to
LUX, and is one step ahead.

Project Outline
There was a new proposal from the current research
representative on the proceedings with the XMASS
going forward. The Committee conducted a hearing
twice to the representative, and based on discussions
among the Committee members, the Committee
understood the present situation and the future
research plan as below.
The XMASS experiment is to search for WIMP, apromising candidate for dark matter, with theworld’s highest sensitivity and thereby aims for its
discovery. It also endeavors to conduct low-energy
solar neutrino observation and discover neutrinolessdouble beta decay. As a pioneer of single-phase
experiments using liquid xenon, the team haspropelled the construction and experiment of
XMASS-I, yielding valuable results with numerous
technological developments and new insights into
physics. At the same time, further improvement insensitivity was required for the WIMP search. Basedon this, XMASS-1.5 (6 tons of xenon liquid to be used)that has about 10-47 cm2 in sensitivity to the nucleonWIMP scattering cross-section was proposed as anext generation detector. However, the constructionof XMASS-1.5 has not been realized, and in adifferent direction, projects that are called the second
generation (G2), such as the XENONnT experimentand the LZ experiment（10 tons of liquid xenon is to
be used each by the 2-phase detector, with searchsensitivity of approximately 1048cm2, can improve
the sensitivity even further—by one order ofmagnitude. These experiments are planned to belaunched in 2019 and 2020, and will allow searches
in a wide parameter area of the supersymmetricmodel. Moreover, another future experiment, called
the third generation (G3), is in the planning. In thistype of detector, the ultimate sensitivity, which is
restricted by the background of solar andatmospheric neutrinos, can be achieved. Here, highparticle discrimination capability is a requirement.
While the single-phase XMASS experiment has been
dependent on the advantage of easy size
enlargement, its particle discrimination capability is
too poor to achieve the desired sensitivity. As the
problem of enlargement has become less of an issue
for the two-phase type—which has higher particle
discrimination capabilities—it has become clearer
the superiority of the two-phase type, indicating thatthe time has come to reevaluate the directionality of
the next generation experiment of XMASS.

Furthermore, since XENONnT has not finalized thedetector design yet, it is possible for Japan to makegreat contributions with resources such as
knowledge and experience gained from the XMASS
experiment. Specific contributions that can be
expected include (1) research and development ofliquid xenon purification technology, (2) low
background technology, (3) personnel who will stayat the site and conduct research (4), utilization ofJapan’s photomultiplier, (5) lending of up to 1.9 tonsof xenon gas, (6) adopt XMASS-I at Kamioka’s
underground laboratory C as a test bench, andleverage the leading edge development and researchof dark matter experiments, as well as to form a basethat can support these operations.
Committee Evaluation
It has become a common understanding among theCommittee members that even with the XMASS-1.5
being realized as an extension of the current singlephase, it remains impossible to keep up withresearch carried out by other countries. It isappropriate that the research group has proposed tochange the research plan. Also, taking part in theongoing G2 experiment overseas, particularly in the
XENONnT project, is a promising plan. The
Committee recommends the research group to
promptly investigate the possibilities in thisdirection and work towards its materialization, andto continue the efforts to gain understanding fromthe researcher community.

10

its experiment overseas gains understanding, it isessential to take into account the research
representative’s intentions, especially regarding theresources purchased with external funding.

In promoting this plan, it is desirable to thoroughly
examine the following points.
1. Scientific Rationality
The outline of the proposal consists of: (1) terminate
the current XMASS experiment in two years time,
and withdraw its next phase, XMASS1.5; (2)participate in overseas G2 experiments. It is anappropriate decision to terminate the current
XMASS experiment and withdraw XMASS1.5,
considering the competition with the ongoing
researches overseas. The Committee recognizes theplan to join overseas G2 experiment to be a promising
policy that can initiate cutting edge physics. As G3 isin the planning following G2, it should be noted thatconsideration must be taken about the transferringof experience gained from G2 and the activities in
Kamioka to G3.

4. As a joint research in Kamioka
The XMASS experiment has been positioned as acollaborative research promoted by Laboratory C of
Kamioka Observatory. In the proposal, Laboratory C
is used as a place for research and development for
G3, however, even after joining the international G2
experiment, it is more preferable to provide thefacility as a place for substantial research to advancetheir shared part of the collaborative experiment.
Committee requests the group to continue having
dialogue within the community while addressing the
aforementioned issues, and to finalize a plan which
can lead to the G3 experiment in the future.

2. Development towards international stage
The purpose of XMASS experiment is the search ofdark matter, so it appears only natural that a larger
scale experiment at an international level becomes
more favorable in terms of scientific objectives. By
joining the international effort with the resource forthe current XMASS experiment, it is expected thatthe team will be able to make a great contribution to
the G2 experiment overseas. At the same time,
XMASS experiment is set up as research focused on
the low background technology at Kamioka
Observatory—with the research and personnel costs
funded by the ICRR—that this could possibly
restrain some research activities overseas. For
example, the researchers for XMASS were hired on
the premise that they work in Kamioka, and this
could lead to problems as to the researchers’ place of
employment. It is desirable to compose a researchgroup capable of promoting the research while
maintaining consistency with the project at Kamioka
Observatory, once the participation requirements for
the G2 experiment are clarified.

References[1] Institute for Cosmic Ray Research, The
University of Tokyo Future Plan Discussion
Committee Report (September 26, 2013)
http://www.icrr.utokyo.ac.jp/report/futureplan/futureplan2013.pdf[2] KEK Project Implementation Plan<s0/>(KEKPIP) (June 30, 2016)
https://www.kek.jp/ja/About/OrganizationOverview/
Assessment/Roadmap/KEK-PIP.pdf[3]<s0/>High Energy Physics Future Plan Discussion
Subcommittee <s1/>(February, 11, 2012)
http://www.jahep.org/office/doc/201202_hecsubc_tou
shin.pdf[4] CRC Future Plan Discussion Subcommittee
Annual Report 2013-2014
http://www.icrr.utokyo.ac.jp/CRC/townmeeting/doc/CRC_FPSC_report_2013-14-v3.pdf

3. Ownership of the resources
The resources of the XMASS project includes not only
those purchased with the ICRR’s budget but alsowith external funding such as the Kakenhi scientific
grants. Even if the research group’s plan to expand

11