INSTITUTEFORCOSMIC RAY RESEARCH
THE UNIVERSITY OF TOKYOANNUAL REPORT(APRIL 2018 – MARCH 2019)

Editorial Board
SAKO, Takashi
OBAYASHI, Yoshihisa
TAKENAGA, Yumiko
NAKAMURA, Makio
ITO, Yorikoc!Institutefor Cosmic Ray Research, The University of Tokyo
5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8582, Japan
Telephone: (81) 4-7136-3102
Facsimile: (81) 4-7136-3115
WWW URL: http://www.icrr.u-tokyo.ac.jp/

TABLE OF CONTENTS
PrefaceResearch Divisions.........................................................................................

1

Neutrino and Astroparticle Division2

High Energy Cosmic Ray Division32

Astrophysics and Gravity Division59

Observatories and a Research Center.........................................................................

75

Norikura Observatory

76

Akeno Observatory

84

Kamioka Observatory

87

Kagra Observatory

88

Research Center for Cosmic Neutrinos

89

Appendix A.

ICRR Workshops and Ceremonies.............................................................

91

Appendix B.

ICRR Seminars..............................................................................

94

Appendix C.

List of Publications..........................................................................

94(a) Papers Published in Journals(b) Conference Papers(c) ICRR Reports
Appendix D.

Doctoral Theses............................................................................

102

Appendix E.

Public Relations............................................................................

102(a) ICRR News(b) Public Lectures(c) Visitors
Appendix F. Inter-University Researches..................................................................

108

Appendix G.

.................................................................

112

List of Committee Members(a) Board of Councillors(b) Advisory Committee(c) Inter-University Research Advisory Committee

Appendix H.

List of Personnel...........................................................................

113

PREFACE
This report summarizes the scientific activities of the Institute for Cosmic Ray Research (ICRR) of The University of Tokyo in the Japanese FY 2018.
ICRR is an inter-university research institute for studies of cosmic rays. The headquarters of ICRR is locatedin Kashiwa, Chiba prefecture, Japan. In order to promote various cosmic-ray-related studies efficiently, ICRR has
three research divisions; Neutrino and Astroparticle division, High Energy Cosmic Ray division, and Astrophysics
and Gravity division. ICRR have four observatories in Japan; Kamioka and KAGRA Observatories (Kamioka
underground, Gifu prefecture), Norikura Observatory (2770 meters above sea level, Mt. Norikura, Gifu prefecture),
and Akeno Observatory (Yamanashi prefecture), together with 1 research center; Research Center for Cosmic
Neutrinos (Kashiwa, Chiba prefecture). In addition, there are two major experimental facilities outside of Japan.
They are located in Utah in USA, and Yangbajing in Tibet, China. The La Palma island in Spain is also recognized
as an important site for the ICRR activities.
Many researchers from various Japanese institutions as well as those from overseas are involved in the research
programs of ICRR. It should be noted that most of the scientific outputs from this institute are the results of thecollaborative efforts by many researchers from various institutions. In order to produce outstanding results, it isvery important to carry out an experiment by an international collaboration composed of top-level researchers allover the world. Hence, all of the experimental collaborations that ICRR is involved are international ones. With
these activities, ICRR has been selected as one of the international inter-university research institute by Ministryof Education, Culture, Sports, Science and Technology -Japan (MEXT) in November 2018.
Many exciting scientific activities of ICRR are described in this report. For example, the Super-Kamiokande
and T2K (which uses Super-Kamiokande as the far detector) experiments have been continuously producing impressive results on neutrino oscillation physics. The discovery of neutrino oscillations by the Super-Kamiokande
experiment was recognized by the 2015 Nobel Prize in Physics.
Some of the other recent highlights are mentioned here: the KAGRA gravitational wave project is going tobegin the observation run in early 2020. TA (Telescope Array) has been producing impressive data on highest
energy cosmic rays, including the indication that these cosmic rays arrive from a particular direction of the sky.
ICRR is playing a major role in CTA, which is a global TeV gamma ray astronomy project. ICRR is contributing
substantially to the construction of Large-Sized Telescopes (LST) of the CTA-North observatory. The constructionof the first LST has been completed in October 2018.
As an inter-university research institute, ICRR is reviewed by the top-level researchers in the field. In particular,
future projects of ICRR are evaluated by a committee composed of top-level researchers from various nearby fields.
The ICRR Future Project Evaluation Committee released its report in October 2017. Several projects have been
recommended highly. Among them, Hyper-Kamiokande, which is the next generation neutrino detector, has been
recommended as the top priority ICRR future project. ICRR is working hard toward the realization of HyperKamiokande.
We hope that this report is useful for the understanding of the current research activities of ICRR. Finally, we
appreciate the strong support of our colleagues in this research field, The University of Tokyo and MEXT. They
are indispensable for the continuing, and exciting scientific outcome of ICRR.

Takaaki Kajita,
Director,
Institute for Cosmic Ray Research,
The University of Tokyo

The ICRR building at Kashiwa,
Chiba, Japan.

The Super-Kamiokande detector(the photo was taken during refurbishment work in 2018).

The XMASS detector (the photo
was taken during the refurbishment
work in 2013).

The first Large Size Telescopeof CTA installed at Observatory
Roque de los Muchachos inaugurated on October 10 2018.

Tibet-III air shower array (37000
m2 ) at Yangbajing, Tibet (4300 m
in altitude).

Air fluorescence telescopes (left)
and a scintillator surface detector(right) of the Telescope Array experiment in Utah, USA to explore
the origin of extremely high energy
cosmic rays.

KAGRA s (left) 3km-long arm tunnel and (right) a sapphire mirror installed in a cryogenic suspension
system.

A public lecture co-organized with
the Institute for the Physics andMathematics of the Universe (Kavli
IPMU).

Organization

Number of Staff Members (As of May 1, 2018)

Neutrino and Astroparticle Div.
High Energy Cosmic Ray Div.
Astrophysics and Gravity Div.
Administration
Total

Scientific
Staff

Technical
Staff

Research
Fellows

26
17
16
0
59

5
13
7
0
25

3
1
6
0
10

Administrators
andSecretaries
17
2
11
16
46

Total
51
33
40
16
140

FY 2012-2018 Budget
Personnel
expenses
Nonpersonnel
expenses
Total

2012

2013

2014

2015

2016

2017

2018

658 000

687 000

706 000

684 000

683 000

779 000

823 000

1 172 000

1 095 000

1 282 000

1 595 000

1 288 000

1 514 000

1 729 000

1 830 000

1 782 000

1 988 000

2 279 000

1 971 000

2 293 000

2 552 000

(in 1 000 yen)

1

RESEARCH DIVISIONS
Neutrino and Astroparticle Division
Overview
Super-Kamiokande
T2K Experiment
XMASS ExperimentHyper-KamiokandeHigh Energy Cosmic Ray Division
Overview
Cherenkov Cosmic Gamma-Ray Group
TA: Telescope Array Experiment
Tibet ASγ Project
ALPACA ProjectHigh Energy Astrophysics Group
Other Activities

Astrophysics and Gravity Division
Overview
Gravitational Wave Group
KAGRA Project

Observational Cosmology Group
Theory Group
Particle Phenomenology
Particle Cosmology

2

NEUTRINO AND ASTROPARTICLE DIVISION
Overview
This division aims to study particle physics with prime interests in physics of neutrinos and proton decay, and astroparticle physics with the use of underground experimental facilities.
The Super-Kamiokande (SK) detector is a 50 kton water
Cherenkov detector using 11,129 50 cm-diameter photomultipliers (PMTs) for its inner detector and 1,885 20 cm-diameter
PMTs for its outer detector. The data taking of SK started
in April 1996 and 20th anniversary was celebrated in 2016.
The neutrino oscillations in atmospheric neutrinos were discovered in 1998 and thereby it was demonstrated that neutrinos have a finite mass. In 2001, the accurate measurements of
the 8 B solar neutrino flux by SK and SNO discovered that neutrino oscillations are the solution of the solar neutrino problem
beyond doubt. These findings became the research achievement for which the Nobel Prize in Physics was awarded in2015. After the epoch-making discoveries, precise measurements of atmospheric neutrinos and solar neutrinos have been
performed and they unraveled various phenomena of neutrino
oscillations. The evidence of tau neutrino appearance in atmospheric neutrinos was confirmed in 2013 and atmospheric
neutrino anomaly has been finally concluded. The indicationof day-night asymmetry of the solar neutrino flux, which is
expected from the matter effect of neutrino oscillations, wasreported in 2014. At present, the most interesting subjects
in those observations are the determination of neutrino mass
hierarchy using atmospheric neutrinos and the consistency
check of solar(νe ) and reactor(ν̄e ) oscillations.
A high intensity neutrino beam experiment using the JPARC accelerator (T2K) was started in 2009. The T2K experiment uses the SK detector as the far detector. Search for
leptonic CP violation and the high precision measurement ofoscillation parameters are main physics subjects in T2K. An
indication of electron neutrino appearance was found in June
2011, and later the electron appearance has been establishedwith greatly improved significance. Since 2014, anti-neutrino
beam data also have been taken in order to search for CP violation.
The search for nucleon decay is another important subject
at SK because it gives a direct evidence for the Grand Unified Theories (GUTs). SK gives the current best limit whichstrongly constrains various GUT models.
If a supernova happens in our galaxy, thousands of neutrino interactions are expected at SK and they will reveal detailed mechanism of the supernova explosion. SK is the only
detector in the world which can identify the direction of thesupernova neutrinos. So, SK has been operated almost all thetime with small dead time and if a supernova is observed at
SK, we will send burst information to astronomers as soon
as it is detected. In addition, SK aims to observe supernova
relic neutrinos, which is an accumulated supernova burst neutrinos from the beginning of the universe. For this purpose, itis planned to add 0.1% of gadolinium into the Super-K tank(called SK-Gd project) in order to tag neutrons for ν̄e detection. A feasibility study for the SK-Gd project is being performed using a 200 ton tank which mimics the Super-K detector. Refurbishment of the Super-K tank and upgrade of thewater circulation system had been conducted in 2018 to become ready for Gd doping.
Another activity of the Neutrino and Astroparticle division is a multi-purpose experiment using liquid xenon aiming
at the detection of cold dark matter, neutrino absolute massusing neutrinoless double beta decay, and low energy solar
neutrinos. A 800 kg liquid xenon detector was constructed in
an experimental hall near the SK site. Data taking continued for more than five years and finished in February 2019.
Searches for dark matter interactions and rare phenomena inliquid xenon are being conducted.
The Hyper-Kamiokande (Hyper-K or HK) experiment is
proposed as a joint project of the university of Tokyo and
KEK by combining a next generation underground water
Cherenkov detector and upgraded J-PARC neutrino beam.
The Hyper-K detector is an order of magnitude larger in detector fiducial mass than Super-K and has discovery potentialof leptonic CP violation and proton decays. The detector construction has been endorsed by the ICRR future project committee’s report in March 2017. Hyper-K has been selected as
one of the seven large scientific projects in the Roadmap of
the Japanese Ministry of Education, Culture, Sports, Scienceand Technology (MEXT) in 2017. We are eagerly waiting forthe approval by the Japanese government.

SUPER-KAMIOKANDE[Spokesperson : Masayuki Nakahata(Kamioka Observatory, ICRR, The University of Tokyo)]
Search for nucleon decay
Proton decays and bound neutron decays (nucleon decays
in general) is the most dramatic prediction of Grand Unified
Theories (GUTs) in which three fundamental forces of elementary particles are unified into a single force. Super-Kam
iokande (SK) is the world’s largest detector to search for nucleon decays. Various nucleon decay modes have been looked
for, but we have found no significant signal excess so far.
A proton decay into one charged lepton and one neutralpion (p → e+ π 0 , p → µ + π 0 ) is one of the popular decay
modes which most of GUT models predict and have relatively
large detection efficiency. On the other hand, a proton decay
into K + and ν is suggested by GUTs collaborated with Supersymmetry theories (SUSY-GUTs). These bench mark decay

3modes have been analyzed with 365 kton·year exposure. SK
data was consistent with background estimation and obtained
lower limit of proton lifetime were 2.0, 1.2, and 0.8×1034
years for p → e+ π 0 , p → µ + π 0 , and p → K + ν , respectively.
These analysis has been done with 22.5 kton fiducial volume
which is defined by 2 m inward from the inner tank wall. If we
can expand the fiducial volume to 1 m from the inner wall, forexample, we can gain 20% more data than the current analysis. To expand the fiducial volume, we need to understandbackground contamination from the wall and reconstruction
performance close to the wall. The new attempt to expand
fiducial volume has just started and it is important for the otherneutrino oscillation analysis.
It was pointed out that observation of a proton decay into three charged leptons (e or µ ) would provide energy scale of order 100 TeV and explanation
to recent b → sµ µ anomalies as the relevant effective
operators arise at dimension 9 or 10 [1].
All possible decay modes with combination of e and µ , p→e+ e+ e− , e+ e+ µ − , e+ e− µ + , e+ µ + µ − , e− µ + µ + , µ + µ + µ −are started to investigate at the first time in SK. Typical
proton lifetime for these modes is predicted as order of 1033
years which could be accessible with exposure of SK. Even
though there are no significant signal observed for thesemodes, proton lifetime limits would be largely improved
because the current world records for these modes were set
by IMB about 20 years ago [2]. All final particles above
Chrenkov threshold for these decay modes are detectable in
SK and higher selection efficiencies are expected. There areno pions in the final particles and these decay modes are freefrom π interactions in nucleus (π -FSI) which often provides
the largest uncertainty in signal selection of nucleon decay
searches.
To select signal, following cuts are required: (1) fully contained and event vertex locates more than 2 m apart from theinner tank, (2) three rings are reconstructed, (3) proper number of e-like and µ -like rings (for example, three e-like rings
are required for p→ e+ e+ e− ), (4) require the same number ofMichel electron as number of µ in particles from proton decay, but accept two Michel electron case for p→ µ + µ + µ − ,(5) no tagged neutron only for SK-IV, (6) reconstructed invariant mass should be in between 800 and 1050 MeV/c2 andtotal momentum should be less than 250 MeV/c. Figure 1
shows number of ring distributions for each mode. Fraction of
three ring events decreases if number of µ in final particles increases because there are some amount of µ below Cherenkov
threshold. Figure 2 shows number of Michel electron distributions for p→ e+ µ + µ − , e− µ + µ + . Less fraction of two Michelelectrons is observed in p→ e+ µ + µ − than p→ e µ + µ + because Michel electron can not be emitted in case of µ − capture in water.
Table 1 summarizes selection efficiencies and backgroundestimations only for SK-IV period. Signal region is divided
into lower (Ptot < 100 MeV /c) and higher (100 ≤ Ptot <250 MeV /c) total momentum region as same as p→ e+ π 0analysis [3]. Total efficiency for p→ e+ e+ e− exceeds 60%
and expected background is well suppressed and there areno background events remained in lower momentum region.
Number of expected background for p → µ + e+ e− and p →0.9Area normalized

82%

0.8

69%

0.7

Background0.6

p → e+ e+ e-

53%

p → μ+ e+ e-

0.5

p → e+ μ+ μ-

37%

0.4

p → μ+ μ+ μ-

0.3
0.2

8%

0.1
0
0

1

2

3

4

5

6
7
8
Number of Ring

Number of ring distribution after fiducial volume
cut. Red green, blue, cyan corresponds to signal MCof p→ e+ e+ e− , µ + e+ e− , e+ µ + µ − , µ + µ + µ − , respectively.
Black histogram shows atmospheric neutrino background.
Only three ring events are selected.

Fig. 1.

SK4

1

3 rings
2 μ-like & 1 e-like
Area normalized

0.8

0.6

p→e+μ+μp→e-μ+μ+

0.4

0.2

0
0

0.5

1

1.5

2

2.5

3
3.5
4
4.5
5
Number of decay electrons

Number of Michel electron distribution for signal
MC. Black corresponds to p→ e+ µ + µ − and red showsp→ e− µ + µ + . Due to µ − capture in water, p→ e+ µ + µ −events have less Michel electron than p→ e− µ + µ + .

Fig. 2.

µ − e+ e+ , p → e+ µ + µ − p → e− µ + µ + are common because
the same selection criteria is applied to background MC forthese two modes, while efficiency is different due to µ − capture. The study will be expanded to all SK era and after systematic uncertainty estimation and side band check, data will
be opened.

Bibliography[1] T. Hambye and J. Heeck, PRL 120, 171801 (2018).
[2] C. McGrew et al. [IMB Collaboration], Phys. Rev. D 59,052004 (1999).
[3] K. Abe et al. [Super-Kamiokande Collaboration], Phys.
Rev. D 95, 012004 (2017).

4

p → e+ e+ e−
p → µ + e+ e−
p → µ − e+ e+
p → e+ µ + µ −p → e− µ + µ +p → µ−µ+µ+Efficiency (%)
low Ptot high Ptot
22.4
41.1
17.6
30.3
14.9
25.9
11.7
20.9
14.4
24.2
12.2
20.4

Backgroundlow Ptothigh Ptot<0.01 0.24±0.07<0.01 0.17±0.05<0.01 0.17±0.05<0.01 0.08±0.03<0.01 0.08±0.03<0.01 0.22±0.06

Table 1. Summary of efficiencies and backround for SK-IV.

Low Ptot and high Ptot are defined as Ptot < 100MeV/c and100≤ Ptot < 250MeV/c, respectively.

Atmospheric neutrinos
Atmospheric neutrinos are produced from the decays ofsecondary particles produced in the collision of primary cosmic rays with nuclei in the atmosphere. Atmospheric neutrinos have several remarkable features:• The flavor ratio, (νµ + ν̄µ )/(νe + ν̄e ) is 2 ( > 2 for above
few GeV)• Zenith angle distribution is up/down symmetry above a
few GeV
These features are realized without neutrino oscillations, andprovide a useful constraint in the study of these neutrinos.
Super-Kamiokande has been observing atmospheric neutrinos since 1996 and has accordingly made several importantmeasurements, including the discovery of neutrino oscillations [1].
Three flavor oscillations and the neutrino mass hierarchy
The SK atmospheric neutrino data are described at leading order by two-flavor νµ → ντ oscillations with maximal
mixing (θ23 =π /4). However, sub-leading contributions via νµ→ νe oscillations induced by the mixing angle θ13 as wellas the “solar” mixing parameters (∆m212 , θ12 ) provide the ability to probe currently unknown aspects of the standard neutrino oscillation paradigm, such as the existence of leptonic
CP violation and the neutrino mass ordering (hierarchy). Understanding these open questions may bring important insight
into larger questions, such as the origin and evolution of today’s matter-dominated universe.
Several sub-leading oscillation effects are expected to appear in atmospheric neutrinos:• Resonant enhancement of νµ → νe oscillations due tothe effects of matter is expected to occur at energies between 2 and 10 GeV and will manifest as an excess of
upward-going electron-like events (e-like) in the atmospheric sample.
• This enhancement exists for either νe or ν̄e dependingon the mass hierarchy. Therefore the mass hierarchy
can be probed by understanding the relative amount of
neutrino and antineutrino interactions in the detector.

• The combination of the solar oscillation parameters andthe octant of sin2 θ23 , may enhance or suppress theevent rate, and to some extent alter the spectral shape,of Sub-GeV electron-like data the due to the νµ ↔ νeoscillations they induce.
• The standard oscillation paradigm includes a CPviolating factor, δcp , which is expected to induce several sub-dominant oscillation effects in many of the SK
atmospheric neutrino samples, even if CP is conserved.
Super-Kamiokande has studied the effects of these oscillations on atmospheric neutrino data separated into fullycontained (FC) events, partially-contained (PC) events, andupward-going muon topologies. Fully-contained events arecharacterized by a primary interaction vertex that is locatedinside the fiducial volume of the detector and whose visible
particles stop within the inner detector. On the other hand,though the primary vertex position of PC events is within thefiducial volume, they are characterized by having at least one
charged particle escaping the inner detector and depositing
light in the outer detector. In most cases the escaping particle
is a muon. Upward-going muons originate from high energy
muon-neutrino interactions in the rock surrounding the detector. Since all other particles are lost to interactions in the rock,
only the muon is penetrating enough to reach the detector
and be identified. The FC sample is separated into electronlike and muon-like (µ -like) subsamples by applying a particle
identification algorithm to the most energetic Cherenkov ringof each event. Since PC and upward-going events are predominantly produced by muon neutrinos, no particle identification is applied. Though SK cannot distinguish on an eventby-event basis neutrino and antineutrino interactions, statistical separation of multi-GeV electron-like subsamples is performed to improve sensitivity to the mass hierarchy. A likelihood method designed to enhance the kinematic differences
between neutrino and antineutrino interactions is applied toseparate events into νe -like and ν̄e -like subsamples.
A new reconstruction algorithm based on a maximum likelihood method has been developed for Super-K. Comparedto the conventional reconstruction algorithm, the new algorithm shows improved performance in a variety of metrics
including event vertex resolution, particle momentum resolution, and particle identification. Fig. 3 shows an improved
ability to discriminate between electrons and muons with lessthan a 1% mis-identification rate for visible energies less than
1330 MeV. The new algorithm has further demonstrated reliable performance over a larger volume of the detector thanthe previous volume. Accordingly, in the present analysis thefiducial volume definition has been expanded to include all
events reconstructed more than 50 cm from any ID wall. This
represents a 32% increase in the number of events available
for analysis relative to the 200 cm threshold used in previous
SK analyses [3, 4]. Zenith angle distributions for each analysis sample using the expanded FV are shown in Fig. 4.
Using the new algorithm with its expanded fiducial volume definition an analysis of a 253.9 kton-year exposure of
the SK-IV atmospheric data has yielded oscillation parameter estimates consistent with both previous Super-K measurements and results from other experiments [2]. Parame-

5

Mis-PID rate [%]ter values and their 1 σ errors are summarized in Table 2.
Fig. 5shows the χ 2 value as a function of the atmospheric
neutrino mixing parameters and δCP in the θ13 -constrained
fit. The data show a weak preference for the normal mass
hierarchy, disfavoring the inverted mass hierarchy at 74.0%
assuming oscillation parameters at the analysis best-fit point.
No strong preference for the θ23 octant is observed.
Fig. 6 shows the expected sensitivity to mass hierarchy as
the function of livetime for both reconstruction algorithms andtheir fiducial volumes. The expected improvement in sensitivity with the new algorithm becomes more apparent as datais accumulated, even assuming the conventional FV. Whilethe new reconstruction has only been applied to the 3118.5
day SK-IV fully contained data set, future efforts will expand
this study to include other Super-K samples and run periods,
which constitute an additional 2,800 days of livetime.
3.5

fiTQun

3

APFit

2.5
2
1.5
1
0.5

Mis-PID rate [%]

0
0

200

400

600

800

1000

1200
1400
Evis [MeV]

600

800

1000

1200
1400
Evis [MeV]

3.5

fiTQun

3

APFit

2.5
2
1.5
1
0.5
0
0

200

400

Fig. 3. Mis-identification rate of single-ring electron (top) and muon(bottom) events from FC CCQE atmospheric neutrino MC events
plotted as a function of visible energy. The open circles indicate
the performance of APFit and the triangles are for fiTQun. The
reconstructed event vertex is required to be at least 200 cm awayfrom the ID wall.

Measurement of the tau neutrino cross section
The deficit of the muon neutrinos observed in the SuperK data is predominantly explained by the oscillation of theoriginal muon neutrinos into tau neutrinos. For this reason,
the direct detection of tau neutrinos is important to confirm
the standard three-flavor neutrino oscillation scheme. SuperKamiokande collaboration previously published a measurement of atmospheric tau neutrino appearance consistent withthree-flavor neutrino oscillation with data collected in SK-I
through SK-III [5]. The analysis has been updated to use fit-

Hierarchysin2 θ23 (1st oct.)sin2 θ23 (2nd oct.)|∆m232,31 | (×10−3 eV2 )δCPNormal
0.425+0.046−0.0370.600+0.013−0.0302.53+0.22−0.12
3.14+2.67−1.35

Inverted+0.0550.425−0.036+0.022
0.588−0.037+0.14
2.53−0.31
4.89+1.51−3.46

Table 2. Parameter estimates for each analysis and hierarchy hypothesis considered. Here “Normal” (“Inverted”) refers to the normal(inverted) hierarchy fit. The value of sin2 θ13 is constrainted to be
0.0210±0.0011 according to reactor measurements. The terms
Unconstrained and Constrained refer to fits without andwith a constraint on sin2 θ 13, respectively. For sin2 θ23 parameter ranges are shown for both octants, with the best-fit octant
enclosed in a box.

ting technique and includes new data from the SK-IV period
for a total livetime of 5,326 days [6].
The ντ signal is characterized by an isotropic distribution of hadrons produced when the τ lepton emerging fromthe original neutrino interaction decays. Background eventsare mainly are result of charged current CC νµ and νe and
NC interactions which produce multiple hadrons. As shownin Fig. 7, CC ντ events typically produce multiple rings in
the detector. Multiple-ring events are relatively easy to separate from single-ring atmospheric neutrino events. However, the multiring background events, resulting from multipion/DIS atmospheric neutrino interactions, are difficult to distinguish from the tau signal. Simple selection criteria based onkinematic variables do not identify CC ντ events efficiently.
Therefore we employ a neural network (NN) technique tohelp distinguish CC ντ events from background events. Seven
kinematic variables are sampled from simulated signal andbackground events to build and train the NN before applying
it to the data. When selecting tau-like events from the events
after reduction by requiring the NN output be greater than 0.5,
76% of the signal events and only 28% of the background remain.
To search for atmospheric tau neutrino appearance, thedata is fit to a combination of the expected tau signal resulting from neutrino oscillations and atmospheric neutrino
background with neutrino oscillations. In order to extract
maximum information from the sample, the analysis uses a
two-dimensional unbinned maximum likelihood fit. Using
two-dimensional histograms of the neural network output andthe reconstructed zenith angle of the events, two-dimensional
probability distribution functions (PDFs) are built for background and tau signal. The PDFs for each event is a combination of that for background and signal:
PDFs = PDFsbg + α × PDFstau + ∑ εi · PDFssys ,iwhere the last term represents the change of the PDFs due tosystematic error effects. The likelihood is calculated as the
product of PDFs for each events and the parameters α andεi ’s are adjusted to maximize the likelihood.
Figure 8 shows the zenith angle distribution of tau-likeand non-tau-like samples after fitting. The NN output distributions for upward- and downward-going events are shown in
the right panels. Relative to the expectation for standard three-6

Sub-GeV e-like 1-dcy e

Sub-GeV µ -like 0-dcy e

Sub-GeV e-like 0-dcy e

Sub-GeV µ -like 1-dcy e

1000

1000

200

500

100

Up Stop µ

200
200
500

100

0

0

0

0

0

1.2
1
0.8

1.2
1
0.8

1.2
1
0.8

1.2
1
0.8

1.2
1
0.8

Multi-GeV e-like ν e

Sub-GeV µ -like 2-dcy e
200

Multi-GeV e-like ν e

200

Non-showering µ

PC Stop

50
50

500

100

100

0

0

2

1.2
1
0.8

1
0
Sub-GeV π 0 -like M-R

200

0

0

0

1.2
1
0.8

1.2
1
0.8

1.2
1
0.8

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

400Showering µ

PC Thru
100

500

100

100

200
50

0

500

1.2
1
0.8

500

1000

1000

0

0

0

0

1.2
1
0.8

1.2
1
0.8

1.2
1
0.8

1.2
1
0.8

200

Multi-GeV µ -like

Multi-Ring µ -like

100

0
-1

0

1

1.2
1
0.8

Multi-Ring Unclassified

400

200

200

100

0
-1

0

1

1.2
1
0.8

-1

0

lepton momentum (MeV)

1

0
-1

0

1

0

1

1.2
1
0.8

-1

0

cos zenith

1-1

cos zenith

cos zenith

cos zenith

Fig. 4. Data and MC comparisons for the SK-IV data divided into 18 analysis samples. The expanded FV, where dwall>50 cm, is shown here.
Samples with more than one zenith angle bin are shown as zenith angle distributions (second through fifth column) and other samples
are shown as reconstructed momentum distributions (first column). Cyan lines denote the best−fit MC assuming the normal hierarchy.
Narrow panels below each distribution show the ratio relative to the normal hierarchy MC. In all panels the error bars represent the statisticaluncertainty.

10

10
SK-IV 3118.5 days
(FiTQun analysis)

8

10
SK-IV 3118.5 days
(FiTQun analysis)

8

99%

99%

99%

4

95%

 2

6

 2

6

 2

6

4

90%

95%

4

90%

2

0
0.0015

2
68%

0.002

0.0025| m223| eV 2

0.003

0.0035

95%
90%

2
68%

SK-IV 3118.5 days
(FiTQun analysis)

8

0
0.3

68%

0.4

0.5

0.6

sin223

0.7

0

0

2

4

6

CP

Fig. 5. Constraints on neutrino oscillation parameters from SK-IV atmospheric neutrino data using the expanded FV and assuming
sin2 θ13 =0.0210±0.0011. The solid blue and dashed orange lines denote the normal and inverted hierarchy fit results, respectively. The
latter has been offset from the former by the difference in their minimum χ 2 values.

flavor neutrino oscillations the fitted number of tau events inthe data is α = 1.47 ± 0.32 (stat+syst.). This corresponds toa statistical significance 4.6σ to reject the no-tau-appearance
hypothesis.
Because of the difficulties in tau neutrino production anddetection, charged-current tau neutrino cross sections have notbeen well measured. DONUT [7] is the only experiment thatreported a measurement of the cross section. We wish to compare the ντ cross section measured with atmospheric neutrinos by Super-K at relatively low energies to that measured

by DONUT with a neutrino beam at higher energies. A fluxaveraged charged current tau neutrino cross section is measured to be (0.94±0.20)×10−38 cm2 for neutrino energy between 3.5 and 70 GeV in Super-K, to be compared with theflux-averaged theoretical cross section of 0.64×10−38 cm2 , asshown in Fig. 9. Our result is consistent with the previous
DONUT result, and is consistent with the Standard Model prediction to within 1.5σ .

7Sensitivity to reject wrong hierarchy Δχ2

Super-Kamiokande IV
Run 999999 Sub 2 Event 7

3.5

16-04-13:05:43:18

fiTQun Expanded FVInner: 8104 hits, 30188 pe
Outer: 3 hits, 2 pe
Trigger: 0x07

3

fiTQun Conventional FV

2.5

APFit Conventional FV

D_wall: 1130.7 cm
Evis:

3.3 GeV

Charge(pe)>26.7
23.3-26.7
20.2-23.3
17.3-20.2
14.7-17.3
12.2-14.7
10.0-12.2
8.0-10.0
6.2- 8.0
4.7- 6.2
3.3- 4.7
2.2- 3.3
1.3- 2.2
0.7- 1.3
0.2- 0.7
< 0.2

2
1.5
1
0.5

3400
0 mu-edecays

2720

0

3000

4000

5000

6000

7000

2040

8000
9000
Livetime [days]

1360
680
0

Fig. 6. Expected sensitivity to the normal mass hierarchy as a function of livetime assuming sin2 θ23 =0.5. Grey and blue bands show
the sensitivity of the atmospheric neutrino sample reconstructedwith fiTQun in the conventional FV and expanded FV, respectively.
Orange lines denote the sensitivity when events are reconstructedwith APFit in the conventional FV.

0

500

1000

1500

2000

Times (ns)Super-Kamiokande IV
Run 999999 Sub 1 Event 179
16-03-10:18:48:26
Inner: 5770 hits, 19150 pe
Outer: 4 hits, 3 pe
Trigger: 0x07
D_wall: 723.9 cm
Evis:

2.2 GeV

Charge(pe)Search for boosted dark matterWith the properties of dark matter so uncertain, various
possibilities must be considered. One possibility is that somedark matter is, in fact, not cold but is highly relativistic and has
been produced at late times, thus denoted “boosted” dark matter [8]. Boosted dark matter could exist as a subdominant dark
matter component, with a dominant cold dark matter component accounting for most of the dark matter energy densityof the Universe. In this way, boosted dark matter can remain
consistent with ΛCDM. The sub-dominant boosted dark matter can be the same particle as the dominant cold dark matter,or it can be a different, lighter particle. Boosted dark mattercan be produced from the dominant cold dark matter through avariety of processes, including annihilation, semiannihilation,
number-changing 3→2 self-annihilation, and decay. Boosted
dark matter can then be observed through its scattering off
electrons or nuclei in large-volume terrestrial detectors. Current direct detection limits can be evaded in multicomponent
models by having only the boosted dark matter species couple
directly to standard model particles or in boosted dark mattersingle-component models by invoking a spin-dependent dark
matter-nucleon cross section.
The results of a search for boosted dark matter coupling toelectrons in Super-Kamiokande (SK) was reported [9], withthe boosted dark matter originating in the Galactic center or
the Sun and with scattered electron energies ranging from 100
MeV to 1 TeV. This is the first time that this class of highenergy electron elastic scatter- like events has been studied
at SK. The search is performed on 2628.1 days of SK-IV data,
which corresponds to 161.9 kt yr exposure. The analysis isdesigned to be independent of the particular model of the coupling between boosted dark matter and electrons. This way,
the results can be applied to any model that predicts a sourceof particles from the Galactic center or Sun which would scatter electrons to energies greater than 100 MeV.
The search begins with the fully contained fiducial volume(FCFV) data set. This is a standard SK data set used to study
atmospheric neutrinos. From this data set, we search for elastically scattered electrons by applying the following analysis>26.7
23.3-26.7
20.2-23.3
17.3-20.2
14.7-17.3
12.2-14.7
10.0-12.2
8.0-10.0
6.2- 8.0
4.7- 6.2
3.3- 4.7
2.2- 3.3
1.3- 2.2
0.7- 1.3
0.2- 0.7< 0.2
1100
2 mu-edecays

880
660
440
220
0
0

500

1000

1500

2000

Times (ns)Fig. 7. Simulation of a CC ντ event (upper) with 3.3 GeV visible energy
in the ID and a multiring NC background event (lower) with 2.2 GeV
visible energy in the ID. The tau signal event produces multiple
rings, making it different from the single-ring background event.
The background event with multirings has a similar pattern to thesignal event, and requires more effort to statistically distinguish.

cuts: (i) one-ring (if Evis<100 GeV), (ii) e-like, (iii) zero decay electrons, and (iv) zero tagged neutrons. The first two cuts
search for a single relativistic electron, while the final two cuts
remove events with a signature of a nuclear interaction. Decay
electrons in e-like events are the result of the π →µ →e decay
chain with the π coming from a neutrino-nucleus interaction.
Tagged neutrons originate from neutrons being knocked outof the nucleus following a neutrino-nucleus interaction, thermalizing, and capturing on hydrogen. Neutron captures are
particularly numerous following neutrino deep inelastic scattering. Neither decay electrons nor neutron captures should
occur following the elastic scatter of an electron by a boosted
dark matter particle.
Since boosted dark matter is expected to originate in regions of high dark matter density, this search looks for a signal
coming from the Galactic center or the Sun. Cones are drawn
around the signal source, and the number of events passing
the analysis cuts in each cone is counted. When the source ofthe signal is the Galactic center, the optimal size of the searchcone is dependent on both the distribution of the dominant
dark matter species in the Galaxy and the production methodof the boosted dark matter. Production of boosted dark matter
through both dark matter annihilation and decay were consid-

8300

BG after fit

1000

Tau after fit
Data

800
Events

200
600
400

100

200

Tau-like
0-1

-0.5

0

0.5

1

0
1200

Events600

Upward
0

0.2

0

0.2

0.4

0.6

0.8

1

0.8

1

1000
800

400

600
400

200

200

Non tau-like
0
-1

-0.5

0
CosΘ

0.5

1

0

Downward
0.4 0.6
NN output

Cross Section (10-38 cm2)

Fig. 8. Fit results, assuming the normal hierarchy, showing binned projections in the NN output and zenith angle distribution for tau-like(NN>0.5), upward-going [cos θ < − 0.2], non-tau-like (NN<0.5) and downward-going [cos θ >0.2] events for both the two-dimensional PDFs
and data. The PDFs and data sets have been combined from SK-I through SK-IV. The fitted tau signal is shown in gray.

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

Bibliography

SK measured CC tau cross section

10-3
0

20

nario, the 40◦ cone was used for all three galactic halo models.
Ninety-percent confidence intervals were computed in the εvs mA plane for the annihilation scenario, and the ε =τ /decay
vs mA plane for the decay scenario, with representative values of mB =200 MeV, m(γ =420 MeV,and g( =0.5, respectively.
These limits are shown for the Moore, NFW, and Kravtsov
halo models in Fig. 10.

4060

Neutrino Energy (GeV)

Fig. 9. Comparison of the Super-K measured (marker with error bars)
and expected tau neutrino cross sections (solid lines) with σ (E)inferred from DONUT (dashed lines).

[1] Y. Fukuda et al. [Super-Kamiokande Collaboration],
Phys. Rev. Lett. 81, 1562 (1998).
[2] M. Jiang et al. [Super-Kamiokande Collaboration],
arXiv:1901.03230 [hep-ex].
[3] R. Wendell et al. [Super-Kamiokande Collaboration],
Phys. Rev. D 81, 092004 (2010).

ered for three dark matter halo models: Moore [10], NavarroFrenk-White (NFW) [11], and Kravtsov [12]. The optimal
half-opeining angle of the search cone was found to range
from less than 5◦ to around 40◦ . We therefore used eightsearch cones around the Galactic center, ranging from 5◦ to
40◦ in steps of 5◦ . When the Sun is the signal source, the situation is much simpler, since it is effectively a point source.
Therefore, a single search cone of 5◦ around the Sun was used
for the solar search.
The results of the search are shown in Table 3. The observed data are consistent with expected background for boththe Galactic center and Sun searches. In the highest energy sample, the search is essentially background-free, andno candidates were found in any of the search cones. Limits
were calculated separately for the Moore, NFW, and Kravtsov
galactic halo models, using the results from a different conefor each fit. For the annihilation scenario, the 5◦ cone wasused for the Moore model, the 10◦ cone for the NFW model,
and the 40◦ cone for the Kravtsov model. For the decay sce-

[4] K. Abe et al. [Super-Kamiokande Collaboration], Phys.
Rev. D 97, no. 7, 072001 (2018).
[5] K. Abe et al. [Super-Kamiokande Collaboration],
Phys.Rev.Lett 110, 181802 (2013).
[6] Z. Li et al. [Super-Kamiokande Collaboration], Phys.
Rev. D 98, no. 5, 052006 (2018).
[7] K. Kodama et al. [DONUT Collaboration], Phys. Lett. B
504, 218 (2001).
[8] See references in [9][9] C. Kachulis et al. [Super-Kamiokande Collaboration],
Phys. Rev. Lett. 120, no. 22, 221301 (2018).
[10] B. Moore, T. R. Quinn, F. Governato, J. Stadel, and
G. Lake, Mon. Not. R. Astron. Soc. 310, 1147 (1999).
[11] J. F. Navarro, C. S. Frenk, and S. D. M. White, Astrophys. J. 462, 563 (1996).

9
SearchCone
GC 5◦
GC 10◦GC 15◦GC 20◦GC 25◦GC 30◦
GC 35◦
GC 40◦Sun 5◦

100 MeV< Evis < 1.33 GeV
Expected
Bckg

Data

8.4 ± 0.7
32.0 ± 1.9
72.5 ± 3.5
126.5 ± 5.4
196.8 ± 7.6
283.7 ± 10.1
384.8 ± 12.8
499.6 ± 15.9
7.59 ± 0.18

5
24
69
125
202
285
375
494
5

Sig
Rate
Limit
(kT-y)−1
0.017
0.023
0.078
0.123
0.201
0.214
0.187
0.249
0.017

1.33 GeV < Evis < 20 GeV
Expected
Bckg

Data

1.6 ± 0.3
6.3 ± 0.84
13.6 ± 1.6
23.3 ± 2.3
35.4 ± 3.3
49.3 ± 4.3
68.1 ± 5.4
90.2 ± 6.9
1.25 ± 0.07

1
5
11
18
31
48
67
90
1

Evis > 20 GeV

Sig
Rate
Limit
(kT-y)−1
0.018
0.026
0.032
0.028
0.049
0.081
0.101
0.124
0.020

Expected
Bckg

Data

0.016 ± 0.005
0.060 ± 0.018
0.14 ± 0.04
0.25 ± 0.07
0.37 ± 0.11
0.53 ± 0.16
0.70 ± 0.21
0.90 ± 0.27
0.015 ± 0.004

0
0
0
0
0
0
0
0
0

Sig
Rate
Limit
(kT-y)−1
0.015
0.015
0.014
0.014
0.013
0.012
0.011
0.011
0.015

Table 3. Estimated backgrounds,numbers of events in data, and signal event rate limits for each cone and each energy sample. The event rate
limits are at the 90% confidence level.

Annihila'on

pep, 7 Be, 8 B and hep neutrinos1 , whose predicted fluxes by
the standard solar model [1] are shown in the left panel of
Figure 11. Among these, 8 B solar neutrino can be detected
relatively easily by many detectors due to its higher energy
which extends up to ∼20 MeV.

up-turn

Decay

Fig. 10. 90% confidence interval upper limits for mB =200 MeV,m(γ =20 MeV, and g( =0.5, for boosted dark matter produced by annihilation (top) and decay (bottom).

[12] A. V. Kravtsov, A. A. Klypin, J. S. Bullock, and J. R. Primack, Astrophys. J. 502, 48 (1998).
Solar Neutrinos
Introduction
Solar neutrinos constitute by far the largest componentof neutrino flux on Earth among those produced from natural sources. Most of solar neutrinos are produced by protonproton reaction, p + p → d + e+ + νe , and its subsequent fusions and β -decays. They are categorized into so called pp,

Fig. 11. Left: Several predictions of the survival probability of electron neutrinos emitted from the Sun as well as the fluxes of eachsolar neutrino; Right: The visual explanation of the day/night flux
asymmetry.

Past observations of solar neutrino flux by SuperKamiokande (SK) [2] and the Sudbury Neutrino Observatory (SNO) [3] led to the discovery of solar neutrino flavor
conversion. Our current interest for solar neutrino measurement with the SK detector [4] is to make a precision test of
the Mikheyev-Smirnov-Wolfenstein (MSW) effect [5, 6], so
called the matter effect. The MSW effect leads to a resonant
conversion of the solar neutrinos within the Sun and results inan about 30% level of the survival probability above ∼5 MeV
as shown in the left panel of Figure 11. The survival probability of lower energy neutrinos are described by vacuum oscillation probability of ∼ 50%, and the transition region between
the two lies at a few MeV region. This transition from the matter dominant oscillation to the vacuum dominant oscillation isoften called as the “up-turn” of the solar neutrino spectrum.
This “up-turn” have not been directory demonstrated by thepast experimental data.
SK aims to directly test the “Spectrum up-turn” by precisely measuring energy spectrum of 8 B solar neutrino in thistransition region. This measurement is not only important totest the MSW effect but also to test several alternative theoretical models, such as sterile neutrinos [7, 8], mass-varying
neutrinos [9], non-standard interactions [10, 11] and so on.
*1 As well as CNO neutrinos

10

0.55

250
SK data(Stat. only)
Systematic error

0.5

SK combined flux

0.45

150

0.4

100



0.35

50

    

8 B solar neutrino flux measurement200

Sun spot number

Sun spot number

Figure 13 shows the SK yearly flux measured throughout thedifferent phases of SK together with the corresponding sun
spot number (Source: WDC–SILSO, Royal Observatory of
Belgium, Brussels [17]). The χ 2 between observed data andno time dependence hypothesis is calculated with the total experimental uncertainties as χ 2 = 21.57/21 d.o.f., which corresponds to a probability of 41.4%. The SK solar rate measurements are fully consistent with a constant solar neutrino
flux emitted by the Sun.
Data/MC(Unoscillated)

The matter effect can also be tested with the matter in interior of the Earth. Electron flavor neutrinos are regenerated
due to the matter effect of the Earth as shown in the right panelof Figure 11, making the neutrino flux in night is larger thanthat in day by about a few % level depending on the neutrino
oscillation parameters. In 2014, SK reported an indication of
the terrestrial matter effects by about 2.7σ [12].
In 2016, the Super-Kamiokande collaboration released apaper reporting a solar neutrino analysis results using 1664
days of SK-IV data [13]. In this annual report, updated results
using data taken through the end of January, 2018 (SK-IV
2,860 days data set) with improved energy reconstruction arepresented. The total livetime throughout the different phasesof SK [2, 14, 15] is 5,695 days.
In addition, we tested effects of non-standard interactions
with the solar neutrino spectrum. These results are also shownin this report.

0.3 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 0
MC* &$'$(*1



q+-(

8

6

B Flux = 5.25× 10 /cm2/sec

* &$'$(*1

Date

Fig. 13. The ratio of 8 B solar neutrino flux in SK over the SNO’s NC
current flux from 1996 to 2018. The red points show the yearlyflux measured by SK (statistical uncertainty only), the gray bands
 +,2!$,$"(&$+$("))"* ' (,/$,#,
show the systematic uncertainties for each SK phase, the black-horizontal line shows the combined measured flux with the uncertainty drawn as the red horizontal band. The back points show thesun spot number provided by [17].

-(,
 +,!$,%"*)-( +,!$,$"(&Energy spectrum analysis
Because of the installation of new front-end electronics for
SK-IV [18], the improvements of the water circulation systemand +,,3
the upgrade of calibration),&(-' *)! . (,+$( 1+  3
+1+, methods. (,+ [19], solar neutrino
interactions
as
well
as
the
backgroundThe solar angle distribution in SK-IV. The
horizontal axis
Fig.
12.-,*$()&-0)(&1
3
+,,3 +1+,4 ' +  event in multi-MeV
shows the cosine of the solar angle cos θSun and the vertical axis
regions has been well understood in SK-IV. Owing to theseshows the number of the observed events. The red points shows upgrades above, SK has achieved the lowest background (inthe observed data, the black (blue) histogram shows the best-fit
duced by radioisotopes in pure water, especially 214 Bi) among(background-shape).
all SK phases [20]. Finally, the energy threshold in SK-IV
The SK detector observes solar neutrinos via elastic scat- have been lowered at 3.5 MeV in recoil energy kinetic enterings with electrons in pure water. The direction of a re- ergy (SK-I: 4.5 MeV, SK-III: 4.0 MeV) and this enables SK
coiled electron is highly correlated with the direction of the to measure the solar neutrino energy spectrum with high senincident neutrino. Figure 12 shows the distribution of cosine sitivity. In addition, on May 2015, the trigger threshold was
between the reconstructed direction of observed recoiled elec- changed from 34 observed PMT signals within 200 nsec totrons and the direction of the Sun. With 2860 days of data in 31 hits [21, 22]. Because of this lower threshold, the detecSK-IV, more than 55,000 events are observed over the back- tion efficiency between 3.5 MeV and 4.0 MeV was improved
ground. Adding the solar neutrino events observed in other from ∼86% to ∼100%. This improvement leads the further
phases, the total number of the solar neutrino events is more reduction of the statistics uncertainty below 5 MeV in SK-IV.
In this analysis, we further improved energy reconstructhan 93,000 events. Based on this data, the 8 B solar neution
method with an improved correction for the detector time
62
trino flux is determined to be (2.33 ± 0.04) × 10 /cm /secvariation.
For low-energy events in SK, their energy is reconassuming a pure electron neutrino flavor content. The ratio
structedbased
on number of PMT hits. The gains of PMTs
between the SK result and the SNO NC current flux (5.25 ×
have
been
gradually
increasing at a rate of 1-2%/year and62
10 /cm /sec) [16] becomes 0.4432 ± 0.0084.
causingtime-dependentbias to the reconstructed energies.
The solar activity cycle is the 11 years periodic change ofThe
newenergyreconstruction
method for low-energy events
sun spots releasing the magnetic flux at the surface of the Sun.
also
corrects
for
this
gain
increase
effect. Figure 14 shows
The number of the sun spots strongly correlated with the sotheeffective
number
of
hits
after
gain
correction for decaylar activity cycle. SK has observed solar neutrinos for more
electrons
fromstoppedcosmicmuons
asa function of time.
than 21 years, which covers about two solar activity cycles.

279

0.6
0.5

6

8

10

12

14

16

0.2

18

2012

2014

2016

2018
Year

Fig. 14. Time variation of averaged effective number of hits after gain
correction for decay-electrons from stopped cosmic muons. Black
points show the data with statistical uncertainties. The horizontal
red line shows the average for the entire SK-IV period and the bluelines show the ±0.5% range.

Figure 15 shows the energy spectrum obtained among all
SK phases, its best-fits with polynomial functions and the prediction of the MSW up-turn assuming the current oscillation
parameters described in the next section. Figure 16 shows
the combined energy spectrum from SK-I to SK-IV with thepredictions of the MSW effect assuming the current oscillation parameters described later. Note that all SK phases are
combined without regard to energy resolution or systematic
uncertainties in Figure 16, but those uncertainties are taken
into account in the χ 2 calculation between the data and theprediction. Comparing χ 2 between the data (black) and thepredictions (green or blue), the energy spectrum of SK is consistent within ∼1σ with the MSW up-turn for the solar bestfit parameters (green in Figure 18) and marginally consistent
within ∼2σ with the MSW up-turn for KamLAND best-fit
parameters (blue in Figure 18).
Oscillation parameter extraction
The oscillation parameters were extracted using the results from SK [2, 14, 15, 13], SNO [16], radiochemical solar
neutrino experiments [23, 24, 25] and Borexino [26, 27, 28]
as well as the anti-neutrino measurement by KamLAND[29, 30]. Figure 17 shows the allowed parameter region fromSK as well as KamLAND data. SK uniquely selects the large
mixing angle MSW region by more than 3σ and SK significantly contributes to the measurement of the solar angle θ12 .
The SK energy spectrum and day/night data favors a lower∆m221 value than KamLAND’s by more than 2σ .
When combining with results from the other solar neutrino
experiments, the mixing angle is determined to be sin2 θ12 =
0.310 ± 0.014 and the mass difference is determined to be−5 eV−2 as shown in Figure 18. Adding∆m221 = 4.82+1.20−0.60 ×10
the KamLAND result, the oscillation parameters are deter−5
mined as sin2 θ12 = 0.310 ± 0.012, ∆m221 = 7.49+0.19−0.17 × 10
2eV . These parameters are mostly determined by the solar
neutrino oscillation fit and further precise measurements are
required in future.

8

Ekin in MeVSK III Spectrum

0.7
0.6
0.5

0.2

2010

0.5

0.3
10

12

14

16

18

Ekin in MeVSK IV Spectrum

0.7
0.6
0.5
0.4

0.3

274
2008

0.6

0.3

0.4

275

0.7

0.4

278

276

SK II Spectrum

0.4

0.2

277

Data/MC (unoscillated)

0.7

Data/MC (unoscillated)

280

Data/MC (unoscillated)#of Effective hit

The time variation of energy scale is well corrected with the
new method and is within ±0.5%.

Data/MC (unoscillated)

11
SK I Spectrum

0.3
4

6

8

10

12

14

16

18

Ekin in MeV

0.2

4

6

8

10

12

14

16

18

Ekin in MeV

Fig. 15. SK-I, II, III and IV recoil electron spectra divided by thenonoscillated expectation. The green (blue) line represents thebest-fit to SK data using the oscillation parameters from the fit toall solar (solar plus KamLAND) data. The orange (black) line is thebest-fit to SK data of a general exponential or quadratic (cubic) Pee
survival probability. Error bars on the data points give the statistical plus systematic energy-uncorrelated uncertainties while theshaded purple, red, and green histograms give the energy-correlated systematic uncertainties arising from energy scale, energy
resolution, and neutrino energy spectrum shift.

Super-K

SNO
quad. fit:

KamLAND Δm2
Solar Δm2

Preliminary

Fig. 16. The energy spectrum combining SK-I through SK-IV as afunction of the recoil electron energy. The red points show the ratioof the data to the expected flux using a non-oscillated 8 B solar
neutrino spectrum. The green (blue) curve shows the expectedenergy spectrum assuming the MSW effect inputting oscillation
parameters of solar global (solar plus KamLAND). The black curveis the result of a fit with the quadratic function shown in the inset.

Search for non-standard interactions
If there are Non-Standard Interactions (NSI) between neutrinos and matter beyond the Standard Model of Particle
Physics, they would alter the matter effect. The 8 B solar neutrino data at Suker-Kamiokande have the potential to constrain
the strength of NSI thanks to high matter density at the center of the sun and high data statistics at SK. Such NSI could
also flatten or invert the “up-turn”, and could provide a better
agreement with the observed data.
The effective Hamiltonian for the matter potential for the
2-flavor mass basis can be expressed using effective parameters ε11 and ε12 as!
"∗
GF ne cos2 θ13 + ε11
ε12
ef f
Hmat
= √
(1)
ε12
− cos2 θ13 − ε112

12PRELIMINARY

2d-quark

PRELIMINARY

sin2(O12)=0.316+0.034-0.026sin2(O12)=0.332+0.027-0.022sin2(O12)=0.324+0.021-0.018-526m221=(7.54+0.19
sin2(O13)=0.0219±0.0014-0.18) 10 eV-52
6m221=(4.73+1.32-0.80) 10 eV-52
6m221=(7.49+0.19-0.17) 10 eV

PRELIMINARYFor NSI of solar neutrinos with down-quarks in solar and terrestrial matter using Super-K

Fig.
19. andObserved
and best
fit solar
survival
probabilities.
SpectralIntegrated Day/Night
Asymmetry,
as wellneutrino
as including the
SNO-Polynomial
Data,
theThe2-D confidence
levelsare: the result for ν -down quark interactions whileleft panel
show
the right panel shows the result for ν -up quark interactions. Solidblue curves shows the best fit with NSI, and the dashed curves
show the fit without NSI.

15

6m

u-quark

1m2-5
2
in 10 eV
21

6r

8 3m
6
4 2m
2

PRELIMINARY

10

5
Very Preliminary
1m

0.1

0.2

0.3

2m

3m

0.4
0.5 2 4 6 822sin (e )6r
12

For NSI of solar neutrinos with up-quarks in solar and terrestrial matter using only SuperK Spectral and Integrated Day/Night Asymmetry (and constraining the 8B flux to the SNO
Neutral Current measurements), the 2-D confidence levels are:Fig. 17. The allowed contours for ∆m221 vs. sin2 θ12 from the SK combined analysis (green) as well as the allowed region from KamLAND (blue). The combined allowed region is shown in red.

2

where the minimum is at (𝜖11 , 𝜖12 ) = (-1.2, -2.0), marked by the black star. The case of No NSI isstill incl ded at 1.23 . The 1-D confidence levels are:

1m

2
-5
2
in 10 eV
21

6r

8 3m
6
4 2m
2
sin2(O12)=0.316+0.034-0.026sin2(O12)=0.310±0.014
sin2(O12)=0.310±0.012-526m221=(7.54+0.19
sin2(O13)=0.0219±0.0014-0.18) 10 eV-52
6m221=(4.82+1.20-0.60) 10 eV-52
6m221=(7.49+0.19-0.17) 10 eV

6m

15
where
minimumat (𝜖11 , 𝜖12 ) = (2.1,-3.3), marked
by εthe black
star. The case of No NSI is
Fig.the20.
The isconfidence
intervals
in the12 vs. ε11 planes for ν -down
still included at 1.03 . T 1-D confidence levels are:quark (top panel) ν -up quark (bottom panel) interactions. The best
fit points are marked with black stars.

10

5
Very Preliminary
1m

0.1

0.2

0.3

2m

3m

0.4
0.5 2 4 6 822sin (e )6r
12Fig. 18. The allowed contours for ∆m221 vs. sin2 θ12 from all solar neutrino data (green solid line). The allowed contour from KamLAND
is also shown in blue. The combined allowed region is shown in
red.

where where GF is the Fermi coupling constant and ne is thenumber density of electrons [10].
We tested ε11 and ε12 for ν -up quark interactions and ν down quark interactions using SK-I, II, III, and IV(1670 days)of data with Day/Night asymmetry and the SNO data. Figure 19 shows the observed spectra with the best fit curves
with NSI. The obtained constraints to ε11 and ε12 are shownin Fig. 20.
Summary
In summary, Super-Kamiokande has precisely measured
the 8 B solar neutrino flux, its time variation and recoil electron
spectrum. Using 5,695 days data, more than 93,000 solar neutrino interactions are extracted over the background. SK has

stably measured the solar neutrino flux for more than 21 years.
No significant correlation between the observed solar neutrino
flux and the sun spot number is found. The measured energy spectrum of 8 B solar neutrino is consistent within ∼1σwith the MSW up-turn for the solar best-fit parameters andmarginally consistent within ∼2σ with the MSW upturn for
KamLAND best-fit parameters. In the solar neutrino oscillation analysis combined with the KamLAND result, the oscil−5
lation parameters are determined as ∆m221 = 7.49+0.19−0.17 × 10
22eV , sin θ12 = 0.310±0.012. We also tested non-standard interactions with the observed solar neutrino energy spectrum,and obtained constraints to the effective parameters ε11 andε12 for ν -up quark and ν -down quark interactions.

Bibliography[1] John N. Bahcall and Roger K. Ulrich, Rev. mod, Phys.
60, 297 (1988).
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[3] Q.R. Ahmad et al., Phys. Rev. Lett. 87, 071301 (2001).
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13[5] S.P. Mikheyev and A. Y. Smirnov, Sov. Jour. Nucl. Phys.
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[10] A. Friedland et al., Phys. Rev. B 594, 347 (2004).
[11] O.G. Miranda et al., J. High Energy Phys. 10 008 (2006)[12] A. Renshaw et al., Phys. Rev. Lett. 112, 091805 (2014).
[13] K. Abe et al., Phys. Rev. D 94, 052010 (2016).
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[16] B. Aharmin et al., Phys. Rev. C 88, 025501 (2013).
[17] WDC-SILSO, Royal Observatory of Belgium, Brussels.
http://www.sidc.be/silso/datafiles[18] H. Nishino et al., Nucl. Instrum. Meth. A 610, 710(2009).
[19] K. Abe et al., Nucl. Instrum. Meth. A 737, 253 (2014).
[20] Y. Nakano, J. Phys. Conf. Ser. 888, 012191 (2017).
[21] S. Yamada et al., IEEE Trans. Nucl. Sci. 57, 428 (2010).
[22] Y. Nakano, PhD thesis, The Univ. of Tokyo (2016).
[23] R. Davis, Jr., D. S. Harmer, and K. C. Hoffman et al.,Phys. Rev. Lett. 20, 1205 (1968).
[24] J.N. Adburashitov et al., Phys. Rev. C 80, 015807(2009).
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[30] A. Gando et al., Phys. Rev. D 88, 033001 (2013).

Supernova neutrinos
In 1987, the observation of supernova 1987a by
Kamiokande and IMB etc, opened the neutrino astronomy.
This observation confirmed that the energy released by neutrinos is about several×1053 ergs. However, the core collapse supernova (ccSN) mechanism is not yet fully understood. SuperKamiokande (SK) would be able to detect several thousand
neutrino events if a ccSN happened near the center of ourgalaxy. Such an observation would enable us to investigate
in detail the mechanics of the ccSN explosion.
On average, 1-2 ccSNe per century are expected in ourgalaxy and therefore we must be prepared for these events.
An online program called SNWATCH searches for time clustered events [1]. Events with total energy greater than 7 MeV
and vertex position within the 22.5-kton fiducial volume in
SK are selected. Cosmic ray muons and their subsequent decay electron events are removed. For each selected event, a
20-second time window is opened backwards in time, and thenumber 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 an automatic phone-calling and emails to experts. Then, the experts check whether it is a real supernova
signal or not by looking at various plots which are uploaded toa secured site accessible from the Internet. These alarms areusually due to the accidental coincidence of two cosmic ray
induced clusters. We have supernova drills several times per
year. So far, no real supernova neutrino burst signal has been
observed at SK.
In a drill, the SNWATCH conveners and the executive
committee members meet via TV conference system, and discuss to make a decision for a prompt announcement (within
1 hour) to outside researchers and the press. We practice thisdrill as if a real supernova happened. We also have SK shifttraining by illuminating an LD in the SK detector a few times
every month. SK shift members are notified by a dummy
alarm that SNWATCH makes when the LD is illuminated.
The shift members then cal l to the SNWATCH experts andgive a report. The SK collaborators will be ready for the real
supernovae through the drill and the training.
For a single SN far from our galaxy, the neutrino flux is notlarge enough to be detected at SK. However, there is about one
ccSN in the universe every second which creates a copious andisotropic neutrino flux. Supernova Relic Neutrinos (SRN) are
this diffuse supernova neutrino background from all the past
ccSNe in the universe. This signal has never been detected,
but it is expected to be detectable in the 16-30 MeV energy
region, which is the gap between the energy ranges of solar
neutrinos and atmospheric neutrinos. Our published result [2]utilizes SK-I, II and SK-III data with analysis energy threshold of 16 MeV. A maximum likelihood search was performed
in multiple regions of the Cherenkov angle distribution to extract the most accurate flux limit. The obtained flux limit isbetween 2.7 and 3.0 ν̄ cm−2 s−1 (positron energy > 16 MeV),
which in fact depends on the shape of the neutrino spectrum
assumed. This result currently provides the world’s best limit

9
8
7
6

νe flux upper limit (cm-2 s-1 MeV-1)

Entries / 25µs

Entries / 1 MeV

14
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. 21. 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 the delayed 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.

on SRN flux. (Figure 22).
In SK-IV, a new result of the SRN search using the neutron tagging technique was also published [3]. In this analysis, neutrons from inverse beta decay (IBD) reactions (ν̄e , p →e+ , n) are captured on hydrogen. After a neutron is captured,a single 2.2 MeV gamma is emitted. Thus, by detecting theprompt positron signal and the delayed 2.2 MeV gamma signal, we can reduce backgrounds, most of which are not accompanied by neutrons.
Figure 21 shows the energy spectrum of prompt signal, thetime difference between a prompt signal and a delayed signal, and the detection efficiency of SRN for each energy bin.
Figure 22 shows the obtained flux limit comparing with otherresults. The neutron detection efficiency is very low becauseof the low energy of the gamma from the neutron capture onhydrogen (to compare the 2.2 MeV gamma with the analysis
kinetic energy threshold for solar neutrino of 3.5 MeV). However, we with this method we could obtain the world best limit
below 16 MeV. This result shows a high potential of neutron
tagging techniques, which can be a strong tool for SRN detection.

Bibliography[1] Abe, K. et al., Astropart. Phys. 81, 39 (2016)[2] K.Bays et al., Phys. Rev. D 85, 052007 (2012)[3] H.Zhang et al., Astropart. Phys. 60, 41 (2015)
EGADS: From R&D for the gadolinium project tostandalone Supernova monitor
As mentioned above, although at SK a few SRN events ayear are expected, SRNs have not been detected yet because

10-2

14

16

18

20

22

24 26 28 30
Neutrino energy (MeV)

Fig. 22. 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.

Fig. 23. In the new cavern, the 200-ton tank (a) with currently 240 photomultipliers installed, the Gd pre-mixing and pre-treatment 15-tontank (b), the selective filtration system (d), Gd removal resins (c)for test and a device to measure the water transparency (e) havebeen installed.

of the large backgrounds that constrain our search. The main
goal of our research is to reduce these backgrounds and beable to detect SRNs. The observation of SRNs in general or
neutrinos from distant supernovae in particular, would give
us some information about the universe, for example the corecollapse rate from SRNs, and about the neutrino itself too, forexample its lifetime.
As shown in the previous section, the current SK detector
can only detect positrons efficiently but not neutrons. In orderto achieve a high detection efficiency for neutrons, it is proposed to add 0.2% of gadolinium (Gd) sulfate by mass into
SK. Since Gd has a 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, neutrons could
be easily detected at SK (in space, vertices within tens of cm

15

and in time, with the neutron capture delayed about 20 µ sec).
EGADS (Evaluating Gadolinium’s Action on Detector
Systems) project was funded in 2009. The main motivationof EGADS is to show that by adding Gd, SK will be able todetect anti-neutrinos using the delayed coincidence technique,while keeping all its capabilities in the other analyses like solar and atmospheric neutrinos. Since then, a new hall near the
SK detector has been excavated and a 200-ton tank with its ancillary equipment has been installed, see Fig.23, to mimic theconditions at SK. The selective water filtration system filtersout water impurities while keeping the Gd in the water.
From January 2010 to July 2011 we circulated pure water
through the 200-ton tank and proved that our water systemis stable and achieves a high water quality. In 2013, fromFebruary 6th to April 20th, the 200-ton tank has been stepwise
loaded with Gd sulfate until the final 0.2% concentration wasreached. Two values are monitored at three different depths:
the Cherenkov light left after travelling 15 m (LL15) in waterand the Gd concentration. Both values confirmed that we canachieve and maintain a good water quality and that Gd sulfate
homogeneously and quickly dissolves in the 200-ton tank.
In summer 2013, we installed 240 photomultipliers andthe data taking started from September without Gd and with a
DAQ based on old SK ATM modules. In April 2015, the target
concentration of 0.2% Gd2 (SO4 )3 was achieved. Figure 24
shows the time variation of LL15. The blue band in the figure
shows typical values for SK-III and SK-IV. As shown in the
figure, the transparency of 0.2% Gd2 (SO4 )3 water is within
the SK range. In addition to the good water transparency, no
Gd loss has been detected since the EGADS detector reached
the final concentration until it was emptied again after about 2
years and 5 months, in October 2017
Detailed studies have evaluated the impact on current analyses at SK. These studies show that current analyses will bebasically unharmed after adding Gd in SK and all other tests
and studies conducted have shown no showstoppers. As a consequence, the SK collaboration decided in spring 2015 to approve the SuperK-Gd project. The SK tank has been opened
for refurbishment work from last year (2018) April 1st.
In order to test the future Gd-loading in SK, forseen for
2020, the detector was refilled with pure-water in November 2017, and loaded with 0.02% Gd2 (SO4 )3 in the end of
March 2018. This loading was performed using only the pretreatment and the fast recirculation systems, with one pass,similar to what is expected for SK 0.02% loading.
Since then, different configurations of the water systemhave been tested in order to determine the water systemneeded to ensure high water transparency in SK in case theband-pass system could not be installed in time. Figure 25
shows the time variation of LL15 since this loading until thepower cut of May 2019. With the fast recirculation system
only, from March 2018 to June 2018 a slow but continous decreased of the water transparency has been observed. From
June 2018 to August 2018 the use of the band-pass systemallowed to recover high water transparency. There-after the
fast-recirculation system only has been used.
A special cation exchange resin, developed to keep Gd in
the water while removing other cations, has been installed,
in addition to the usual anion exchange resin, in January-

February 2019 (in January, the new cation exchange resin wasused alone, leading a water transparency drop). Preliminary
results indicate an improvement of the water transparency
compared than with only anion exchange resin. This increase
was enhanced following the replacement of the TOC and UV
lamps in March 2019, which stressed the importance of theionisation lamps for the water transparency. This study is stillon-going.
In June 2017, EGADS electronics has been upgraded fromthe ATM electronics to SK’s QBee electronics, the DAQ system was also upgraded to use these new electronics. Thanks
to this upgrade, the stability of the DAQ has been greatly
improved, specially at high rate. Several SN tests were performed, and demonstrated that EGADS is able to detect andprocess a 10 second long burst of more than 100k Hz without trouble. This is much more than what is expected from avery close galactic SN burst: in case of Betelgeuse, about 25k
events are expected according to Nakazato’s model.
We developed an online fitter, HEIMDALL, based on SK
WIT. It is able to reconstruct all the events online during a SN
burst and then provides an alarm within few seconds. HEIMDALL is looking for IBD candidates, therefore taking advantage of the delayed coincidence to reduce the background.
Thanks to this, EGADS is able to look for SN in the far endof our galaxy. This enables us to instantly and autonomously
detect a galactic SN and report to local experts and neutrino
and astronomy community. This system was running during
the SK refurbishment work last year.
Tank Open Work 2018-2019
From May 31 to January 29, 2018, the tank open work to
upgrade the Super-Kamiokande (SK) detector was performed
after 12 years of continuous operations, Figure 26 shows aphoto of the tank on August 18, 2018.
There were five main tasks:
1. Fix a small water leak in the Super-K tank.
2. Clean any rust and other dirt that has accumulated in thedetector since its original completion in 1996.
3. Install additional water piping to increase the total water flow for increased water purification, and to enable
better control of the flow direction in the tank.
4. Replace the photomultiplier tubes (PMTs) that havefailed (a few hundred out of 13,000) since the previous
in-tank refurbishment in 2006.
5. Maintenance of detector calibration devices.
Leak fixing in the SK tank
A so-called “floating floor” was used to work at the wallsof the detector. During this phase of the refurbishment effort we drained two meters of water from the tank every threedays, and worked to seal potential water leakage points on theouter wall, as well as replace those failed PMTs that we could
reach from the floor. The Super-K tank is 40 m in diameterand 42 m in height and contains 50,000 tons of pure water. Its
sides form an icosagon comprised of 400 pieces of 4 mm thick16

Fig. 24. Cherenkov light left at 15 m for Gd loaded water in EGADS until September 2017. The horizontal blue band are the typical values for
SK-III and SK-IV. The vertical lines shows the injection dates where we also indicate the concentration (% in mass) in the 200-ton tank. The
black dashed line shows the final expected concentration.

Fig. 25. Cherenkov light left at 15 m for Gd loaded water in EGADS since December 2017. The horizontal blue band are the typical values for
SK-III and SK-IV. The vertical lines shows the injection dates where we also indicate the concentration (% in mass) in the 200-ton tank. The
black dashed line shows the final expected concentration.

stainless steel plates, each 4 m wide and 2 m tall. The bottom
also consists of an assembly of stainless steel plates. The total
submerged surface area is more than 6,000 m2 and the total
length of welding lines is more than 6.2 km. More than 3,200
bolts penetrate the sides. Because of this complexity, a small
amount of water leaked from the SK tank, which we could not

fix for a long time. This is not serious as long as we use only
pure water. Although there is no official regulation regarding
Gd-loaded water, it is necessary to stop the SK water leak to
avoid release even in the case of a large earthquake.
To achieve a reliable fix, Super-K colleagues have developed a special sealant material. It needed to be soft, flexible,

17

then degrade the water after restarting operation of Super-K.
The water team tried, investigated, combined, and developed
many kinds of tools.
We maintained excellent quality of sealant attachment andremoved a large amount of radioactivity; at least 100 Bq ofradium was removed during the cleanup according to a measurement by a germanium detector. As the radon gas emitted
from this radium is comparable to that arising from the PMTs
themselves, the in-tank shiftworkers contributed to lowering
the background for future solar neutrino observations.

Fig. 26. Working inside the Super-Kamiokande tank on August 18,
2018.



Fig. 27. Sealant work on the SK tank wall being conducted from thefloating floor.

and able to withstand external stretching forces. It had to meetrequirements regarding small elution into water, low naturalradioactivity, long-time stability during water exposure, andsufficient attachment strength to stainless steel plates.
Figure 27 shows the actual sealant work. Workers put
thick tape along both sides of the welding line to hold thesealant material on the line. They used trowels and paintbrushes to spread the sealant material over the weld with thesame thickness as the surrounding tape. A double layer ofsealant was required to allow for unexpected gaps or pinholes
in one of the two layers.
Cleaning and rust removal
The dirty wall and structures inside the Super-K tank required a large amount of person power to clean up. In particular, the top and outermost structures were covered by aheavy residue of exhaust gas and dust left over from when
we constructed the detector in 1991-1996. We also aimed toremove the most potential sources of rust during the cleaning work. As for tools, it was dangerous to choose arbitrary
ones since most of them leave other types of dust which may

Water system upgrade
The SK water system, including in-tank piping, was upgraded during the tank open period. The water purification
system was originally designed with a supply of 30 m3 /h andcirculation of 60 m3 /h, and the piping inside the tank wasmade without separate lines for the inner detector (ID) andthe outer detector (OD). This time, we decided to improve thewater flow in the tank by separating not only the pipes of theinner volume and the outer volume, but also the pipes on theside, top, and bottom of the outer volume so that all the flow
rates can be adjusted independently. Up until now the effective detector volume for the solar neutrino analysis has been
limited by the inflow of radon background, but this plumbing
improvement makes it possible to control and optimize theflow of water in the tank.
We started water filling on October 13, 2018. Previous water fills have taken place at a rate of 30 m3 /h using the original
SK water purification system; this took 2.5 months to fill thetank. Since the water that had been fed to the tank was nevercirculated and repurified until after the tank became full, it
took another two months or so after the tank was full before
the water quality improved to the point that useful data could
be collected. This time, as shown in Figure 28, the new SK-Gd
water system is to be temporarily operated as an ordinary water purification/recirculation system by installing conventional
ion exchange resins made for producing ultrapure water into
the modules in the system. Water filling was stopped in the
middle of December for the remaining work in the top regionof the tank. After the final work, water filling was restarted
in January, 2019 and completed on January 29. The watertransparency was measured as soon as data taking started andconfirmed that the transparency was actually good and similar
to that of the end of SK-4.
PMT exchange and other work
During this tank open period, in parallel with sealing
work, the exchange of dead PMTs was also done. In total,
136 ID PMTs and 224 OD PMTs were exchanged.
For ID PMT replacement, new PMTs under developmentfor Hyper-Kamiokande (HK) were used. In addition to the recovery of SK performance, another purpose of this exchange
is to check the performance and long term stability of HK
PMTs under the expected actual operating environment. Similarly to other SK ID PMTs, anti-implosion covers made offibre-reinforced plastic (FRP) and acrylic were attached tothese PMTs by shift workers. The exchange of ID and OD
PMTs (Figure 29), including cable connections inside the tank

18

SK-Gd Water system

SK Water system

Fig. 30. OD Tyvek installation work.
Fig. 28. New filling with circulation system using the SK-Gd system.

and the following signal checks, was also performed by shift
workers with a well coordinated time schedule to avoid interference with the sealant work. Another important task carried out during the tank open work is related to the calibration
system inside the tank and studies for more precise detector
understanding, such as replacement of laser injectors and optical fibers, installation of a new light injection system, precise
magnetic field mapping, and recording all PMTs dynode directions. The installation work of the white Tyvek sheet thatcovers the OD region for high light detection efficiency was
also a significant job. With much effort by many workers, all
the sheets covering the tank wall were removed for the sealantwork and re-installed afterwards (Figure 30).

Fig. 29. Exchange work of the ID (top two panels) and OD (bottom
two panels) PMTs.

Water leak check
After filling the tank completely with pure water, a waterleakage measurement was conducted. The method monitors
the water level to 0.1 mm precision while water circulation is
stopped. The water circulation system was stopped between
11:30 on January 31, and 15:52 on February 7, 2019; the total
measurement time was 7 days, 4 hours, 22 minutes. Over
this period, we could not see any decrease of the water level.

Therefore, we did not observe any water leakage from the SK
tank within the precision of our measurement, which is lessthan 0.017 tonne per day. When the same measurement was
performed one year ago, the water level decreased at a rate of∼3 mm per day, corresponding to a leak rate of ∼4 tonne perday. As a result, we have concluded that the current SK waterleakage has been reduced to less than 1/200 of the rate during
the period before the tank upgrade work.

T2K EXPERIMENT[Spokesperson : Atsuko Ichikawa] Kyoto University
The Tokai to Kamioka (T2K) experiment [1] is a long
baseline neutrino oscillation experiment: a man-made beamof neutrinos is used to do precise studies of the oscillations ofneutrinos. Accelerated protons are used to produce the neutrinos in the J-PARC center in the Ibaraki prefecture, which thentravel 295 km to reach the Super-Kamiokande (Super-K) detector in the Gifu prefecture where they 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 was the first long baseline experiment to use the off-axis beam technique [2]: the beam is not
aimed directly at Super-K, but in a direction making a 2.5◦ angle with the far detector direction. This gives increased sensitivity to neutrino oscillations while reducing the backgrounds
by producing a narrow band neutrino beam centered on theenergy corresponding to the first maximum of the νµ → νeoscillation probability.
T2K realized the first observation of the appearance of aflavor of neutrinos through oscillation by detecting electron
neutrinos in a beam of neutrinos produced in the muon flavor [3]. After achieving this milestone, the experiment has
been using its ability to produce a beam of either neutrinos
or anti-neutrinos to compare the oscillations of neutrinos andtheir antiparticles. This allows to study the main remaining
open questions in neutrino oscillations (CP symmetry and or-19dering of the neutrino mass states) by looking at the differences between the oscillations νµ → νe and ν̄µ → ν̄e . At
the same time, T2K’s physics goals include the precise measurement of the neutrino oscillation parameters θ23 and ∆m232
through a precise study of the pattern of disappearance of themuon neutrinos in conjunction with the data used to study theoscillation to the electron flavor. The experiment additionally performs searches for physics beyond the standard model,such as oscillations due to sterile neutrinos and CPT violation.
Finally, the near detectors are used to perform a wide range ofneutrino and anti-neutrino cross-section measurements.
In J-PARC, protons are accelerated to 30 GeV by a seriesof three accelerators, and hit a 1.9 interaction-length graphitetarget. 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, whilehigh energy muons (5 GeV/c or higher) can pass through thisbeam dump and are measured to provide a first, indirect monitoring of the neutrino beam. The horns can be used either witha positive current (ν -mode), in which case the beam is mainly
made of νµ , or with a negative current (ν̄ -mode) which gives
a mainly ν̄µ beam.
The near detectors are separated into two groups. On theaxis of the beam, the INGRID detector, made of fourteen
identical modules is used to monitor the beam direction andrate stabilities. Each module is made of an succession of ironplates to provide large target mass ( 7.1 tons per module) alternating with scintillator planes for detection. Using the number of events reconstructed in 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 moredetailed measurements of the unoscillated neutrino beam. In
neutrino oscillation analyses, the ND280 is used to provide
information on the νµ and ν̄µ unoscillated spectra directed at
SK, constrain the dominant backgrounds, and constrain the
combination of flux and interaction cross sections.
The far detector, Super-K, is a 50 kton water Cherenkov
detector, shielded from atmospheric muons by 1000 m ofrock, 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 and electron-likeevents based on the light pattern of this ring, and additional
selection cuts are applied to produce samples enriched in certain interaction modes.
To study neutrino oscillations, the data observed at the far
detector are compared to the predictions of the three-flavor
oscillation model for different values of the oscillation parameters. To this end, a model of the experiment is constructed:
the fluxes of the different flavors of neutrinos reaching the detectors are predicted by a series of simulations, and the in-Table 4. T2K data taking periods and integrated numbers of protons
on target (POT) used in the far detector analysis.

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

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

teractions of ν and ν̄ in the detectors are simulated using the
NEUT Monte Carlo event generator [7]. First, the flux andproperties of the proton beam reaching the target are measured by the proton beam line monitors. Interactions of the
protons in the graphite target and production of secondary
hadrons are then simulated using the FLUKA package [4].
Measurements from hadron production experiments, in particular NA61/SHINE [5], are used to tune this part of the simulation and the out-of-target interactions. The propagation anddecay in flight of the hadrons in the decay tunnel are then simulated using the GEANT3 and GCALOR [6] packages. The
predictions from this model are compared to the data observedin the near detectors to tune the predictions for the far detector by constraining the model parameters. The result of thisnear detector fit provides the initial values and uncertainties
of the flux and interaction model parameters used in the far
detector analysis to measure parameters describing neutrino
oscillations.
T2K started collecting physics data in January 2010, andhas now completed its 9th run, accumulating a total of 1.51 ×
1021 protons on target (POT) in ν -mode and 1.65 × 1021 POT
in ν̄ -mode. The details of the data used in the far detector
analysis (slightly lower due to data quality cuts) can be foundin table 4. Over this period, the event rates and the beam direction were found to be consistent with the expectations andstable by the measurements of the muon monitor and the onaxis near detector. In particular, the beam direction remained
stable well within the ±1mrad target. During run 9, stable operation at 485 kW was achieved, and operation at more than
500 kW was successfully demonstrated.
Search for CP violation in Neutrino and Antineutrino
Oscillations by the T2K experiment with 2.2×1021
protons on target
T2K published new neutrino oscillation results using datacollected from January 2010 to May 2017 (corresponding to
Run 1 to 8 on table 4) [10]. The data sets include a beam exposure of 14.7×1020 POT in neutrino mode and 7.6×1020 POT
in antineutrino mode for the far-detector (SK) analysis and an
exposure of 5.8×1020 POT in neutrino mode and 3.9×1020
POT in antineutrino mode for the near-detector (ND280) anal-

20
T2K Run1-830
Normal25

Inverted-2Δln(L)ysis. T2K has previously reported that the CP conservation
hypothesis (δCP = 0, π ) is excluded at 90% C.L. using thedata collected up to May 2016 [8, 9]. Since then, the neutrino
mode data set has doubled, and the electron neutrino and antineutrino event selection efficiencies have increased by 30%
and 20%, respectively, improving the ability of the experiment
to study CP symmetry in neutrino oscillations. A significant
improvement compared to previous T2K publications comesfrom the use of a new Cherenkov-ring reconstruction algorithm, previously used only for neutral current (NC) π 0 background suppression, and optimized event selection criteria. As
a result, the acceptance for signal events in the muon-like samples (corresponding to charged current quasi-elastic (CCQE)
interactions) increased by 15% with a 50% reduction of themain neutral current background (NC1π + ). The charged current events acceptance also increased by 20% for the nominal (CCQE-like) electron-like samples with similar purity toprevious analyses. Finally, the acceptance of signal events increased by 33% with a 70% reduction in background causedby particle mis-identification for the electron-like sample enriched in resonant interactions (CC1π + ).
A particular effort was done to improve the neutrino interaction uncertainty model, and to understand the possible
biases in the oscillation measurements coming from those
uncertainties. The multi-nucleon contribution to CCQE-likeevents is of particular importance in T2K, as it leads to abias in the neutrino reconstructed energy used to measure theparameters describing neutrino oscillations. Additional systematic parameters describing different uncertainties on thosemulti-nucleon processes, as well as parameters designed tocover the theoretical uncertainty in the transferred momentum dependence of the random phase approximation (RPA)
corrections to the CCQE interaction model were added to theanalysis. Some systematic uncertainties are not easily implemented by varying model parameters. A new procedure, “simulated data” studies, was used to study their impact on the oscillation measurements: data generated from a variant model
are analyzed under the assumptions of the default model, andthe results obtained with both models are compared to estimate the potential bias coming from this model uncertainty.
Fits to these simulated data sets showed no significant biases
in the parameter measuring CP asymmetry in neutrino oscillation , δCP . However, biases in some of the other parametersdescribing the oscillations (∆m232 and sin2 θ23 ) were seen, andadditional uncertainties were added in the fits to take those
into account.
The fit of the run 1-8 data yielded best-fit values and+0.0301σ errors for sin2 θ23 and ∆m2 of 0.526+0.032−0.036 (0.530−0.034 )+0.0712 4−3−3
and 2.463−0.070 ×10 (2.432±0.070×10 ) eV /c respectively for normal (inverted) ordering. The result is consistent
with maximal disappearance and the posterior probability forθ23 to be in the second octant (sin2 θ23 > 0.5) is 78%. The∆m2 value is consistent with the Daya Bay reactor measurement [11]. The measurement of sin2 2θ13 using only T2K data
was found to agree well with the results of the reactor experiments.
Confidence intervals for δCP were calculated using theFeldman–Cousins method [12] from a fit using the result of
the reactor measurement as a prior for sin2 2θ13 . The best fit

20
15
10
5
0

−3

−2

−1

0

δCP

1

2

3

Fig. 31. Results of the fit of T2K run 1-8 data for δCP assuming normal (black) and inverted (red) mass ordering, using the reactor
measurement prior on sin2 (2θ13 ). The vertical lines show the corresponding allowed 2σ confidence intervals, calculated using theFeldman-Cousins method.

value is δCP = −1.87(−1.43) for the normal (inverted) ordering, which is close to maximal CP violation (Fig. 31). The
δCP confidence intervals at 2σ (95.45%) are (−2.99, −0.59)
for normal ordering and (−1.81, −1.01) for inverted ordering.
Both intervals exclude the CP-conserving values of 0 and π ,meaning that the significance of the exclusion of the conservation of CP symmetry in neutrino oscillations has increased
from the 90% CL level previously reported to the 2σ level.
Concerning the mass hierarchy, the normal ordering is preferred with a posterior probability of 87%.
Search for light sterile neutrinos with the T2K far detector Super-Kamiokande at a baseline of 295 km
The data from most experiments studying neutrino oscillations are consistent with the three flavor paradigm where thethree weakly interacting neutrino flavors are related to threeneutrino mass states by the Pontecorvo-Maki-NakagawaSakata mixing matrix. However, deviations from the threeflavor scheme have been reported, and could be explained bya fourth neutrino state with a mass difference ∆m2 ∼ 1 eV2 /c4
with respect to the three Pontecorvo-Maki-Nakagawa-Sakata
states [13, 14, 15]. While T2K is designed for studying standard three flavor oscillation at ∆m2 ∼ 10−3 eV2 /c4 , it also has
the potential to search for oscillation signatures due to sterile neutrinos around this ∆m2 range. T2K performed a longbaseline search for sterile neutrinos in the “3 + 1” frameworkusing the run 1 to 8 data [16].
Neutral-current (NC) neutrino interactions are also collected at the far detector. These events have previously only
been used in publications for systematic uncertainties [10] andcross-section studies [17], but they can also be used to enhance the sensitivity to sterile mixing as the sterile neutrinos,
unlike other active neutrinos, do not interact through CC or
NC scattering. In this analysis, NCπ 0 and NC γ -deexcitation
samples are used in the oscillation fit to enhance the sensitivity to sterile mixing parameters. The NCπ 0 samples selectneutrino events with single π 0 production, where π 0 → 2γ decay produces two visible Cherenkov rings in the detector. The

21

Fig. 32. The T2K 90% exclusion limits on sin2 θ24 as a function of∆m241 , with results from other experiments [18, 19, 20]. The areas
on the right are excluded.

NC γ -deexcitation sample targets NC quasi-elastic events inwhich γ rays are emitted from residual nucleus deexcitation
and when knocked-out nucleons interact with other nuclei inwater. The overall analysis strategy is similar to that used in
the standard T2K three flavor analysis, but with the addition of
the three NC samples described previously to the 5 CC samples used in the standard analysis.
We considered the parameter space of ∆m241 > ∆m221 which
is most sensitive in T2K. The “3 + 1” best fit differs from thestandard three flavor best fit by ∆χ 2 = 1.0 (4.7) assuming normal (inverted) mass hierarchy. From 2500 sets of MC studies
with statistical fluctuations, this level of disagreement is expected with the standard three flavor hypothesis in 50% (30%)
of the studies. Fig. 32 shows the T2K 90% exclusion limits inthe (sin2 θ24 , ∆m241 ) plane together with results from other experiments, where we can see that T2K has set the most stringent limit on sin2 θ24 for ∆m241 < 3 × 10−3 eV2 /c4 . The NC
samples allow us to constrain θ34 in conjunction with θ24 . Because these samples have low statistics and large cross-section
uncertainties, the sensitivity is limited, but our results are consistent with other measurements. We constrain sin2 θ24 < 0.1
and |Uτ 4 |2 = cos2 θ24 sin2 θ34 < 0.5 at 90% C.L if ∆m241 = 0.1
eV2 /c4 is assumed.
In summary, the data are consistent with the standard threeflavor oscillation hypothesis. Limits have been set on thesterile mixing parameters, with the world’s best constraint on
sin2 θ24 for 10−4 eV2 /c4 < ∆m241 < 3 × 10−3 eV2 /c4 . Our current precision is restricted by statistics and the uncertainty on
the NC interaction cross section. Apart from future updates of
the analysis as we take more data, dedicated systematic studies are required for further improvements to the precision. Another possible extension is to perform a joint analysis of nearand far detector data that would expand the range of constraint
to ∆m241 ! 1 eV2 /c4 with additional data at smaller L/E.

Measurement of inclusive double-differential νµcharged-current cross section with improved acceptance in the T2K off-axis near detector
In addition to the oscillation measurements, T2K has an
ongoing program to study neutrino interactions using the neardetector complex in order to improve the understanding andmodeling of these interactions. Previously, T2K reported themeasurement of the flux-integrated double-differential crosssection for muon neutrino charged-current interactions on carbon as a function of the muon momentum and angle [21].
Since that time, many improvements have been made in theanalysis. New results, obtained with more data, reduced neutrino flux uncertainties (thanks to new NA61/SHINE measurements [5]), increased angular acceptance, reduced backgroundcontamination, and a different unfolding method were presented this year [22].
This new analysis uses data collected in the near detector in neutrino running mode between November 2010 and
May 2013. The total sample comes from 5.7 ×1020 protons on target, which is a factor of 5 larger than that usedin the similar previously published analysis from T2K [21].
Events containing muons emanating from interactions that occur in the fiducial volume of the first Fine Grained Detector(FGD1) are selected, and the cross section results are presented based on the kinematics of this outgoing muon. In previous T2K work on this topic, the analysis was optimized to
select forward-going muons originating from FGD1 and making a long track. This new work aims to include the so-called
high-angle tracks, as well as long backward-going tracks. The
addition of backward-going muon candidates in the event selection only became possible with the introduction of timing
information correlated between sub-detectors.
Two event generators, NEUT 5.3.2 [7] and GENIE
2.8.0 [23], are used to simulate the interaction of neutrinosin the near detector and the effect of the nuclear medium on
the produced particles. The efficiency as calculated in NEUT
and GENIE is generally in agreement. However, the predicted efficiency is different for low momentum muons goingvery forward with respect to the neutrino direction. The fluxintegrated total cross section is computed by integrating boththe number of signal events and the signal efficiency over themuon phase space,
σDATA FIT W /NEUT = (6.950 ± 0.049[stat] ± 0.123[syst] ±0.608[flux]) × 10−39 cm2 nucleon−1
σDATA FIT W /GENIE = (6.850 ± 0.048[stat] ± 0.121[syst] ±0.599[flux]) × 10−39 cm2 nucleon−1 .
This is compatible with predictions from the two event generators: σNEUT = 7.108 × 10−39 cm2 nucleon−1 and σGENIE =6.564 × 10−39 cm2 nucleon−1 .
The flux-integrated, double-differential cross section iscomputed as a function of the outgoing muon kinematics andthe result is compared to the NEUT and GENIE predictions(Fig. 33). In the new regions of phase space (high-angle andbackward-going muons), there is good agreement, but uncertainties are still large. For forward-going muons, the binningis finer, and interesting structures are observed.

22

0.01
10

pµ [GeV/c]

4
2
0
10-1

6
4
2
0
10-1

1

1

10

pµ [GeV/c]

-39

[10

10

-39

cm2
]
nucleon GeV/c

1210pµ [GeV/c]

8

x100

6
4
2
0
10-1

1

10

pµ [GeV/c]

-39

[10

4

cm2
]
nucleon GeV/c

0.985 < cos θµ < 1x20

d2 σ f.i.

2
1

x150

8

0.96 < cos θµ < 0.985

66

0.92 < cos θµ < 0.96

[10

cm2
]
nucleon GeV/c
-39

[10

d2 σ f.i.

10pµ [GeV/c]

8

0
10-1

12
10

dpµ dcos θµ1

10pµ [GeV/c]

14dpµ dcos θµ

cm2]
nucleon GeV/c
-39

[10

d2 σ f.i.

dpµ dcos θµ

[10

-39

cm2
]
nucleon GeV/c

410dpµ dcos θµd2 σ f.i.

x200

6
2
0
10-1

1

x200

8

0.87 < cos θµ < 0.92

14

8

cm2
]
nucleon GeV/c

2
0
10-1

10

pµ [GeV/c]

10d2 σ f.i.

4

0.8 < cos θµ < 0.87

10pµ [GeV/c]

12dpµ dcos θµ

2

x300

6

1

14

-39

8

-39

cm2
]
nucleon GeV/c10d2 σ f.i.

d2 σ f.i.

dpµ dcos θµ

4

12
10

2

0.71 < cos θµ < 0.8

12

[10

[10

x500

1

10

pµ [GeV/c]

14dpµ dcos θµ

8

-39

cm2
]
nucleon GeV/c

10

0
10-1

1

x300

4

0.6 < cos θµ < 0.71

12

6

6

0
10-1

d2 σ f.i.

10.45 < cos θµ < 0.6x200dpµ dcos θµ

0
10-1

4
3
2
1
0
10-1

8

d2 σ f.i.

d2 σ f.i.

d2 σ f.i.

0.02

10

dpµ dcos θµ[10
dpµ dcos θµdpµ dcos θµ

[10

0.03

0.25 < cos θµ < 0.45

8
7
6
5

-39

-39

0.04

cm2
]
nucleon GeV/c-0.25 < cos θµ < 0.25

0.05

[10

-1 < cos θµ < -0.25
cm2]
nucleon GeV/c

cm2]
nucleon GeV/c

0.06

10

pµ [GeV/c]

8
7
6
5
4
3
2
1
0
10-1

DATA FIT W/ NEUT (SYST)x10

DATA FIT W/ NEUT (SYST+STAT)DATA FIT W/ GENIE (SYST+STAT)
NEUT 5.3.2 (χ2 = 218.7)

110pµ [GeV/c]

GENIE 2.8.0 (χ2 = 192.0)

Fig. 33. The flux-integrated, double-differential cross section per nucleon for NEUT (continuous red line), for GENIE (dashed red line),
and the unfolded-data result using as a prior either NEUT or GENIE. The bin of highest momentum is scaled by the factor shownin each plot to make it visible. χ 2 values are computed with theunfolded-data result using NEUT as a prior.

Characterization of nuclear effects in muon-neutrino
scattering on hydrocarbon with a measurement of
final-state kinematics and correlations in chargedcurrent pionless interactions at T2K
Experiments studying neutrino oscillations such as T2K
use neutrino interactions on nuclei to provide evidence of neutrino oscillations and to search for leptonic CP-symmetry violation. The systematic uncertainties arising from neutrinonucleus interactions, especially those related to nuclear effects, are currently one of the limiting factors for oscillation measurements in T2K and NOvA, and will become thedominant uncertainties for future long-baseline experiments,
such as DUNE and Hyper-Kamiokande. T2K presented measurements of final-state proton multiplicity, muon and proton
kinematics, and their correlations in charged-current pionlessneutrino interactions, measured by the T2K ND280 near detector in its plastic scintillator target [24]. Thanks to their
exploration of the proton kinematics and of imbalances between the proton and muon kinematics, the results offer anovel probe of the nuclear-medium effects most pertinent tothe (sub-)GeV neutrino-nucleus interactions that are used inaccelerator-based long-baseline neutrino oscillation measurements.
The measurements use data taken between years 2010 and2013, corresponding to approximately 6×1020 protons on target. Three different analyses were performed, each of themmeasuring differential cross sections as a function of different
observables to study the kinematics of the outgoing muon andprotons in charged-current events without pions in the final
state (CC0π ). In particular, one of them uses single transverse
variables (STV) [25], which are built specifically to characterize, and minimize the degeneracy between, the nuclear effects
most pertinent to long-baseline oscillation experiments. Eachof the analyses take different approaches when extracting across section from the selected events. All of these methods

Fig. 34. The extracted differential cross section as a function of the
STV δ pT compared to different initial state models in the NuWro
11q simulation. The NuWro 11q RFG + RPA prediction shownis similar to the NEUT model used as a starting point for T2K’s
oscillation analyses. The inlays show the same comparisons on alogarithmic scale.

involve an effective background subtraction; an efficiency correction; and the deconvolution of detector effects either by abinned likelihood fit for two of them, or an iterative unfolding
procedure for the last one.
Many differential cross-sections are measured, and the results are compared to different neutrino-nucleus interaction
models which all fail to describe at least part of the observed
phase space. In particular the relativistic Fermi gas (RFG)
model, widely used for the analysis of the data of neutrino oscillation experiments, is able to describe only a very limited
region of phase space and is categorically disfavored when
considering the results as a function of one of the STV, δ pT
(Fig. 34). In case of events without a proton above the detection threshold in the final state, a fully consistent implementation of the local Fermi gas (LFG) model with multinucleon interactions gives the best description of the data. In the case ofat least one proton in the final state, the spectral function (SF)model agrees well with the data, most notably when measuring the kinematic imbalance between the muon and the protonin the plane transverse to the incoming neutrino. Within themodels considered, only the existence of multinucleon interactions are able to describe the extracted cross section withinregions of high transverse kinematic imbalance. The measurement of neutrino-nucleus interactions with a pionless final state with protons clearly shows the potential to provide
an even more detailed characterization of nuclear effects inneutrino-nucleus scattering in the future. To this aim, larger
statistics are needed, alongside more robust predictions of outgoing proton kinematics in multi-nucleon interactions (2p2h)
and final state interaction models.

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23

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XMASS EXPERIMENT[Spokesperson : Shigetaka Moriyama]
Kamioka Observatory, ICRR, the University of Tokyo
Introduction
The XMASS project is designed to detect dark matter,neutrinoless double beta decay, and 7 Be/pp solar neutrinos using highly-purified liquid xenon (LXe) scintillator in an ultralow radioactivity environment [1]. The advantages of using
LXe are a large amount of scintillation light yield, scalabilityof the size of the detector mass, an easy purification to reduce internal radioactive backgrounds (BGs), shielding ability against radiations from outside of the detector due to a highatomic number (Z = 54). The detector with ∼830 kg of LXe
has been constructed in September 2010. After completionof the detector, commissioning data was taken from December 2010 to May 2012. We published results from searches
for some dark matters [2] [3] [4], solar axions [5], and twoneutrino double electron capture on 124 Xe [6]. We also studied
a possibility to detect galactic supernova neutrinos via coherent elastic neutrino-nucleus scattering [7].
During the commissioning data-taking, we found that amajority of events at low energy originated from radioactive
contamination in the aluminum seal of the photomultipliertube (PMT) window. In order to minimize the BG contribution, detector refurbishment was conducted. The contaminated parts of PMTs were covered by copper rings and plates
in order to stop scintillation lights and radiations caused by its
contamination. PMT windows were cleaned by nitric acid andcopper parts were electropolished in order to remove possible surface contamination. After a year of detector refurbishment, data-taking resumed in November 2013 and continued
for more than five years. We then completed the data taking in
February 2019. We have published results from the update ofsearches for annual modulation from dark matters [8] [9], forsolar Kaluza-Klein axions [10], and for two-neutrino double
electron capture on 124 Xe [11].
In the following sections, we introduce the XMASS-I detector briefly and report the latest physics results from the
XMASS data collected after the refurbishment.
The XMASS-I detector
XMASS-I is a single phase LXe scintillator detector located underground (2700 m water equivalent) at the Kamioka
Observatory [12]. Fig. 35 shows a schematic drawing of the
XMASS-I detector. It contains ∼830 kg of LXe in an active region. The volume is viewed by 630 hexagonal and24Electronics
hut

Water tank

OFHC copper vessel
Refrigerator
10.5m
642
PMTs
~830kg
xenon
10m

~1.2m

72 50cm PMTs (veto)Fig. 35. Schematic drawing of the XMASS-I detector.

12 cylindrical Hamamatsu R10789 PMTs arranged on an 80
cm diameter pentakis-dodecahedron support structure. These
PMTs were developed to achieve low background requirement [13]. The largest contributions to the reduction of radioactivity came from the stem and the dynode support. The
glass stem was exchanged to the Kovar alloy one and the ceramic support were changed to the quartz one. R10789 is
the first model of Hamamatsu Photonics K. K. that adopted
these materials for low background purposes and provided
a groundbreaking step for further reductions of radioactivity
in PMTs. A total photocathode coverage of more than 62%
is achieved. The spherical arrays of PMTs are arranged ina double wall vessel made of oxygen free high conductivity(OFHC) copper. The waveforms in each PMT are recordedwith CAEN V1751 waveform digitizers with 1 GHz sampling
rate and 10 bit resolution. The detector is calibrated regularly
with a 57 Co source inserted along the central vertical axis of
the detector and external 60 Co source. By the data taken withthe 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 were used: keVee represents an electron equivalent energy, and keVnr denotes the nuclear recoil
energy. Scintillation decay time constant was investigated inliquid xenon with the XMASS detector. These are summarized at [14] and [15].
In order to shield the LXe detector from external gammas, neutrons, and muon-induced BGs, the copper vessel wasplaced at the center of a φ 10 m× 10.5 m cylindrical tank filledwith pure water. The water tank is equipped with 72 Hamamatsu R3600 20-inch PMTs to provide both an active muon
veto and passive shielding against these BGs. XMASS-I isthe first direct detection dark matter experiment equipped withsuch an active water Cherenkov shield. The LXe and water
Cherenkov detectors are hence called an Inner Detector (ID)and an Outer Detector (OD), respectively.
Dark matter search in a fiducial volume [16]Weakly Interacting Massive Particles (WIMPs) are one of
the well-motivated dark matter candidates and are thought tobe observable through nuclear recoils in the target material. A
number of experimental searches have been carried out, however, no indication of such a signature has been observed yet.
Considering the latest experimental constraints on the WIMP-

nucleon cross section, the detectors have to have a low energy
threshold, ultra-low BG, and a large target mass.
We performed a search for WIMP dark matter in a fiducial volume using 705.9 live days of data collected between
November 2013 and March 2016. The event vertex was reconstructed with two different methods: one based on hits’ timing
and the other based on the PE distribution. The reconstructedradial positions by these methods are referred to as R(T ) andR(PE), respectively. The position resolution of R(T ) is worse
than that of R(PE) since traveling time of the scintillation
light across the detector volume is not much larger than the
scintillation time constant or PMT timing response. However,requiring R(T ) < 38 cm eliminated some surface events thatwere often mis-reconstructed by the PE-based reconstruction.
Then, a fiducial volume containing 97 kg of xenon was established by requiring R(PE) < 20 cm. The event rate after theselection was (4.2 ± 0.2) × 10−3 event/day/kg/keVee around
5 keVee with the signal efficiency of 20%.
In order to extract the WIMP signals, the amount of BG
and its systematic error were evaluated from a detailed detector simulation verified by various detector calibrations. The
assumed radioactive BG was classified as either radioisotopes(RIs) dissolved in the LXe, 210 Pb in the detector’s inner surface, or RIs in other detector material.
For the RIs in the LXe, 222 Rn, 85 Kr, 39 Ar, and 14 C areconsidered. The 222 Rn amount was measured to be 10.3 ±0.2 µ Bq/kg using the 214 Bi-214 Po coincidence. The 85 Kr contamination was found to be 0.30±0.05 µ Bq/kg by the β -γ coincidence in its decay. The 39 Ar and 14 C concentrations were
evaluated by the spectrum fitting in a larger fiducial volumeof R(PE) < 30 cm in the 30–250 keVee energy range where
no contribution from possible WIMP-induced nuclear recoils
appear. The 210 Pb contamination at the detector surface was
evaluated by α -ray events extracted from the full volume of
the LXe detector. The α -ray events were selected based on
their shorter scintillation decay time. Then, the ratio of the
maximum number of PEs on a single PMT to the total number of PEs in the event was used to discriminate α -ray events
originating from the PMT’s quartz window surface, the copper
surface, or the bulk of the copper. The estimated concentrationof 210 Pb in the copper bulk was 25 ± 5 mBq/kg, which wasconsistent with the measured value of 17–40 mBq/kg with a
low-BG α -ray counter [17]. The RIs in other detector material
were evaluated by the spectrum fitting using the full volume of
the LXe detector. All the detector components except for thecopper and the LXe were assayed with high-purity germanium(HPGe) detectors and the results of these measurements wereused as initial values and their uncertainties as constraints forthe full volume spectrum fit. The spectrum above ∼400 PE
was fit to determine the activities of the RIs. Fig. 36 shows acomparison of the full volume spectrum and the expected BG
spectrum corresponding to the best-fit. The γ -rays from the
PMTs were the largest BG source in the full volume data.
Fig. 37 shows the expected BG spectrum in the fiducial
volume. The dominant contribution comes from 210 Pb in thebulk of the copper and the RIs in the PMTs. The size of thesystematic uncertainty is also shown in the figure. The largest
contribution comes from the uncertainty of the detector geometry such as gaps between the copper plates, the roughness of

500

20
18

LXe

[/day/keVee/kg]

event/day/kg/800PE

25Energy scaled by light yield[keVee]
1000
1500
2000
PMT aluminumseal

16
Pb210 surface

Detector vessel,

14

holder and filler

1210Pb210 copper plate

PMT (except

and ring

aluminum seal)

0.006

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

0.005
0.004
0.003
0.002
0.001

WIMP

60 GeV/c2, 2.2 × 10-44 (90% CL)

0
0

8

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20

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25
30
Energy [keVee]

4

5000

1.6
1.4
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10000

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20000

15000 20000

25000

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35000

25000 30000 35000

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40000
PE

[/day/kg/keVee]

PMT aluminumseal

Pb210 surface

0.004

Detector vessel,

10

-42

10

-43

10

-44

10

10-45
-46

10

10-47

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

XMASS (2013)

-41

ork

1

10

8)
0 (201Side-5 17)
rk
a
0
D
0
0
36
(2
rCDM
LUX
DEAP
S
17)
-II (20
X
a
d
n
Pa
017)
N1T (2XENOThis w

Supe

2

3

10
10
2
WIMP mass [GeV/c ]410holder and filler
Pb210 copper plateand ring

0.003

PMT (except
aluminum seal)

Fig. 39. 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.

0.002
0.001

[/day/kg/keVee]

10-40

.

LXe

0.005

-39

10

d
mo
S )
AS017
XM (2

Fig. 36. Number of PE spectra in the full volume of data and the best–
fit simulation. The black line represents the data and each coloredhistogram is a different BG RI. In the bottom plot, the ratio of thebest-fit simulation to the data is shown as a red points. The blueand green bands indicate the ±1σ and ±2σ systematic uncertainties, respectively.

Fig. 38. Data spectrum (filled dots) overlaid with the best-fit BG
estimate (blue) with the ±1σ error band (green). The expected
60 GeV/c2 WIMP spectrum (red) at a WIMP-nucleon cross sectionof 2.2 × 10−44 cm2 is also shown.
WIMP-nucleon cross section [cm2]MC/data

2

0
0.005
0.004
0.003
0.002
0.001

5

10

15

20

0

5

10

15

20

25

30

25
30
Energy[keVee]

Fig. 37. Expected BG energy spectrum in the fiducial volume. The
top figure illustrates a breakdown of the BG while the bottom figure
shows the same energy spectrum with systematic error evaluation.

the copper rings, and the copper reflectivity.
To search for the WIMP dark matter signal in our fiducial volume data, we performed a chi-square fitting of theobserved energy spectrum with the expected signal and BG
spectra in the 2–15 keVee energy range The best-fit result was
obtained in the case of no existence of a WIMP signal, with
χ 2 /NDF=8.1/12. Fig. 38 shows the energy spectrum of thedata and the best-fit BG estimate with the ±1σ systematic error band. All the remaining events are consistent with our BGevaluation. Hence, the 90% confidence level (CL) upper limiton the spin-independent WIMP-nucleon cross section was derived as a function of mass between 20 GeV/c2 and 10 TeV/c2
as shown in Fig. 39. Our lowest limit is 2.2 × 10−44 cm2 for a
60 GeV/c2 WIMP.

Search for dark matter in the form of hidden photons
and axion-like particles [18]Hidden Photons (HPs) and Axion-like Particles (ALPs),
which are respectively vector and pseudo-scalar realizationsof bosonic super-WIMPs, are alternative cold dark matter candidates with expected masses <1 MeV/c2 if they were produced non-thermally in the early universe. XMASS alreadycarried out searches around this mass region for HPs and
ALPs using commissioning data taken in 2010–2012, and had
given limits in the mass range of 40–120 keV/c2 [4].
This time we conducted a improved search for both ofthese bosons using 800 live-days of data with 327 kg of liquid xenon in the fiducial volume. The R(PE) < 30 cm cut
is applied in order to remove BG events originating from γ rays or β -rays from RI in/on the detector’s inner surfaces andbulk materials. We searched for a signal from the data bycomparing the observed energy spectrum with the combined
predictions of signal and BG simulation including their respective uncertainties. The energy spectrum after applying all
the selections is shown in Fig. 40. The peak around corrected
number of photoelectron (NPEcor ) = 2400 came from residual 131m Xe after calibration with 252 Cf, which was useful asa reference for the global energy scale of the simulation. Theenergy range between NPEcor =590–1760 (corresponding to γ ray energies of 40–120 keV) was used for the signal search.
In this region the spectrum is almost flat with an event rateof 5×10−4 count/day/kg/keV. No significant signal was ob-

80

100

120

140

160

180

Ae

60

10-11

(ALPs)

g

40

rana

Majo

E [keV a ]131mXesignal125I

XENON100(best ﬁt)prev. work
LUX

RI in/ondet. comp.

10

0.5

RI in Xe
2000

10

-23

10

-24

10

-25

10

-26

102

(HP)

Fig. 40. Best fit NPEcor distributions. The upper scale is translated
into the corresponding γ -ray energies. The black dots represent
the data. The stacked histograms show the BG Monte Carlo simulation (MC) for RIs in/on the detector components (green), RIs in
the liquid xenon (red), and xenon isotopes activated by neutrons(light blue). The dark hatched blue area shows the estimated contribution from dead PMTs. The magenta part of the histogramshows the best fit HP signal for a HP mass of mHP = 85 keV/c2 .

served, and thus we set constraints on the α ( /α parameter
related to kinetic mixing of HPs and the coupling constant gAeof ALPs in the mass range from 40 to 120 keV/c2 , resulting
in α ( /α <6×10−26 and gAe < 4 × 10−13 . These limits are themost stringent over this mass range derived from both direct
and indirect searches to date as shown in Fig. 41.
Search for WIMP-129 Xe inelastic scattering with particle identification [19]A WIMP search was conducted through inelastic scattering in which 129 Xe nuclei were excited. An observation of
WIMP-nuclei inelastic scattering would be direct evidence ofa spin dependent (SD) interaction mechanism as well as that
WIMPs have spin since nuclear excitation in inelastic scattering can be led only by SD interaction. We have studied
the inelastic scattering in 2014 [3]. This is the update of ourprevious study. The inelastic excitation sensitivity was improved by detailed evaluation of background, event classification based on scintillation timing that distinguished γ -rays
and β -rays, and simultaneous fitting of the energy spectra of
γ -like and β -like samples.
The data used for the analysis was collected from November 20, 2013 to July 20, 2016. The samples were separated
into β -depleted and β -enriched samples. This separation was
performed with a particle identification technique based on the
different LXe scintillation time profiles. The time constant of
scintillation from a β -ray becomes longer as the energy becomes larger [14]. Since a γ -ray is converted into lower energy electrons in LXe, its time constant is shorter than that ofa β -ray. The scintillation light from the nuclear recoil (NR)
has a shorter time constant than that of a β -ray and a γ -ray

10

a
ran

jo

Ma
HB

XENON100

rk

o
v. w

pre

10-27

o.

2
m
ALPs mass
De [keV/c ]!

2500
NPEcor

10-14
10-22

se

1500

this work

ffu

1000

PandaX-II

-13

di

0
500

o.

Dem

10-12

dead
PMTs

RG

1

_’/_

event rate [10-3/keV a /kg/day]26

this work

-28

102

HP / ALPs mass [keV/c 2]

Fig. 41. Constraints on gAe of the ALPs (top) and α ( /α of the HP(bottom). The red line shows the 90% CL constraint limit. The
black line shows our previous result [4]. The blue, magenta, green,and orange lines are limits reported by the XENON100, the Majorana Demonstrator, the LUX, and the PandaX-II, respectively. The
dotted, dashed, and dash-dotted lines in light blue color are constraints from indirect searches derived from red giant stars (RG),
diffuse γ -ray flux, and horizontal branch stars (HB), respectively.

since its ionization density is higher and the ion-electron pairs
recombine faster. Therefore, inelastic scattering, which has
contributions from both NR and a γ -ray, has a shorter time
constant than a pure β -ray event. This classification reduced
the number of events by about one order of magnitude in thesignal region. For a 200 GeV/c2 WIMP, the signal efficiency
in the fiducial volume region is approximately 51%. The β ray events classified as β -depleted was typically about 10%
for a 200 GeV/c2 WIMP search. The data was divided into
four periods, 1–4, because background condition is different.
The energy spectra were fitted with WIMP + BG MC spectrain the energy range from 30 to 200 keVee . Fig. 42 shows theenergy spectra for period 1. No evidence of a WIMP signal
was found. Thus, we set the upper limits of the inelastic channel cross section at 90% CL. This result provides the moststringent limits on the SD WIMP-neutron interaction and isbetter by a factor of 7.7 at 200 GeV/c2 than the existing experimental limit as shown in Fig. 43.

Bibliography[1] Y. Suzuki et al., hep-ph/0008296.
[2] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 719(2013) 78.
[3] H. Uchida et al. (XMASS Collaboration), Prog. Theor.

β-enriched sample [events/day/kg/keVee]×10-4(a) β-depleted sample

6

4

2

0

×10-3

Bi sample [events/day/kg/keVee]

SD WIMP-neutron cross section [cm2]β-depleted sample [events/day/kg/keVee]

27

50

100

150
200
Energy [keVee]

10-35

10-36

)

14

SS

(20

A
XM

17)

100

XE

10-37

(20

N
NO

is

Th

rk

wo

10-38

10-39

10

102

103
104
WIMP mass [GeV/c 2]

Fig. 43. 90% CL upper-limit for the WIMP-neutron cross section obtained by inelastic scattering searches. The result of this analysis
is shown as a solid bold line. The results of other experimental SD
inelastic scattering searches are shown with solid lines: XMASS(2014) [3], XENON100 (2017).

(b) β-enriched sample

1

0.5

89 (2017) 51.
[8] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 759(2016) 272.

0

×10-4

50

100

150
200
Energy [keVee][10] N. Oka et al. (XMASS Collaboration), Prog. Theor. Exp.
Phys. 2017 (2017) 103C01.

(c) 214Bi sample

2[11] K. Abe et al. (XMASS Collaboration), Prog. Theor. Exp.
Phys. 2018 (2018) 053D03.

1.5[12] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 716 (2013) 78.

1
0.5[13] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 922 (2019) 171.

214

0[9] K. Abe et al. (XMASS Collaboration), Phys. Rev. D 97(2018) 102006.

50

100

150
200
Energy [keVee]

Fig. 42. Energy spectra for period 1 of 200 GeV/c2 WIMP (90% CL upper limit). β -depleted, β -enriched, and 214 Bi samples are shown in(a), (b), and (c), respectively. The observed data is shown as black
points with error bars over the MC histograms. WIMP (red filled),
125 I (green hatched), 14 C (orange filled), 39 Ar (magenta filled), 85 Kr(blue filled), 214 Pb (cyan filled), 136 Xe (brown filled), external γ -rays(gray filled), 131m Xe (red hatched), 133 Xe (blue hatched), and 214 Bi(green filled) are shown as stacking histograms.

Exp. Phys. (2014) 063C01.
[4] K. Abe et al. (XMASS Collaboration), Phys. Rev. Lett.
113 (2014) 121301.
[5] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 724(2013) 46.
[6] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 759(2016) 64.
[7] K. Abe et al. (XMASS Collaboration), Astropart. Phys.

[14] H. Takiya et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 834 (2016) 192.
[15] K. Abe et al. (XMASS Collaboration), JINST 13 (2018)
P12032.
[16] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 789(2019) 45.
[17] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 884 (2018) 157.
[18] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 787(2018) 153.
[19] T. Suzuki et al. (XMASS Collaboration), Astropart.
Phys. 110 (2019) 1.

28

HYPER-KAMIOKANDE[Project Leader: Masato Shiozawa(Kamioka Observatory, ICRR, The University of Tokyo)]Introduction
The Hyper-Kamiokande (Hyper-K or HK) experiment is
proposed as a joint project of the university of Tokyo and
KEK by combining a next generation underground water
Cherenkov detector and upgraded J-PARC (Japan Proton Accelerator Research Complex, Ibaraki, Japan) neutrino beam.
In FY2016 ICRR and KEK have set first priority to the HyperK and upgrade of J-PARC for the project. The Hyper-K
project has been chosen for inclusion in the Ministry of Education, Culture, Sports, Science and Technology (MEXT)
Large Project Roadmap, “Roadmap 2017”. A ‘seed funding’towards the construction of Hyper-K has been allocated by
MEXT within its budget request for the 2019 fiscal year.
Figure 44 shows a schematic drawing of the Hyper-K
cylindrical detector. The detector is filled with 0.258 million
metric tons of ultra pure water, which serves an order of magnitude larger fiducial mass of 0.187 million metric tons thanSuper-K. It provides an enormous potential to discover leptonic charge-parity (CP) violation by observing neutrino andanti-neutrino beams from J-PARC. A search for nucleon decays is an important subject to explore new paradigm beyond
the standard model of particle physics. Hyper-K will also havefar better capabilities to observe atmospheric neutrinos, solar
neutrinos, and neutrinos from other astronomical sources thanthose of predecessor experiments.

Fig. 44. Schematic view of the Hyper-K water tank.

The Hyper-K international proto-collaboration consists ofabout 300 researchers 17 countries. Technical details were
published as a design report in May 2018 with various physics
reaches [1]. The detector technology has been developed
based upon the successful Super-K experiment. The candidate
site has been pinned down by geological surveys. In conclusion the feasibility study has been completed and the projectis technically ready to start construction. The internationalgroup aims to start the detector construction in 2020 and to
start its operation in 2027.
Photosensors
A Cherenkov light in a ultra pure water is detectedby 40,000 newly developed photomultiplier tubes (PMTs),
R12860 by Hamamatsu Photonics K.K., with 40% photocoverage. It was significantly upgraded from the R3600 PMT
used in Super-K due to an improved dynode structure using a
box-and-line type and optimized curvature glass as shown in
Fig. 45.

Fig. 45. New 50 cm photomultiplier tube with a box-and-line dynode(R12860, Hamamatsu Photonics K.K.).

The peak quantum efficiency is about 30%, that is 1.4
times higher than that of the Super-K PMT. In total, the
new PMT achieved twice higher single photon detection efficiency because the photoelectron collection efficiency was
also greatly improved. The timing and charge resolutions
at single photoelectron 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. These outstanding improvements enhance Hyper-K detector performance and its physics
reaches.
Moreover, pressure tolerance of PMT was improved up to
the 125 m water depth, so that new PMT can be used underabout 70 m depth of the detector water. To prevent a chain
reaction of imploding PMTs caused by the unlikely event ofa single PMT implosion, every PMT in the Hyper-K watertank will be housed in the shockwave prevention cover. The
first prototype cover consists of an acrylic front window and astainless steel backside cover with conical shape. The prototype have been produced and tested in 2016 and 2018 using along vertical shaft at Kamisunagawa town, Hokkaido, whichis naturally filled with spring water. The first prototype testing
was successful and confirmed the cover prevents the chain implosion at 80 m water depth. Alternative cover designs havealso been studied to reduce production cost and its weight.
One of the new designs employs back-side cover made of aresin, which largely reduce the total weight of the cover. A
prototype of resin cover has been produced and tested in 2018at the same testing site in Hokkaido.
More than 100 of R12860 PMTs with the protective covers were installed in Super-Kamiokande in 2018. Figure 46
shows photos of R12860 PMT installation in Super-K. Figure 47 shows photos of R12860 PMTs with new protective
covers, which have been developed for Hyper-K and were installed in Super-K.

29Hyper-K PMTnificance of the CP violation discovery by ten years operation.
Hyper-K covers the 76% of δCP parameter space with 3σ ormore significance, and 57% of the parameter space with 5σ ormore. Hyper-K will go beyond 7σ significance if δCP = −90◦
as suggested by T2K [2] and NOν A [3] results.

m3r 

10

6LJQLILFDQFHIRUVLQѝCP=0 exclusion
Normal mass hierarchy

HK

8

DUNE

6
Fig. 46. Photos of box-and-line dynode PMT (R12860) installation inSuper-K.

Hyper-K PMT w/
SUS coverѫ

T2K-II

4ѫ

20

T2K+NOvA(~2020)

-150

-100

-50

0

50

100
150
ƣCP [degree]

Fig. 48. Significance of the CP violation discovery in lepton sector
with a 10-year observation in Hyper-K (HK) as a function of theunknown CP phase. The normal neutrino mass hierarchy is assumed. Ongoing and planned long baseline experiments are superimposed [4, 5].

Hyper-K PMT w/
resin coverFig. 47. Photos of R12860 PMT with Hyper-K PMT cover installationin Super-K.

Detailed in-situ calibrations of R12860 PMTs installed inSuper-K have been done, and preliminary results of the calibration confirmed R12860 PMTs have approximately twice
better photo-detection efficiency and twice better timing resolution than Super-K PMTs. The averaged dark rate of R12860
PMTs is evaluated to be about 6 kHz.
Physics
Hyper-K will be able to measure the magnitude of the CP
violation with high precision, which could explain the baryon
asymmetry in the Universe. Figure 48 shows an expected sig-

A proton decay p→e+ π 0 is an important signal favored bymany Grand Unified Theory models with a prediction close to
the current limit of the proton decay life time. Figure 49 showsreconstructed invariant mass distributions for p→e+ π 0 search
after applying the event selection cuts. Thanks to the highsensitive PMTs of Hyper-K, background events (atmospheric
neutrinos) can be largely eliminated and p→e+ π 0 search inHyper-K is expected to be “background-free” particularly inthe free-proton enhanced signal region. Figure 50 shows the3σ discovery potential for the p→e+ π 0 mode as a functionof year. Hyper-K is an only realistic proposal which can go
beyond the proton lifetime of 1 × 1035 years.
As well as the supernova burst neutrino that was successfully observed in Kamiokande at once, undiscovered supernova relic neutrinos, accumulated by past all supernovae since
the beginning of the universe, are interesting events to explore
the history of heavy elements and the onset of stellar formation. Figure 51 shows the expected number of events as afunction of year.
In January 2015, ICRR and the Institute of Particle andNuclear Studies (IPNS) of KEK signed a memorandum of understanding (MoU) for cooperation on the Hyper-K project.
In accordance of the MoU, the Hyper-K Advisory Committee (HKAC) has been formed under the directors of ICRR and
KEK-IPNS. The HKAC is composed of international members outside the Hyper-K proto-collaboration, and its mandate
is to advise on the scientific, technical and managerial strategy of the Hyper-K project with a view to further developinga highly effective neutrino programme based in Japan. Tworounds of the HKAC meetings were held so far, the first in
winter 2015-2016 and the second in summer 2017. In the official report by the HKAC, the committee endorses Hyper-K

III.2NUCLEON DECAYS

7
6
5
4
3
2
1
0

600

800

1000

1200

500

Number of Events

2

Total mass (MeV/c )
14
12
10
8
6
4
2
0

Number of SRN events in FV

Number of Events

30

HK
SK-Gd
JUNO

400
350
300

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

250
200
150
100
50
0

600

800

1000

1200
2

Total mass (MeV/c )

2020

2025

2030

2035

2040

2045
Year

Fig. 51. The accumulated number of supernova relic neutrino events
as a function of year. The JUNO experiment and SK-Gd are alsoplotted [7, 8, 9, 10]. Effective temperature of neutrinos inside supernova is assumed to be 6 MeV. The solid line assumes no blackhole formation while the dashed line assumes that 30% of core–collapse stars form a black hole.

Fig. 49. Reconstructed invariant mass distribution of events passing
KEK Project Implementation Plan (KEK-PIP) released inall steps of the p→e+ π 0 event selection except the invariant mass
June 2016 [11] has put the first priority to the upgrade of the
cut after a 10 year exposure of Hyper-K.
+ 0 The hatched histograms
J-PARC accelerator for the Hyper-K experiment. The ICRR
vents passing
all
steps
of
the
p
!
e
⇡
event
selection
exceptshow the atmospheric neutrino background and the solid crosses
denote the sum of the background and proton decay signal. Here
Future Project Committee has concluded in March 2017 thatthe proton lifetime
assumed Reconstructed
to be, 1.7×1034 years,invariant
just beyond massf a single Hyper-K
tankis(left).
Hyper-K should be the laboratory’s next main project.
current Super-K limits. The free and bound proton-enhanced sigThe Hyper-K project is listed in “Roadmap 2017” by Minnal windows are the upper and lower panels of the plot. The figselection except the invariant mass cut after! e+ ⇡ 0 event
istrya of Education, Culture, Sports, Science and Technolures are quoted from “Hyper-Kamiokande Design Report” [1].

ogy (MEXT) as one of the seven highest-priority large-scaleght). The hatched histograms show the atmospheric neutrino
projects in Japan.
τ/ β [years]

In October 2017, the University of Tokyo launched Nextsum of the backgroundand proton decay signal. Here the
p→e π
+

0

Generation Neutrino Science Organization (NNSO), where
ICRR, Kavli IPMU, and the School of Science cooperate forars, just beyond current
Super-K limits. The free and boundHK 186 kton HD , 3σ35pioneering the future of neutrino physics through the develop10n the left plot, and are the upper and lower panels of the rightment of neutrino research techniques and detector technologies. In particular, it aims to promote what will become its
flagship facility, the Hyper-Kamiokande project. The devel1034
opment of the organization framework for the Hyper-K detector construction centered on NNSO is starting.
A ‘seed funding’ towards the construction of Hyper-K has
been allocated by MEXT within its budget request for the1033
2019 fiscal year. A statement by the president of the Univer2020
2030
2040
sity of Tokyo was released [12], which addresses that the UniYear
versity of Tokyo pledges to ensure construction of the HyperKamiokande
detector commences as scheduled in April 2020.
ral pionFig.
is 50.
a The
favoredmany
models.
Exdiscovery of
reachin protonGUT
lifetime with
3σ sigp→e+ π 0 mode
Hyper-Kamiokande Financial Forum (HKFF) has been esnificance as a function of year. It shows Hyper-K (HK) planing totablished, in which foreign agency representatives and laborastart in 2026,
superimposedwith the ongoing
Super-K in(SK) theand final
event topology,
withno invisible
particles
tory directors gathered to engage in discussion with Hyper-K
planned DUNE experiments. The DUNE project assumes 10 kton
operation from 2024, toward full 40 kton by increasing 10 kton ev- host organizations and project members, and the first meeting
nstruct theeryproton’smass from its decay products wasandheld on January 2019 [13]. Hyper-Kamiokande collabyear [6].
oration firmly makes all efforts towards the realization of theum of the recoiling system should be small. The event
as a very important experiment for Japan and for the world- experiment.
wide withinprogram offundamental
science,and encourages
ned events
theHyper-K
fiducial
volumeICRR
with twoand IPNS to fully support it.
Bibliography
The Hyper-K project are strongly supported by research[1] “Hyper-Kamiokande Design Report,”hough thedecay of the pion is expected to produce two
communities. The Japan Association of High Energy PhysiarXiv: 1805.04163.
cists (JAHEP) and the Cosmic Ray Researchers Congressecays thetwo photons may be close enough in space[2]toK. Abe et al. [T2K Collaboration],(CRC) in Japan endorse Hyper-K as the main future project.
Phys. Rev. Lett. 118, 151801.
DUNE 40 kton, staged , 3σ
SK 22.5 kton , 3σheric neutrino events with a muon below threshold areelectrons in the event. For those events with three rings,

sest to the ⇡ 0 mass are labeled the ⇡ 0 candidate. An231[3] P. Adamson et al. [NOν A Collaboration],
Phys. Rev. Lett. 118, 231801.
[4] K. Abe et al. [The T2K Collaboration],
arXiv:1609.04111 [physics.ins-det].
[5] K. Abe et al. [The T2K Collaboration],
PTEP 2015, no. 4, 043C01 (2015).
[6] R. Acciarri et al. [The DUNE Collaboration],
arXiv:1601.05471 [physics.ins-det].
[7] H. Sekiya [Super-K Collaboration],
PoS(ICHEP2016) 982 (2016).
[8] ICRR News (2016),
http://www.icrr.u-tokyo.ac.jp/2016/06/30101400.html.
[9] F. An et al. [JUNO Collaboration],
J. Phys. G 43, no. 3, 030401 (2016).
[10] Yu-Feng LI, Presentation at ”Workshop on Supernova at
Hyper-Kamiokande” (2017).
[11] KEK Project Implementation Plan (2016),
https://www.kek.jp/ja/About/
OrganizationOverview/Assessment/
Roadmap/KEK-PIP.pdf.
[12] “Statement from the President of The University ofTokyo Concerning the Start of Hyper-Kamiokande,”
http://www.hyper-k.org/doc/2018/
Concerning%20the%20Start%20of%
20Hyper-Kamiokande.pdf.
[13] “The First Meeting of the Hyper-Kamiokande Experiment Financial Forum,”https://www.u-tokyo.ac.jp/focus/en/
articles/z0208_00026.html.

32

HIGH ENERGY COSMIC RAY DIVISION
Overview
There are three major experimental research activities inthe High Energy Cosmic Ray Division, the study of high energy gamma rays and the development of the next generation
gamma-ray telescopes by the Cherenkov Cosmic Gamma Ray
group, the study of extremely high energy cosmic rays by theTelescope Array (TA) group, and the study of very high energy cosmic rays and gamma rays by the Tibet ASγ group.
Other activities, such as experiments utilizing the Akeno
observatory, the Norikura observatory, the Mt. Chacaltaya observatory (jointly operated with Bolivia) are closely related tointer-university joint research programs. Also an all-sky highresolution air-shower detector (Ashra) is in partial operationon the Hawaii island. The High Energy Astrophysics group
created in the fiscal year 2009 aims to explore various highenergy astrophysical phenomena, through theoretical and observational approaches.
The CANGAROO telescopes had been in operation inSouth Australia since 1992, with a 3.8 m small telescope andthen with four 10 m telescopes. The major scientific objective was the study of Very High Energy (VHE) gamma-ray
sources in our galaxy in the southern hemisphere. The mission of these telescopes was completed and the CANGAROO
observation site was closed in 2011.
For further development of VHE gamma-ray astronomy,
the Cherenkov Cosmic Gamma Ray group is working on thedesign study and development and construction of the next
generation international ground-based gamma-ray observatory CTA which will offer an order of magnitude better sensitivity than currently running Cherenkov telescopes, threetimes better angular resolution, and wider energy coverage
from 20 GeV to 100 TeV or higher. The construction of thefirst Large-Sized Telescope (LST) was completed on the CTANorth site in La Palma in 2018.
At the Akeno observatory, a series of air shower arrays of
increasing geometrical sizes were constructed and operated toobserve extremely high energy cosmic rays (EHECRs). The
Akeno Giant Air Shower Array (AGASA) was operated from1991 to January 2004 and covered the ground area of 100 km2
as the world largest air shower array. In 13 years of operation, AGASA observed a handful of cosmic rays exceeding
the theoretical energy end of the extra-galactic cosmic rays
(GZK cutoff) at around 1020 eV.
The Telescope Array (TA), a large plastic scintillator array with air fluorescence telescopes, has been constructed in
Utah, USA, which succeeds AGASA and measures the EHECRs with an order of magnitude larger aperture than that of
AGASA for the further study of EHECRs. The full-scale TA
is accumulating data as the largest array viewing the northern sky and observed the energy spectrum with high statistics,
which is in good agreement with the GZK suppression andfound evidence for anisotropy of arrival directions of EHECRs. The TAx4, which is aimed at quadrupling TA, was par-tially constructed by March of 2018.
An air shower experiment aiming to search for celestial
gamma-ray point sources started in 1990 with Chinese physicists at Yangbajing (Tibet, 4,300 m a.s.l.). This international
collaboration is called the Tibet ASγ Collaboration. An extension of the air shower array was completed in 1995 and an
emulsion chamber has been combined with this air shower
array since 1996 to study the primary cosmic rays aroundthe knee energy region. After successive extensions carriedout in 1999, 2002 and 2003, the total area of the air shower
array amounts to 37,000 m2 . The sun’s shadow in cosmic
rays affected by the solar magnetic field was observed forthe first time in 1992, utilizing its good angular resolution at
multi-TeV energy region. The group added underground water Cherenkov muon detector to detect cosmic gamma rays
above 100 TeV. The group is planning to construct a new air
shower array, called ALPACA, to cover the sky in the Southern hemisphere, and started the construction of its partial arrayin Bolivia in 2019.
The High Energy Astrophysics group is conducting theoretical researches on fundamental processes responsible fornon-thermal particle acceleration in various astrophysical environments, including first-order diffusive shock acceleration,
second-order stochastic acceleration in shock downstream regions, modification of shock structure by pick-up interstellar
neutrals, as well as injection processes of suprathermal particles. In addition to these theoretical works, R/D studies forradio observations of pulsars and cosmic ray air showers arealso being made.

Cherenkov Cosmic Gamma-Ray GroupCTA Project (Cherenkov Telescope Array)
CTA-Japan Consortium[Spokespersons : M.Teshima and H.Kubo]
Collaboration list:
Institute for Cosmic Ray Research, The University of Tokyo,
Chiba, Japan; Department of Physics, Aoyama Gakuin University, Tokyo, Japan; Department of Physics, Hiroshima
University, Hiroshima, Japan; Hiroshima Astrophysical Science Center, Hiroshima University, Hiroshima, Japan; Faculty of Science, Ibaraki University, Ibaraki, Japan; Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Ibaraki, Japan; Department of Physics, Konan University, Hyogo, Japan; Facultyof Medical Engineering and Technology, Kitasato University,
Kanagawa, Japan; Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan; Departmentof Physics, Kyoto University, Kyoto, Japan; Department of

33

Fig. 1. Artist view of the CTA observatory. CTA consists of three types of telescopes, Large Size Telescopes (23m diameter), Mid Size
Telescopes (12m) and Small Size Telescopes (4m), and covers the broad energy band from 20GeV to 100TeV.

Applied Physics, University of Miyazaki, Miyazaki, Japan;
Department of Physics, Nagoya University, Aichi, Japan;
Solar-Terrestrial Environment Laboratory, Nagoya University,
Aichi, Japan; Kobayashi-Maskawa Institute, Nagoya University, Aichi, Japan; Department of Earth and Space Science,
Osaka University, Japan; Department of Physics, Kinki University, Osaka, Japan; Astrophysical Big Bang laboratory,
RIKEN, Wako, Japan; Department of Physics, Rikkyo University, Tokyo, Japan; Department of Physics, Saitama University, Saitama, Japan; Institute of Space and Astronautical
Science, JAXA, Kanagawa, Japan; Department of Physics,
Tokai University, Kanagawa, Japan; Faculty of Integrated Arts
and Sciences, The University of Tokushima; Department of
Astronomy, The University of Tokyo, Tokyo, Japan; Department of Physics, The University of Tokyo, Tokyo, Japan; Faculty of Science and Engineering, Waseda University, Tokyo,
Japan; Department of Physics, Yamagata University, Yamagata, Japan; Faculty of Management Information, Yamanashi
Gakuin University, Yamanashi, Japan; Center for Cosmology
and AstroParticle Physics, Ohio State University, Ohio, USA;
Max-Planck-Institute for Physics, Munich, Germany [1].
CTA Project
During the past several years, Very High Energy (VHE)
gamma-ray astronomy has made spectacular progress and has
established itself as a vital branch of astrophysics. To advance
this field even further, we are constructing the Cherenkov
Telescope Array (CTA) [6], the next generation VHE gamma
ray observatory, in the framework of a worldwide, international collaboration. CTA is the ultimate VHE gamma ray observatory, whose sensitivity and broad energy coverage willattain an order of magnitude 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 theorigin of the highest energy cosmic rays in our Galaxy and beyond, the physics of energetic particle generation in neutronstars and black holes, as well as the star formation history of
the Universe. CTA will also address critical issues in fundamental physics, such as the identity of dark matter particles
and the nature of space and time.
VHE gamma rays from 100GeV to 10TeV can be observed with ground-based IACTs. The history of VHE gamma
ray astronomy began with the discovery of VHE gamma raysfrom the Crab Nebula by the Whipple Observatory in 1989.
To date, 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.
CTA is designed to achieve superior sensitivity and performance, utilizing established technologies and experience
gained from the current IACTs. The project is presently
in its pre-construction ( prototyping) phase, with international efforts from Japan, US, and EU countries. It will consist of several 10s of IACTs of three different sizes (Large
Size Telescopes, Mid Size Telescopes, and Small Size Telescopes). With a factor of 10 increase in sensitivity (1m Crab∼ 10−14 erg s−1 cm−2 ), together with a 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 is anticipated with CTA.
CTA will allow us to explore numerous diverse topics inphysics and astrophysics. The century-old question of the origin of cosmic rays is expected to be finally settled through detailed observations of supernova remnants and other Galactic

34

objects along with the diffuse Galactic gamma ray emission,
which will also shed light on the physics of the interstellar
medium. Observing pulsars and associated pulsar wind nebulae will clarify physical processes in the vicinity of neutronstars and extreme magnetic fields. The physics of accretion
onto supermassive black holes, the long-standing puzzle ofthe origin of ultra-relativistic jets emanating from them, 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 most energetic explosions in the Universe. Detailed studies of both AGNs and GRBs can also reveal the origin of thehighest energy cosmic rays in the Universe, probe the cosmic
history of star formation including the very first stars, as wellas provide high precision tests of theories of quantum gravity. Finally, CTA will search for signatures from elementary
particles constituting dark matter with the highest sensitivity
yet. Realisation of the rich scientific potential of CTA is verymuch feasible, thanks to the positive experience gained fromthe current IACTs.
The CTA-Japan consortium [1] is contributing in particularly to the construction of the Large Size Telescopes (LSTs)
and is involved in their development. The LST covers thelow energy domain from 20GeV to 3000GeV and is especially
important for studies of high redshift AGNs and GRBs. The
diameter and area of the mirror are 23m and 400m2 , respectively, in order to achieve the lowest possible energy thresholdof 20GeV. All optical elements/detectors require high specifications, for example, high reflectivity, high collection efficiency, high quantum efficiency and ultra fast digitisation of
signal, etc. For this purpose, CTA-Japan is developing highquantum efficiency photomultipliers, ultrafast readout electronics and high precision segmented mirrors. On the strengthof their experience gained from construction of the MAGIC
telescope, the Max-Planck-Institute for Physics in Munich is
responsible for the design of the 23m diameter telescope structure, based on a carbon fiber tube space frame. The LSTs require very fast rotation (180 degrees/20seconds) for promptly
observing GRBs. The first LST has been built in the CTA
North, La Palma, Spain in 2018 and now in the commissioning, and three more LSTs will be built until 2022, then four
more LSTs in the CTA South are expected to built in the
ESO site in Paranal, Chile. The location of the LST array in
the CTA North will overlap with MAGIC telescopes, which
will allow us to operate CTA-LSTs and MAGIC telescopes
together in the early phase of the construction.
The Cherenkov Cosmic Gamma Ray group is also operating the MAGIC Telescopes [10] on La Palma, Canary Islands.
This facility is used not only for scientific observations but
also for technological development toward the future observatory CTA.

Bibliography[1] CTA
Consortium
website:cta-observatory.jp/ andcta-observatory.org/.
[2] Science

with

the

Cherenkov

http://www.
http://www.
Telescope

Array,

Fig. 2. Large Size Telescope (23m diameter) designed by Max–
Planck-Institute for Physics. CTA Japan is contributing to the design and and construction of the imaging camera at the focal plane,
ultrafast readout electronics, and high precision segmented mirrors.

Fig. 3. Camera cluster for the Large Size Telescope (LST) developedby CTA-Japan. This cluster consists of seven high quantum efficiency photomultipliers (R11920-100), CW High Voltages, pre-amplifier, Slow Control Board, DRS4 Ultra fast waveform recording
system and Trigger. The LST camera can be assembled with 265of these clusters, cooling plates and camera housing.

Fig. 4. The high precision segmented mirrors for the Large Size Telescope (LST) developed by CTA-Japan in cooperation with Sanko
Co.LTD. The mirror is made of a 60mm thick aluminum honeycomb
sandwiched by 3mm thin glass on both sides. A surface protection coat consisting of the materials SiO2 and HfO2 is applied toenhance the reflectivity and to elongate the lifetime.

35

Fig. 6. MAGIC Stereo System with two Cherenkov telescopes of 17m
diameters, so far achieved the threshold energy of 25GeV. It locates near the mountain top of the Roque de los Muchachos on
the Canary Island of La Palma. Two telescopes are located withthe distance of 85 meters.
Fig. 5. The Large Size Telescope prototype (LST-1) under construction at Observatory de Roque de los Muchachos. The diameter ofdish and mirror surface area are 23m and 400m2 .

arXiv:1709.07997[3] The Cherenkov Telescope Array potential for the studyof young supernova remnants, Astropart. Phys. 62(2015) 152-164.
[4] Introducing the CTA concept, The CTA Consortium, Astropart. Phys. 43 (2013) 3-18.
[5] Gamma-ray burst science in the era of the Cherenkov
Telescope Array, S. Inoue et al., Astropart. Phys. 43(2013) 252-275.
[6] Design Concepts for The Cherenkov Telescope Array,
The CTA Consortium, Exper. Astron. 32 (2011) 193316.
[7] Status of Very High Energy Gamma Ray Astronomy and
Future Prospects, M. Teshima, The Astronomical Herald, 104 (2011) 333-342.
[8] Design Study of a CTA Large Size Telescope,Proc. of ICRC2012 at Beijing China, M. Teshima,
arXiv:1111.2183.
[9] Introducing the CTA Concept, B. Acharya et al., Astroparticle Physics, 34 (2013) 3.
[10] MAGIC Collaboration website:
mppmu.mpg.de/.

http://magic.

MAGIC
The MAGIC Collaboration has built in 2002 / 2003 a firstlarge atmospheric imaging Cherenkov telescope, MAGIC-I,
with a mirror surface of 236 sq.m. and equipped with photomultiplier tubes of optimal efficiency. In 2009, a second
telescope of essentially the same characteristics was added;
MAGIC-II was installed at a distance of 85m from MAGICI. With the accent of these instruments on large mirror surface and best light collection, cosmic gamma-rays at an energy threshold lower than any existing or planned terrestrial gamma-ray telescope have become accessible. So far
achieved has been a threshold of 25 GeV. The Japanese grouphas joined the MAGIC collaboration since 2010, and contributed to the operation, observations and data analysis. The
MAGIC telescopes are upgraded with new cameras, electronics and partially new mirrors in 2012, and are now operated
with an unprecedented sensitivity by an international collaboration of 17 institutes from 8 countries.
The recent highlights from MAGIC are, 1) Observation of
sub-TeV gamma rays from the IceCube170922A [1], [2] , 2)
the successful observation of pulsed gamma ray signal fromthe Crab pulsar up to TeV regime [3], 3) the discoveries of themost distant blazers 3S 0218 + 35 with the redshift of 0.944[4] and PKS 1441 + 25 with the redshift of 0.939 [5], 4) the
observation of the very fast flare of 1min time scale from theblazar inside Perseus cluster, IC310 [6]. These results brought
new questions on the pulsar emission mechanism, the EBL
energy density, and gamma ray emission mechanism from thesupermassive blackholes or vicinity of them. 5) In 2019 January, the MAGIC telescopes, for the first time, observed the
TeV Gamma Ray Burst GRB 190114C [?]ith the redshift of
0.42 [7]. The observation shows about the 100 times higher
flux of gamma rays than Crab Nebula. The highest-energy
photon reaches above 1000GeV, and clearly, the new gammaray emission mechanism is required. The light curve shows
the power-law decay t −1.6 .

Bibliography[1] First-time detection of VHE gamma rays by MAGIC
from a direction consistent with the recent EHE neutrino
event IceCube-170922A, ATel #10817.
[2] Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A, Science 12 July 2018, Science.eaat1378.
[3] Phase-resolved energy spectra of the Crab pulsar in therange of 50-400 GeV measured with the MAGIC telescopes, the MAGIC Collaboration, Aleksić et al. A&A
540 (2012) A69.
[4] ATEL # 6349.
[5] Discovery of Very High Energy Gamma-Ray Emission
from the distant FSRQ PKS 1441+25 with the MAGIC
telescopes, ATEL # 7416.

36

of which is to make a battery-powered data acquisition system for a future mobile imaging atmospheric Cherenkov telescope array [2]. The system consists of 32 PMTs, of which
detected Cherenkov light signals are read out by only four
GHz-sampling analog memory ASICs.
After test observations of atmospheric Cherenkov light
events in 2016, we performed observations of the Crab Nebula using this system in 2017. The Crab Nebula is the standard
candle in the TeV gamma-ray energy region and utilized forchecking performance of telescope systems. We could accumulate about 10 hr Crab data after two observation periods ofabout one week each. The data are under analysis for detecting the gamma-ray signal from the Crab and for estimating
performance of the system.

Fig. 7. Sky position of IceCube-170922A in the equatorial coordinate with contour of 50% and 90% confidence level overlaying the
gamma-ray colour contour map observed by MAGIC (7 sigma inthe peak).

Bibliography[1] M. Ohishi et al., Proc. of 33rd Internat. Cosmic Ray
Conf. (Rio de Janeiro), 587 (2013).
[2] T. Yoshikoshi et al., Proc. of 34th Internat. Cosmic Ray
Conf. (The Hague), 887 (2015).

TA: Telescope Array Experiment
Spokespersons (S. Ogio1 , C.C.H. Jui2 )1 : Graduate School of Science, Osaka City University
2 : Dept. of Physics and Astronomy, University of Utah

Collaborating Institutions:Fig. 8. Akeno atmospheric Cherenkov telescope of 3 m diameter, located in the Akeno Observatory.

[6] Black hole lightning due to particle acceleration at subhorizon scales, the MAGIC collaboration, Science 346(2014) 1080-1084.
[7] The first detection of a GRB at sub-TeV energies;
MAGIC detects the GRB190114C., ATEL # 12390.
Other Activities
As a test bench for domestic R & D activities of future
ground-based gamma-ray observatory projects, an old atmospheric Cherenkov telescope of 3 m diameter was repaired
and then placed at the Akeno Observatory in November 2010.
This telescope shown in Figure 8 (Akeno telescope, hereafter)
is currently the only atmospheric Cherenkov telescope locatedin Japan [1]. We have developed an R & D imaging camera system with the Akeno telescope since 2009, the purpose

Chiba Univ., Chiba, Japan; Chubu Univ., Kasugai, Japan;
Earthquake Research Institute, Univ. of Tokyo, Tokyo, Japan;
Ehime Univ., Matsuyama, Japan; Ewha W. Univ., Seoul,
Korea; Hiroshima City Univ., Hiroshima, Japan; Hanyang
Univ., Seoul, Korea; ICRR, Univ. of Tokyo, Kashiwa, Japan;
INR, Moscow, Russia; Inst. of Phys, Czech Acad. of Sci.,
Prague, Czech; IPMU, Univ. of Tokyo, Kashiwa, Japan;
Kanagawa Univ., Yokohama, Japan; KEK/ IPNS, Tsukuba,
Japan; Kindai Univ., Higashi-Osaka, Japan; Kochi Univ.,
Kochi, Japan; Kyushu Univ., Fukuoka, Japan; Moscow M.V.
Lomonosov State University; Moscow, Russia, Nat. Inst. ofRad. Sci., Chiba, Japan; Osaka Electro-Comm. Univ. Osaka, Japan Osaka City Univ., Osaka, Japan; RIKEN, Wako,
Japan; Ritsumeikan Univ., Kusatsu, Japan; Rutgers Univ., Piscataway, NJ, USA; Saitama Univ., Saitama, Japan; Shinshu
Univ., Nagano, Japan; SKKU, Suwon, Korea, Tokyo City
Univ., Tokyo, Japan; Tokyo Inst. of Tech., Tokyo, Japan;
Tokyo Univ. of Science, Noda, Japan; ULB, Brussels, Belgium; UNIST, Ulsan, Korea; Univ. of Utah, Salt Lake City,
USA; Univ. of Yamanashi, Kofu, Japan; Waseda Univ.,
Tokyo, Japan; Yonsei Univ., Seoul, Korea

37

Introduction
The Telescope Array (TA) is the largest Ultra-High Energy Cosmic Ray (UHECR) observatory in the northern hemisphere. The main aim of TA is to explore the origin and natureof UHECRs by measuring the energy spectrum, arrival direction distribution and mass composition. The TA collaborationconsists of approximately 130 researchers from US, Russia,
Korea, Belgium, Czech and Japan.
The TA detector consists of a surface array of 507 plastic
scintillator detectors (SD) and three stations of fluorescence
detectors (FD). It is located in the desert, approximately 200
km south from Salt Lake City in Utah in the U.S.A. The SDs
were deployed on a square grid with 1.2-km spacing, and the
SD array covers an area of ∼700 km2 . Each SD has two layers of 1.2-cm-thick scintillator with an area of 3 m2 . The fulloperation of SDs started in March 2008. The duty cycle isgreater than 95%. The FDs view 3◦ - 31◦ or 33◦ above horizon. One northern FD station at the MD site uses refurbished
HiRes telescopes. Two southern FD stations at the BR andLR sites were buit newly. All three FD stations started theobservation 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 to clarify the expectedtransition from galactic cosmic rays to extragalactic cosmic
rays and the comparison of the data with Monte Carlo (MC)simulation that takes into account the results of the LHC experiments. The TALE detector is located north of the TA site
(MD site). This FD station consists of refurbhised HiRes telescopes. The TALE FD views 31◦ - 59◦ in elevation angle. The
TALE SD is described later.
We report the results from TA below.
Energy Measurement
TA energy spectrum
The preliminary result of cosmic-ray spectrum for nine
years of the TA SD data is shown in Fig. 9 [1]. The systematic
uncertainty of energy determination is 21%. TA confirmed
the ankle at 1018.69±0.02 eV and the flux suppression above
1019.81±0.04 eV. The statistical significance of having the same
spectral index above the ankle (no suppression) is ∼7σ .

Fig. 9. 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 abroken power law.

TALE energy spectrum
The energy spectrum using the TALE FD data between ∼2
PeV and 100 PeV is shown in Fig. 10 [2]. The events observedwith the TALE FD are placed into three subsets: Cherenkov
dominated events, fluoresence dominated events, and mixed
signal events. We see two clear breaks at 1016.22±0.02 eV and
1017.04±0.03 eV. We possibly see the knee feature at around1015.6 eV. It becomes of great importance to measure the composition precisely using hybrid events by adding the TALE SD
array to understand this spectral feature. The TALE SD is described later.

Fig. 10. a) TALE cosmic-ray energy spectrum. The gray band indicates the size of the systematic uncertainties.

TA combined energy spectrum
Fig. 11 shows the TA combined spectrum (TA spectrum(Fig. 9) and the TALE spectrum (Fig. 10) together with the
Pierre Auger Observatory (Auger) energy spectrum with energy shifts by +10.2% and the KASCADE-Grande energy
spectrum. Good agreement between the TA combined (actually TALE) energy spectrum and the KASCDE-Grande energy spectrum is seen below 1018 eV. Significant discrepancy
between the TA combined energy spectrum and the Auger
combined energy spectrum is seen above around 1019.5 eV,
whereas good agreement is seen 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. 12a. The cutoff (or break) energies agree at 0.5σ level.
Based on this study, we compared TA energy spectra with δ> 25.8◦ and δ < 25.8◦ and found the difference of the cutoff energies as shown in Fig. 12b. The chance probability ofobtaining this difference is 3.5 σ [3].
The TA collaboration performed the linearity check of SD
with FD using hybrid events [4]. There was no evidence of
nonlinearity. For energy estimation, we use MC energy table
as the standard method. We compared it with the method of
constant intensity cuts that is based on the idea that showers
generated by primary cosmic rays with the same energy andcomposition would arrive at the detector with the same frequency due to the isotropy of the primary cosmic rays, assuming 100% detection efficiency. The results from both meth-38

log10 (E) > 19.0, TA has insufficient statistics to distinguishthe difference between different composition.

Fig. 11. The TA combined spectrum in black together with Auger combined energy spectra with energy shifts by +10.2% in red and KASCADE-Grande energy spectrum in green, respectively.

Fig. 13. 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. For data, stars, triangles and circles denote the results
using the BR/LR hybrid, MD hybird and BR/LR/MD stereo events,
respectively.

Fig. 12. a) The TA (black) and Auger (red) energy spectra of UHECR
events in the common declination band after energy shift. b) The
TA energy spectra for the declination angles below 24.8◦ in red
and above 24.8◦ in black

ods are consistent. The TA energy estimation nonlinearity
was finally evaluated as −0.3 ± 9% per decade above 1019 eV.
The Auger energy estimation nonnlineary was estimated to be±3% per decade above 1019 eV [5].
Mass Composition
Xmax
The result of the depth of shower maximum Xmax using
the first 8.5 years of hybrid events using the surface detectors and the BR and LR FD telescopes was published [6].
Fig. 13 shows the evolution of the average Xmax as a function of energy together with MC expectations for the BR andLR FD hybrid events, the MD FD hybrid events [7] and stereo
FD only events [8]. All TA measurements are in agreementwith each other. Here the BR/LR systematic errors are shown(±17.4g/cm2 ). The result is in agreement with light composition within systematic uncertainty.
The dependence of another parameter σ (Xmax ), the widthof Xmax , on energy is shown in Fig. 14. Below 1019 eV, the TA
result is consistent with QGSJet II-04 proton model, whereas
TA has insufficient exposure to make a careful measurementof the widths of Xmax distributions above 1019 eV.
We checked the shape of Xmax distributions, too. After allowing for systematic shfting of the data Xmax and performing
the likelihood test on the data and MC distributions, we findthat we fail to reject QGSJet II-04 protons as being compatible
with the data for all energy bins at the 95% confidence level asshown in Fig. 15. QGSJet II-04 helium, nitrogn and iron are
rejected for log10 (E) < 19.0, 19.2, and 19.4, respectively. For

Fig. 14. The RMS of reconstructed Xmax (σ (Xmax )) as a function of energy. The black, red, magenta, orange and blue colors denote thedata, pure proton, helium, nitrogen and iron QGSJet II-04 predictions, respectively. For data, stars, triangles and circles denote theresults using the BR/LR hybrid, MD hybird and BR/LR/MD stereo
events, respectively.

Fig. 15. Unbinned maximum likelihood test on observed and simulated QGSJet II-04 Xmax distributions after systematic shifting of
the data to find the best log likelihood.

TA SD composition
The result on UHECR mass composition obtained with
the TA SD is presented [9] using the boosted decision tree

39

multivariate analysis based on 14 observables sensitive to theproperties of the shower front and the lateral distribution function. The multivariate classifier is trained with MC simulations with the QGSJET II-03 model. Within the errors, theaverage atomic mass of primary particles shows no significant
energy dependence and corresponds to < lnA >= 2.0 ± 0.1(stat.) ± 0.44 (syst.). The obtained composition is qualitatively consistent with the TA hybrid results as shown in Fig. 16
while all the points lie higher than the pure proton composition..

Fig. 17. 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 andyellow represent QGSJET II-03, QGSJET II-04, EPOS1.99 and
SIBYLL2.1, respectively.
Fig. 16. The result of average atomic mass < lnA > from the TA SD
events in black in comparison with the TA hybrid results in green.

TA muon studies
The number of muons in inclined air showers observedwith water Cherenkov surface detectors at the Pierre Auger
Observatory is approximately 1.8 times that of the proton prediction with QGSJET II-03 model at 1019 eV [10]. We studied muons in air showers using seven years of the TA SD
data [11, 12]. Air shower events are classified using θ , φ
and R parameters to search for the condition of high purityof muons. 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 distanceof the location of the surface detector from the shower axis.
The direction of zero degrees of φ is opposite to the cosmicray 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 purityof ∼65% from the MC expectation at E ∼1019 eV. Typicalratios 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 and3.14 ± 0.36 (stat) ± 0.72 (syst) at 2760 m < R < 3120 m for
QGSJET II-03 model together with other hadronic models asshown in Fig. 17. A similar excess is seen in comparison withother hadronic models.
Search for the diffuse photons above 1018 eV
We present the search for photons using the first nine yearsof the TA SD data, employing multivariate analysis with theclassifier based on the Boosted Decision Tree [13]. There areno photon candidates found in the data set for 1018.0 , 1018.5 ,1019.0 , 1019.5 and 1020 eV, and the diffuse flux limits for photons are compared to the results of other experiments as shownin Fig. 18.

Fig. 18. The photon flux limit presented in [13] (TA SD, red arrows)
compared with the results from AGASA (light blue), Pierre Auger
Observatory SD (black) and hybrid data (gray), Yakutsk (magenta)
and previously published TA SD result (TA 3yr, dark blue) and thepredictions of some models.

Arrival Directions of UHECRs
TA hotspot for the highest-energy cosmic rays
We have searched for intermediate-scale anisotropy of 72
cosmic-ray events above 5.7×1019 eV using the first five yearsof the TA SD data [14]. We reported a cluster of events that
we call the hotspot, found by oversampling using circles 20◦
in radius. The hotspot has a Li-Ma pre-trial statistical significance of 5.1σ . The probability of such a hotspot appearing by
chance in an isotropic cosmic-ray sky is estimated to be 3.4σ .
For the first ten years of the TA data, 157 events above
5.7×1019 eV were observed. We scanned by circles with different radii and obtained the maximum Li-Ma pre-trial significance of 5.0σ at right ascension of 144.3◦ and declinationof +40.3◦ with oversampling using circles with a radius of

40

25◦ [15]. The preliminary Li-Ma significance plot is shown in
Fig. 19.

scales of TA and Auger. Fig. 21 shows the arrival directions of
the TA events and Auger events above 1020 eV [15]. The distance of arrival
directions of two cosmic rays is required to be√less than 2 degrees. We observed two doublets in the sample of nine years of the TA events and six years of the Auger
events. The preliminary probability√ to have greater than andequal to two doublets at less than 2 degrees is 0.30% (2.8σ ).

Fig. 19. Preliminary aitoff projection of Li-Ma significance map using
cosmic-ray events above 5.7×1019 eV for the first ten years of the
TA SD data in equatorial coordinates. Our FoV is defined as theregion above the dashed curve at decl. = -10◦ .
Fig. 21. The arrival directions of the TA events (red) and Auger events(blue) above 1020 eV in the equatorial coordinates.

Hot/cold spot
An energy dependent intermediate-scale anisotropy wasstudied using UHECRs above 1019.2 eV for seven years of the
TA SD data [16]. The energy distributions inside oversampled
circles are compared to that outside using the Poisson Likelihood Ratio test. The maximum pre-trial significance was obtained to be 6.17σ at right ascension of 139◦ and declinationof 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. 20.
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.

Fig. 20. 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).

Small-scale clusters above 1020 eV
We searched for small-scale clusters for the highestenergy cosmic rays observed with the Telescope Array and
the Pierre Auger Observatory. Here we used 1020 eV as theenergy threshold although we have not adjusted the energy

Search for correlation with starburst galaxies
Auger reported a correlation between the arrival directionsof cosmic rays with energies above 3.9×1019 eV and the flux
pattern of 23 nearby starburst galaxies (SBGs) [17]. It was
found that the starburst model fits the data better than the
hypothesis of isotropy with a statistical significance of 4.0σ ,the highest value of the test statistic being for energies above
3.9×1019 eV with SBG fraction of 9.7% and the search radiusof 12.9◦ .
TA tested the Auger SBG hypothesis. Here we used theenergy threshold of 4.3×1019 eV when taking into account theenergy-scale difference between two experiments. We foundthat the TA data is compatible with isotropy to within 1.1σ
and with the Auger SBG hypothesis to within 1.4σ .
Interdisciplinary research
TA SD burst events in coincdence with lightning
The TA SD observed several short-time bursts of air
shower like events for the first five years. The expectationof chance coincidence is less than 10−4 . We found evidence
for correlations between these bursts of the TA SD eventsand powerful lightning data obtained with the National Lightning Detection Network in timing and position [20]. Afterinstallting a 3-D lightning mapping array and electric fieldchange instrument at the TA SD site, detailed features wereobserved [21]. The observations provide a general description of downward-directed terestrial gamma flashes associatedwith downward negative lightning leaders. The result indicates that the showers consist primarily of downward-beamed
gamma radiation, which is supported by GEANT simulation
studies. We conclude that the showers are terrestrial gamma
ray flashes, similar to those observed by satellites.
TA extension
TAx4
As mentioned above, TA found evidence for intermediatescale anisotropy of arrival directions of cosmic rays withgreater than 5.7×1019 eV. With enhanced statistics, we expect

41

to observe the structure of the hotspot along with other possible excesses and point sources along with the correlations withextreme phenomena in the nearby universe. We proposed toquadruple the effective area of the TA SD aperture including
the existing TA SD array (TAx4) by installing additional 500
counters of the current TA SD design on a square grid withwider, 2.08-km spacing between each [22, 23]. The new array would need two FD stations overlooking it to increase thenumber of hybrid events for the measurement of Xmax and to
confirm the energy scale. These FDs will be formed using
additional refurbished HiRes telescopes.
The proposal of the SD part of TAx4 that was approved
for the Japan Society for the Promotion of Science (JSPS)
Grant-in-Aid for Scientific Research on Specially Promoted
Research in Japan in 2015 as a five-year project. The TAx4
scintillator counters were assembled at MEISEI ELECTRIC
CO., LTD in 2016 and at the Akeno Observtory in 2016, 2017and 2018 as shown in Fig. 23. The assembled counters weresent to Utah, and the final assembly of the TAx4 SDs was partially performed at the Cosmic Ray Center in Delta city near
the TA site. The SD deployment was performed in February
and March of 2019. The FD part of TAx4 was approved by
NSF in the USA in 2016. The TAx4 FD at the northern site
was completed and started the stable operation in February of
2018. The TAx4 FD at the southern site is under construction
in 2018. The layout of TAx4 is shown in Fig. 22 together with
TALE.
TALE
The layout of TALE is shown in Fig. 22. The TALE FD
operation was commenced in the spring of 2013. We saw twoclear breaks in the energy spectrum measured with the TALE
FD as shown in Fig. 10. It is of importance to measure Xmax
precisely adding timing information of surface detectors near
the shower core on the ground. The TALE SD array consistsof 80 plastic scintillation counters, which are identical to thoseof the TA SD array. These counters have graded spacings,
ranging from 400 m near the FD to 600 m further away. The
35 TALE SDs were firstly deployed in 2013. Therefore we
proposed to complete the full TALE SD array. The proposalof adding remaining SDs in the TALE SD array was approved
for the JSPS Grant-in-Aid for Scientific Research (S) in 2015as a five-year project. The full TALE SDs were deployed in
February of 2017. The TALE SD array is in stable operation [24]. Fig. 24 is an example of the footprint of a TALE SD
event.
The observation of cosmic rays with energies down to1015 eV, called the Non-Imaging CHErenkov (NICHE) array [25], was proposed. The plan is to install an array ofsimple Cherenkov counters of PMTs 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 constructed by the JSPS Grant-in-Aid for Young Scientists (A).
R&D for future detectors
To collect much larger statistics at the highest energies,
observatories of significantly larger aperture are needed toFig. 22. The layout of the proposed TAx4. The array of 507 SDs (redfilled circles on the left) is the current TA SD array. There are three
TA FD stations (MD to the north, LR to the west, and BRM to theeast of the TA SD array) with black cross symbols. The array of
surface detectors (green) to the north of the TA SD array is the
TALE SD array. Additional two sub-arrays of 500 surface detectors
in total (blue) for TAx4 are located to the northeast and southeastof the TA SD array. The 257 large blue filled circles denote deploydSDs, whereas remaining blue dots denote SDs to be deployed.
Additional two FD stations with refurbished HiRes telescopes for
the TAx4 are located at the MD and BRM FD sites and view to thenortheast and southeast as denoted each by the blue frame of thefan. Purple diamod symbols denote TAx4 communications towers.

identify sources via anisotropy studies. Because of the prohibitive cost, it may be difficult to build a sufficiently large
detector using existing technologies. Hence, it is important todevelop new detectors. The TA site is used worldwidely for
R&D of future detectors.
The JEM-EUSO is a new type of observatory that will utilize very large volumes of the earth’s atmosphere as a detector of the highest-energy cosmic rays from the universe. KEUSO [26] employs the technologies developed for the JEMEUSO mission and will be mounted to the International Space
Station (ISS) . K-EUSO is a wide-field (40◦ ), fast, and large
telescope, and observes brief UV flashes in the earth’s atmosphere caused by the highest-energy cosmic rays. The advantage is its rather uniform and huge aperture in the whole sky.
EUSO-TA, which is a EUSO prototype installed at the TA
BRM FD site in 2013, has been tested occasionally. The first
result from EUSO-TA for laser shots, star images, and UHECRs in association with those observed with the BRM FD was
published in [27].
New types of fluorescence detector were proposed forlarge area and low-cost detection of UHECRs such as
FAST with reflecting mirrors [28] and CRAFFT with fresnel

42Fig. 23. Assembly of a plastic scintillator counter at the assembly hallof the Akeno Observatory

bly of the TAx4 SDs is under way. One of the TAx4 FDs at
two sites was completed and started its operation in February,
2018.
The TALE SDs were fully deployed at the TALE site,
and the data acquisition system is been checked. TA, TAx4,
TALE, TAx4 and NICHE will provide important measurements of energy spectrum, composition, and arrival directionsof UHECRs from the knee region up to the highest-energy
region of over five to six decades in energy.
The TA BRM FD site is used for R&D of future detectors with larger aperture. The prototype (EUSO-TA) of JEMEUSO has been tested occasionally since 2013. The first result from EUSO-TA was summarized in [27]. New types offluorescence telescope consisting of a small number of PMTs
that aim at a future large and low-cost detector of UHECRs
such as FAST and CRAFFT are tested.

Bibliography[1] J.N. Matthews, “Highlights from the Telescope Array”, The 35th International Cosmic Ray Conference(ICRC2017), 2017/7/17 (invited).
[2] R.U. Abbasi et al. “Cosmic-Ray Energy Spectrum between 2 PeV and 2 EeV Observed with the TALE detector
in monocular mode”, Astrophys. J. 865:74 (18pp), 2018,arXiv:1803.01288.
[3] 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.
[4] D. Ivanov, “TA Spectrum”, Ultra-High-Energy-CosmicRay Conference (UHER2018), 2018/10/08 (oral).
Fig. 24. An example of the footprint of a TALE SD event

lenses [29]. They were installed at the BRM FD site, and theobservations of UHECRs are being performed.
Summary
TA confirmed the ankle at 1018.7 eV and the flux suppression above 1019.8 eV. The statistical significance of having thesame spectral index above the ankle (no suppression) is ∼7σ .
We confirmed the breaks at 1016.2 eV and 1017.0 eV in theenergy spectrum measured with the TALE FD.
The Xmax measurement above 1018.2 eV is consistent withlight composition. We need more statistics to clarify the feature above 1019 eV.
We obtained 143 cosmic-ray events above 5.7×1019 eV
in the nine years of the TA SD data. By scanning the sizesof circles for oversampling these events, we found maximum
Li-Ma significance at ∼5σ . Its significance appearing in an
isotropic cosmic-ray sky is ∼3σ . Evidence for some featuresof spectral anisotropy is seen.
In order to confirm the TA hotspot and understand its feature, we are constructing TAx4, which would quadruple the
TA SD aperture and would add two FD stations. The assem-[5] D. Ivanov, “Auger-TA Energy Spectrum Working Group
Report”, UHECR2018, 2018/10/08 (oral).
[6] 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 Ridge FADC Fluorescence Detectors and Surace Array in Hybrid Mode”, Astrophys. J., 858:76 (27pp), 2018, arXiv:1801.09784.
[7] J. Lundquist, PoS ICRC2015 (2016) 441.
[8] D. Bergman and T. Stroman, PoS ICRC2017 (2018) 538.
[9] R.U. Abassi et al., “Mass composition of ultra-highenergy cosmic rays with the Telescope Array Surface Detector Data”, Phys. Rev. D. 99, 022002, arXiv:1808.03680.
[10] A. Aab et al., (Pierre Auger Collaboration), Phys. Rev.
D 91 032003 (2015).
[11] R. Takeishi, ”Study of muons from ultra-high energy
cosmic ray air showers measured with the Telescope Array experiment”, University of Tokyo (2016).
[12] R.U. Abbasi et al., “Study of muons from ultra-high energy cosmic ray air showers measured with the Telescope
Array experiment”, Phys. Rev. D 98, 022002 (2018).

43[13] R.U Abbasi et al., “Constraints on the diffuse photon
flux with energies above 1018 eV using the surface detectorof the Telescope Array experiment”, Astroparticle Physics
110 (2019) 8-14, arXiv:1811.03920.
[14] R.U. Abbasi et al.,
“INDICATIONS OF
INTERMEDIATE-SCALE ANISOTROPY 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).

[28] T. Fujii, “First results from the full-scale prototype
for the Fluorescence detector Array of Single-pixel Telescopes”, ICRC2017, Busan, Korea, 2017/7/17 (oral).
[29] Y. Tameda, “The Cosmic Ray Air Fluorescence Fresnel lens Telescope (CRAFFT) for the next generation UHECR obsevatory”, ICRC2017, Busan, Korea,
2017/7/18 (poster).

[15] K. Kawata, “TA Anisotropy Summary”, UHECR2018,2018/10/10 (oral).

Tibet ASγ Project[16] R.U. Abbasi. et al., “Evidence of Intermediate-scale Energy Spectrum Anisotropy of Cosmic Rays E ≥ 1019.2 eVwith the Telescope Array Surface Detector”, Astrophys. J.
862:91 (6pp), 2018, arXiv:1802.05003.

[Spokesperson: M. Takita][17] A. Aab et al., Astrophysical Journal Letters, 853,:L29(2018).

1. Experiment[18] R.U. Abbasi et al., “Testing a Reported Correlation
between Arrival Directions of Ultra-high-energy Cosmic
Rays and a Flux Pattern from nearby Starburst Galaxies using Telescope Array Data”, Astrophysical Journal Letters,
867:L27 (5pp), 2018.

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 awide field-of-view (approximately 2 steradian) observation ofcosmic rays and gamma rays in the northern sky.
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 squaregrid covering 36,900 m2 , and 36 density (D) counters aroundthe 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-diameterphotomultiplier tube (FT-PMT), and 52 out of 185 FT counters are also equipped with a wide dynamic range 1.5-inch-indiameter 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 topof each counter in order to increase the counter sensitivity by
converting gamma rays into electron-positron pairs in an electromagnetic shower. The mode energy of the triggered eventsin 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 of the Tibet II
array. We called this high-density array Tibet HD. The modeenergy of the triggered events in Tibet HD is a few TeV.
In the late fall of 1999, the array was further upgraded byadding 235 FT-counters so as to enlarge the high-density areafrom 5,200 m2 to 22,050 m2 , and we call this array and further
upgraded one Tibet III. In 2002, all of the 36,900 m2 area wascovered by the high-density array by adding 200 FT-countersmore. Finally we set up 56 FT-counters around the 36,900 m2
high density array and equipped 8 D-counters with FT-PMT
in 2003. At present, the Tibet air shower array consists of 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[19] 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.
[20] R.U. Abbasi et al. “The bursts of high energy events observed by the telescope array surface detector”, Phys. Lett.,
A381 (2017) 2565-2572.
[21] R.U. Abbasi et al., “Gamma Ray Showers Observed at
Ground Level in Coincidence With Downward Lightning
Leaders”, J. Geophys. Res: Atmos., 123 (2018).
[22] H. Sagawa, “The plan of the Telescope Array Experiment for the Next Five Years”, ICRC2013, 2013/7/2-7/9(poster).
[23] E. Kido, “The TAx4 experiment”, ICRC2017, Busan,
Korea, 2017/7/15 (oral).
[24] S. Udo, “The Telescope Array Low-energy Extension”,
ICRC2017, Busan, Korea, 2017/7/15 (oral).
[25] D. Bergman, “j-NICHE: Prototype detectors of a nonimaging Cherenkov array at the Telescope Array site”,
ICRC2017, Busan, Korea, 2017/7/17 (oral).
[26] M. Casolino “KLYPVE-EUSO: Science and UHECR
observational capabilities”, ICRC2017, Busan, Korea(2017).
[27] G. Abdellaoui et al., JEM-EUSO Collaboration,“EUSO-TA - First results from a ground-based EUSO telescope”, Astrop. Phys. 102 (2018) 98-111.

ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582

44

FT Detector (512)

45.4

−0.3

−0.6

−0.1
−0.2

−0.4

Data
MC

−0.5

−0.7
10

15 m

21.6

0

Displacement (degrees)

36,900 m2

0.1

South

Tibet III Air Shower Array (2003)

North

Energy (TeV) / Z
2.94 4.20 6.46 11.4

Data Best fit

100

1000

Σρ FT

Fig. 2. From [1]. The Moon’s shadow center displacement from theapparent position in the north-south direction as a function of energy, observed by Tibet III.

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

Energy (TeV) / Z
East

0.1

West

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◦ ,as shown in Fig. 2, by displacement of the shadow center fromthe apparent center in the north-south direction, as the eastwest component of the geomagnetic field is very small at theexperimental site. On the other hand, the shadow center displacement in the east-west direction due to the geomagnetic
field enables us to spectroscopically estimate the energy scale
uncertainty at ±12 % level, as shown in Fig. 3. Thus, theTibet air shower experiment introduces a new method for energy scale calibration other than the conventional estimationby the difference between the measured cosmic-ray flux by anair shower experiment and the higher-energy extrapolation ofcosmic-ray flux measured by direct measurements by balloonborne or satellite experiments.

Displacement (degrees)Fig. 1. Schematic view of Tibet III.

2.94 4.20 6.46 11.4

21.6

45.4

0
−0.1
−0.2
−0.3
−0.4

Data
MC
MC Best fit
MC +−10%

−0.5
−0.6
−0.7
10

100

Σρ FT

1000

Fig. 3. From [1]. The Moon’s shadow center displacement from theapparent position in the east-west direction as a function of energy,
observed by Tibet III.

rays.

2.

Physics Results
Our current research theme is classified into 4 categories:(1) TeV celestial gamma-ray point/diffuse sources,(2) Chemical composition and energy spectrum of primarycosmic rays in the knee energy region,(3) Cosmic-ray anisotropy in the multi-TeV region with highprecision,(4) Global 3-dimensional structure of the solar and interplanetary magnetic fields by observing the Sun’s shadow in cosmic

We will introduce a part of the results obtained in this fiscal year[2].
High-energy galactic cosmic rays arriving from the direction behind the Sun are shielded by the Sun and cast a shadow(Sun’s shadow) in the galactic cosmic-ray intensity, whichis affected by the solar magnetic field. Coronal mass ejections (CMEs) are large magnetized clouds of plasma ejectedby solar eruptions going into the interplanetary space. A fast
CME forms a strong shock wave and a magnetic sheath containing the turbulent magnetic field behind the shock front.
CMEs sometimes have rope-like magnetic structures behind45

the magnetic sheath. A CME spreading away from the Sun
influences galactic cosmic rays in many manners. The famous
phenomenon is the so-called “Forbush decrease,” which is observed when an observation appratus is included in a regionof suppressed galactic cosmic-ray density behind the shockcaused by a CME. The CMEs may sometimes trigger major
geomagnetic storms when they reach the Earth.
As the currently feasible measurements on board a satellite provide the one-dimensional distributions of the magnetic
field in the CME along the satellite’s orbit, there exist difficulties to precisely derive the three-dimensional magnetic structure of a CME. Galactic cosmic rays at TeV and sub-TeV energies with large Larmor radii can sense the remote CMEs andgive useful additional information on their structure. Observations of these high-energy GCRs are also relevant to space
weather, because these particles travel nearly at the speed oflight, sense the CME, and escape into the upstream region of
the shock. They can easily overtake the much slower CME,
warning in advance of the Earth-directed CME (ECME) approaching the Earth. The solar modulation of galactic cosmic
rays at sub-TeV energies has been already reported, but the influence of ECMEs has not been measured in the energy region
exceeding 1 TeV. Therefore, using the Sun’s shadow observed
by Tibet-III, we studied the influence of ECMEs on the Sun’sshadow observed in the 3 TeV galactic cosmic-ray intensity.
As regards the solar magnetic field model in the MC simulation, employed is the CSSS model. The CSSS model isa potential field model which reproduces the yearly variation of the Sun’s shadow observed with Tibet-II at 10 TeV .
The potential field models describe the coronal magnetic field
based on the optical measurements of the photospheric magnetic field. We employ the photospheric field observed withthe spectromagnetograph of the National Solar Observatory
at Kitt Peak for each Carrington rotation (CR) period (∼27.3
days) in the simulation. The CSSS model includes four freeparameters, the radius Rcp (= 1.7R# ) of the sphere where themagnetic cusp structure in the helmet streamers appears, andthe length scale la of horizontal coronal electric currents, theradius Rss of the spherical source surface (SS) where the supersonic solar wind starts blowing radially, and the order n of
the spherical harmonic series describing the observed photospheric field. We set la to be one solar radius (la = R# ) andcheck two different cases with Rss =2.5R# and Rss = 10R# .
The former Rss is a standard value used in the original study,
while the latter gained support from some recent evidences.
We set n = 10 which is sufficient to describe fine structures
relevant to the orbital motion of high-energy particles withlarge Larmor radii. The radial component of the coronal magnetic field at Rss is then stretched out forming the Parker’s
spiral interplanetary magnetic field (IMF). For the radial solar wind speed needed in the Parker’s model, we use the solar
wind speed synoptic chart estimated from the interplanetary
scintillation measurement in each CR and averaged over the
Carrington longitude. A dipole model is employed for the geomagnetic field.
Simulating primary galactic cosmic rays causing the Sun’sshadow, we assume the energy spectra and the elemental compositions modeled by compiling various measurements between 0.3 TeV and 1000 TeV. The minimum energy of pri-mary particles is set at 0.3 TeV, below the threshold energy
for triggering our air shower detection. we randomly generate air shower events at the top of the atmosphere along theSun’s orbit using the CORSIKA code with the EPOS-LHC
interaction model so that air shower cores are uniformly distributed within 300 m radius from the center of array. This
area within 300 m from the center sufficiently covers detectors actually hit by air shower particles. We then distributethese simulated events among detectors configuring Tibet-III
by the GEANT4 code and calculate the output of each detector, which can be analyzed for AS reconstruction and event
selections in the same way as the experimental data. Then,an opposite charge is allocated to each primary particle at the
top of the atmosphere and randomly throw it within 4◦ radios
centered at the Sun, and we define it as the initial shooting
direction. Subsequently, the orbital motion of each particle isbacctraced to the Sun in the assumed magnetic fields by the
fourth-order Runge-Kutta method. Finally, its initial shooting direction is regarded as the direction contributing to theintensity deficit in the Sun’s shadow, if a particle hits the photosphere. The final expected Sun’s shadow equivalent to theobservation is obtained, smearing initial shooting directions
by the angular resolution event by event.
Air shower events observed from the direction of the Sun
by Tibet-III in 2000-2009 are analyzed. The modal energy
and the angular resolution of primary galactic cosmic rays
recorded with the array are estimated to be 3 TeV and 0◦ .9,
where we define the angular resolution as an angular radiuscontaining 50% of air shower events. The number of onsource events (Non ) is calculated as the number of events coming from a certain direction within a circular window centered at a certain point on the celestial sphere. The numberof background events, namely, off-source events (-Noff .), on
the other hand, is calculated as the number of events averaged
over within each of the eight off-source windows which arelocated at the same zenith angle as the on-source window, but
apart by ±9◦ .6, ±12◦ .8, ±16◦ .0 and ±19◦ .2 in the azimuthal
direction. The window radius of 0◦ .9 (angular resolution for
3 TeV galactic cosmic rays) is adopted for calculating both
Non and -Noff .. We calculate Non and -Noff . on each gridpointon 0◦ .1×0◦ .1 mesh of the geocentric solar ecliptic (GSE) longitude and latitude surrounding the optical center of the Sun.
We then estimate the deficit relative to the number of background events as Dobs = (Non − -Noff .)/-Noff . at every grid.
As for calibration purpose, the Moon’s shadow in the equatorial coordinate system is calculated in a similar manner.
Figures 4 (a) and (b) show yearly maps of Dobs in the Sun’sand the Moon’s shadows, respectively, observed at 3 TeV in
2000-2009, except for 2006, which is excluded from Figure 4(a) because of insufficient statistics. The Moon’s shadow provides us with a good reference of the detector stabilities, because the Moon has no magnetic field and its diameter observed at the Earth is almost the same as that of the Sun. It
is seen in Figure 1 (a) that the Sun’s shadow is darker (withlarger negative Dobs ) around 2008 when the solar activity wasclose to the minimum, while it becomes faint (with smaller
negative Dobs ) around 2001 when the activity was close to the
maximum. In contrast, the observed Moon’s shadow shown in
Figure 4 (b) is quite stable, ensuring the instrumental stability

46

Fig. 4. From [2]. Year-to-year variation of (a) the Sun’s shadow and(b) Moon’s shadow observed by the Tibet-III array between 2000
and 2009. The upper panels show 2D contour maps of Dobs in
the Sun’s shadow in the GSE coordinate system, while the lower
panels display Dobs in the Moon’s shadow each as a function ofright ascension and declination relative to the apparent center ofthe Moon.

Fig. 5. From [2]. Temporal variations of Dobs in the Sun’s shadow
during ten years between 2000 and 2009. For reference, panel(a) shows the variations of the monthly mean sunspot number andthe number of ECMEs recorded in each year plotted on the rightand left vertical axes, respectively. The gray histogram in panel (a)shows all ECMEs listed in the catalog by [?], while the blue histogram displays only ECMEs each with the transit periods coveredby the observation of the Sun’s shadow by the Tibet-III array. The
solid squares in each panel from (b) to (f) display (b) Dobs in the
Sun’s shadow observed at 10 TeV by the same detector configuration as the Tibet-II array [3], (c) Dobs in the Sun’s shadow observedby the Tibet-III array at 3 TeV during an entire period, (d) Dobs of theSun’s shadow at 3 TeV observed during a period without ECMEs
transit periods, (e) Dobs of the Sun’s shadow at 3 TeV observed
during the ECME transit periods that are excluded in (d), and (f)Dobs of the Moon’s shadow at 3 TeV. The error bars indicate thestatistical errors. The dashed lines in panels (b)-(e) and a dashedcurve in (f) indicate Dobs expected from the apparent angular sizeof the Sun and the Moon. The red open circles and green open
triangles in panels (b)-(e) display Dobs predicted by two different
MC simulations using the CSSS models with Rss = 2.5R# and Rss= 10.0R# , respectively.

during the same period.
Then, analyzed is the yearly variation of Dobs , measured
at the center of the yearly mean 2D map in Figure 4 (a). The
solid curve in Figure 5 (a) displays the monthly mean sunspot
number standing for the solar activities on the right verticalaxis, while the solid squares in Figure 5 (b) show the yearlyvariation of Dobs observed by Tibet-II at the 10 TeV region.
In Figure 5 (b), we notice that the magnitude |Dobs | shows aclear solar-cycle variation decreasing with increasing solar activity. The MC simulations suggest that galactic cosmic rays
passing near the solar limb are scattered by the complex andviolent coronal magnetic field during the solar maximum, andmay appear from the direction of the optical solar disc reducing |Dobs |. By the red open circles and green open triangles in
Figure 5 (b), we show predictions by two different MC simulations employing the CSSS models with Rss = 2.5R# and
Rss = 10.0R# . We see that the yearly variation over the entire
period [3] are reproduced by the models.
We show the yearly variation of Dobs observed at 3 TeV byTibet-III by solid squares in Figure 5 (c) . We see the solarcycle variation with a larger amplitude compared with that in
Figure 5 (b) at 10 TeV, due to the larger magnetic deflection
expected for lower energy galactic cosmic rays in the solar
corona. This energy dependent feature of the solar-cycle variation is overall reproduced by the MC simulations using the
CSSS models. It should be noted that |Dobs | in Figure 5 (c) issignificantly smaller than those predicted by the MC simulations. The tendency is conspicuous in 2000-2002 around thesolar maximum at the statistical significance of 4.6σ and 5.4σ
for the CSSS models with Rss = 2.5R# and Rss = 10.0R# , respectively. For 10 TeV galactic cosmic rays, however, we donot see the deviations from the MC simulations in Figure 5(b).
We estmated the deviations quantitatively by the χ 2 test in
Figure 5 which are summarized in Table 1. The solid squares
in Figure 5 (f) represent the yearly variation of Dobs in the
Moon’s shadow observed by Tibet-III array, while the dashedcurve indicates the variation of Dobs expected from the distance variation between the Moon and the Earth. The systematic error of Dobs in Table 1 are estimated to be 0.178% fromthe deviation of Dobs from the dashed curve in this figure. In
Table 1 we demonstrate that the deviation of Dobs from theMC simulation during the entire period in Figure 5 (c) is not
acceptable.
We then check how the deviation is affected by the
ECMEs that take place during the solar maximum period. For
the purpose, the catalog of CMEs is adopted containing 228
ECMEs recorded in 2000-2009. The 3 TeV galacric cosmic
rays take only ∼8 minutes to reach the Earth after passing
near the Sun, while an ECME arrives at the Earth 2 − 4 days
after the solar eruption. As glactic cosmic rays reach the Earthin the direction of the Sun. they are affected by the ECME
during a transit period between the arrival time at the Earthand the solar eruption time observed by the Solar and Heliospheric Observatory (SOHO)/Large Aperture Solar Coronagraph (LASCO). The influence of ECMEs is examined byanalyzing Dobs observed during a period with/without thesetransit periods of ECMEs. Four days for the transit period areassumed, which is an average of all ECMEs covered by the
SOHO/LASCO observation, with respect to ECMEs lacking
in relevant eruption time information.
In Figure 5 (a), we show the yearly number of ECMEs.
The gray histograms represent all the 228 ECMEs. The blue
histograms indicate the 118 ECMEs with a transit period
covered by the observation of Dobs by Tibet-III. W see that

47
Table 1. From [2]. Summary of the χ 2 tests of the agreement between Dobs and predictions by MC simulations based on the statistical andsystematic errors
During an Entire Period
Models3 TeV CSSS Rss =2.5R#
3 TeV CSSS Rss =10R#
a 10 TeV CSSS R =10R#ssχ 2 /d.o.f.
21.3(32.1)/1030.1(46.9)/10
8.3(10.3)/14

Probability
0.019(3.9 × 10−4 )8.2 × 10−4 (9.8 × 10−7 )0.87(0.74)

the number of ECMEs changes positively correlated with thesunspot numbers. Approximately, half of the analysis period
is occupied by 20 − 35 ECMEs per year in 2000-2002 aroundthe solar maximum due to the correlation. On the contrary,
more than 90% of the analysis period does not includes any
ECME in 2007-2009 correspoinding to the solar minimum
period. Dobs observed during periods without ECMEs transit periods is shown by solid squares in Figure 5 (d). In the
meanwhile, the solid squares in Figure 5 (e) indicate Dobs during the ECME transit periods which are excluded in Figure 5(d). We see that the deviation of Dobs from the MC simulationsin 2000-2002 is significantly reduced in Figure 5 (d) than in
Figure 5 (c), while it is increased in Figure 5 (e), as confirmedby the χ 2 tests in Table 1. The Dobs at 10 TeV observed by
the Tibet-II array during the ECME transit period seem to besomehow inconsistent with the MC simulation at the chanceprobability of 0.044.
Thus, the present result indicates the first observational
evidence for the ECMEs which affect the Sun’s shadow incosmic rays in the 3 TeV energy region.

3.

Other Activities

The emulsion-pouring facilities can meet the demands formaking any kind of nuclear emulsion plates which are used
for cosmic ray or accelerator experiments. The thermostatic
emulsion-processing facilities are operated in order to develop
nuclear emulsion plates or X-ray films. Using these facilities, it is also possible to make and develop emulsion pellicle in 600-micron thickness each. In this way, these facilities
have been open to all the qualified scientists who want to carryout joint research programs successfully. Of recent, however,the shrinking demand for the facilities let us decide that weshould suspend calls for joint research programs to utilize theemulsion-pouring facilities, starting from 2012.

4.

Ongoing Plans(1)Chemical composition of primary cosmic rays making theknee in the all-particle energy spectrum
We have measured the energy spectra of primary cosmicray protons, helium, all particles around the knee 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. We

Without ECME Transit Periodsχ 2 /d.o.f.
8.6(12.2)/10
14.3(21.0)/10
7.5(8.9)/14

Probability
0.57(0.27)
0.16(0.021)
0.91(0.84)

During ECME Transit Periodsχ 2 /d.o.f.
19.2(23.9)/7
24.0(29.4)/7
20.1(21.1)/11

Probability
0.0076(0.0012)
0.0011(1.2 × 10−4 )0.044(0.032)

Fig. 6. YAC2 set up at Yangbajing.

have a plan to install an Yangbajing Air shower Core detector array (YAC) near the center of Tibet III to distinguish thechemical component making the knee. We completed construction of YAC2 (124 detectors over ∼500m2 in area), as isshown in Fig. 6, and started data-taking in 2014. YAC2 aims
at mainly studying the energy spectra of proton and helium
components in the knee energy region.
(2) 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[4] is very effective to reduce cosmic-ray 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 the present Tibet air
shower array (Tibet AS). By Tibet AS + MD, we aim atbackground-free detection of celestial point-source gamma
rays around 100 TeV with the world-best sensitivity and at
locating the origins (PeVatrons) of cosmic rays accelerated
up to the knee (PeV) energy region in the northern sky. The
measurement of cut off energies in the energy spectra of suchgamma rays in the 100 TeV region may contribute significantly to understanding of the cosmic- ray acceleration limit
at SNRs. Search for extremely diffuse gamma-ray sources byTibet AS + MD, for example, from the galactic plane or fromthe Cygnus region may be very intriguing as well. Above 100
TeV, the angular resolution of Tibet AS with 2-steradian wide
field of view is 0.2◦ and the hadron rejection power of Tibet MD is 1/1000.
In addition to unknown point-like sources, we expect

48
10Integral Flux (photons cm-2 s-1)

10

-9Tibet AS
Tibet AS+MD 1/3 scale
Tibet AS+MD full scaleMAGIC
VERITAS
HESS
HAWC
CTA
HEGRA Crab

-10

10-11
10-12
10-13
10

-14

10-15
10-16
10-1

Fig. 7. The 5 shaded squares represent the constructed 5 MD pools.

100

101
Energy (TeV)

102

103

Fig. 9. Sensitivity to point-like gamma-ray sources with Tibet AS+MD(see,Tibet AS+MD 1/3 scale) by pink curve.

comparison with real data. Various analysis tools are also extensively being developed. According to the simulation, thesensitivity of the current configuration (Tibet AS + MD) isdemonstrated in Fig. 9.

Bibliography
Papers in refereed journals

Fig. 8. PMTs mounted in a MD cell filled with water.

to detect established sources in the 100 TeV region: TeV
J2032+4130, HESS J1837-069, Crab, MGRO J2019+37,
MGRO J1908+06, Milagro candidate sources, Mrk421,
Mrk501 are sufficiently detectable and Cas A, HESS J1834087,LS I+63 303, IC443 and M87 are marginal.
Furthermore, our integral flux sensitivity to diffuse gamma
rays will be very high. We hope that the diffuse gamma raysfrom the Cygnus region reported by the Milagro group andalso diffuse gamma-rays from the galactic plane will be detected. Diffuse gamma-rays of extragalacitic origin may be aninteresting target as well.
In 2007, a prototype 100-m2 underground water
Cherenkov muon detector was successfully constructed in Tibet to demonstrate the technical feasibility, cost estimate, validity of our Monte Carlo simulation. Data analyses demonstrate that our MC simulation reproduces real data quite reasonably.
In 2014, construction of the ∼4,000 m2 MD, as is shownin Fig. 7, was successfully completed and data-taking started.
We have accumulated approximately three-year data. One ofthe detector cell filled with water is demonstrated in Fig. 8.
Development of Monte Carlo simulation is under way for[1] “Multi-TeV Gamma-Ray Observation from the Crab
Nebula Using the Tibet-III Air Shower Array Finely
Tuned by the Cosmic-Ray Moon’s Shadow”, M.
Amenomori et al., Astrophysical Journal, 692 , 61-72(2009).
[2] “Influence of Earth-Directed Coronal Mass Ejections on
the Sun’s Shadow O bserved by the Tibet-III Air Shower
Array”, M. Amenomoriet al., Astrophysical Journal, 860:13-1-7 (2018).
[3] “Probe of the Solar Mangetic Field Using the “CosmicRay Shadow” of the Sun”, M. Amenomoriet al., Physical Review Letters, 111 011101-1-5 (2013).
[4] “Exploration of a 100 TeV gamma-ray northern sky using the Tibet air-shower array combined with an underground water-Cherenkov muon-detector array”, T.K.
Sako et al., Astroparticle Physics, 32 , 177-184 (2009).
The Tibet ASγ Collaboration
M. Amenomori1 , X. J. Bi2 , D. Chen3 , T. L. Chen4 ,W. Y. Chen2 , S. W. Cui5 , Danzengluobu4 , L. K. Ding2 ,C. F. Feng6 , Zhaoyang Feng2 , Z. Y. Feng7 , Q. B. Gou2 ,Y. Q. Guo2 , H. H. He2 , Z. T. He5 , K. Hibino8 , N. Hotta9 ,Haibing Hu4 , H. B. Hu2 , J. Huang2 , H. Y. Jia7 , L. Jiang2 ,F. Kajino10 , K. Kasahara11 , Y. Katayose12 , C. Kato13 ,K. Kawata14 , M. Kozai13,15 , Labaciren4 , G. M. Le16 ,A. F. Li17,6,2 , H. J. Li4 , W. J. Li2,7 , C. Liu2 , J. S. Liu2 ,49

M. Y. Liu4 , H. Lu2 , X. R. Meng4 , T. Miyazaki13 ,K. Munakata13 , T. Nakajima13 , Y. Nakamura13 , H. Nanjo1 ,M. Nishizawa18 , T. Niwa13 , M. Ohnishi14 , I. Ohta19 ,S. Ozawa11 , X. L. Qian6,2 , X. B. Qu20 , T. Saito21 ,T. Y. Saito22 , M. Sakata10 , T. K. Sako23,14 , J. Shao2,6 ,M. Shibata12 , A. Shiomi24 , T. Shirai8 , H. Sugimoto25 ,M. Takita14 , Y. H. Tan2 , N. Tateyama8 , S. Torii11 ,H. Tsuchiya26 , S. Udo8 , H. Wang2 , H. R. Wu2 , L. Xue6 ,Y. Yamamoto10 , K. Yamauchi12 , Z. Yang2 , A. F. Yuan4 ,L. M. Zhai3 , H. M. Zhang2 , J. L. Zhang2 , X. Y. Zhang6 ,Y. Zhang2 , Yi Zhang2 , Ying Zhang2 , Zhaxisangzhu4 , andX. X. Zhou7
1 Department of Physics, Hirosaki University, Hirosaki 036-

8561, Japan
2 Key Laboratory of Particle Astrophysics, Institute of HighEnergy Physics, Chinese Academy of Sciences, Beijing
100049, China
3 National Astronomical Observatories, Chinese Academy of
Sciences, Beijing 100012, China
4 Department of Mathematics and Physics, Tibet University,
Lhasa 850000, China
5 Department of Physics, Hebei Normal University, Shijiazhuang 050016, China
6 Department of Physics, Shandong University, Jinan 250100,
China
7 Institute of Modern Physics, SouthWest Jiaotong University,
Chengdu 610031, China
8 Faculty of Engineering, Kanagawa University, Yokohama
221-8686, Japan
9 Faculty of Education, Utsunomiya University, Utsunomiya
321-8505, Japan
10 Department of Physics, Konan University, Kobe 658-8501,
Japan
11 Research Institute for Science and Engineering, Waseda
University, Tokyo 169-8555, Japan
12 Faculty of Engineering, Yokohama National University,
Yokohama 240-8501, Japan
13 Department of Physics, Shinshu University, Matsumoto
390-8621, Japan
14 Institute for Cosmic Ray Research, The University of Tokyo,
Kashiwa 277-8582, Japan
15 Institute of Space and Astronautical Science, Japan
Aerospace Exploration Agency (ISAS/JAXA), Sagamihara
252-5210, Japan
16 National Center for Space Weather, China Meteorological
Administration, Beijing 100081, China
17 School of Information Science and Engineering, Shandong
Agriculture University, Taian 271018, China
18 National Institute of Informatics, Tokyo 101-8430, Japan
19 Sakushin Gakuin University, Utsunomiya 321-3295, Japan
20 College of Science, China University of Petroleum, Qingdao
266555, China
21 Tokyo Metropolitan College of Industrial Technology, Tokyo
116-8523, Japan
22 Max-Planck-Institut für Physik, Munich D-80805, Germany
23 Escuela de Ciencias Fı́sicas y Nanotechnologı́a, YachayTech, Imbabura 100115, Ecuador
24 College of Industrial Technology, Nihon University,

Narashino 275-8576, Japan

25 Shonan Institute of Technology, Fujisawa 251-8511, Japan

26 Japan Atomic Energy Agency, Tokai-mura 319-1195, Japan

ALPACA Project[Spokesperson: M. Takita]
ICRR, The Univ. of Tokyo, Kashiawa, Chiba 277-8582

Cosmic rays are supposed to be accelerated up to the knee
energy (PeV) region at supernova remnants (SNRs) in ourgalaxy. Therefore, we naturally expect gamma rays at 100
TeV energies, which originate in π 0 decays produced by theaccelerated cosmic rays interacting with matter surrounding
the SNRs. However, on-going experiments focus on measuring gamma rays in the 1 - 10 TeV region. The gamma-ray
emission of electron origin might be highly suppressed above
10 TeV due to rapid decrease of inverse-Compton cross section by the Klein-Nishina effect as well as synchrotron radiation energy loss in the strong magnetic field around the SNRs.
The detection and spectral measurement of gamma rays in
the 100 TeV region from their celestial sources, together
with multi-wavelength (radio, X-ray, gamma-ray) observations, will be an important experiment enabling us to discriminate between the two processes (cosmic-ray/electron origins),
to locate the acceleration site (PeVatron which accelerate cosmic rays up to PeV energies) of cosmic rays and to verify
the standard acceleration model of cosmic rays. Furthermore,diffuse gamma rays from the Fermi bubbles recently reportedby the Fermi-LAT group and sub-PeV neutrino events[1] detected by IceCube suggests that the Fermi bubbles be a PeVatron candidate. Similarly, the energy spectrum of diffuse
gamma rays from the extended region around the galactic center marginally measured by HESS up to approximately 10
TeV also strongly indicates existence of PeVatron[2], fromwhich we expect to detect gamma rays at 100 TeV energies.
Thus, a wide field-of-view gamma-ray imaging at 100 TeV energies in the southern sky, where the HESS sources, the Fermi
bubbles and the galactic center are located within field of view,
will be a key experiment.

1. Experiment
The ALPACA[3],[4] (Andes Large area PArticle detector
for Cosmic ray physics and Astrophysics) is a cosmic-ray experiment with a large surface air shower array with a large
underground muon detector array The experimental site (approximately 500 m×500 m ∼250,000 m2 in total area) is located on a flat high land called Chacaltaya Plateau (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 partin 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) andan 83,000 m2 air shower array (AS), shown in Fig. 2. MD of

50

Fig. 1. From Ref.[3]. Experimental site for the ALPACA experiment,
Chacaltaya Plateau ia (4,740 m above see level, 16◦ 23’ S, 68◦ 08’W), near Mount Chacaltaya, in Bolivia.

2.

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 around the Kneeenergy region (100 TeV – 100 PeV)
3. Measurement of cosmic ray anisotropy > 5 TeV at siderealtime 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 (anorder of magnitude better than any previous/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 located in the southern sky, the ALPACA experimentis sensitive to the source with ∼15 % Crab intensity during
one calendar year, as is demonstrated in Fig. 3.
The AS + MD in the southern hemisphere will be aunique/complementary experiment to on-going experiments

Fig. 2. From Ref.[3]. 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, eachof which contains twelve 56 m2 muon detector units. The total areaof the underground muon detector array is 5,400 m2 .
10Integral Flux x E (eV cm−2 s −1 )water Cherenkov type is composed of eight pools with eachpool (approximately 1 m deep) containing twelve 56 m2 unit
detectors. AS is made up of 401 1 m2 plastic scintillation
counters at 15 m spacing.
The AS field of view is roughly 2 steradian. The expectedangular 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 ∼2025 %. The hadron rejection power of MD is more than 99.9 %at 100 TeV, while keeping most of gamma-ray events. Longterm 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 bright stable TeV
gamma ray sources in the southern sky.

10

3
[GeV]

[TeV]

[PeV]
Past
Present
Future2
Crab flux
Tibet AS

10

1
EGRET

10

CASA−MIAMAGIC
VERITAS
H.E.S.S.
HAWC

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

10

12

10

13

10

14

10

15

10

16

Energy (eV)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.[6].

(FERMI, HESS, VERITAS, MAGIC, CALET, Tibet ASγ ,HAWC) and future projects (LHAASO, CTA) in this field,
which are either located in the northern hemisphere or aiming at gamma-ray astronomy below 10 TeV region, or having narrow field-of-view. Thus, the new energy window in
the 100 TeV region observing gamma rays with wide fieldof-view will be opened first in the southern sky by the ALPACA experiment. We expect to detect established more thana dozen of sources, i.e., young SNRs (SN1006, RX J1713.73946, RX J0852.0-4622), Pulsar Wind Nebulae, the galactic
center, etc) in the 100 TeV region, some of which may becosmic-ray PeVatron candidates. Furthermore, our wide fieldof-view sensitivity to diffuse gamma rays allows us to study
extremely diffuse gamma-ray sources which are difficult to51[3] M. Takita for the ALPACA Collaboration, THE EUROdetect by IACTs. The diffuse gamma rays from the Fermi
PEAN PHYSICAL JOURNAL, 145, 01002-1-3, (2017).
bubbles recently reported by the Fermi-LAT group may beclearly detected, if they extend up to the 100 TeV region. Sim[4] T. Asaba et al, Proc. of ICRC2017, ID=827, Busan, Koilarly, detection of diffuse gamma rays above 100 TeV fromrea, July 12-20, (2017).
extended region from the galactic center is promising, where
the gamma-ray energy spectrum strongly suggests existence[5] F. Aharonian et al., ApJ, 614, 897 (2004)
of PeVatron. Detection and spectral measurement of gamma[6] T. K. Sako et al., Astroparticle Physics, 32, 177 (2009).
rays in the 100 TeV region from these celestial sources, together with multi-wavelength (radio, X-ray, gamma-ray) ob[7] C. Y. Kenny et al., arXiv:1508.06276v1.
servations, are key points enabling us to discriminate between
the two processes (cosmic-ray/electron origins), to locate the[8] T. Asaba et al, Proc. of ICRC2017, ID=437, Busan, Koacceleration site of cosmic rays and to examine the standard
rea, July 12-20, (2017).
acceleration model of cosmic rays. In astronomical pointof view, we pioneer the ultra-high energy (above 100 TeV) The ALPACA Collaboration as of April 2018
gamma-astronomy in the southern sky. Besides, gammaray emission from near-by extragalactic sources, e.g. M87, T. Asaba,a K. Hibino,b N. Hotta,c M. Kataoka,a Y. Katayose,a
Cen A, gamma rays of dark matter origin, those from the Sun C. Kato,d K. Kawata,e H. Kojima, f g R. Mayta,h P. Miranda,idisk recently obseved by Fermi[7] may be interesting subjects. K. Munakata,d Y. Nakamura,d M. Nishizawa, j S. Ogio,hWe also aim at measuring energy spectra of proton, he- M. Ohnishi,e A. Oshima,k M. Raljevich,i H. Rivera,i T. Saito,llium and iron components separately around the knee energy T. Sako,e T. K. Sako,me T. Sasaki,a S. Shibata,k A. Shiomi,nregion with the new AS + MD. The standard cosmic-ray accel- M. Subieta,i M. Suzuki,a N. Tajima,o M. Takita,e Y. Tameda, peration model at SNR predicts the knee energy of each nucleus K. Tanaka,q R. Ticona,i H. Tsuchiya,r Y. Tsunesada,h S. Udob
component being proportional to Z (atomic number). We can and M. Wakamatsua (The ALPACA Collaboration)
discriminate proton and iron components by MD, as an iron a Faculty of Engineering, Yokohama National University, Japan
nucleus produces approximately 2 times more muons than a b Faculty of Engineering, Kanagawa University, Japan
proton with the same energy. Thus, the cosmic-ray accelera- c Faculty of Education, Utsunomiya University, Japantion scenario (SNR shock acceleration) will be verified by ob- d Department of Physics, Shinshu University, Japan
serving the linearly Z(atomic number)-dependent knee(=bent) e Institute for Cosmic Ray Research, The University of Tokyo,
positions of proton, helium, iron components around the knee Japanenergy region.
f Faculty of Engineering, Aichi Institute of Technology, Japan
Precise cosmic-ray anisotropy measurement at sidereal g Chubu Innovative Astronomical Observatory, Japan
time frame in the TeV energy region in the southern sky pro- h Graduate School of Science, Osaka City University, Japanvides unique data for the community to understand the mag- i Instituto de Investigaciones Fı́sicas, Universidad Mayor denetic field structure in the heliosphere. The ALPACA exper- San Andrés, Bolivia
iment gives complementary data in the TeV region to those j National Institute of Informatics, Japan
from IceCube above a few tens of TeV.
k College of Engineering, Chubu University, Japan
Furthermore, measurement of the Sun shadow in cosmic l Tokyo Metropolitan College of Industrial Technology, Japanrays above the TeV energy region in the southern hemisphere m Escuela de Ciencias Fı́sicas y Nanotechnologı́a, Yachay Tech,also helps understand the modeling of the magnetic fields be- Ecuador
tween the Sun and the Earth, complementary to the observa- n College of Industrial Technology, Nihon University, Japan
tions in the northern hemisphere.
o RIKEN, Japan

3.

ALPAQUITA

p Faculty of Engineering, Osaka Electro-Communication Uni-

versity, Japanq Graduate School of Information Sciences,

As a proto-type experiment, the ALPAQUITA[8] air r University, Japan
shower array without MD, which is ∼25 % of the ALPACA Japan Atomic Energy Agency, Japan
air shower array in area will be constructed at the experimental site, Chacaltaya Plateau, in 2019. The containers including materials and equipments necessary to construct the ALPAQUITA array arrived at La Paz, Bolivia in early 2019.

Bibliography
Papers and references[1] C. Lunardini et al., PRD, 92, 021301-1-5 (2015).
[2] A. Abramoswski et al., Nature, 531, 476 (2016).

Hiroshima City

52

High Energy Astrophysics Group[Spokesperson: K. Asano]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582

Overview
The high energy astrophysics group has been making theoretical and observational studies of violent astrophysical phenomena, in which nonthermal cosmic ray particles are being
accelerated. Targets of the group’s study include high energy
astrophysical objects such as supernova remnants, pulsar, pulsar wind nebulae, black hole/neutron star mergers, jets from
active galactic nuclei (AGNs), and gamma-ray bursts (GRBs).
We especially study the formation of relativistic outflows, particle acceleration in jets, emission mechanisms of electromagnetic waves or neutrinos, and electromagnetic counterparts forcompact binary mergers. Our research supports the multimessenger astronomy, which probes astronomical phenomena
through collaborating observations of electromagnetic waves,cosmic rays, neutrinos, and gravitational waves.
Research Topic 1: Blazar Emission Model with Turbulence Particle Acceleration
The double-peak structure of blazar spectra is well explained by leptonic models, in which gamma-ray emission
is generated via inverse-Compton (IC) scattering. The synchrotron self-Compton (SSC) process upscatters synchrotron
photons produced by the same electron population in jets. For
flat spectrum radio quasars (FSRQs), external photons surrounding the disk dominate the seed photons for the IC scattering. In most models, the nonthermal electrons are assumed
to be accelerated via shocks with the Fermi process. However,some blazar spectra imply that the electron power-law index
is smaller than 2, the value from the simplest shock acceleration model. The change of the index at the spectral break isvery large, which is inconsistent with the cooling break. An
alternative acceleration mechanism candidate is the stochastic
acceleration by turbulence.
We consider the case of so-called hard-sphere scattering
as an interaction model between turbulences and electrons, inwhich the acceleration timescale is independent of the electron energy. We numerically simulate broadband emission
from blazar jets with a one-zone time-dependent code, taking
into account the turbulence acceleration. Our model spectra
have reproduced the curved photon spectra of the five representative blazars from the radio to the high-energy gamma-ray
bands well (see Figure 1).
The fast mode is most likely the dominant wave mode ofthe electron energy source in low-magnetized plasma. The energy of the turbulence is injected at a large scale as fast waves,and cascades to shorter scales following the Kolmogorov law.
If the Larmor radius of electrons is significantly shorter than
the wavelength, the pitch angle diffusion via gyroresonanceis not responsible for particle scattering. In this case, themain mechanism of the energy exchange between electrons
and waves is transit-time damping (TTD); electrons are accelerated when their velocity along the magnetic field equals

Fig. 1. Model photon spectra for Mrk 421.

the parallel component of the phase velocity. This non-gyroresonant scattering may provide a mechanism to realize thehard-sphere-like acceleration in blazars, as required by ourmodels.
Research Topic 2: Pulsar Wind Nebulae with Particle Diffusion
Pulsar wind nebulae (PWNe) are extended objects arounda rotation-powered pulsar with a size of about a few pc,
and their emission spectrum extends from radio to gammaray. The emission is due to synchrotron radiation and inverse Compton scattering by electrons and positrons accelerated at the termination shock generated by the interaction
between the supernova remnant (SNR) and the pulsar wind.
Based on this idea, Kennel & Coroniti (1984) established a
1-dimensional steady magnetohydrodynamical (MHD) model(hereafter the KC model) of PWNe. Although the KC model
has been accepted as a standard model of PWNe, some problems in the KC model have been raised by morphology research with high angular resolution observations in X-ray.
The X-ray photon spectrum becomes gradually softer as theradius increases. Such gradual softening is incompatible with
the KC model, which predicts sudden softening at a certain
radius. Our paper, Ishizaki et al. (2017), showed that the KC
model has severe difficulty reproducing both the entire spectrum and the surface brightness profile simultaneously.
In the KC model, particles are simply advected with thespherical wind. Some authors have pointed out that the particle diffusion effect resolves the above discrepancy. We
present a new PWN model that solves both advection and diffusion of nonthermal particles in a self-consistent way to satisfy the momentum and energy conservation laws. Assuming
spherically symmetric steady outflow, we calculate the emission spectrum integrating over the entire nebula and the radial
profile of the surface brightness. We find that the back reaction of the particle diffusion modifies the flow profile. The
photon spectrum and the surface brightness profile are different from the model calculations without the back reaction of
the particle diffusion.
Our model is applied to the two well-studied PWNe, 3C
58 and G21.5-0.9. By fitting the spectra of these PWNe, we
determine the parameter sets and calculate the radial profiles

53of X-ray surface brightness. For both the objects, obtained
profiles of X-ray surface brightness and the photon index are
well consistent with observations (see Figure 2).

Fig. 2. Radial profiles of the surface brightness of X-rays, radio, opticaland gamma-rays for G21.5–0.9. The red line represents the modelwith the diffusion effect, and the thin black solid line represents the
X-ray profile of the KC model.

Our model suggests that particles that have escaped fromthe nebula significantly contribute to the gamma-ray flux. A
gamma-ray halo larger than the radio nebula is predicted in
our model.
Research Topic 3: Hadronic Cascade in Gamma-Ray
Bursts
The high-energy (> 100 MeV) emission observed by the
Fermi Large Area Telescope during the prompt phase of someluminous GRBs could arise from the hadronic cascade induced by the photomeson production. The cascade efficiency
largely depends on the maximum proton energy. In previous
studies, the maximum energy is assumed as the theoreticallyhighest one. Although the maximum proton energy can reachsuch a high energy when considering the acceleration by a relativistic shock in the Bohm condition, such a condition may
not be achieved in real shocks. In the case that the maximum proton energy falls below the threshold energy of thephotomeson process to interact with the photons of energy ofthe spectral peak, the photomeson process may not be efficient to produce the observed high-energy flux in a GRB. On
the other hand, the Bethe–Heitler (BH) pair production process of protons has a lower threshold energy. Thus, protons
may still induce an electromagnetic cascade via this process,producing high-energy emission during the prompt emission
phase.
We revisit the hadronic model for the prompt high-energy
emission of GRBs with a smaller maximum proton energy.
We show that with a relatively low maximum proton energy
with a Lorentz factor of 105 in the comoving frame, the cascade emission can still reproduce various types of high-energy
spectra of GRBs as shown in Figure 3. The adopted parameters in the spectrum fittings are consistent with the constraints
from the null detection of GRB-correlated neutrino events by
the IceCube neutrino telescope. The cascade emission may
also lead to a low-energy excess below a few keV and might
be used as an indication of the baryon component in the GRB

ejecta.

Fig. 3. Spectral fitting for GRB 090926A with a suppressed maximum proton energy. The photon energy unit is keV. The purple
curves represent the emissions of cascaded electrons originatingfrom the BH process, while the red curves represent that fromthe photomeson process. The red and purple dotted and dashedcurves show the synchrotron and IC radiation of the electrons fromthe dominant process. The green dashed curves show the fitting of
the burst’s spectrum by the Fermi-LAT collaboration. The shaded
region shows the uncertainty in the spectrum fitting at the high-energy end.

Research Topic 4: Kilonova Emission
A kilonova/macronova is the emission that has been expected to be associated with a neutron star-neutron star or
black hole-neutron star merger as the consequence of themass ejection from the system. The previous studies showedthat lightcurves of kilonovae/macronovae depend on the mass,
velocity, and element abundance of ejecta. Since thoseejecta profiles reflect the merger process and the late time
evolution of the merger remnant, detailed property of kilonova/macronova lightcurves, such as those brightness andcolor evolution, provide the physical information of themerged binary and the post-merger evolution of the system as complementary information to that inferred by thegravitational-wave data analysis.
Electromagnetic (EM) counterparts were observed simultaneously with the first gravitational wave event from a binary
neutron star merger (GW170817) over the entire wavelength
range, from gamma-ray to radio wavelengths. In particular, acounterpart in optical and infrared wavelengths is identified as
the emission from a kilonova/macronova. A number of studies have shown that the optical and infrared EM counterparts
found in GW170817 is consistent with kilonova/macronova
models composed of multiple ejecta components with different lanthanide fraction. However, in most these studies, contribution from each ejecta component to the lightcurves is separately calculated and composited. In reality, the lightcurves
are determined through the non-trivial radiation transfer ofphotons in both ejecta components.
We perform an axisymmetric radiative transfer simulation for kilonovae/macronovae taking the interplay of multiple
ejecta components of non-spherical morphology into account.
We show that the photon interplay of multiple ejecta components is important for the case that the post-merger ejecta

54
D=40 Mpc, 20!)e<28!
-17

g
r
i
z
J
H
K

AB absolute magnitude

-16
-15
-14
-13
-12
-11

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
t [day]Fig. 4. Optical and infrared lightcurves of EM counterparts in
GW170817 compared with the kilonova/macronova model observed from 20◦ ≤ θ ≤ 28◦ . We assume that GW170817 is at a
distance of 40 Mpc. All the magnitudes are given in AB magnitudes.

is surrounded by a high-opacity dynamical ejecta, predictedby numerical-relativity simulations. In particular, we showthat the optical and infrared lightcurves of EM counterparts
in GW170817 can be reproduced by smaller ejecta masses
than the previous studies by a factor of ≈ 1.5–2. This implies that taking the interplay of multiple ejecta components
into account is crucial for the ejecta mass estimation.
Research Topic 5: CALET Project
We have joined CALET, CALorimetric Electron Telescope, which is a mission for the Japanese Experiment
Module-Exposed Facility (JEM-EF) on the International
Space Station. The CALET mission aims at revealing unsolved problems in high energy phenomena of the Universeby carrying out accurate measurements of high energy spectraof electrons, gamma-rays and nuclei. HTV5 equipped withCALET was successfully launched by the H-IIB at 8:50:49
p.m. on August 19 2015 (JST) from the Tanegashima Space
Center, and CALET is observing cosmic rays without apparent problems.
Major scientific objectives are to search nearby cosmic ray
sources and dark matter signatures by carrying out accurate
measurements of cosmic ray electrons in 1 GeV – 20 TeV andgamma-rays in 4 GeV – 10 TeV. Since proton background isvery large, high proton rejection power is mandatory for highenergy electron and gamma-ray measurements. CALET has
an imaging and deep calorimeter with 30 radiation length forelectromagnetic particles, which provides high proton rejection and excellent energy resolution.
First CALET results of a cosmic-ray electron and positron
spectrum from 10 GeV to 3 TeV was published in 2017. Extended results on the cosmic-ray electron–positron spectrum
from 11 GeV to 4.8 TeV, utilizing the data up to November
2017, are shown in Figure 5. In the energy region from 40 to
300 GeV, the power-law index is found to be −3.12 ± 0.02,
which is consistent with other experiments within errors. Our
results agree with the AMS-02 results. However, the spectrum
is considerably softer from 300 to 600 GeV than the spectra

Fig. 5. Cosmic-ray all-electron spectrum measured by CALET from
10.6 GeV to 4.75 TeV, where the gray band indicates the quadratic
sum of statistical and systematic errors (not including the uncertainty on the energy scale). Also plotted are direct measurementsin space and from ground-based experiments.

measured by DAMPE and Fermi-LAT. The CALET results
exhibit a lower flux than those of DAMPE and Fermi-LAT
from 300 GeV up to near 1 TeV.
On the other hand, the flux in the 1.4 TeV bin of DAMPE
spectrum, which might imply a peak structure, is not compatible with CALET results at a level of 4σ significance, including the systematic errors from both experiments.

Bibliography
Papers in refereed journals
1. Kai Wang, Ruo-Yu Liu, Zi-Gao Dai, Katsuaki Asano“Hadronic Origin of Prompt High-Energy Emission ofGamma-Ray Bursts Revisited: In the Case of a Limited Maximum Proton Energy”, Astrophys. J., 857,
24(12pp) (2018).
2. 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 the International Space Station”, Phys. Rev. Lett., 120, 261102(7pp) (2018).
3. 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”, Astropart.
Phys., 100, 29-37 (2018).
4. Katsuaki Asano, Masaaki Hayashida “Blazar Spectrawith Hard-Sphere-like Acceleration of Electrons”, Astrophys. J., 861, 31(7pp) (2018).
5. 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).
6. N. Cannady, Y. Asaoka, F. Satoh, et al. “Characteristics
and Performance of the CALorimetric Electron Telescope (CALET) Calorimeter for Gamma-Ray Observations”, Astrophys. J. Supp., 238, 5(16pp) (2018).

55

7. Kyohei Kawaguchi, Masaru Shibata, Masaomi
Tanaka “Radiative Transfer Simulation for the Opticaland Near-Infrared Electromagnetic Counterparts to
GW170817”, Astrophys. J. Lett., 865, L21(6pp)(2018).
8. Yutaro Tachibana, Makoto Arimoto, Katsuaki Asano,et al. “Late Engine Activity of GRB 161017A Revealedby Early Optical Observations”, Pub. Astron. Soc. Jap.,
70, 92(9pp) (2018).
9. G. Abdellaoui, et al. “EUSO–TA — First Results froma Ground-Based EUSO Telescope”, Astropart. Phys.,
102, 98-111 (2018).
10. Wataru Ishizaki, Katsuaki Asano, Kyohei Kawaguchi“Outflow and Emission Model of Pulsar Wind Nebulaewith the Back Reaction of Particle Diffusion”, Astrophys. J., 867, 141(12pp) (2018).

Fig. 6. Concept of imaging observation of PeV ν ’s, γ -rays, and nuclei
with Ashra NTA summit array. For example, Ashra-1 and NTA
detectors can simultaneously observe our galactic bulge in their
FOV, checking the coincidence of ν ’s with γ -rays originating fromthe same objects or regions.

11. Tomoya Kinugawa, Yuichi Harikane, Katsuaki Asano“Long Gamma-Ray Burst Rate at Very High Redshift”,
Accepted for Astrophys. J., (2019).
Textbook
1. Nobuyuki Kawai, Katsuaki Asano “Gamma-Ray
Burst”, NIPPON HYORON SHA, Tokyo, (2019).
ISBN: 978-4-535-60744-6

Other Activities
Ashra NTA

Combined detection of PeV ν ’s and γ ’s from an accelerator provides indispensable identification of the location andthe physics mechanism i.e. p + γ → ∆+ → π 0 + p, π + +n; p + nucleus → π ±, 0 + X, which can clearly reveal the longstanding unresolved origin(s) of cosmic rays. Recently several observations suggesting cosmic ray accelerators havebeen independently made [1, 2, 3]. Such a “multi-particle”
paradigm [4] can be performed by Ashra NTA with the single
unique detector system [5].
Since 2001, we have been developing the Earth-skimmingtau ν (ES-ντ ) air-shower technique [6], as a promising potential. which can enjoy a large target mass by detecting airshowers (ASs) produced by τ decays in the air. The τ ’s, produced by ντ ’s that interact with the Earth matter, traverse, andemerge out of a mountain or the ground decaying and generating ASs. Adding to that, the advantages are perfect shieldingof cosmic ray secondaries, precise arrival direction determination, and negligible background from atmospheric ν ’s [7].
The detectors of Ashra-1 and its extention plan NTA can precisely image AS Cherenkov (CE) and fluorescence (FL) light
generated from ES-ντ and γ ASs in the huge effective volume
of air around the mountain in the field of view (FOV) (Figure 6) [8].
The Ashra Phase 1 (Ashra-1) [9] light collector (LC) (Figure 7left) achieves the total resolution of ∼ 3 arcmin covering42◦ (Figure 7right). The key feature is the use of electrostatic

Fig. 7. Left: The Ashra-1 light collector (LC) facing Mauna Kea. Right:
Boundary (large red circle) between the inside (open circle) andoutside (hatched area) of the FOV of the LC and the layout oftrigger pixel FOVs (blue boxes) for Cherenkov τ shower observation. Attached array of the trigger pixel FOVs (upper four blueboxes) to check the detection sensitivity with ordinary cosmic-ray
air showers at a higher elevation. A simulated image of a cosmic-ray air shower readout along the trigger (points). The trajectory of GRB081203A counterpart (circular arc), the ridge lines of
Mauna Kea (red) and Mauna Loa (green) mountains, the horizon,
and Mauna Kea access road are shown.

rather than optical lenses to generate convergent beams withthe 20 inch Photoelectric Lens Imaging tube (PLI) [10] (Figure 7) demagnifying to 1 inch at focal surface, enabling highresolution over a wide FOV [11]. The following trigger
readout Photoelectric Image Pipeline (PIP) [12] can image
and read out three independent phenomena on different timescales, i.e. AS CE emission (ns), AS FL (µ s), and starlight (s),without sacrificing the S/N ratios. Figures 8 and 9 show thecosmic ray energy spectrum and the ES-ντ limit from updated
the Ashra-1 3rd observation period (Obs3) of 1863 hours [13].
The observed cosmic ray spectrum agrees reasonably wellwith the Monte Carlo simulation based on the cosmic ray flux
observations in the knee region by the conventional air shower
arrays as used in our first ES-ντ search paper [14]. The estimation of the detection sensitivity of the Ashra-1 LC andthe validity of the reconstruction procedure were well demonstrated.
Our results are the most stringent in the PeV–EeV region
and complementary to other observational results for the sub-

56

Fig. 8. Observed cosmic-ray flux spectrum (filled circles) with bars
indicating statistical and systematic errors and the MC predictions for proton primary (red histogram) and iron primary (bluehistogram) assumptions.

Fig. 9. Ashra 90% CL limit (thick continuous line (red)) on ντ flux fromsources in the PeV–EeV region (see the text). For comparison,
MAGIC [15], IceCube [16], Auger [17], LUNASKA [18] limits aswell as model predictions [19, 20] are shown. Plots adopted from
[15].

Fig. 10. Neutrino flux sensitivity of NTA with only fluorescence mode,that including far-Cherenkov mode and other experiments, neutrino flux predictions, and existing flux constraints. Figure adaptedfrom [8].

the half of the adjacent ones. It results in total rate of thestereoscopic observation can be more than 70% of the trajectory in the sky. The estimated annual observable time of1150 hours ×εw during nights without moon in the south ismore than 50 times better than HESS achieved i.e. 227 hours
for Sgr A∗ in 10 years [2], assuming the weather efficiency
εw ∼ 90% according to the Ashra-1 operation.
Another fascinating is the detection of γ -rays with thelarge zenith-angle (LZA) ethod. The galactic bulge has the
trajectory in the southern night sky with LZA more than 50 degrees, corresponding to the shower max distance larger than
9 km and the detection threshold energies higher than 16 TeV.
Our situation studies check the cut-off energy in the γ -rayspectrum in the galactic bulge or central region. We confirm
the LZA method is promising particularly for the PeV γ -raydetection. Once the northward NTA units detect ν ’s from thesame γ -ray objects observed by Ashra-1 LCs, we can argue,
more concretely than ever, the physics of the occurrence ofγ -rays and ν ’s [8].

Bibliography
PeV and over-EeV energy regions, and indicate the advanced
sensitivity of the system.
By optimizing the layout of the NTA stations to enhance
the sensitivity for ES-ντ ’s around 1 PeV from the simulation
studies [5, 7], four NTA stations will be served on Mauna Loa
at 3000 - 3500 m asl (Summit Array), which watch the air volume surrounding the mountain including the surface. Mauna
Loa is the world largest volcano suitable for detecting CE andFL light from τ ASs with both short and long decay lengths
and γ ASs as shown Figure 6. Figure 10 shows the neutrino
flux sensitivity of NTA with only fluorescence mode, that including far-Cherenkov mode and other experiments, neutrino
flux predictions, and existing flux constraints. The NTA neutrino sensitivity can be fairly competitive in PeV-EeV.
The combination between Ashra-1 and NTA detector units
is planned to realize the comprehensive observation both with
TeV-PeV γ -rays and PeV ν ’s. Six combined Ashra-1 LCs
will be realigned for the FOV centers to be on the arc of the
Galactic Center trajectory maximizing the stereoscopic observation efficiency. Each LC FOV will be overlapped with

[1] M. Aartsen et al., PRL 113, 101101 (2014).
[2] HESS Collab., Nature 531, 476 (2016).
[3] IceCube Collab., Science 361, 147 (2018).
[4] M. Sasaki, ICRR2000 Sat. Sympo., 109 (2000).
[5] M. Sasaki, G. Hou, arXiv:1408.6244 (2014).
[6] M. Sasaki, et al., Astropart. Phys. 19, 37 (2003).
[7] Y. Asaoka, M. Sasaki, Astropart. Phys. 41, 7 (2013).
[8] M. Sasaki (Ashra-1/NTA), PoS (ICRC2019) 1003.
[9] M. Sasaki, Prog. Theo. Phys. Suppl. 151, 192 (2003).

[10] Y. Asaoka, M. Sasaki, NIMA 647, 34 (2011).
[11] M. Sasaki, et al., NIMA 492, 49 (2002).
[12] M. Sasaki et al., NIMA 501, 359 (2003).

57[13] S. Ogawa (Ashra-1), PoS (ICRC2019) 970.
[14] Y. Aita, et al., ApJL 736, L12 (2011).
[15] M. Ahnen, et al., Astropart. Phys. 102, 77 (2018).
[16] R. Abbasi, et al., ApJ, 732 (2011) 18.
[17] P. Abreu, et al., ApJ Lett., 755 (2012) L4[18] C. James, et al., MNRAS, 410 (2011) 885.
[19] A. Cuoco, S. Hannestad, PRD, 78 (2008) 023007.
[20] M. Kachelriess, et al., New J. Phys., 11, (2009) 065017.

γ I Groupγ I Consortium[Spokesperson : R.Enomoto]Collaboration list:
ICRR, The University of Tokyo, Chiba, Japan; National Institute of Technology, Sendai College, Miyagi, Japan College of
Science, Ibaraki University, Ibaraki, Japan; Faculty of Medical Engineering and Technology, Kitasato University, Kanagawa, Japan; Department of Radiological Sciences, Tokyo
Metropolitan University, Tokyo, Japan; IPNS, High Energy
Accelerator Research Organization, Ibaraki, Japan; National
Cancer Center, East, Chiba, Japan;
A portable radioactive detection system for outdoor
measurement of concentration of radioactive caesium ina mushroom bed log at a low-level contaminated environment
Log cultivation of shiitake mushrooms has been declining in the Fukushima Prefecture and its surrounding areas
due to the Fukushima Dai-ichi Nuclear Power Plant accident. Shiitake mushrooms by log cultivation can be rarely
found in Kanto area nowadays. The Forestry Agency has
set a maximum concentration limit for radioactive caesium of
50 Bq/kg for bed logs to cultivate safe shiitake mushrooms(<100 Bq/kg). The reason why the latter level is higher than
the former is due to the high absorption feature by shiitake
mushrooms[1]. In addition the difficulty is that this level ofradioactivity is too small to be separated from the backgroundby the natural radioactivity. In addition, broad area in East
Japan is weakly polluted and its level is nearly that of natural background. The instruments to measure this level of radioactivity are available only for indoor measurement, however, they are too heavy to be portable with thick radiation
shield[2]. Also they are expensive[2]. The demands to thedetector to be used at the polluted area, i.e., cultivation area
(outdoor), is high.
In this study, we developed a low-cost portable radioactive detection system without heavy radiation shield. The safe
bed logs were selected using a screening test with the portableradioactive detection system even at a for levels of contamination (∼0.1 Sv/h). The detail can be found in Kagaya et al[3].

Fig. 11. Principles of measuring radioactive caesium concentrations
in a bed log for the cultivation of shiitake mushrooms.

We developed the following method to measure low level
radioactivity even in the polluted area as shown in Fig. 11.
The inorganic scintillators made of CsI(Tl) are used to detect 662 keV γ -rays from 137 Cs. The size of crystals is 5cm
cube. 4 Crystals are used. The 2-inchi PMTs readout the lightyields. The total procedure is consisted of two measurements.
First, the polluted mushroom bed log is placed on the detector
and the γ -ray’s energy spectrum is taken. Second, the polluted
log is replaced with un-polluted one which was bought from
West Japan area. the 662-keV γ -ray peak is obtained fromthe difference of those two spectra. The reason why the log
is placed above the crystals is that the 662-keV γ -rays rarely
radiated from the sky direction compared with the other directions. Also the meaning to place un-polluted log is to compensate the effect of absorption by the wood itself of γ -ray fromthe direction of sky. All through two measurements, the detector is set at the same place in order to cancel the backgroundradiations mainly from the ground, woods, and etc.
The photograph of the detector system is shown in Fig.
12. The electronic system is as same as the γ I Compton

Fig. 12. A portable radioactive detection system.

Camera[4]. The total weight of the system is 5 kg. A single Li-ion battery pack can operate the system for 12 hours.
Portability is, therefore, well guaranteed.
The obtained energy spectra are shown in Fig. 13. The
solid histograms were obtained for the polluted bed log andthe hatched ones were for the un-polluted bed log. The energy region only around 605, 662, and 795 keV show the differences, and the spectrum of the other region remains same.
After the outdoor measurements of 28 polluted bed logs,
they were milled and measured by the Ge detector. The correlation between two data is shown in Fig. 3. The correlation+0.08
coefficient is 0.83−0.17. The total error including the systematic ones are considered to be 10 Bq/kg. The size differenceof the measured and reference logs is one of those errors. We,
therefore, concluded that our detector is suitable for screening
the polluted bed log over 50 Bq/kg even at outside of 0.1µ Sv/h

58[4] M.Kagaya, H.Katagiri, R.Enomoto et al, Nucl. Instrum.
Meth. A804, 25-32 (2015).

Fig. 13. An energy spectrum obtained from the measurement of a bed
log outdoors. This energy spectrum was obtained from four scintillation counters and the exposure time was 20 minutes. The figureon the left side shows the energy spectrum from 0 to 1000 keV.
The figure on the right side displays the enlarged, energy spectrum near the 662 keV peak. Areas with solid lines represent dataobtained by measuring a contaminated bed log, whereas shaded
areas represent data measured on a bed log without radioactive
contamination.

Fig. 14. Plotted gamma-ray events detected by a portable radioactive
detection system and the concentrations of radioactive caesium
measured by a Ge detector. Radioactive caesium concentrations
were corrected using density and water content of the bed logs
used for shiitake mushroom cultivation.

area. The prefectures of Tochigi, Ibaraki, and Miyagi corresponds to this level.
We developed a portable detector to measure the low-level
radioactivity from the shiitake mushrooms bed log. The performance was proven by the comparison with the results of
Ge detector. The screening of polluted bed log can be carried out even at the low-level polluted area such as prefectures
surrounding Fukushima.
This detector only measures bed logs. The next target isto measure standing trees. The project is well going on using
32 keV γ -rays from 137 Cs, i.e., K α of 133 Ba. The advantage isthat a thinner and lighter shield is applicable than in the caseof 662-keV γ -rays.

Bibliography[1] http://www.rinya.maff.go.jp/j/press/tokuyou/120830.html[2] For example, Gamma Spotter F, Furukawa Electric.
[3] M.Kagatya, H. Katagiri, R.Enomoto et al, RADIOISOTOPES, 68, 305-315 (2019), doi: 10.3769/radioisotopes.68.305

59

ASTROPHYSICS AND GRAVITY DIVISION
Overview
Astrophysics and Gravity Division consists of Gravitational Wave Group, The Observational Cosmology Group,
Primary Cosmic Ray Group and Theory Group.
The Gravitational Wave Group conducts experimental research of gravitational wave with researchers of gravitational
wave experiment and theory in Japan. The main items are
the construction of the large scale cryogenic interferometer(KAGRA) at Kamioka underground and the operation of
CLIO. For this purpose, KAGRA observatory was establishedat the beginning of the fiscal year of 2016 to assist the construction of KAGRA gravitational wave telescope.
The Observational Cosmology Group studies the cosmic
history based on deep multi-wavelength observations in collaboration with worldwide researchers. This group has started
a new optical deep survey project with the wide-field imagerof Hyper Suprime-Cam mounted on the Subaru telescope.
Theory Group conducts both theoretical study of the Universe and astroparticle physics.

Fig. 1. Schematic view of the bKAGRA interferometer[4]. Type-A,
Type-B, Type-Bp, and Type-C are the names of vibration isolation
system for each mirror.

Gravitational Wave Group
KAGRA Project Status[Spokesperson : Takashi UCHIYAMA]
ICRR, The Univ. of Tokyo, Hida, Gifu 506-1205

Overview
KAGRA, Large-scale Cryogenic Gravitational wave Telescope, aims at detecting gravitational waves and developing
gravitational wave astronomy, which was established by the
first detection of gravitational waves by LIGO. KAGRA employs a 3 km L-shaped laser interferometer with a cryogenic
mirror system placed underground at Kamioka[1]. The KAGRA development is divided into two stages: the initial KAGRA (iKAGRA) and baseline KAGRA (bKAGRA). The iKAGRA interferometer is a simple Michelson interferometerwith a 2-Watt laser, room-temperature mirrors, and a simple
vibration isolation system. We completed the iKAGRA interferometer with a test run in April 2016[2]. Then we proceeded
to bKAGRA.
Figure 1 and 2 show a schematic view of optical layout of
the bKAGRA interferometer and the KAGRA vibration isolation systems. Table 1 shows design parameter of the bKAGRA interferometer[4]. The bKAGRA interferometer will
employ a Resonant Sideband Extraction (RSE) interferometerwith 180-Watt laser, cryogenic Sapphire mirrors, and several
kinds of vibration isolation systems. The bKAGRA interferometer should attain the sensitivity high enough for the detection of gravitational waves with the help of the high power

Fig. 2. KAGRA vibration isolation systems[4]. KAGRA equips fourkinds of vibration isolation systems such as Type-A, Type-B, Type-Bp, and Type-C.

laser and RSE interferometer to reduce the quantum noise, thecryogenic Sapphire mirrors to reduce the thermal noise, andthe vibration isolation systems to reduce the seismic noise.
Figure 3 shows designed sensitivities of bKAGRA in case of
Broadband RSE (BRSE) and of Detuned RSE (DRSE), where
incoherent sum of the fundamental noise sources is assumed.
Observation range for an in-spiral and merger of neutron-starbinary reaches 135 Mpc in BRSE and 153 Mpc in DRSE withthe same definition of the observation range as LIGO and
Virgo.
Figure 4 shows the international collaborative observation scenario[3]. LIGO conducted Observation 1 (O1) from

60
Table 1. The design parameters of the bKAGRA interferometer[4].

Arm cavity length
Laser wave length
Input power at PRM
Arm intra-cavity power
ITM transmittancePRM transmittance
SRM transmittance3000 m1064 nm
67W
340 kW
0.4 %
10 %
15 %

Test mass size
Mass of test mass
Temperature of test mass
Beam radius at test mass
PRC/SRC lengths
Detuning angle
Homodyne angle

Fig. 3. The designed sensitivity of the bKAGRA interferometer[4]. ”total”, ”seismic”, ”mirror thermal”, ”suspension thermal”, ”quantum”,
and ”SQL” mean total sum of fundamental noise sources shownin this figure, seismic noise including gravity gradient noise, mirror thermal noise, suspension thermal noise, quantum noise, andstandard quantum limit, respectively. The figure shows ”total” and”quantum noise” in both Broadband RSE (BRSE) and Detuned
RSE (DRSE) case. Observation range for an in-spiral and mergerof neutron-star binary reaches 135 Mpc in BRSE and 153 Mpc in
DRSE with the same definition of the observation range as LIGO
and Virgo.

September 12th, 2015 to January 19th, 2016 and Observation 2 (O2) from November 30th, 2016 to August 25th, 2017.
Virgo joined O2 from August 1st, 2017. LIGO and Virgo
started Observation 3 (O3) from April 1st, 2019 and O3 willcontinue by the end of April in 2020. KAGRA is aiming tojoin O3 in 2019.
In FY2018 we started with an operation of KAGRA interferometer as bKAGRA phase 1 which is 3 km Michelsoninterferometer with two sapphire mirrors suspended by theType-A vibration isolation systems. One sapphire mirror wascooled at 18 K. The operation was done from April 28 to
May 6 in 2018 and it was the first demonstration of operating
km-class interferometer at cryogenic temperature. Figure 5
and Figure 6 shows a summary of daily status of the operation and a strain sensitivity comparing with noise sources[4],respectively. Duty factor in the first half of the operation
reached 88.6%. The observation range for an in-spiral andmerger of neutron-star binary and BH binary reached 17 pc
and 100 pc, respectively. The longest continuous operation
time was 11.1 hour.
After the bKAGRA phase 1 operation, we started con-

φ 22 cm × 15 cm
22.8 kg
22 K
3.5 cm
66.6 m
3.5 deg
135.1 deg

Fig. 4. International observation scenario[3]. Virgo was joined in Observation 2 (O2) from August 1st in 2017. LIGO and Virgo started
Observation 3 (O3) from April 1st in 2019. O3 will continue by theend of April in 2020. KAGRA is aiming to join O3 in 2019.

Fig. 5. Daily status of bKAGRA phase 1 operation[4]. The figure
shows daily duty factor (Top panel), inspiral range (Middle panel),and seismic noise level (Bottom panel) during the operation. The
operation was done from April 28 to May 6 in 2018.

struction of the bKAGRA interferometer with 40 W laser
power. What we have installed were an infrared laser system with the maximum power of 40 W, two sets of arm lengthstabilization system using a green laser, calibration systems
using photon radiation pressure, large beam baffles, transmission monitor systems, some optics consisting a signal recycling cavity and output optics, two input test masses called
ITMX and ITMY in Figure 1, and so on. Physical Environmental Monitor (PEM) is a sensor network consisting several kinds of environmental sensors such as accelerometers,

61[5] ”An arm length stabilization system for KAGRA and future gravitational-wave detectors”, KAGRA collaboration, to be published in Class. Quantum Grav. (2019)[6] http://gwcenter.icrr.u-tokyo.ac.jp/en/

Integrated DAQ/control system using real time computers[Spokesperson : Osamu MIYAKAWA]
ICRR, The Univ. of Tokyo, Hida, Gifu 506-1205

Fig. 6. Strain sensitivity of KAGRA in phase 1 operation[4].

seismometers, magnetometers, thermometers, acoustic soundmonitors, power monitors and so on. Purpose of PEM is
to check the detector health, noise sources, and data quality
in cooperation with the detector characterization group. We
placed many sensors in the KAGRA site and monitoring has
already started.
We have tried lock acquisition of the X-arm cavity forthe first time in parallel with the installation works mentioned
above. The lock acquisition of the X-arm cavity was successfully achieved with helps of the arm length stabilization system. Then we carried out charcterization of the X-arm cavity.
Table2 shows a summary of optical parameters of the X-arm
cavity comparing with designed and measured values.
We also enhanced the international collaborations with the
Einstein Telescope (ET) project, LIGO, Virgo, Korean andother Asian groups mainly based on the JSPS core-to-coreprogram.
The rapidly progressing status of KAGAR were presented
in many international conferences. Many papers about the
progress of KAGRA were also published [1], [4], [5]. We
also presented activities on our web-page.[6]Bibliography[1] ”KAGRA: 2.5 generation interferometric gravitational
wave detector”, KAGRA collaboration, Nature Astronomy, Vol. 3, January 2019, 35-40[2] ”Constructionof
KAGRA:
anundergroundgravitational-wave observatory”, KAGRA collaboration, Prog. Theor. Exp. Phys. 2018, 013F01 (2018)[3] ”Prospects for observing and localizing gravitationalwave transients with Advanced LIGO, Advanced Virgoand KAGRA”, Abbott, B.P., Abbott, R., Abbott, T.D. et
al. arXiv:1304.0670v9 [gr-qc][4] ”First cryogenic test operation of underground km-scale
gravitational-wave observatory KAGRA”, KAGRA collaboration, Class. Quantum Grav. 36 165008 (2019)

The 2018 fiscal year, we started from 9 days short operation with a 3km simple Michelson configuration with a single
end mirror in low temperature. By the short operation in May
of 2018, basically we established the whole integrated control/data acquisition system for KAGRA.
Stable operation with the real time control system The interferometer configuration was just a Michelson type and it
was still much simpler than the same as the final one. However as a control system, it was almost the same as the final configuration. The difference was only that we had no
input test masses. We had already prepared the control system for the two input test masses. So we did not increase thenumber of control computers in this FY, even the input test
masses were installed after the short operation in the rest of
this FY. On the other hand, we spent a lot of time to stabilized
the whole control/data acquisition system. At this stage we
had 25 real time control computers, 8 data acquisition computes, 10 servers as camera, script etc. and 15 workstations asuser clients. Especially connecting such a many numbers of
the real time control computers were the first experience forus. We had carefully performed many tests for this system in
the independent test bench but the number of computers had
been limited in the test. We experienced several serious issues
caused by them number of computers working at a time.
The biggest issue was that we noticed some glitches happened in the signal. They happened when the control computers had a heavy load. Basically the control computers use areal time operating system. Some delay due to the heavy task
causes a serious problem for control loops and it emerges as
jumps or glitches on many signals. These glitches increased
in proportional with the number of operated computers. Then
we realized that this issue can be solved by replacing the slow
computer to the faster one after some experiments at the test
bench. We have just started replacing the slow computerswhich are being used in KAGRA. Once the new faster computer runs processes in the KAGRA system, it never happens
glitches or some other serious problem. We keep replacing
existing slow computers in the next year and we will finish it
before the observation starts in the end of 2019.
Guardian: automatic operation system One of the big topics of this fiscal year was that some automatic control system implemented drastically. It was called ’Guardian’. The
Guardian is always looking at all important channels and puts
proper commands according to a script that was written by human before. Almost of subsystems in KAGRA are controlled

62
Table 2. Optical parameters of the X-arm cavity[5].

Parameter name
Cavity length
Finesse for 1064 nm
Roundtrip loss for 1064 nm
Finesse for 532 nm

under this Guardian, and they are automatically operated as awhole gravitational wave detector. KAGRA is the huge plant
which consists of a large number of subsystems. We need toconnect the subsystems, and the KAGRA should work as awhole combined device as a gravitational wave detector.
Stability of data acquisition system This system also has animportant role as a data acquisition system. This data acquisition system consists of a collecting data server, data providing
servers, and data writing servers. Actual chunk of data comesfrom the control computers and it was collected by the datacollecting server. As increasing number of control computers, amount of data increased drastically in this year, and wenoticed that the streaming data sometimes failed once or several times in a week. We had to replace some of the weakest
servers to the faster one with more memories. We had prepared two paths for data acquisition as redundancy, so we did
not miss data yet. We need to keep watching the stability ofthese data transfers until the actual observation starts.
In the FY of 2019, we plan to join an observation with
LIGO and VIRGO. It is expected that this DAQ/control system performs stable automatic operations for commissioning
work, noise hunting, and the observation.
Data Analysis[Spokesperson : Hideyuki Tagoshi]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba, 277-8582

There are variety of data related activities in KAGRA. The
main data server of KAGRA is located at ICRR Kashiwa. It
has a 2.5PiB data storage. All KAGRA data taken at Kamioka
are packed into one file for every 32 seconds, and are transferred continuously to the main data server at Kashiwa. Beside this, low latency data transfer is also done by packing
only main interferometer data into one file for every 1 seconds. For low latency data transfer, the latency of about 3 seconds is achieved from Kamioka to Kashiwa (this time include
the time necessary for calibration).
KAGRA detector is producing several hundreds thousandsof channels of data which record signals from various sensors,
signals to control instruments, signals to monitor environmentof the detector. Those data are used to check the status of detector and to improve the sensitivity. It is important to introduce convenient tools to visualize the data in order to accelerate the installation and commissioning works. Web based
visualization tools are now being developed. Some of tools
developed by LIGO group are also installed. These tools arealso useful when gravitational wave signals are detected. In

Designed
3000 m1530¡ 100 ppm
49.2

Measured
29999.990(2) m
1410(30)
86(3) ppm
41.0(3)order to have a confidence of detection of gravitational wave
signals, it is important to investigate environmental channels
whether there are any noise sources which might produce data
which are similar to real gravitational wave signals. These visualization tools can be used to check various environmental
channel data.
In order to detect gravitational wave signals, several
pipelines have been developed in KAGRA. Among them, apipeline to search for gravitational waves from compact binary coalescences (CBC) are developed in KAGRA Algorithmic Library (KAGALI). KAGALI is a common data analysis library written mainly in C. The CBC pipeline have beenused to analyze KAGRA data during iKAGRA operation. Improvement of the CBC pipeline are now ongoing in order totreat multiple detectors and to introduce the spin parametersin the waveform. These tasks will be continued in 2019. The
improvement of the parameter estimation pipeline for CBC
signals based on the Markov Chain Monte Carlo method wascontinued from the last year. This work is lead by Hyung Won
Lee (Inje Univ).
There are several efforts to introduce new data analysis
methods in the analysis of gravitational wave data. Amongthem, the performance of Non-Harmonic Analysis (NHA)
in visualizing the time-frequency behavior of the data wasevaluated. NHA is a method to evaluate the spectrum ofdata by evaluating multiple instantaneous frequencies and amplitudes of data in a way which is different from discrete
Fourier transform. We find that there are various advantage in
NHA in visualizing CBC signals compared with the methodof short time Fourier transform. We apply NHA to publicdata of LIGO-Virgo events, like GW150914, GW170817, anddemonstrated the visualization of the signal on time-frequency
plane. This work has been done in collaboration with thegroup of Shigeki Hirobayashi (Univ. Toyama).
Ref. Kenta Yanagisawa , Dongbao Jia, Shigeki Hirobayashi,
Nami Uchikata , Tatsuya Narikawa, Koh Ueno, Hirotaka
Takahashi, Hideyuki Tagoshi, PTEP 2019 (2019) no.6,
063F01.

Observational Cosmology Group[Spokesperson : Yoshiaki Ono]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582

63

SFR at the center. Under the assumption of a constant SFR,
a compact stellar distribution in z ∼1–2 compact quiescentgalaxies (cQGs) is well reproduced, while a spheroidal stellar
morphology of cQGs (nopt = 4) is not, suggestive of otherimportant mechanism(s) such as dynamical dissipation.

Bibliography[1] Fujimoto, S., Ouchi, M., Kohno, K., Yamaguchi, Y., Hatsukade, B., Ueda, Y., Shibuya, T., Inoue, S., Oogi, T.,
Toft, S., Gómez-Guijarro, C., Wang, T., Espada, D., Nagao, T., Tanaka, I., Ao, Y., Umehata, H., Taniguchi, Y.,
Nakanishi, K., Rujopakarn, W., Ivison, R. J., Wang, W.H., Lee, M. M., Tadaki, K., Tamura, Y., & Dunlop, J. S.,
2018, The Astrophysical Journal, 861, 7
Fig. 7. Example of summary page

ALMA 26 arcmin2 Survey of GOODS-S at One millimeter (ASAGAO): Average Morphology of High-z
Dusty Star-Forming Galaxies is an Exponential-Disk(n 0 1) [1]

In collaboration with the members of The University ofTokyo, Kyoto University, Niels Bohr Institute, National Astronomical Observatory of Japan, Ehime University, Chinese Academy of Sciences, The Open University of Japan,
RIKEN, Chulalongkorn University, National Astronomical
Research Institute of Thailand, University of Edinburgh, European Southern Observatory, Academia Sinica, and Nagoya
University.

We present morphological properties of dusty starforming galaxies at z =1−3 determined with the highresolution (FWHM∼0.((19) Atacama Large Milllimeter/submilimeter Array (ALMA) 1mm map of our ASAGAO
survey covering a 26-arcmin2 area in GOODS-S. In conjunction with the ALMA archival data, our sample consistsof 45 ALMA sources with infrared luminosity (LIR ) rangeof ∼ 1011 − 1013 L1 . To obtain an average rest-frame
far-infrared (FIR) profile, we perform individual measurements and careful stacking of the ALMA sources using theuv-visibility method that includes positional-uncertainty andsmoothing-effect evaluations through Monte Carlo simulations. We find that our sample has an average FIR-wavelengthSérsic index and effective radius of nFIR = 1.2 ± 0.2 andRe,FIR =1.0–1.3 kpc (Figure 8), respectively, additionallywith a point-source component at the center, indicative of
the existence of AGN. The average FIR profile agrees witha morphology of an exponential-disk clearly distinguishedfrom a de Vaucouleurs spheroidal profile (Sérsic index of4). We also examine the rest-frame optical Sérsic index noptand effective radius Re,opt with deep Hubble Space Telescope(HST) images. Interestingly, we obtain nopt = 0.9 ± 0.3(0nFIR ) and Re,opt = 3.2 ± 0.6 kpc (> Re,FIR ), suggestingthat the dusty disk-like structure is embedded within a larger
stellar disk. The rest-frame UV and FIR data of HST and
ALMA provide us a radial profile of the total star-formation
rate (SFR), where the infrared SFR dominates over the UVSILVERRUSH. V. Census of Lyα , [O III]λ 5007, Hα , and[C II ]158µ m Line Emission with ∼ 1000 LAEs at z =4.9 − 7.0 Revealed with Subaru/HSC [2]

In collaboration with the members of The University ofTokyo, Kitami Institute of Technology, Osaka Sangyo University, The European Space Agency, California Institute ofTechnology, Ehime University, National Astronomical Observatory of Japan, University of California Santa Barbara,
Carnegie Observatories, Universidad Nacional Autonoma
de Mexico, Institut d’Astrophysique de Paris, Laboratoired’Astrophysique de Marseille, and Niels Bohr Institute.
We investigate Lyα , [O III ]λ 5007, Hα , and [C II ]158µ m
emission from 1124 galaxies at z = 4.9–7.0. Our sample iscomposed of 1092 Lyα emitters (LAEs) at z = 4.9, 5.7, 6.6,
and 7.0 identified by Subaru/Hyper Suprime-Cam (HSC) narrowband surveys covered by Spitzer Large Area Survey with
Subaru/HSC (SPLASH) and 34 galaxies at z = 5.148 − 7.508
with deep ALMA [C II ]158µ m data in the literature. Fluxes ofstrong rest-frame optical lines of [O III] and Hα (Hβ ) are constrained by significant excesses found in the SPLASH 3.6 and4.5µ photometry. At z = 4.9, we find that the rest-frame Hαequivalent width and the Lyα escape fraction fLyα positively
0 .
correlate with the rest-frame Lyα equivalent width EWLy
α0The fLyα − EWLyα correlation is similarly found at z ∼ 0 − 2,suggesting no evolution of the correlation over z 0 0 − 5.
The typical ionizing photon production efficiency of LAEs islog ξion /[Hz erg−1 ] 0 25.5, significantly (60%–100%) higher
than those of LBGs at a given UV magnitude. At z = 5.7−7.0,
there exists an interesting turnover trend that the [O III]/Hα0 0 0–30 Å and then decreases
flux ratio increases in EWLy
α
0 0 130 Å (Figure 9). We also identify an anticorout to EWLy
αrelation between a ratio of [C II] luminosity to star formation
0
rate (L[CII] /SFR) and EWLy
α at the > 99% confidence level.
We carefully investigate physical origins of the correlations
with stellar-synthesis and photoionization models, and find0that a simple anticorrelation between EWLy
α and metallicity
explains self-consistently all of the correlations of Lyα , Hα ,[O III]/Hα , and [C II] identified in our study, indicating detec0 0 200Å.
tions of metal-poor (∼ 0.03Z1 ) galaxies with EWLy
α64

Fig. 8. Rest-frame FIR properties of nFIR (left) and Re,FIR (right) as a function of LIR . The red filled circle is obtained from the visibility-based
stacking for the 33 ASAGAO sources at z = 1–3. The LIR error bar represents the 16th–84th percentiles of the LIR distribution for the
ASAGAO sources, while the nFIR and Re,FIR error bars are evaluated by the bootstrap method and the MC simulations. The red open
squares present the additional sample of the 12 individual bright ALMA sources at z = 1–3. The red filled square indicates the median valueof the 12 individual ALMA sources, where the error bars denote the 16th–84th percentiles of the distribution. The black open triangles andsquares are the previous ALMA results. In the right panel, the black open squares are estimated by fixing nFIR = 1, which we do not present
in the left panel. The red shaded regions are the best estimates of the constant nFIR (left) and the Re,FIR –LIR relation (right). The constantnFIR is estimated from the stacked ASAGAO and the median value of the 12 individual ALMA sources, while the best-fit Re,FIR –LIR relation is
obtained from the literature.

Bibliography[2] Harikane, Y., Ouchi, M., Shibuya, T., Kojima, T., Zhang,
H., Itoh, R., Ono, Y., Higuchi, R., Inoue, A. K., Chevallard, J., Capak, P. L., Nagao, T., Onodera, M., Faisst, A.
L., Martin, C. L., Bruzual, G. A., Charlot, S., Davidzon,
I., Fujimoto, S., Hilmi, M., Ilbert, O., Lee, C.-H., Matsuoka, Y., Silverman, J. D., & Toft, S. 2018, The Astrophysical Journal, 859, 84
CHORUS II. Subaru/HSC Determination of the Lyα
Luminosity Function at z = 7.0: Constraints on Cosmic Reionization Model Parameter [3]In collaboration with the members of The University ofTokyo, Osaka Sangyo University, Kitami Institute of Technology, National Astronomical Observatory of Japan, Ehime
University, and The Open University of Japan.
We present the Lyα luminosity function (LF) derived from34 LAEs at z = 7.0 on an area of sky of 3.1 deg2 , the largest
sample of those in the literature to date obtained at a redshift z ! 7. The LAE sample is compiled from deep largearea narrowband observations with Subaru conducted by the
Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) project. The z = 7.0 Lyα LF of our project is consistent
with those of the previous Dark Energy Camera and Subaru
studies at the bright and faint ends, respectively, while having uncertainties that are significantly smaller than those of
the previous study results. Exploiting the small errors of ourmeasurements, we investigate the shape of the Lyα LF fromthe faint end to the bright end. We find that the shape of

Fig. 9. [O III ]λ 5007/Hα flux ratios as a function of rest-frame
Lyα EW. The blue, red, and green circles and squares are the[O III ]λ 5007/Hα flux ratios at z = 5.7, 6.6, and 7.0, respectively.
The squares represent the results of the EWLyα > 20Å subsamples. The upward and downward arrows represent 2σ lower andupper limits, respectively. The open gray diamonds and circles are
the ratios of z = 2.5 and 0.3 galaxies obtained in the literature. We0plot the median and the 1σ scatters of the ratios in EWLy
α sub0samples. We also plot the fitting result of the (Z, logU, Age)–EWLy
αrelations with the dark gray curve with the shaded region representing the 1σ uncertainty.

the z = 7.0 Lyα LF can be explained by the steep slope of
α 0 −2.5 suggested at z = 6.6, and that there is no clear signature of a bright-end excess at z 0 7 claimed by the previous

65

Fig. 10. Redshift evolution of the Lyα and UV LDs obtained with LAE
and LBG samples. The red filled star represents the Lyα LDs at
z = 7.0 from this study in the case of α = −2.5. The red filled circles
indicate the Lyα LDs at z = 5.7, 6.6, and 7.3 in the literature. The
blue squares are the UV LDs for z = 5.9, 6.8, 7.9, 9.0, and 10.4
obtained in the literature. In this figure, we adopt the integration
limits of log LLyα [erg s−1 ] = 42.4 and MUV = −17 for the Lyα and UV
LD estimates.

work, which was thought to be created by the ionized bubbles
around bright LAEs, whose Lyα photons could easily escapefrom the partly neutral IGM at z 0 7. We estimate the Lyαluminosity densities (LDs) with Lyα LFs at z 0 6 − 8 given
by our studies and the previous ones, and compare the evolution of the UV-continuum LD estimated with dropouts (Figure 10). The Lyα LD monotonically decreases from z ∼ 6 to
8, and evolves stronger than the UV-continuum LD, which isindicative of the Lyα damping wing absorption of the IGM
towards the heart of the reionization epoch.

Bibliography[3] Itoh, R., Ouchi, M., Zhang, H., Inoue, A. K., Mawatari,K., Shibuya, T., Harikane, Y., Ono, Y., Kusakabe, H.,
Shimasaku, K., Fujimoto, S., Iwata, I., Kajisawa, M.,
Kashikawa, N., Kawanomoto, S., Komiyama, Y., Lee, C.H., Nagao, T., & Taniguchi, Y., 2018, The AstrophysicalJournal, 867, 46
Morphologies of ∼ 190, 000 Galaxies at z = 0 − 10 Revealed with HST Legacy Data. III. Continuum Profile
and Size Evolution of Lyα Emitters [4]In collaboration with the members of Kitami Institute of Technology, The University of Tokyo, and National Astronomical
Observatory of Japan.
We present the redshift evolution of the radial surface
brightness (SB) profile of the rest-frame UV and optical stellar
continua for 9119 LAEs at z 0 0 − 8 and 0 − 2, respectively.
Using HST data and the LAE catalogs taken from the literature, we derive the structural quantities of the 9119 LAEs
and of 0 180, 000 photo-z star-forming galaxies (SFGs) andLyman break galaxies (LBGs) for comparison, by the welltested profile fitting. From 936 well-fitted LAEs, we care-fully define a homogeneous sample of LAEs falling in thesame ranges of UV-continuum luminosity and Lyα equivalent
width over z 0 0 − 8, and evaluate the redshift evolution. We
find that the distribution of effective radius re is represented
by a log-normal function, and that the median Sérsic index isalmost constant at n 0 1 − 1.5 for the LAEs over z 0 0 − 7,suggesting that typical LAEs have a stellar-disk morphology.
The size–luminosity relation of the LAEs decreases monotonically towards high-z, following size–luminosity relations of
SFGs and LBGs. The median re values of the LAEs evolve
significantly as re ∝ (1 + z)−1.37 , similar to those of the SFGs
and LBGs in the same luminosity range (Figure 11), in contrast with the claims of no evolution made by previous studies, whose LAE samples are probably biased to faint sources
at low-z. The re distribution, star formation rate surface densities, and stellar-to-halo size ratios of the LAEs are comparable with those of the SFGs and LBGs, indicating that LAEs
have stellar components similar to SFGs and LBGs with a
Lyα emissivity controlled by the non-stellar physics such as
the geometry, kinematics, and ionization states of the interstellar/circumgalactic medium.

Bibliography[4] Shibuya, T., Ouchi, M., Harikane, Y., & Nakajima, K.,
2019, The Astrophysical Journal, 871, 164
The onset of star formation 250 million years after
the Big Bang [5]In collaboration with the members of Osaka Sangyo University, National Astronomical Observatory of Japan, University
College London, Université de Toulouse, Uppsala University,
Johns Hopkins University, Nagoya University, Pontificia Universidad Católica de Chile, Millennium Institute of Astrophysics, Space Science Institute, The University of Tokyo,
European Southern Observatory, Hokakido University, Heidelberg University, Osaka University, The Open University ofJapan, and RIKEN.
A fundamental quest of modern astronomy is to locate theearliest galaxies and study how they influenced the intergalactic medium a few hundred million years after the Big Bang.
The abundance of star-forming galaxies is known to decline
from redshifts of about 6 to 10, but a key question is the extent
of star formation at even earlier times, corresponding to theperiod when the first galaxies might have emerged. Here we
report spectroscopic observations of MACS1149-JD1, a gravitationally lensed galaxy observed when the Universe was lessthan four per cent of its present age. We detect an emission
line of doubly ionized oxygen at a redshift of 9.1096±0.0006,
with an uncertainty of one standard deviation. This precisely
determined redshift indicates that the red rest-frame opticalcolor arises from a dominant stellar component that formed
about 250 million years after the Big Bang, corresponding toa redshift of about 15. Our results indicate that it may be possible to detect such early episodes of star formation in similar
galaxies with future telescopes. (Figure 12)

66∗ . TheFig. 11. Redshift evolution of re in the LUV range of 0.12 − 1 Lz=3
red filled diamonds with and without an open circle represent reUV
Opt
and re values measured for our LAEs, respectively. The red open
Opt
diamond at z 0 0.5 is re for LAEs at z 0 0−1 which is inferred fromthe extrapolation of the size–luminosity relation. The red open diamond at z 0 7.5 represents reUV measured from the three LAEs at
z 0 7−8. The error bars of reUV at z 0 7−8 are not reliably estimatedbecause of the small statistics. The small and large cyan filled circles indicate the SFGs and LBGs, respectively. The magenta solidand cyan dashed lines present the best-fit (1 + z)β functions for the
LAEs and SFGs/LBGs, respectively. The best-fit β value for reUVof the LAEs is −1.37 ± 0.65, which is obtained from the three reUV
data points at z 0 2 − 7. The gray symbols present LAEs in theliterature. The measurement technique is noted in the parenthesisof the legend (S: SExtractor; G: GALFIT; P: PHOT). For the previous studies with “LUV ” in the parenthesis of the legend, the median∗ . The data
re value is calculated in the range of LUV = 0.12 − 1Lz=3
points are slightly shifted along the x-axis for clarity. The error barsof some data points are smaller than the size of symbols.

Bibliography[5] Hashimoto, T.,; Laporte, N., Mawatari, K., Ellis, R. S.,
Inoue, A. K., Zackrisson, E., Roberts-Borsani, G., Zheng,W., Tamura, Y., Bauer, F. E., Fletcher, T., Harikane, Y.,
Hatsukade, B., Hayatsu, N. H., Matsuda, Y., Matsuo,
H., Okamoto, T., Ouchi, M., Pelló, R., Rydberg, C.-E.,
Shimizu, I., Taniguchi, Y., Umehata, H., Yoshida, N.,
2018, Nature, 557, 7705

Theory Group
Overview
The theory group is active in elementary particle physics
focusing on particle phenomenology, and in astroparticle
physics focusing on particle cosmology. In particle physics,
the main topics are theoretical studies of dark matter, inflation and extensions of the standard model. In astroparticle
physics, the main topics are theoretical studies of inflation,
thermal history of the early universe, dark matter, baryogenesis and big-bang nucleosynthesis.

Fig. 12. ALMA [O III] contours and spectrum of MACS1149-JD1. a)Magnification of an HST image (F160W), with the ALMA [O III] contours overlaid. Contours are drawn at −3σ (dashed line), 3σ , 4σ ,5σ , and 6σ , where σ = 17.5 mJy km s−1 per beam. The ellipse at
the lower left corner indicates the synthesized beam size of ALMA.
RA is the right ascension and dec. denotes the declination, both
expressed in J2000 coordinates. b) The ALMA [O III] 88µ m spectrum in frequency space, obtained with a resolution of about 42
km s−1 .

After the discovery of the Higgs boson, the LHC has
shown no strong hints on new physics. In this situation, we
need to reconsider many ideas of new physics models as wellas conventional strategies to search for them. Accordingly,
our studies on beyond the Standard Model physics becomes
more and more diverse.
Since the LIGO detectors detected the first confirmed
gravitational waves (GWs) from colliding black holes on
September 14, 2015, new era of GW astronomy began. So
far, more than 10 GW events were detected by LIGO. Among67

them is the first detection of GW from binary neutron star.
The discovery suggested strongly the existence of kilonova
which can produce r-process elements. If they continue tofind GW events from black holes and neutron stars, their origin becomes one of the most interesting topics.
The supersymmetric (SUSY) extension of the standard
model (SM) in the particle physics is considered to be oneof the most promising models beyond the standard model. It
solves the naturalness problem for the Higgs boson mass termin the standard model, and it is also compatible with the grandunified theories (GUTs). Although no hints of the superparticles have been indicated from the LHC yet, the SUSY models
are the most attractive candidates beyond the Standard Model.
Our group has been studying phenomenological and cosmological aspects of the SUSY models.
Recent cosmological observations including the Planck
data determine precisely the mean densities of matter andbaryon in the Universe, and existence of non-baryonic dark
matter is established. Weakly interacting massive particles(WIMPs) are considered to be good candidates of the dark
matter. They act as the cold dark matter in the structure formation of the universe. Our group has been studying modelbuilding for dark matter and detectability in direct and indirect
search experiments.
For understanding of the early universe, a role of the elementary particle physics is crucial. Recent progress in the particle physics such as grand unification theories and supersymmetry leads us to a more deeper insight into the fundamental
aspects of the early universe. In the inflationary universe, thequantum fluctuations of the scalar field which drives the inflation become the density fluctuations and lead to formation of
the structure observed in the present universe. On the other
hand cosmology and astrophysics are used to test new theories in particle physics. Such particle cosmology is one ofmain subjects of our group.
Big Bang Nucleosynthesis (BBN) is one of the most important subjects in modern cosmology. Predicted abundancesof the light elements are very sensitive to the cosmologicalscenario. On the other hand, physics beyond the standard
model predicts the new particles which would have existed
at the BBN epoch. Such particles may spoil the success of
BBN, which leads to constraints on the new particles and theparticle physics models.
The grand unified theories predict that our universe undergoes several vacuum phase transitions. In the course of phasetransitions topological defects (monopoles, cosmic strings anddomain walls) are generally produced depending on symmetries of the vacua. Our group has studied evolution of various
topological defects.

Particle Phenomenology[Spokesperson : M. Ibe]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582

Dark Matter Search• Foreground effect on the J-factor estimation of ultrafaint dwarf spheroidal galaxies [1]
In collaboration with the members of ICRR and Kavli IPMU
Dwarf spheroidal galaxies (dSphs) are promising targets
for the gamma-ray dark matter (DM) search. In particular,
DM annihilation signal is expected to be strong in some of
the recently discovered nearby ultra-faint dSphs, which potentially give stringent constraints on the O(1) TeV WIMP
DM. However, various non-negligible systematic uncertainties complicate the estimation of the astrophysical factors relevant for the DM search in these objects. Among them, theeffects of foreground stars particularly attract attention because the contamination is unavoidable even for the future
kinematical survey. In this article, we assess the effects ofthe foreground contamination on the astrophysical J-factor
estimation by generating mock samples of stars in the four
ultra-faint dSphs and using a model of future spectrographs.
We investigate various data cuts to optimize the quality of thedata and apply a likelihood analysis which takes member andforeground stellar distributions into account. We show that theforeground star contaminations in the signal region (the regionof interest) and their statistical uncertainty can be estimatedby interpolating the foreground star distribution in the control region where the foreground stars dominate the member
stars. Such regions can be secured at future spectroscopic observations utilizing a multiple object spectrograph with a large
field of view; e.g. the Prime Focus Spectrograph mounted on
Subaru Telescope. The above estimation has several advantages: The data-driven estimation of the contamination makes
the analysis of the astrophysical factor stable against the complicated foreground distribution. Besides, foreground contamination effect is considered in the likelihood analysis.
• Dark halo structure in the Carina dwarf spheroidal
galaxy: joint analysis of multiple stellar components [2]
In collaboration with the members of ICRR and NAOJ, INAF,
ASI, Nicolaus Copernicus Astronomical Center, Sapienza
Univ. of Roma, IAC, Dominion Astrophysical Observatory
Photometric and spectroscopic observations of the Carina
dSph revealed that this galaxy contains two dominant stellar
populations of different age and kinematics. The co-existenceof multiple populations provides new constraints on the dark
halo structure of the galaxy, because different populations
should be in equilibrium in the same dark matter potential
well. We develop non-spherical dynamical models including such multiple stellar components and attempt to constrain
the properties of the non-spherical dark halo of Carina. We
find that Carina probably has a larger and denser dark halo
than found in previous works and a less cuspy inner dark matter density profile, even though the uncertainties of dark haloparameters are still large due to small volume of data sample. Using our fitting results, we evaluate astrophysical factors
for dark matter annihilation and decay and find that Carina
should be one of the most promising detectable targets among68

classical dSph galaxies. We also calculate stellar velocity
anisotropy profiles for both stellar populations and find thatthey are both radially anisotropic in the inner regions, while in
the outer regions the older population becomes more tangentially biased than the intermediate one. This is consistent withthe anisotropy predicted from tidal effects on the dynamical
structure of a satellite galaxy and thereby can be consideredas kinematic evidence for the tidal evolution of Carina.
• Effects of mass models on dynamical mass estimate:
the case of ultra diffuse galaxy NGC1052-DF2 [3]In collaboration with the members of ICRR, Kavli IPMU, The
University of Tokyo
NGC1052-DF2 was recently discovered as the darkmatter deficient galaxy claimed by van Dokkum et al. (2018a,
vD18). However, large uncertainties on its dynamical massestimate have been pointed out, concerning the paucity ofsample, statistical methods and distance measurements. In
this work, we discuss the effects of the difference in modelingof the tracer profile of this galaxy on the dynamical mass estimate. To do this, we assume that the tracer densities are modeled with power-law and Sérsic profiles, and then we solve thespherical Jeans equation to estimate the dynamical mass. Applying these models to kinematic data of globular clusters in
NGC1052-DF2, we compare 90 per cent upper limits of dynamical mass-to-light ratios estimated between from this analysis and from vD18. We find that the upper limit obtained by
the power-law is virtually the same as the result from vD18,
whilst this limit estimated by the Sérsic is significantly greaterthan that from vD18, thereby suggesting that NGC1052-DF2
can still be a dark-matter dominated system. Consequently,
we propose that dynamical mass estimate of a galaxy is largely
affected by not only small kinematic sample but the choice oftracer distributions, and thus the estimated mass still remains
quite uncertain.
• Non-sphericity of ultralight axion dark matter haloes in
the Galactic dwarf spheroidal galaxies [4]In collaboration with the members of ICRR
Ultralight-axion (ULA) dark matter is one of the possible
solutions to resolve small-scale problems, especially the corecusp problem. This is because ULA dark matter can create acentral soliton core in all dark matter haloes stemmed fromthe quantum pressure against gravity below the de Broglie
wavelength, which becomes manifest on astrophysical scaleswith axion mass range ∼ 10−22 eV. In this work, we apply
our non-spherical dynamical models to the kinematic data ofeight classical dwarf spheroidals (dSphs) to obtain more reliable and realistic limits on ULA particle mass. This is motivated by the reasons that the light distributions of the dSphs
are not spherical, nor are the shapes of dark matter haloes
predicted by ULA dark matter simulations. Compared withthe previous studies on ULA dark matter assuming spherical mass models, our result is less stringent than those constraints due to the uncertainties on non-sphericity. On the   ( -  ) (   )   
 



(Ursa Minor & Fornax)

MW Substructures(Sextans)

Excluded by
UV Luminosity function    
10-24

10-23

10-22

- 

10-21m [eV]

10-20

Constraints on the ULA mass from our study−22 eV, 2σ level) with red contour com(mψ = 1.05+4.98−0.80 × 10
pared with the other mass bands favoured by several measurements such as previous Jeans analysis works (orange), presences of MW substructures (cyan), rotation curves of LSB and
SPARC galaxies (purple). The lower mass regions less than blueand green line are disfavoured by dynamical heating of stellar
streams in MW and Lyman-α forest (mψ ! 10−21 eV, green)at around 2σ levels. The lower mass region mψ " 10−23 eV
is severely excluded by the UV Luminosity function of high-z
galaxies at more than 8σ level.

Fig. 13.

other hand, remarkably, we find that the dSphs would prefer to have a flattened dark matter halo rather than a sphericalone, especially Draco favours a strongly elongated dark matter halo caused naively by the assumption of a soliton-core
profile. Moreover, our consequent non-spherical core profiles
are much more flattened than numerical predictions based on
ULA dark matter, even though there are still uncertainties on
the estimation of dark matter halo structure. To alleviate thisdiscrepancy, further understanding of baryonic and/or ULA
dark matter physics on small mass scales might be needed.

Bibliography[1] K. Ichikawa, S. i. Horigome, M. N. Ishigaki,
S. Matsumoto, M. Ibe, H. Sugai and K. Hayashi,Mon. Not. Roy. Astron. Soc. 479 (2018) no.1, 64
doi:10.1093/mnras/sty1387 [arXiv:1706.05481 [astroph.GA]].
[2] K. Hayashi, M. Fabrizio, E. L. Łokas, G. Bono,
M. Monelli, M. Dall’Ora and P. B. Stetson, Mon.
Not. Roy. Astron. Soc. 481 (2018) no.1, 250
doi:10.1093/mnras/sty2296 [arXiv:1804.01739 [astroph.GA]].
[3] K. Hayashi and S. Inoue, Mon. Not. Roy. Astron.
Soc. 481 (2018) no.1, L59 doi:10.1093/mnrasl/sly162[arXiv:1808.10116 [astro-ph.GA]].
[4] K. Hayashi and I. Obata, Mon. Not. Roy. Astron.
Soc. 491 (2020) no.1, 615 doi:10.1093/mnras/stz2950[arXiv:1902.03054 [astro-ph.CO]].

69

Beyond Standard Model• Composite Asymmetric Dark Matter with a Dark Photon Portal [1]Asymmetric dark matter (ADM) is an attractive framework relating the observed baryon asymmetry of the Universeto the dark matter density. A composite particle in a newstrong dynamics is a promising candidate for ADM as thestrong dynamics naturally explains the ADM mass in the GeV
range. Its large annihilation cross section due to the strong
dynamics leaves the asymmetric component to be dominant
over the symmetric component. In such composite ADM scenarios, the dark sector has a relatively large entropy density
in the early Universe. The large dark sector entropy resultsin the overclosure of the Universe or at best contradicts withthe observations of the cosmic microwave background andthe successful Big-Bang Nucleosynthesis. Thus, composite
ADM models generically require some portal to transfer theentropy of the dark sector into the Standard Model sector.
In this paper, we consider a dark photon portal with a massin the sub-GeV range and kinetic mixing with the Standard
Model photon. We investigate the viable parameter space ofthe dark photon in detail, which can find broad applications todark photon portal models. We also provide a simple working example of composite ADM with a dark photon portal.
Our model is compatible with thermal leptogenesis and B − L
symmetry. By taking into account the derived constraints, weshow that the parameter space is largely tested by direct detection experiments.

 [ / ]In collaboration with the members of ICRR and IBS Korea   >  -       =    >  










 [ / ]Fig. 14. The constraint on the VEVs of φ and φ ( . The gray shaded

region is excluded by ∆θ < 10−10 for the non-SUSY model. Theorange lines are the contours of the effective decay constant FA .
In the blue shaded region, -φ . > -φ ( ..

The gauged Peccei-Quinn (PQ) mechanism provides asimple prescription to embed the global PQ symmetry into
a gauged U(1) symmetry. As it originates from the gauged
PQ symmetry, the global PQ symmetry can be protected fromexplicit breaking by quantum gravitational effects once appropriate charge assignment is given. In this paper, we identify
the gauged PQ symmetry with the B − L symmetry, which is
obviously attractive as the B − L gauge symmetry is the mostauthentic extension of the Standard Model. As we will show, anatural B − L charge assignment can be found in a model motivated by the seesaw mechanism in the SU(5) Grand Unified
Theory. As a notable feature of this model, it does not require
extra SU(5) singlet matter fields other than the right-handed
neutrinos to cancel the self and the gravitational anomalies.

jecture, the observed dark energy cannot be explained by thecosmological constant. The local maximum of the Higgs potential at the symmetric point also contradicts with the conjecture. In order to make the Standard Model consistent with theconjecture, it has been proposed to introduce a quintessence
field, Q, which couples to the cosmological constant and the
local maximum of the Higgs potential. In this paper, we showthat such a modified Higgs potential generically results in a
Q-dependent Higgs vacuum expectation value (VEV). The Qdependence of the Higgs VEV induces a long-range force,
which is severely excluded by the tests of the equivalenceprinciple. Besides, as the quintessence field is in motion, theHiggs VEV shows a time-dependence, which is also severely
constrained by the measurements of the time-dependence ofthe proton-to-electron mass ratio. Those constraints require an
additional fine-tuning which is justified neither by the swampland conjecture nor the anthropic principle. We further show
that, even if such an unjustified fine-tuning condition is imposed at the tree level, radiative corrections upset it. Consequently, we argue that most of the habitable vacua in thestring landscape are in tension with the phenomenologicalconstraints.

• The swampland conjecture and the Higgs expectation
value [3]• Quintessence Axion Revisited in Light of Swampland
Conjectures [4]In collaboration with the members of ICRR and University ofTokyo

In collaboration with the members of ICRR and IPMU

• B − L as a Gauged Peccei-Quinn Symmetry [2]In collaboration with the members of ICRR and IPMU

The recently proposed de Sitter swampland conjecture excludes local extrema of a scalar potential with a positive energy density in a low energy effective theory. Under the con-

We point out that the swampland conjectures, forbidding
the presence of global symmetries and (meta-)stable de Sitter vacua within quantum gravity, pick up a dynamical axion
for the electroweak SU(2) gauge theory as a natural candidate

70

for the quintessence field. The potential energy of the electroweak axion provides an attractive candidate for the darkenergy. We discuss constraints from the weak gravity conjecture, from the conjecture of no global symmetry, and fromobservations, which can be satisfied elegantly in a supersymmetric extension of the standard model.
• Ultraviolet Completion of a Composite Asymmetric
Dark Matter Model with a Dark Photon Portal [5]In collaboration with the members of ICRR and IBS (Korea)
Composite asymmetric dark matter scenarios naturally explain why the dark matter mass density is comparable withthe visible matter mass density. Such scenarios genericallyrequire some entropy transfer mechanism below the composite scale; otherwise, their late-time cosmology is incompatible with observations. A tiny kinetic mixing between a dark
photon and the visible photon is a promising example of thelow-energy portal. In this paper, we demonstrate that grandunifications in the dark and the visible sectors explain theorigin of the tiny kinetic mixing. We particularly consider
an ultraviolet completion of a simple composite asymmetricdark matter model, where asymmetric dark matter carries a
B − L charge. In this setup, the longevity of asymmetric dark
matter is explained by the B − L symmetry, while the dark
matter asymmetry originates from the B − L asymmetry generated by thermal leptogenesis. In our minimal setup, theStandard Model sector and the dark sector are unified intoSU(5)GUT × SU(4)DGUT gauge theories, respectively. Thismodel generates required B − L portal operators while suppressing unwanted higher-dimensional operators that could
wash out the generated B − L asymmetry.
• Automatic Peccei-Quinn symmetry [6]Fig. 15. The summary plots for the collider constraints and thedark matter candidate. In the orange (yellow) region, ∆aµ is
explained within 1 sigma (2σ ). The gray shaded region is excluded by the negative slepton masses. On the right (left) side of
the black dashed line, the neutralino (the sneutrino) is the LSP.
The blue shaded regions are excluded from the collider searches
at 95% CL. On the green line, the observed current dark matter
abundance Ωh2 0 0.12 is achieved due to the bino-wino coannihilation.

solving a relativistic field equation, with which the decay rateof the I-ball/oscillon is obtained. We also perform a classicallattice simulation and confirm the validity of our analyticalformula of the decay rate numerically.
• Muon g − 2 in Split-Family SUSY in light of LHC Run II[8]

In collaboration with the members of ICRR, IPMU and IFT(Spain)In collaboration with the members of ICRR, IPMU, and Tohoku Univ.

We present a dynamical (composite) axion model where
the Peccei-Quinn (PQ) symmetry arises automatically as aconsequence of chirality and gauge symmetry. The Standard
Model is simply extended by a confining and chiral SU(5)gauge symmetry. The PQ symmetry coincides with a B − L
symmetry of the exotic sector. The theory is protected by construction from quantum gravitational corrections stemmingfrom operators with mass dimension lower than nine.

The Split-Family supersymmetry is a model in which
the sfermion masses of the first two generations are inO(100-1000) GeV while that of the third one is in O(10)TeV.
With such a hierarchical spectrum, the deviation of the muong − 2 and the observed Higgs boson mass are explained simultaneously. In this paper, we revisit the Split-Family SUSY
model in light of the updated LHC constraints. We also study
the flavor changing neutral current problems in the model.
As we will show, the problems do not lead to stringent constraints when the Cabibbo-Kobayashi-Maskawa matrix is the
only source of the flavor mixing. We also study how large
flavor mixing in the supersymmetry breaking parameters isallowed.

• Decay of I-ball/Oscillon in Classical Field Theory [7]In collaboration with the members of ICRR

I-balls/oscillons are long-lived and spatially localized solutions of real scalar fields. They are produced in various contexts of the early universe in, such as, the inflaton evolution
and the axion evolution. However, their decay process haslong been unclear. In this paper, we derive an analytic formula of the decay rate of the I-balls/oscillons within the classical field theory. In our approach, we calculate the Poynting
vector of the perturbation around the I-ball/oscillon profile by• More about Q-ball with elliptical orbit [9]In collaboration with the members of ICRR and Seoul Natl.
Univ.
Q-balls formed from the Affleck-Dine field have rich cosmological implications and have been extensively studied
from both theoretical and simulational approaches. From the71[6] M. B. Gavela, M. Ibe, P. Quilez and T. T. Yanagida, Eur.
Phys. J. C 79 (2019) no.6, 542 doi:10.1140/epjc/s10052019-7046-3 [arXiv:1812.08174 [hep-ph]].
[7] M. Ibe, M. Kawasaki, W. Nakano and E. Sonomoto,
JHEP 1904 (2019) 030 doi:10.1007/JHEP04(2019)030[arXiv:1901.06130 [hep-ph]].
[8] M. Ibe, M. Suzuki, T. T. Yanagida and N. Yokozaki, Eur.
Phys. J. C 79 (2019) no.8, 688 doi:10.1140/epjc/s10052019-7186-5 [arXiv:1903.12433 [hep-ph]].
[9] F. Hasegawa, J. P. Hong and M. Suzuki, Phys. Lett. B
798 (2019) 135001 doi:10.1016/j.physletb.2019.135001[arXiv:1903.07281 [hep-ph]].
Fig. 16. Formation of excited Q-balls in 3D lattice simulation att = t f . We plotted an isosurface of the energy density for theillustration. We set εini = 0.2.

theoretical point of view, the exact solution of the Q-ball was
obtained and it shows a circular orbit in the complex planeof the field value. In practice, however, it is reported that
the Q-ball that appears after the Affleck-Dine mechanism has
an elliptical orbit, which carries larger energy per unit U(1)charge than the well-known solution with a circular orbit. We
call them ”elliptical” Q-balls. In this paper, we report the firstdetailed investigation of the properties of the elliptical Q-ballsby 3D lattice simulation. The simulation results indicate thatthe elliptical Q-ball has an almost spherical spatial profile withno nodes, and we observed a highly elliptic orbit that cannot
be described through small perturbations around the groundstate Q-ball. Higher ellipticity leads to more excitation of theenergy, whose relation is also derived as a dispersion relation.
Finally, we derive two types of approximate solutions by extending the Gaussian approximation and considering the timeaveraged equation of motion and we also show the consistency
with the simulation results.

Bibliography[1] M. Ibe, A. Kamada, S. Kobayashi and W. Nakano,
JHEP 1811 (2018) 203 doi:10.1007/JHEP11(2018)203[arXiv:1805.06876 [hep-ph]].
[2] M. Ibe, M. Suzuki and T. T. Yanagida, JHEP
1808 (2018) 049 doi:10.1007/JHEP08(2018)049[arXiv:1805.10029 [hep-ph]].
[3] K. Hamaguchi, M. Ibe and T. Moroi, JHEP 1812 (2018)023 doi:10.1007/JHEP12(2018)023 [arXiv:1810.02095[hep-th]].
[4] M. Yamazaki, arXiv:1910.08691 [hep-ph].
[5] M. Ibe, A. Kamada, S. Kobayashi, T. Kuwahara and W. Nakano, JHEP 1903 (2019) 173
doi:10.1007/JHEP03(2019)173[arXiv:1811.10232[hep-ph]].

Particle Cosmology[Spokesperson : M. Kawasaki]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582

Inflation, CMB, Thermal History in the Early
Universe• Exploring compensated isocurvature perturbations
with CMB spectral distortion anisotropies [1]
In collaboration with the members of Tokyo Institute of Technology, Utrecht Univ., and Università degli Studi di Padova
We develop a linear perturbation theory for the spectral
y-distortions of the cosmic microwave background (CMB).
The y-distortions generated during the recombination epoch
are usually negligible because the energy transfer due to theCompton scattering is strongly suppressed at that time, butthey can be significant if there is a considerable amount ofcompensated isocurvature perturbation (CIP), which is nottightly constrained from the present CMB observations. The
linear y-distortions explicitly depend on the baryon density
fluctuations, therefore y anisotropies can completely resolve
the degeneracy between the baryon isocurvature perturbations
and the cold dark matter ones. This novel method is freefrom lensing contaminations that can affect the previous approach to the CIPs based on the nonlinear modulation of the
CMB anisotropies. We compute the cross correlation functions of the y-distortions with the CMB temperature and the Emode polarization anisotropies. They are sensitive to the correlated CIPs parameterized by f ( ≡ PCIPζ /Pζ ζ with Pζ ζ
and PCIPζ being the auto correlation of the adiabatic perturbations and the cross correlation between them and the CIPs.
We investigate how well the y anisotropies will constrain f (in future observations such as those provided by a PIXIE-likeand a PRISM-like survey, LiteBIRD and a cosmic variancelimited (CVL) survey, taking into account the degradation inconstraining power due to the presence of Sunyaev Zel’dovich
effect from galaxy clusters. For example, our forecasts showthat it is possible to achieve an upper limit of f ( < 2 × 105 at
68LiteBIRD, and f ( < 2 × 104 with CVL observations.

72• Big Bang Nucleosynthesis Constraint on Baryonic
Isocurvature Perturbations [2]
In collaboration with the members of IPMU and UCLA
We study the effect of large baryonic isocurvature perturbations on the abundance of deuterium (D) synthesized in
big bang nucleosynthesis (BBN). We found that large baryonic isocurvature perturbations existing at the BBN epoch(T ∼ 0.1 MeV) change the D abundance by the second order effect, which, together with the recent precise D measurement, leads to a constraint on the amplitude of the power spectrum of the baryon isocurvature perturbations. The obtained
constraint on the amplitude is " 0.016 (2σ ) for scale k−1 !
0.0025 pc. This gives the most stringent one for 0.1 Mpc−1 "k " 4 × 108 Mpc−1 . We apply the BBN constraint to the relaxation leptogenesis scenario, where large baryon isocurvature
perturbations are produced in the last Nlast e-fold of inflation,
and we obtain a constraint on Nlast .
• Hunting for Statistical Anisotropy in Tensor Modes with
B-mode Observations [3]
In collaboration with the members of Rikkyo University,
Nagoya University, Kyoto University
We investigate a possibility of constraining statisticalanisotropies of the primordial tensor perturbations by using future observations for the Cosmic Microwave Background (CMB) B-mode polarization. By parameterizing astatistically-anisotropic tensor power spectrum as Ph (k) =Ph (k) ∑n gn cosn θk , where θk is an angle of the direction ofk̂ = k/k from a preferred direction, we find that it would
be possible for future B-mode observations such as CMBS4 to detect the tensor statistical anisotropy at the level ofgn ∼ O(0.1).
• Circular polarization of the cosmic microwave background from vector and tensor perturbations [4]In collaboration with the members of Johns Hopkins University
Circular polarization of the cosmic microwave background (CMB) can be induced by Faraday conversion of
the primordial linearly polarized radiation as it propagates
through a birefringent medium. Recent work has shown thatthe dominant source of birefringence from primordial density
perturbations is the anisotropic background CMB. Here we
extend prior work to allow for the additional birefringencethat may arise from primordial vector and tensor perturbations. We derive the formulas for the power spectrum of theinduced circular polarization and apply those to the standard
cosmology. We find
√ the root-variance of the induced circularpolarization
tobe< V 2 > ∼ 3 × 10−14 for scalar perturba√
2
tions and < V > ∼ 7 × 10−18 (r/0.06) for tensor perturbations with a tensor-to-scalar ratio r.

• Chiral photons from chiral gravitational waves [5]In collaboration with the members of Johns Hopkins University
We show that a parity-breaking uniform (averaged overall directions on the sky) circular polarization of amplitude V00 0 2.6 × 10−17 ∆χ (r/0.06) can be induced by chiralgravitational-wave (GW) background with tensor-to-scalar ratio r and chirality parameter ∆χ (which is ±1 for a maximallychiral background). We also show, however, that a uniform
circular polarization can arise from a realization of a nonchiral GW background that spontaneously breaks parity. The
magnitude of this$ polarization is drawn from a distribution ofroot-variance

2 > 0 1.5 × 10−18 (r/0.06)1/2 implying< V00

that the chirality parameter must be ∆χ ! 0.12(r/0.06)−1/2
to establish that the GW background is chiral. Althoughthese values are too small to be detected by any experimentin the foreseeable future, the calculation is a proof of principle that cosmological parity breaking in the form of a chiralgravitational-wave background can be imprinted in the chirality of the photons in the cosmic microwave background. It
also illustrates how a seemingly parity-breaking cosmologicalsignal can arise from parity-conserving physics.

Bibliography[1] T. Haga, K. Inomata, A. Ota and A. Ravenni, JCAP
1808 (2018) 036 doi:10.1088/1475-7516/2018/08/036[arXiv:1805.08773 [astro-ph.CO]].
[2] K. Inomata, M. Kawasaki, A. Kusenko and
L. Yang, JCAP 1812 (2018) 003 doi:10.1088/14757516/2018/12/003 [arXiv:1806.00123 [astro-ph.CO]].
[3] T. Hiramatsu, S. Yokoyama, T. Fujita and
I. Obata, Phys. Rev. D 98 (2018) no.8, 083522
doi:10.1103/PhysRevD.98.083522 [arXiv:1808.08044[astro-ph.CO]].
[4] K. Inomata and M. Kamionkowski, Phys. Rev. D 99
(2019) no.4, 043501 doi:10.1103/PhysRevD.99.043501[arXiv:1811.04957 [astro-ph.CO]].
[5] K. Inomata and M. Kamionkowski,
Phys.
Rev.
Lett.
123
(2019)
no.3,
031305
doi:10.1103/PhysRevLett.123.031305[arXiv:1811.04959 [astro-ph.CO]].

Axions• Optical Ring Cavity Search for Axion Dark Matter [1]In collaboration with the members of Tokyo University(Hongo), Kyoto University
We propose a novel experiment to search for axion dark
matter which differentiates the phase velocities of the left
and right-handed polarized photons. Our optical cavity measures the difference of the resonant frequencies between two
circular-polarizations of the laser beam. The design of our

73

cavity adopts double-pass configuration to realize a null experiment and give a high common mode rejection of environmental disturbances. We estimate the potential sensitivity
to the axion-photon coupling constant gaγ for the axion mass
m " 10−10 eV. In a low mass range m " 10−15 eV, we canachieve gaγ " 3 × 10−16 GeV−1 which is beyond the current
bound by several orders of magnitude.

GeV−1 ) at the axion mass m 0 3 × 10−13 eV (2 × 10−15eV) and remains at around this sensitivity for 3 orders ofmagnitude in mass. Furthermore, its sensitivity has a sharp
peak reaching gaγ 0 10−14 GeV−1 (8 × 10−17 GeV−1 ) at
m = 1.563 × 10−10 eV (1.563 × 10−11 eV). This sensitivity
can be achieved without loosing any sensitivity to gravitational waves.

• Long-term dynamics of cosmological axion strings [2]Bibliography

In collaboration with the members of ICRR, University ofTokyo and Tokyo Institute of Technology[1] I. Obata, T. Fujita and Y. Michimura, arXiv:1811.12051[physics.ins-det].

We present results of new field-theoretic simulation ofcosmological axion strings, which are eight times longer thanprevious ones. We have upgraded our simulation of physicalstrings in Hiramatsu et al. (2011) in terms of the number ofgrids as well as the suite of analysis methods. These improvements enable us to monitor a variety of quantities characterizing the dynamics of the physical string network for the longest
term ever. Our extended simulations have revealed that global
strings do not evolve according to the scaling solution but its
scaling parameter, or the number of long strings per horizon,
increases logarithmically in time. In addition, we have also
found that the scaling parameter shows nontrivial dependence
on the breaking scale of the Peccei-Quinn symmetry.

[2] M. Kawasaki, T. Sekiguchi, M. Yamaguchi and
J. Yokoyama, PTEP 2018 (2018) no.9, 091E01
doi:10.1093/ptep/pty098 [arXiv:1806.05566 [hep-ph]].

Primodial Black Holes• Mixed Non-Gaussianity from Axion-Gauge Field Dynamics [3]• Formation of primordial black holes in an axionlike curvaton model [1]In collaboration with the members of Kyoto University,
McGill University

In collaboration with the members of ICRR

We study scalar-tensor-tensor cross correlation -ζ hh. generated by the dynamics of interacting axion and SU(2) gauge
fields during inflation. We quantize the quadratic action andsolve the linear equations by taking into account mixing terms
in a non-perturbative manner. Combining that with the inin formalism, we compute contributions from cubic interactions to the bispectrum Bζ hh . We find that the bispectrum ispeaked at the folded configuration, which is a unique feature
encoded by the scalar mixing and localized production of tensor modes. With our parameter choice, the amplitude of thebispectrum is k6 Bζ hh ∼ 10−16 . The unique shape dependence,
together with the parity-violating nature, is thus a distinguishing feature to search for in the CMB observables.
• Axion Dark Matter Search with Interferometric Gravitational Wave Detectors [4]In collaboration with the members of ICRR, Tokyo University(Hongo), Kyoto University
Axion dark matter differentiates the phase velocities of thecircular-polarized photons. In this Letter, a scheme to measure
the phase difference by using a linear optical cavity is proposed. If the scheme is applied to the Fabry-Prot arm of Advanced LIGO-like (Cosmic-Explorer-like) gravitational wave
detector, the potential sensitivity to the axion-photon coupling constant, gaγ , reaches gaγ 0 8×10−13 GeV−1 (4×10−14[3] T. Fujita, R. Namba and I. Obata, JCAP 1904
(2019)
044
doi:10.1088/1475-7516/2019/04/044[arXiv:1811.12371 [astro-ph.CO]].
[4] K. Nagano,
T. Fujita,
Y. Michimura and
I. Obata, Phys. Rev. Lett. 123 (2019) no.11,
111301
doi:10.1103/PhysRevLett.123.111301[arXiv:1903.02017 [hep-ph]].

We performed the detailed analysis of the primordial blackhole (PBH) formation mechanism in an axionlike curvaton
model with a coupling to inflaton. The phase direction of
the complex scalar works as a curvaton and produces enough
PBHs to explain the black hole binaries (∼ 30M1 ) observed in
the LIGO-Virgo Collaboration or PBHs as dark matter (DM)(∼ 10−12 M1 ). We examine whether our model satisfies thecurrent constraints on the PBH mass spectrum, the curvature perturbation and the secondarily produced gravitational
waves. We also take into account ambiguity about the choiceof the window functions and effect of the non-Gaussianity.
• Primordial Black Holes from Affleck-Dine Mechanism[2]
In collaboration with the members of ICRR
The recent observations of the gravitational waves (GWs)by LIGO-Virgo collaboration infer the increasing possibilityof the primordial black holes (PBHs). Recently it was pointed
out that sufficient PBHs are produced by the Affleck-Dine
mechanism where inhomogeneous baryogenesis takes place
due to change of the Hubble induced mass during and after inflation and forms high baryon bubbles (HBBs). The produced
HBBs have large density contrasts through the QCD phasetransition or stable Q-ball formation, which leads to formation of the LIGO PBHs. Furthermore, in this model stringent
constraints from CMB µ -distortion and pulsar timing array

74(PTA) experiments are completely absent. In this paper, westudy the model in full details based on gravity and gauge mediated supersymmetry breaking scenarios and show that themodel can explain the current GWs events evading observational constraints.
• Footprint of Two-Form Field: Statistical Anisotropy in
Primordial Gravitational Waves [3]
In collaboration with the members of Kyoto University
We study the observational signatures of two-form field in
the inflationary cosmology. In our setup a two-form field is kinetically coupled to a spectator scalar field and generates sizable gravitational waves and smaller curvature perturbation.
We find that the sourced gravitational waves have a distinct
signature: they are always statistically anisotropic and theirspherical moments are non-zero for hexadecapole and tetrahexacontapole, while the quadrupole moment vanishes. Since
their amplitude can reach O(10−3 ) in the tensor-to-scalar ratio, we expect this novel prediction will be tested in the next
generation of the CMB experiments.
• Primordial Black Holes and the String Swampland [4]In collaboration with the members of ICRR and UCLA(USA)The ”swampland conjectures” have been recently suggested as a set of criteria to assess if effective field theories (EFTs) are consistent with a quantum gravity embedding.
Such criteria, which restrict the behavior of scalar fields in thetheory, have strong implications for cosmology in the early
universe. As we demonstrate, they will also have direct consequences for formation of primordial black holes (PBHs) anddark matter (DM).
• Gravitational waves induced by scalar perturbations asprobes of the small-scale primordial spectrum [5]In collaboration with the members of The Hong Kong University of Science and Technology
Compared to primordial perturbations on large scales,roughly larger than 1 megaparsec, those on smaller scales
are not severely constrained. We revisit the issue of probing small-scale primordial perturbations using gravitational
waves (GWs), based on the fact that, when large-amplitude
primordial perturbations on small scales exist, GWs with relatively large amplitudes are induced at second order in scalar
perturbations, and these induced GWs can be probed by both
existing and planned gravitational-wave projects. We use accurate methods to calculate these induced GWs and take into
account sensitivities of different experiments to induced GWs
carefully, to report existing and expected limits on the smallscale primordial spectrum.

Bibliography[1] K. Ando,
M. Kawasaki and H.
suka, Phys. Rev. D 98 (2018) no.8,

Nakat083508

doi:10.1103/PhysRevD.98.083508 [arXiv:1805.07757[astro-ph.CO]].
[2] F. Hasegawa and M. Kawasaki, JCAP 1901
(2019)
027
doi:10.1088/1475-7516/2019/01/027[arXiv:1807.00463 [astro-ph.CO]].
[3] I. Obata and T. Fujita, Phys. Rev. D 99 (2019)
no.2,
023513
doi:10.1103/PhysRevD.99.023513[arXiv:1808.00548 [astro-ph.CO]].
[4] M. Kawasaki and V. Takhistov, Phys. Rev. D 98 (2018)
no.12,
123514 doi:10.1103/PhysRevD.98.123514[arXiv:1810.02547 [hep-th]].
[5] K. Inomata and T. Nakama, Phys. Rev. D 99
(2019) no.4, 043511 doi:10.1103/PhysRevD.99.043511[arXiv:1812.00674 [astro-ph.CO]].

75

OBSERVATORIES andA RESEARCH CENTER
Location of the Institute and the Observatories in Japan

Norikura Observatory
Location:
Telephone (Fax):
Telephone (satellite):Telephone (car):

Norikuradake, Nyukawa-cho, Takayama-shi, Gifu Prefecture 506-2100
N 36◦ 06( , E 137◦ 33( , 2770 m a.s.l.
+81-50-3730-3809+81-90-7721-5674+81-90-7408-6224

Akeno Observatory
Location:
Telephone / Fax:

5259 Asao, Akeno-machi, Hokuto-shi, Yamanashi Prefecture 408-0201
N 35◦ 47( , E 138◦ 30( , 900 m a.s.l.
+81-551-25-2301 / +81-551-25-2303

Kamioka Observatory
Location:
Telephone / Fax:

456 Higashi-mozumi, Kamioka-cho, Hida-shi, Gifu Prefecture 506-1205
N 36◦ 25( 26(( , E 137◦ 19( 11(( , 357.5 m a.s.l.
+81-578-85-2116 / +81-578-85-2121

KAGRA Observatory
Location:
Telephone / Fax:

238 Higashi-mozumi, Kamioka-cho, Hida-shi, Gifu Prefecture 506-1205
N 36◦ 25( 26(( , E 137◦ 19( 11(( , 350 m a.s.l.
+81-578-85-2343 / +81-578-85-2346

Research Center for Cosmic Neutrinos
Location:
Telephone / Fax:

5-1-5 Kashiwanoha, Kashiwa, Chiba Prefecture 277-8582+81-4-7136-3138 / +81-4-7136-3115

76

NORIKURA OBSERVATORY
Introduction
Norikura Observatory (36.10◦ N and 137.55◦ E) was
founded in 1953 and attached to ICRR in 1976. It is locatedat 2770 m above sea level, and is the highest altitude mannedlaboratory in Japan (Fig. 1). Experimental facilities of thelaboratory are made available to all the qualified scientists inthe field of cosmic ray research and associated subjects. The
AC electric power is generated by the dynamo and supplied
throughout the observatory. The observatory can be accessedeasily 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.

high-altitude environment, etc..

Cosmic Ray Physics

Fig. 1. Norikura Observatory

Norikura Observatory gave manned operation to the observations 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 order to study the possibilities to reduce
maintenance and labor costs without causing serious inconveniences for the 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, asshown 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 rays in theinterplanetary space associated with the solar activity, the generation of energetic particles by the solar flares, and the particle acceleration mechanism in thunderclouds, all of which
require long-term observation. These researches have been
carried out by the group of user universities, where ICRR provides them with laboratory facility. A part of the facility has
been open for the environmental study at high altitude such as
aerosol-related mechanism in the atmosphere, observation of
total ozone and UV solar radiation, for botanical study in theSpace weather observation
Space weather observation is actively madeby a 25 m2 muon hodoscope at Norikura
Observatory[1],[2],[3],[4],[5],[6],[7],[8],[9],[10] . Mt. Norikura
muon hodoscope has started operation in May, 1998 andsuccessfully observed a clear precursory signature of theinterplanetary shock arrival at Earth. With its improved
angular resolution of muon incident direction, the detector
succeeded for the first time to observe a loss-cone signature
which is an intensity deficit within a narrow cone around theinterplanetary 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 installed a small
muon hodoscope in Kuwait City, Kuwait as the fourth detector in our Global Muon Detector Network (GMDN) with otherthree multidirectional muon detectors in Nagoya (Japan), Hobart (Australia), and São 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 complementary to observations with neutron monitors monitoringa lower energy range below ∼ 10 GeV and the 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 information of the spatial distribution of the cosmic ray density in three dimensions. The Mt. Norikura muon
hodoscope and GMDN have revealed the dynamic variationsof the anisotropy which give us important information of thespace weather. It has been already confirmed that the GMDN
can measure the rapid variation of the anisotropy in the cos-

77

mic ray burst observed in June 2015 in 10 minute time resolution. The Kuwait muon hodoscope was enlarged three times
in March 2016 and one minute data are now available fromall of four detectors in the GMDN enabling us to analyze theanisotropy in 1 minute time resolution.
Recently, we also developed the method of the correctionof the atmospheric temperature effect on muon count rate byusing 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 was possible so far only for cosmic ray below∼ 10 GeV using the neutron monitor data nearly free from thetemperature effect. We have already published the long-term
variation of the anisotropy observed by Nagoya muon detector.
Solar neutron observation
Observation of solar neutrons in solar cycle 24 has continued at Norikura Observatory of ICRR since fiscal 2007 tounderstand the acceleration mechanism of high energy (>100
MeV) ions associated with solar flares[11] . These neutrons areproduced by the interaction between accelerated ions and thesolar atmosphere. Neutrons are not reflected by the interplanetary magnetic field, and thought to be more informative thanaccelerated ions themselves to study the acceleration mechanism at the solar surface. Solar neutron events detected 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 worldwide network of 7 solar neutron telescopes in the world. The
solar neutron telescope operated at Norikura Observatory has
an area of 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 isdetected when a recoil proton is produced in the scintillator,
and the energy of the recoil proton is measures. Proportionalcounters are used both to veto charged particles and measure
the direction of recoil protons. The telescope is operated bysolar power during the winter period when the Norikura observatory is closed.
Solar cycle 24 was its maximum in February 2014 and has
decreased its activity since then. We searched for solar neutron signals from the world-wide network of the solar neutrontelescopes between January 2010 and December 2014 when
the large (≥X1.0 class) solar flare occurred. No solar neutron
event was detected by this search. We statistically studied therelation between upper limits of the neutron flux and the energy of soft 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 is thatthe total energy obtained by neutrons during solar flare does
not exceed 0.1 % of the total energy of soft X-rays.
Relativistic electron acceleration in thunderstorm electricfield and high-energy atmospheric phenomena at lightning
The Gamma-Ray Observation of Winter THundercloud(GROWTH) collaboration is aiming at revealing highenergy atmospheric phenomena occurring at lightning andin thunderstorms[12],[13],[14],[15],[16] . The project started in2006 and has detected bremsstrahlung gamma rays from relativistic electrons accelerated by strong electric fields in winterthunderstorms. This gamma-ray radiation events were named“long burst” by our group (as known as gamma-ray glow),which lasts for a minute time-scale corresponding with passage of a thundercloud above our detectors. Winter thunderstorms observed along the Japan sea are ideal targets for ourobservation campaigns thanks to its low altitude of the cloudbase and frequent energetic lightning, while observations ofsummer thunderclouds at mountain tops are also important tomeasure the phenomena very close to or even inside thunderstorms. Our collaboration has used the Mount Norikura cosmic ray observatory to study the long bursts, and successfullyrecorded events.
The GROWTH collaboration newly launched multi-point
mapping observation campaigns in 2015. The primary purpose is to study life cycle of the electron acceleration sites inthunderstorms comparing with weather-monitoring data. Another purpose of the project is to reveal mysterious “shortburst” events, which have been sometimes detected in ourpast observations, associated with lightning discharges withits 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 detection of “long burst” nor “short burst” eventsfrom summer thunderclouds during the two years. However,
we successfully used these summer campaigns as pilot observations toward the winter campaigns to check our operationand capability of the detectors. The Norikura observations arealso 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 themystery of the short burst. 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 the physics fieldin 2017. We are now trying to develop the “high-energy atmospheric physics” of lightning and thunderstorms, a new interdisciplinary field combining the gamma-ray and radio observations.
Study of Secondary Cosmic Rays from Thundercloud at
Mt. Norikura
In order to study the relativistic electron acceleration
mechanism by thundercloud-derived electric field and the relation between thunder and cosmic-ray air shower, we started
an experiment in 2015, mainly using gamma-ray detectors at
Norikura Observatory of Institute for Cosnic Ray Research,
the University of Tokyo. This experiment consists of gammaray 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 of magnitude from 70
keV to 120 MeV as a whole.

78

Observation period: 22 days from August 24 to September 14,
2015
During this time, the thundercloud did not pass, and no
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 increase in gamma rays of 3 MeV or more. This meansthat gamma rays from radon can be distinguished from contributions from other gamma rays not derived from radon, andit can be expected that these detectors can operate normallyeven when they are installed at an altitude about 4000 m for acertain future project.
Observation period: 30 days from July 19 to September 16,
2016 (excluding the summer season)
No direct lightning strike event occurred during the observation period, but 6 atmospheric electric field fluctuations exceeding 30 kV/m, which seemed to be an influence of thundercloud 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 was nolightning strike nearby, but it was observed that the thundercloud passed several times. Currently, the data at the time of
the thundercloud passing is being analyzed in detail (Fig. 3).

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

Study of gamma ray bursts from mountain-top thunderclouds
We observed gamma ray bursts that arise in relation to
thunderclouds at the Norikura Observatory of ICRR(2,770 m
above sea level)[17],[18],[19],[20] . Measurement was carriedout by placing PANDA64 detector outdoors of the observatory. The detector is made of 64-module plastic scintillators(total mass about 640 kg) developed for reactor operation

monitoring. Our measurement has unprecedented features including high statistics, good energy response, direction sensitivity and neutron identification.
Long-duration persistent bursts were observed 12 times in
54 days from July to September 2014 and their energy spectrum extended up to 25 MeV in the largest burst. The durationof the bursts ranged from a few to ten minutes. Since thesebursts 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 were observed
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 the bremsstrahlung
γ -rays by source electrons with monochromatic energy of 40–
80 MeV falling downwards from altitude of 400–1000 m produced the observed total energy spectra of the bursts well. It
is supposed that secondary cosmic ray electrons, which act as
seed, were accelerated in electric field of thunderclouds andmultiplied by relativistic runaway electron avalanche.
The estimated energy of the source electrons was higher
than that of the bursts we previously observed at Ohi Power
Station at sea level. Additionally, estimated electron flux atthe estimated source height was remarkably lower than that of
the Ohi site. These results give new restrictions to the modelof electron acceleration and multiplication process in electricfield of thunderclouds.
Development of high energy proton irradiation techniquefor devices used in spaceshipSpace exploration is presently interesting in business field.
Ion beam irradiation verification for devices to be mounted on
spaceships is required to simulate cosmic rays expected in theuniverse to estimate lifetime of these devices[21],[22],[23],[24] .
Flux estimation technique of primary ion beam in wide
range from an accelerator is needed In this kind of cosmic ray
simulation field. The desired flux of the ion beam for this kindof field is between 102 and 106 protons·cm−2 ·s−1 in typical
proton cases. Plastic scintillators can be used in lower intense
region to count 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 this whole 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 hasa feature that detector components do not occupy the beam
path and the presence of the detector do not influence the main
simulation field at all.
NaI(Tl) scintillator system for high energetic γ -ray measurement 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 exper-

79

iments using 100 MeV proton beam were carried out at the
Wakasa Wan Energy Research Center (WERC). Backgroundmeasurements were carried out at Norikura Observatory of
ICRR in summer, where one can expect high energetic γ -ray,which has the similar energy region compared to the accelerator field of this study. EFM100 atmospheric electric field
monitor system of Boltek was added at Norikura to measure
accidental high energetic γ -rays caused by thunder lightning.

Fig. 4. Time structure of γ -ray above 3 MeV at WERC 100 MeV proton
beam delivery. Vertical axis shows γ intensity ( events/ms ) whilehorizontal 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, theresult clearly shows that this measurement system can distinguish the beam ON/Off, while this system is still in verification for a quantitative discussion. The dead time of the systemshould be defined.
In the meantime, thunder lightning events were searchedusing data obtained at Norikura. No events have been distinguished. The whole data at Norikura show a stable conditionof the whole system for a couple of 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 toestimate the quantitative property of the system in ion beam
environment.
Development of high energy proton irradiation techniquefor devices used in spaceship
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 aircraft crew have generally
been evaluated by model calculation. It is necessary to verify the calculation with measurements to maintain accuracy
and credibility of dose assessment. The purposes of this study
were to construct a compact and inexpensive cosmic-ray neutron monitoring system which was based on a rem-counter 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 was received in National Institute of Radiological Science in Chibacity via the ICRR network. This monitoring system succeededin continuous monitoring more than ten months twice during
2014 to 2016. The averaged counting rate was about 1 countper minute, which was equivalent to neutron ambient dose
equivalent rate of about 15 nSv/h by the preliminary evaluation. It is a future problem to reveal the cause of the reduction
in counting rate seen over from November to April.
Evaluation of Response to the Gamma-ray of the Emulsion Telescope (2007, 2013)
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), polarization-sensitive, large-aperture-area(∼10 m2 ) balloon-borne telescope using nuclear emulsion
film[25],[26],[27],[28],[29],[30],[31] . 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 to confirm detection performanceusing atmospheric γ -ray.
2007 test was the first trial of the detection to the γ ray spread wide incoming angle. We established configuration of the telescope and its analysis scheme. Based on
this experience, we finalized the design of the first balloonborne 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 toimprove track finding efficiency as well as signal-to-noise ratio. We obtained high (> 97%) track finding efficiency in asingle film and confirmed γ -ray detection capability at 100
MeV energy region (Figure 5).
Based on this experience, we performed 2nd balloon experiment (obs/Norikura/GRAINE 2015) in Japan-Australia JAXA
collaborative balloon experiment at the Alice Springs balloonlaunching station in May 2015.

Environmental Study

Aerosol sampling at Mt. Norikura
Aerosol in the atmosphere has been sampled since 2013at 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 the aerosols sampled by anAndersen 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 withthe 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 in Yamagatacity (Fig.6.). Its ratio is almost consistent to a simulated ratio
8.8 of 7 Be productions due to secondary cosmic rays in the atmosphere by EXPACS. This experimental result is useful foran estimation of altitude distribution of cosmogenic nuclide.

80

ice particles. However, in the 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 the alpine
tree-line

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

Trees at high altitudes experience severe embolism (lossof xylem conductivity for water) during winters, which is attributed to the entrapment of air in xylem conduits during
frost-drought. However, in the sub-alpine fir (Abies mariessii) at Mt. Norikura, air-filled conduits were not observed evenin severely-embolized (complete loss of conductivity) shoots.
Rather, the pits (valves of partitions inter-conduits) closed before the severe frost-drought in mid-winter, thereby resulting
in severe-embolism (complete loss of conductivity). Thus, bypit (valve) cloure, shoots could maintain water in the xylem
throughout the winter, which is thought to be an adaptation
against lethal filling of air in the conduits during severe frostdrought.
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.
Investigation of alpine plants on Mt. NorikuraWestudiedmainlythefollowing
three
researcheson
Mt.
Norikura
after
2012[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46],[47],[48],[49] .
1) Long-term monitoring and community assembly of alpine
plantsFig. 6. 7 Be concentration as a function of aerosol size

Adaptation of alpine plants to severe environmental conditions
Trees in the alpine regions experience harsh conditions
including strong winds, low temperatures, desiccation, andheavy snow. Thus, plants growing in such regions 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[32],[33],[34] .
1. Adaptation of leaf cuticles in sub-alpine fir (Abies mariessii) at the alpine tree-line
Leaf browning and death are frequently observed in evergreen conifers at the alpine tree-line. These are thought to due
to increased transpiration caused by a thinner cuticle and/or
mechanical damage to the cuticle by wind-blown snow andWe made 40 plots for long-term monitoring of alpine vegetation because climate change possibly affects distributionsof alpine plants. We also examined the community assembly
process of alpine plants at the 40 plots from the view points ofhabitat filtering and limiting similarity. Habitat filtering andlimiting 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 (1600m 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 forsoil respiration rates.
3) Genetic differentiation of Solidago virgaurea complex
Plant species distributed along wide altitudinal or latitudinal gradients show phenotypic variation due to their heterogeneous habitats. This study investigated whether phenotypicvariation in populations of the Solidago virgaurea complex
along an altitudinal gradient is caused by genetic differentiation. Population genetic analyses with microsatellite markers were used to infer the genetic structure and levels of geneflow between populations. However, the population genetic

81

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 existenceof gene flow and that phenotypic variation of the S. virgaurea
complex along the altitudinal gradient is maintained by strong
selection pressure.

Bibliography[1] “ Solar neutron events in association with large solar
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84

AKENO OBSERVATORY
Introduction

The Akeno Observatory is situated in Akeno of Hokutocity, 20 km northwest of Kofu and 130 km west of metropolitan Tokyo. The location is at the longitude of 138.5◦ E and thelatitude of 35.8◦ N. The altitude is ∼900 m above sea level.
It was established in 1977 as a research center for air showerstudies in the very high energy region, and it has been administered by the ICRR as a facility of joint-university-use.
The 40th anniversary of the Akeno Observatory was heldin 2017.

Akeno Air Shower Experiments

Fig. 2. One of the muon detector housings with concrete shielding.

log(FLUX * E3 in eV2m-2s-1sr-1)

The Akeno Air Shower Experiment started in 1979 withan array covering 1 km2 area (the 1 km2 array, see Fig.1).
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

26

25.5

25

24.5

24

23.5

23

Fig. 1. Aerial View of Akeno Observatory and 1 km2 Array Area

to detect Ultra-High Energy Cosmic Rays (UHECRs) in theenergy range of 1020 eV.
One of the distinctive features of Akeno experiments isthat the measurements were made over five decades of energies well covering 1015 eV - 1020 eV by using both the surface detector for electromagnetic component, and the shielded
detector for muon component (Fig.2). The wide energy coverage was accomplished by the arrays of scintillation detectors of various inter-detector spacings from 3 m to 1 km andwith different triggering conditions. This feature of Akeno air
shower measurement is well demonstrated in Fig.3, in which
the spectra from Akeno 1 km2 array for 1014.5 eV - 1018.8 eV
2 and AGASA for 1018.5 eV - 1020.3 eV 3 are plotted.
*2 M. Nagano et al., J. Phys. G10, 1295 (1984); M. Nagano et al., J. Phys.
G18, 423 (1992).
*3 M. Takeda et al., Astropart. Phys. 19, 447 (2003).

14

15

16

17

18

19

20

21

log(ENERGY in eV)

Fig. 3. Akeno energy spectrum measurements for 1015 eV - 1020 eV.

AGASA

The AGASA was composed of 111 surface detectors, eachwith plastic scintillator of 2.2 m2 area and 5 cm thickness.
The counters were deployed with ∼1 km spacing covering the
ground area of about 100 km2 in the suburban area of Akeno,
outside of the observatory campus. The AGASA served as thelargest air shower array in the world since its commissioning
in 1990 until it stopped data taking in January 2004, when theconstruction of the succeeding experiment, Telescope Array(TA), started in Utah. The AGASA was dismantled in 2007
together with other Akeno air shower arrays.
An exposure of 5.8 × 1016 m2 s sr above 1019 eV wasaccumulated by AGASA in 13 years of operation. Extensive
air showers with zenith angles smaller than 45◦ and with core

85

locations inside the array area were used for the analysis. The
AGASA reported an extension of the energy spectrum beyond
the predicted Greisen-Zatsepin-Kuzmin (GZK) cutoff in 1998
4 and a total of eleven UHECR events were observed above
1020 eV by 2003.

Measurement of UHECRs

Since the AGASA measurement in 1998, High Resolution
Fly’s Eye (HiRes) 5 , Pierre Auger Observatory (PAO) 6 , andTelescope Array (TA) 7 measured the energy spectra of UHECRs with higher statistics.
The HiRes observed the UHECR using the fluorescencetelescope. The PAO and the TA measure the energy spectrausing the surface array consisting of either water tanks (PAO)or plastic scintillators (TA), but the energy scale of the arrayis determined by the fluorescence telescope using a subset of
events observed by the fluorescence telescope and surface array at the same time. The adoption of the energy scale by thefluorescence telescopes is based on its small dependence on
the air shower simulation.
The energy spectra above 1018 eV by AGASA andother experiments are compiled and compared by the working group represented by UHECR experiments in the
UHECR2012 symposium held in 2012 8 . The result is plottedin Fig.4 with the energy scale of each experiment adjusted toa reference energy, which is set halfway between PAO andTA/HiRes. Following factors were applied for the energy
scale; ×1.10 for PAO, ×0.91 for TA and HiRes, ×0.65 for
AGASA and ×0.56 for Yakutsk.

J(E) × E3/1024 [eV2 m-2 s-1 sr-1]

Recent Research Activities

The study of UHECRs by AGASA in Akeno was succeeded by the TA experiment in Utah, USA since 2008. After the cessation of AGASA, the Akeno Observatory has beenused for small scale cosmic ray experiments, astrophysical observations and as a test and maintenance facility of TA by the
ICRR and university researchers. Fig.5 shows a recent photograph of the main site of the Akeno Observatory.

Fig. 5. The main site 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.

10

1

10-1
18

TA

HiRes-I

PAO

HiRes-II
Yakutsk

AGASA
18.5

19

log (E/eV)

19.5

20

20.5

10

Fig. 4. Compilation of UHECR energy spectra (UHECR2012). Theenergy scale of each experiment is adjusted as described in the
text.

As seen in Fig.4, the overall agreement between experiments is good, and a “dip” structure was seen around 1018.7
eV by all experiments. The HiRes, PAO and TA confirmed
a strong flux suppression above approximately 1019.7 eV. Although the AGASA spectrum does not demonstrate the cutoff
structure, the number of events above 1020 eV became only
two after the energy rescaling, making the claim of the extended spectrum statistically insignificant. The estimate of
*4
*5
*6
*7
*8

systematic uncertainty of the energy measurement is approximately 20% for all the experiments, and rescalings for the
TA/HiRes and PAO are within this limit. Rescaling of thesurface array energy for AGASA and Yaktsuk indicates that
there exist larger systematic uncertainties than originally estimated by running the air shower simulation. This differenceof energy scale obtained by the surface array and by the fluorescence telescope remains as a basic question in the understanding of the air shower phenomena.

M. Takeda et al., Phys. Rev. Lett. 81, 1163 (1998).
R.U. Abbasi et al., Phys. Rev. Lett. 100, 101101 (2008).
J. Abraham et al., Phys. Lett. B685, 239 (2010).
T. Abu-Zayyad et al., Astrophys. J. 768, L1 (2013).
http://indico.cern.ch/conferenceDisplay.py?confId=152124

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 installedin the Akeno Observatory in 2003 (Fig. 6). The telescope has
an aperture of 50 cm, an FOV of 28’ × 28’ and is equippedwith a tricolor CCD camera capable of g( RC IC -bands photometry (g’:400∼550 nm, Rc:570∼730 nm, Ic:730∼ 850 nm). It
is operated remotely from the Tokyo Tech at the Ookayama
Campus. Upon receiving a GRB alert from Swift or Fermi
satellite, it directs the telescope toward the GRB direction
within two minutes, and makes a prompt observation of the
GRB and its afterglow.
The follow-up observation of GRBs was continued by the
MITSuME telescope. In this fiscal year the values of limiting magnitude of 53 GRBs were observed. The objects wereidentified for four GRBs, and light variations were observed
for three objects during observation.
As the part of OISTER (Optical and Infrared Synergetic
Telescopes for Education and Research) program by universities, the MITSuME telescope performed the follow-up observations of three events of supernova explosion afterglow, two

86

X-ray binaries detected by MAXI (Monitor of All-sky X-ray
Image) and Swift, and two dwarf novae.
In order to perform follow-up visible-light observationsof gravitational objects, the MITSuME participated in J-GEM(Japanese collaboration for Gravitational-wave ElectroMagnetic follow-up) and GROWTH(Global Relay of Observatories Watching Transients Happen) to establish worldwide observation network focusing on the electromagnetic identification of gravitational wave sources as one of scientific themes.
In this fiscal year, the system of summarizing observational
information and results for J-GEM was developed. And theobservation of a transient source with the MiTSuME telescope
related to the GROWTH project was reported.

prototype tests with atmospheric Cherenkov observations of
gamma rays on the ground 9 . This telescope is the only
telescope to observe atmospherice Cherenkov light emitted
from air showers induced by TeV gamma rays in Japan. In
JFY2016, an atmospheric cherenkov light event was firstly
observed with this telescope.
One of the R&D performed at the Akeno Observatory is
the development of low-power-consumption data acquisition
system with 32 photomultipliers. It is planned to observe the
Crab Pulsar using the part of this system.
In JFY2018, it succeeded in reading out data continuously
at 100-µ sec sample interval which is sufficiently shorter thanthe period of the Crab Pulsar. Around the end of JFY2018, thefinal tuning of the timing calibration was being performed. It
is planed to introduce the main system into the telescope todetect optical pulse signal by observing the Crab Pulsar.

Fig. 6. 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) etal.
Four layers of proportional counter telescopes, each with 25
m2 area, were installed in three muon houses in Akeno andhave been continuously measuring the cosmic ray muonssince 2003. Fig. 2 shows one of the muon houses. The modeenergy of the primary cosmic rays is approximately 2.5 GeV
corresponding to 2m thick concrete ceiling of the muon houseat the latitude of the Akeno Observatory. The measurement in
Akeno is combined with a simultaneous measurement by the
GRAPES-3 experiment at Ooty in India, and this telescope
aims at measuring the modulation effects and anisotropy ofgalactic cosmic rays in the region of about 100 GeV and detecting trasient phenomena of cosmic rays. It is expected tounderstand cosmic-ray flow in the universe magnetic field andto obtain a clue to the solution of cosmic-ray propagation.
And it is challenging to establish the method of a new space
weather observation by simultaneous multi-directional observations with high statistical precision. In JFY 2018, the maintenance of three muon houses were performed for data servers
and proportional counters, and the observations with all threehouses resumed.
Research and development for a small atmospheric
Cherenkov telescope at the Akeno Observatory by
T. Yoshikoshi (ICRR) et al.
An alt-azimuth telescope with an aperture of three meters(Fig. 7) was setup in the Akeno Observatory for various

Fig. 7. The Cherenkov telescope at the tour of the 40th anniversaryof the Akeno Observatory in 2017.

Research and development for the Telescope Arrayobservation in Utah by the TA collaboration and others
All the TA fluorescence imaging cameras were assembled in
the Akeno Observatory by the TA collaboration team. The
detectors were tested in Akeno and shipped to the Utah observation site for the installation. All the unit mirrors of theTA fluorescence telescope were tested in Akeno and the atmospheric monitoring lidar of TA using YAG laser was developed in Akeno. In JFY 2015, the R&D of the surface detectors
were performed in the large experimental hall of the Akeno
Observatory for the TAx4 project that aims at quadrupling theTA surface detector array in Utah. In August in 2016, 2017and 2018 the TAx4 scintillator counters were assembled in
the Akeno Observatory. They were sent to the Cosmic Ray
Center in Delta City in Utah, USA, where the TAx4 surface
detectors were finally assembled.
The tests using facilities in the Akeno Observatory byother subjects were also performed.

*9 M. Ohishi et al., 33rd ICRC, (Rio deJaneiro), 587 (2013).

87

KAMIOKA OBSERVATORY
Kamioka observatory is located at 1000 m underground(2700 m water equivalent) in the Kamioka Mine, Gifu prefecture, about 200 km west of Tokyo. The observatory wasestablished in 1995 in order to operate Super-Kamiokande
experiment (SK). The underground laboratories are locatedunder Mt.Ikeno-yama and accessible to the experimental sitethrough a 1.7 km horizontal tunnel. The observatory also
has surface research buildings and a dormitory located at the
distance of 15 minutes drive from the entrance of the underground laboratories.
The Super-Kamiokande experiment had discovered neutrino oscillations through the observations of atmospheric andsolar neutrinos (see the section for Neutrino and Astroparticle Division). The atmospheric neutrino oscillation was confirmed by the long baseline neutrino oscillation experiment,
K2K, using accelerator neutrino beam, which was conducted
between 1999 and 2004. A new long baseline neutrino oscillation experiment (the T2K experiment) using a high intensity
beam, 50 times of the K2K neutrino beam, by the J-PARC
proton accelerator has started in 2009. In 2011, the experiment has observed 6 νe appearance events indicating non-zeroθ13 which was as yet determined the last neutrino oscillation
parameter. Anti-neutrino beam data was taken from 2014 to2016 in order to search for CP violation. Based on the whole
data taken until 2016, T2K presented the first CP violation
search result in August 2016.
The low cosmic ray flux and low seismic noise environ-ment in the underground site enables us to conduct various
researches. There is a 100 m long laser interferometer, which
is a proto-type of the 3 km gravitational wave antenna (KAGRA). Using the low radioactive background environment in
the Kamioka Mine, a dark matter experiment, called XMASS
is operated in Lab-C. The XMASS group constructed a 800kg
liquid xenon detector and started data taking from 2010. The
detector has been improved and searches for dark matter interactions and rare phenomena in luquid xenon have been conducted in the last few years. The R&D study of a tracking type detector for dark matter detection led by the Kobe
University group (the NEWAGE experiment) has also been
performed in Lab-B. A double beta decay experiment using
48 Ca (the CANDLES experiment) led by the Osaka University
group has been running in Lab-D. The study to improve theneutrino detection sensitivity by adding gadolinium to SuperKamiokande (called SK-Gd project) has been performed at
Lab-E. A 200 ton test tank dedicated for the R&D study of
the SK-Gd project was constructed and a feasibility study has
been performed. In order to support those experiments andalso related R&D works, the Observatory is equipped withlow background Germanium detector in Lab-1 and Lab-A,
ICP-MS and so on to measure extremely low radioactive backgrounds. Lab-G area was newly excavated in 2015 and the
SK-Gd equipments which include a Gd dissolving system, apre-treatment system and a Gd-water circulation system were
installed in 2016.

G

Fig. 1. Kamioka Underground Observatory.

88

KAGRA OBSERVATORYKAGRA 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 L-shape tunnel facility, and it is locatedmore than 200m under Mt.Ikeno-yama. 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 in the community
center of Hida city located about 5km away from the Atotsu
entrance of KAGRA.
KAGRA aims to observe several gravitational waves(GWs) per a 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 Leading-edge Research Infrastructure Program, and also supported by Program for Promoting
Large-scale Science Projects, Subsidy for Facilities Expenseand Grants-in-Aid for Scientific Research from Ministry of
Education, Culture, Sports, Science and Technology (MEXT).
In KAGRA project, Institute for Cosmic Ray Researchplays a role of a host promoting institute, and National Astronomical Observatory in Japan (NAOJ) and High Energy Accelerator Research Organization (KEK) are the main support
organizations, then more than 297 researchers in 85 institutes
and universities in the world are collaborating for construction
and data analysis of KAGRA.
The tunnel excavation started in May 2012, and finishedin 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 (April 2019), all the interferometer components had been installed to complete the KAGRA Observatory that adopts a power recycled Fabry-Perot Michelson typeinterferometer with the resonant sideband extraction and theinterferometer is under commissioning. We plan to start thejoint observation with LIGO and Virgo within 2019.

Fig. 1. Surface Research Building.

Fig. 2. Atotsu Entrance of KAGRA.

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RESEARCH CENTER FOR COSMIC NEUTRINOS
The Research Center for Cosmic Neutrinos (RCCN) wasestablished in April 1999. The main mission of this center isto promote researches related to neutrinos based on data fromvarious observations and experiments, and we have provided
the occasion to discuss theoretical ideas and experimental results on neutrino physics. Members of this center have been
involved in the Super-Kamiokande and T2K experiments, andcontributing precise measurements of neutrino oscillations.
Also, we have been involved in Hyper-Kamiokande project,
and worked on the calculation of the atmospheric neutrino flux
to have better predictions of the neutrino flux.
RCCN, together with the computer committee, oversees
the operation of the central computer system in ICRR (Fig 1).
The computer facility has high performance to analyze hugeamount of data, and has been operated without any serious
trouble since it was upgraded in 2014. Since 2004, RCCN has

been accepted inter-university programs related to activities
in the low-background underground facility also (Fig 2). In
FY2018, we accepted 9 programs related to these facilities.

Fig. 1. Photo of the central computer facility in ICRR upgraded in 2014.

Fig. 2. Photo of the low-background underground facility in ICRR.

RCCN has been organizing domestic neutrino-related
workshop since it was established. On March 23, 2019, we
hosted one neutrino workshop on Supernova relic neutrino .
Twenty-one physicists participated in this meeting. We havealso contributed holding public lectures. Since JFY2009,
ICRR and the Kavli Institute for the Physics and Mathematicsof the Universe (Kavli-IPMU) have co-sponsored two public
lectures each year. The public lecture held in Spring is coorganized by RCCN and the Public Relation Office of ICRR.
The Spring public lecture in FY2018 was held on April 14,
2018. Two scientists lectured on the supernova neutrino andtheoretical study using LHC accelerator data.

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APPENDICES
A. ICRR Workshops and Ceremonies
B. ICRR Seminars
C. List of Publications(a) Papers Published in Journals(b) Conference Papers (Proceedings)(c) ICRR Report

D. Doctoral Theses
E. Public Relations(a) ICRR News(b) Public Lectures(c) Visitors

F. Inter-University Research Activities
G. List of Committee Members(a) Board of Councillors(b) Advisory Committee(c) Inter University Research Advisory Committee

H. List of Personnel

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A. ICRR Workshops and Ceremonies
The first meeting of the Hyper-Kamiokande Experiment Financial Forum
Date: Jan. 11, 2019
Place: Kojima Hall, Hongo Campus, The University of Tokyo

Outline: Hyper-Kamiokande (HK) is a new world-leading international scientific research project aiming to start its operation in the second half of the 2020s. In October 2017, the Next-Generation Neutrino Science Organization (NNSO) wasestablished in the University of Tokyo by the Institute for Cosmic Ray Research (ICRR), the Kavli Institute for the Physics andMathematics of the Universe (Kavli IPMU), and the School of Science to cooperate in pioneering the future of neutrino physics
programs through the development of neutrino research techniques and detector technologies. Professor Takaaki Kajita, thedirector of NNSO, invited foreign agency representatives and researchers from 14 countries (Armenia, Brazil, Canada, France,
Italy, South Korea, Poland, Russia, Spain, Sweden, Switzerland, UK, USA and Japan) and about 50 people gathered to holddiscussions. The forum was established to offer a place for agencies to find out the details of the Hyper-Kamiokande project
including its goals, timelines and the organizational structure, and to share information on interests and constraints of eachcountry and region. We discussed to identify the necessary tasks to further solidify the international collaboration.
Participants: 50 participants.

Fig. 1. About 50 participants attended the first meeting of HKFF.

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Inauguration Ceremony of the First Large Size Telescope of
Cherenkov Telescope Array
Date: Oct. 10, 2018
Place: IAC s observatory of Roque de los Muchachos on the island of La Palma, Spain

Outline: The Cherenkov Telescope Array (CTA) is the next-generation observatory for gamma-ray astronomy, and is the
project more than 1,400 researchers and engineers from 31 countries have joined. The first of four Large Size Telescopes(CTA-LST1) was completed, and its inauguration ceremony was held on 10th of October, 2018, on the island of La Palma,
Spain. More than 200 guests from around the world gathered on the northern array site of CTA to celebrate its debut.
Professor Takaaki Kajita, Director of ICRR, stated a celebration and CTA is expected to play an essential role not onlyin the gamma-ray astronomy but in the multi-messenger astronomy, in his remarks. Professor Masahiro Teshima, LST work
package leader, appreciated the related countries governments who extended the strong financial support up to this point,
and rewarded the researchers and the engineers involved for their effort, and asked for continuous support to complete theconstruction of both CTA s northern and southern hemisphere arrays.
The ceremony was followed by congratulatory speeches from Professor Masashi Haneda, Executive Vice President of the
University of Tokyo, Mr. Pestana Anselmo, Presidente del Cabildo de La Palma, and Mr. Pedro Duque, Ministro de Ciencia in
Spain.
The ceremony was closed with ribbon-cutting.
Participants: 187 participants.

Fig. 2. CTA-LST1 on the top of the island of La Palma.

Fig. 3. Ribbon-Cutting Ceremony.

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Symposium and Celebration to Commemorate the 10th anniversaryof Telescope Array
Date: Dec. 19, 2018
Place: Media Hall, Kashiwa Campus of the University of Tokyo.

Outline: The symposium and the celebration to commemorate the 10th anniversary of Telescope Array was held on 19thof December 2018, at Kashiwa Campus of the University of Tokyo, and the approximately 90 people involved attended in these
events.
Telescope Array is an international collaborative project, and about 130 researchers from 6 countries have been working
on. The 507 surface detectors (SDs) and the 3 fluorescence detectors (FDs) have been deployed in 700 square kilometers in thedessert of Utah, USA, to study the origin and the nature of Ultra-High Energy Cosmic Rays. Currently TA extension named
TA×4 has been ongoing to quadruple the TA effective area, and about 250 SDs were deployed in February and March of
2019.
Professor Takaaki Kajita, Director of ICRR, greeted the attendee and expressed his appreciation of the kind support andcooperation for those who are involved in this experiment in his opening remarks. Dr. Norio Matsuki, Executive Vice Presidentof the University of Tokyo, stated the importance of the role expected to ICRR and the University of Tokyo to contribute toacademia and even to the humanity. The ceremony was followed by the congratulatory addresses from Hiroyuki Sakuragi,
Executive Director and Vice President of Osaka City University which is the affiliation of Professor and Co-spokesperson of
TA experiment Shoichi Ogio, and Professor Charles Jui, Co-spokesperson of TA experiment in the University of Utah.
In this ceremony, the letters of gratitude were presented by Prof. Kajita to the nine companies for their contribution to TA
project.
Participants: 91 participants.

Fig. 4. Prof. Kajita’s opening remarks.

Fig. 5. Celebration for the 10th anniversary of TA experiment.

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B. ICRR Seminars
1. April 20, 2018: Dr. Takashi Sako (ICRR), ” Forward Particle Measurements at Colliders and Air Shower Development”
2. May 25, 2018: Dr. Koji Noda (ICRR), ” Gamma-ray Burst Physics with MAGIC and CTA LST”
3. June 19, 2018: Dr. Ken Mawatari (ICRR), ” Challenge to the Most Distant Evolved Galaxies: Hint on Star-formation
Activity during the First 500 Million Years of the Cosmic History”
4. July 24, 2018: Dr. Manel Martı́nez (IFAE-BIST, Visiting Prof. of ICRR), ” On Bounds on a Possible Energy Dependenceof the Speed of Light in Vacuum”
5. July 27, 2018: Dr. Kyohei Kawaguchi (ICRR), ” Modelling Gravitational Waveforms and Electromagnetic Signals from
Neutron Star Binary Mergers”
6. October 3, 2018: Dr. Susumu Inoue (RIKEN), ” The Dawning of Electroweak Astronomy: Interpreting Electromagnetic+neutrino Observations of Blazars”
7. November 2, 2018: Dr. Jonathan R. Gair (The University of Edinburgh), ” Gravitational-wave standard siren measurements of the Hubble constant”
8. November 8, 2018: Dr. Kenny C. Y. Ng (Weizmann Institute of Science), ” The Surprising Solar Gamma-ray Emission
from Cosmic-Ray Interactions”
9. November 26, 2018: Dr. A. Nepomuk Otte (Georgia Institute of Technology), ” An instrument to detect cosmogenic
neutrinos with the Earth skimming technique”
10. December 3, 2018: Dr. Angela Olinto (The University of Chicago), ” Space Probes of the Highest Energy Particles:
POEMMA & EUSO-SPB”
11. December 27, 2018: Dr. Toshihiro Fujii (Kyoto University), ” A personal decadal survey in ultrahigh-energy cosmic-ray
observatories”
12. January 10, 2019: Dr. Kimihiro Okumura (ICRR), ” Updated results from the T2K experiment with 3.13×1021 protons
on target”
13. January 25, 2019: Dr. Kohei Hayashi (ICRR), ” Milky Way dwarf spheroidal galaxies as a probe of dark matter properties”
14. February 20, 2019: Dr. Olaf Reimer (University of Innsbruck), ” Diffuse Galactic Gamma-Ray Emission before CTA”
15. February 25, 2019: Dr. Kumiko Kotera (Institut d’astrophysique de Paris), ” Hunting the highest energy astroparticles
with GRAND”
16. February 27, 2019: Dr. Shigeyuki Sako (Institute of Astronomy, U. Tokyo), ” The Tomo-e Gozen project”
17. March 11, 2019: Dr. Jishnu Suresh (ICRR), ” Efficient technique to probe Stochastic Gravitational Wave BackgroundAnisotropy with ground-based detectors”
18. March 25, 2019: Dr. Felix Riehn (Lab. Inst. Fisica Experim. Particulas), ” Interactions of ultra-high energy cosmic
rays”

C. List of Publications(a) Papers Published in Journals
1. ”Search for Neutrinos in Super-Kamiokande Associated with the GW170817 Neutron-star Merger”, The SuperKamiokande Collaboration, Astrophys. J. Lett. 857, L4, 2018.
2. ”Direct dark matter search by annual modulation with 2.7 years of XMASS-I data”, XMASS Collaboration, PhysicalReview D 97, 102006 (2018).

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3. ”Search for Boosted Dark Matter Interacting With Electrons in Super-Kamiokande”, The Super-Kamiokande Collaboration, Phys. Rev. Lett. 120, 221301 (2018).
4. ”Improved search for two-neutrino double electron capture on 124 Xe and 126 Xe using particle identification in XMASSI”, XMASS Collaboration, Prog. Theor. Exp. Phys. 2018 (2018) 053D03.
5. ”Physics Potentials with the Second Hyper-Kamiokande Detector in Korea”, Hyper-Kamiokande proto-collaboration,
Prog. Theo. Exp. Phys. 2018, 063C01 (2018).
6. ”Measurement of the tau neutrino cross section in atmospheric neutrino oscillations with Super-Kamiokande”, Z. Li etal. (Super-Kamiokande Collaboration), Phys. Rev. D 98, 052006.
7. ”Search for dark matter in the form of hidden photons and axion-like particles in the XMASS detector”, XMASS Collaboration, Phys. Lett. B787 (2018) 153-158.
8. ”A direct dark matter search in XMASS-I”, XMASS Collaboration, Phys. Lett. B 789 (2019) 45-53.
9. ”A measurement of the scintillation decay time constant of nuclear recoils in liquid xenon with the XMASS-I detector”,
XMASS Collaboration, JINST 13 (2018) P12032.
10. ”Development of low radioactivity photomultiplier tubes for the XMASS-I detector”, XMASS Collaboration, Nuclear
Inst. and Methods in Physics Research, A 922 (2019) 171-176.
11. ”Search for WIMP-129 Xe inelastic scattering with particle identification in XMASS-I”, XMASS Collaboration, Astroparticle Physics 110(2019), 1-7.
12. ”Development of new radon monitoring systems in the Kamioka mine”, G. Pronost, M. Ikeda, T. Nakamura, H. Sekiya,
S. Tasaka, Progress of Theoretical and Experimental Physics, Volume 2018, Issue 9, 1 September 2018, 093H01.
13. ”Gamma Ray Spectrum from Thermal Neutron Capture on Gadolinium-157”, K. Hagiwara, T. Yano et al., Progress ofTheoretical and Experimental Physics, 2019 - 023D01.
14. ”Atmospheric neutrino oscillation analysis with external constraints in Super-Kamiokande I-IV ”, The SuperKamiokande Collaboration, Phys. Rev. D 97, 072001 (2018).
15. ”Search for CP Violation in Neutrino and Antineutrino Oscillations by the T2K Experiment with 2.2×1021 Protons on
Target”, T2K Collaboration, Phys.Rev.Lett. 121 (2018) no.17, 171802.
16. ”Characterization of nuclear effects in muon-neutrino scattering on hydrocarbon with a measurement of final-state kinematics and correlations in charged-current pionless interactions at T2K”, T2K Collaboration, Phys.Rev. D98 (2018)
no.3, 032003.
17. ”Measurement of inclusive double-differential νµ charged-current cross section with improved acceptance in the T2K
off-axis near detector”, T2K Collaboration, Phys.Rev. D98 (2018) 012004.
18. ”First measurement of the νµ charged-current cross section on a water target without pions in the final state”, T2K
Collaboration, Phys.Rev. D97 (2018) no.1, 012001.
19. ”Deep observations of the globular cluster M15 with the MAGIC telescopes”, V.A.Achari et al (MAGIC Collaboration),
Mon.Not.Roy.Astron.Soc.484 (2019) no.2, 2876-2885.
20. ”A Fast Very High Energy γ -ray Flare from BL Lacertae during a Period of Multiwavelength activity in June 2015”,
V.A.Achari et al (MAGIC Collaboration), Astron.Astrophys. 623 (2019) A175.
21. ”Development of three silicon photomultiplier detector modules for the MAGIC telescopes for a performance comparison to PMTs”, A.Hahn, A. Dettlaff, D. Fink, D. Mazin, M. Razmik, M.Teshima, Nucl.Instrum.Meth. A912 (2018)259-263.
22. ”Discovery of TeV gamma ray emission from the neighbourhood of the supernova remnant G24.7+0.6 by MAGIC”,
V.A.Achari et al (MAGIC Collaboration), Mon.Not.Roy.Astron.Soc. 483 (2019) no.4, 4578-4585.
23. ”Periastron Observations of TeV Gamma-Ray Emission from a Binary System with a 50-year Period”, A.E Abeysekara
et al. (VERITAS and MAGIC Collaborations), Astrophys.J.Lett. 867 (2018) no.1, L19.
24. ”Constraining very-high-energy and optical emission from FRB 121102 with the MAGIC telescopes”, V.A.Achari et al(MAGIC Collaboration), Mon.Not.Roy.Astron.Soc. 481 (2018) 2479-2486.

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25. ”Extreme HBL behavior of Markarian 501 during 2012”, M.L.Ahnen et al (MAGIC, FACT and VERITAS collaborations), Astron.Astrophys. 620 (2018) A181.
26. ”Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A”, IceCube,
Fermi, MAGIC,,, collaborations, Science 361 (2018) no.6398, eaat1378.
27. ”The blazar TXS 0506+056 associated with a high-energy neutrino: insights into extragalactic jets and cosmic ray
acceleration”, S.Ansoldi et al (MAGIC Collaboration), Astrophys.J. 863 (2018) L10.
28. ”The broad-band properties of the intermediate synchrotron peaked BL Lac S2 0109+22 from radio to VHE gamma-ray”,
S.Ansoldi et al (MAGIC Collaboration), Mon.Not.Roy.Astron.Soc. 480 (2018) no.1, 879-892.
29. ”Multi-wavelength characterization of the blazar S5 0716+714 during an unprecedented outburst phase”, M.L.Ahnen etal (MAGIC, MWL, Fermi Collaborations), Astron.Astrophys. 619 (2018) A45.
30. ”Constraining Dark Matter lifetime with a deep gamma-ray survey of the Perseus Galaxy Cluster with MAGIC”,
V.A.Achari et al (MAGIC Collaboration), Phys.Dark Univ. 22 (2018) 38-47.
31. ”Detection of persistent VHE gamma-ray emission from PKS 1510-089 by the MAGIC telescopes during low states
between 2012 and 2017”, V.A.Achari et al (MAGIC and Fermi Collaboration), Astron.Astrophys.619 (2018) A159.
32. ”Gamma-ray flaring activity of NGC1275 in 2016-2017 measured by MAGIC”, S.Ansoldi et al (MAGIC Collaboration),
Astron.Astrophys. 617 (2018) A91.
33. ”A technique for estimating the absolute gain of a photomultiplier tube”, M. Takahashi et al , Nucl.Instrum.Meth. A894
(2018) 1-7.
34. ”Detection of the blazar S4 0954+65 at very-high-energy with the MAGIC telescopes during an exceptionally highoptical state”, M.L.Ahnen et al (MAGIC Collaborations), Astron.Astrophys. 617 (2018) A30.
35. ”Indirect dark matter searches in the dwarf satellite galaxy Ursa Major II with the MAGIC Telescopes”, M.L.Ahnen etal (MAGIC Collaborations), JCAP 1803 (2018) no.03, 009.
36. ”Hadronic Origin of Prompt High-energy Emission of Gamma-ray Bursts Revisited: in the Case of a Limited Maximum
Proton Energy”, Kai Wang, Ruo-Yu Liu, Zi-Gao Dai, and Katsuaki Asano, ApJ, 857:24(2018)12pp.
37. ”Extended Measurement of Cosmic-ray Electron and Positron Spectrum from 11 GeV to 4.8 TeV with the CalorimetricElectron Telescope on the International Space Station”, O. Adriani, Y. Akaike, K. Asano, et al. (CALET Collaboration),
Phys. Rev. Lett. 120, 261102.
38. ”On-Orbit Operations and Offline Data Processing of CALET onboard the ISS”, Y. Asaoka, S. Ozawa, S. Torii et al.,Aph 100, 29-37.
39. ”Blazar Spectra with Hard-sphere-like Acceleration of Electrons”, Katsuaki Asano, Masaaki Hayashida, ApJ 861
31(2018).
40. ”Radiative Transfer Simulation for the Optical and Near-infrared Electromagnetic Counterparts to GW170817”, Kyohei
Kawaguchi, Masaru Shibata, Masaomi Tanaka, ApJL 865 L21(2018).
41. ”Search for GeV Gamma-ray Counterparts of Gravitational Wave Events by CALET”, O. Adriani, Y. Akaike, K. Asano,et al., ApJ 863 160(2018).
42. ”Late Engine Activity of GRB 161017A Revealed by Early Optical Observations”, Yutaro Tachibana, Makoto Arimoto,Katsuaki Asano et al., PASJ (2018) 70 (5), 92 (1-9).
43. ”EUSO-TA ― First Results from a Ground-Based EUSO Telescope”, G. Abdellaoui, et al., Aph 102, 98-111.
44. ”Characteristics and Performance of the CALorimetric Electron Telescope (CALET) Calorimeter for Gamma-Ray Observations”, N. Cannady, Y. Asaoka, F. Satoh, et al., ApJS, 238, 5(16pp).
45. ”Outflow and Emission Model of Pulsar Wind Nebulae with the Back-reaction of Particle Diffusion”, Wataru Ishizaki,
Katsuaki Asano, and Kyohei Kawaguchi,, ApJ 867 141(2018).
46. ”Depth of Ultra High Energy Cosmic Ray Induced Air Shower Maxima Measured by the Telescope Array Black Rockand Long Ridge FADC Fluorescence Detectors and Surface Array in Hybrid Mode”, R.U. Abbasi et al. (The Telescope
Array Collaboration), The Astrophysical Journal, 858:76 (27pp).

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47. ”Gamma Ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders”, R.U. Abassi etal. (Telescope Array Collaboration), Journal of Geophysical Research: Atmospheres, 123 (2018).
48. ”Study of muons from ultrahigh energy cosmic ray air showers measured with the Telescope Array experiment”, R.U.
Abbasi et al. (Telescope Array Collaboration), PHYSICAL REVIEW D 98, 022002 (2018).
49. ”Evidence of Intermediate-scale Energy Spectrum Anisotropy of Cosmic Rays E ≥ 1019.2 eV with the Telescope Array
Surface Detector”, R.U. Abbasi et al. (the Telescope Array Collaboration), Astrophys. J. 862:91 (2018).
50. ”The Cosmic Ray Energy Spectrum between 2 PeV and 2 EeV Observed with the TALE Detector in Monocular Mode”,
R.U. Abbasi et al. (Telescope Array Collaboration), The Astrophysical Journal, 865:74 (18pp).
51. ”Testing a Reported Correlation between Arrival Directions of Ultra-high-energy Cosmic Rays and a Flux Pattern fromnearby Starburst Galaxies using Telescope Array Data”, R.U. Abbasi et al. (Telescope Array Collaboration), The Astrophysical Journal Letters, 867:L27 (5pp),2018.
52. ”Mass composition of ultra-high-energy cosmic rays with the Telescope Array Surface Detector Data”, R.U. Abbasi etal. (Telescope Array Collaboration), Physical Review D 99, 022002 (2019).
53. ”Constraints on the diffuse photon flux with energies above 1018 eV using the surface detector of the Telescope Arrayexperiment”, R.U.Abbasi et al. (Telescope Array Collaboration), Astroparticle Physics 110(2019) 8-14.
√
54. ”Measurement of inclusive forward neutron production cross section in proton-proton collisions at s = 13 TeV with
the LHCf Arm2 detector”, The LHCf collaboration, JHEP11(2018)073.
55. ”Influence of Earth-directed Coronal Mass Ejections on the Sun’s Shadow Observed by the Tibet-III Air Shower Array”,
M. Amenomori et al. (The Tibet ASγ Collaboration), The Astrophysical Journal, 860, 13 (2018).
56. ”Development of an all-sky gamma-ray Compton camera based on scintillators for high-dose environments”, Hideaki
Katagiri et al., JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY 2018, VOL. 55, NO. 10, 1172-1179 .
57. ”Remote measurement of urinary radioactivity in 18 F-FDG PET patients using Compton camera for accuracy evaluationof standardized uptake value”, Takara Watanabe et al., Biomed. Phys. Eng. Express, 4, 065029 (2018).
58. ”On the stability of a superspinar”, Ken-ichi Nakao, Pankaj S. Joshi, Jun-Qi Guo, Prashant Kocherlakota, Hideyuki
Tagoshi, Tomohiro Harada, Mandar Patil, Andrzej Krolak, Phys.Lett. B780 (2018) 410-413.
59. ”Time-frequency-based non-harmonic analysis to reduce line-noise impact for LIGO observation system ”, D.Jia, K.
Yanagisawa, M. Hasegawa, S. Hirobayashi, H. Tagoshi, T. Narikawa, N. Uchikata, H. Takahashi, Astronomy and Computing, 25, (2018) 238-246.
60. ”Multiwindow Nonharmonic Analysis Method for Gravitational Waves”, Dongbao Jia, Kenta Yanagisawa, Yuta Ono,
Kanna Hirobayashi, Masaya Hasegawa, Shigeki Hirobayashi, Hideyuki Tagoshi, Tatsuya Narikawa, Nami Uchikata,
Hirotaka Takahashi, IEEE Access., 6, 48645 (2018).
61. ”Detectability of the 21cm-signal during the epoch of reionization with 21cm-Lyman- α emitter cross-correlation. I. ”,
Kenji Kubota, et al, MNRAS, 479(2018), 2, p.2754-2766.
62. ”SILVERRUSH. V. Census of Lyα , [O iii] λ 5007, Hα , and [C ii] 158 µ m Line Emission with ∼1000 LAEs at z =4.9 − 7.0 Revealed with Subaru/HSC”, Yuichi Harikane, et al., Astrophys.J, 859:84(21pp).
63. ”SPLASH-SXDF Multi-wavelength Photometric Catalog”, Vihang Mehta, et al, ApJ Supplement Series,235:36(18pp).
64. ”SILVERRUSH. VI. A simulation of Lyα emitters in the reionization epoch and a comparison with Subaru Hyper
Suprime-Cam survey early data”, Akio K. Inoue et al., PASJ, (2018) 70 (3), 55 (1-30).
65. ”A Candidate z ∼ 10 Galaxy Strongly Lensed into a Spatially Resolved Arc”, Brett Salmon et al., ApJL 864:L22(6pp).

66. ”ALMA 26 Arcmin2 Survey of GOODS-S at One-millimeter (ASAGAO): Average Morphology of High-z Dusty StarForming Galaxies is an Exponential-Disk (n 0 1)”, Seiji Fujimoto et al., ApJ 861:7(12pp).

67. ”Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) III. Star formation properties of the host galaxies
at z ! 6 studied with ALMA”, Takuma Izumi et al., PASJ, (2018) 70 (3), 36 (1-21).
68. ”Large Scale Environment of a z = 6.61 Luminous Quasar Probed by Lyα Emitters and Lyman Break Galaxies”, Kazuaki
Ota et al., ApJ 856:109(27pp).

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69. ”Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IV. Discovery of 41 Quasars and Luminous
Galaxies at 5.7 #z# 6.9”, Y. Matsuoka et al., ApJ S, 237:5(17pp).
70. ”RELICS: Strong Lensing analysis of the galaxy clusters Abell S295, Abell 697, MACS J0025.4-1222, and MACS
J0159.8-0849”, Nathália Cibirka et al., ApJ 863:145.
71. ”The onset of star formation 250 million years after the Big Bang”, Takuya Hashimoto et al., Nature 557, 392-395(2018).
72. ”CHORUS II. Subaru/HSC Determination of the Lyα Luminosity Function at z=7.0: Constraints on Cosmic Reionization Model Parameter”, Ryohei Itoh et al., Astrophys.J. 867 (2018) no.1, 46.
73. ”The distribution and physical properties of high-redshift [OIII] emitters in a cosmological hydrodynamics simulation”,
Kana Moriwaki et al., MNRAS 481, L84-L88(2018).
74. ”Detection of the Far-infrared [O III] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in theHeart of the Reionization Era”, Yoichi Tamura et al., ApJ 874 27(2019).
75. ”Alma Twenty-six Arcmin2 Survey Of Goods-s At One-millimeter (asagao): Source Catalog And Number Counts”,
Hatsukade, Bunyo et al., PASJ(2018) 70 (6), 105 (1-20).
76. ”Morphologies of ∼190,000 Galaxies at z = 0 − 10 Revealed with HST Legacy Data. III. Continuum Profile and Size
Evolution of Lyα Emitters”, Shibuya, Takatoshi; et al., ApJ 871 164(2019).
77. ”RELICS: High-Resolution Constraints on the Inner Mass Distribution of the z=0.83 Merging Cluster RXJ0152.7-1357
from strong lensing”, Ana Acebron et al., ApJ 874 132(2019).
78. ”Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution
to Cosmic Reionization at z=6”, Yoshiki Matsuoka, et al., ApJ 869 150(2018).
79. ”The infrared-luminous progenitors of high-z quasars”, Michele Ginolfi, et al, MNRAS 483,1, 1256-1264 .
80. ”Near-Infrared Survey and Photometric Redshifts in the Extended GOODS-North field”, Li-Ting Hsu, et al, ApJ 871
233(2019).
81. ”Environmental impacts on molecular gas in protocluster galaxies at z ∼ 2”, Ken-ichi Tadaki, et al., PASJ.
82. ”Discovery of the First Low-Luminosity Quasar at z > 7”, Yoshiki Matsuoka, et al, ApJL 872 L2(2019).
83. ”Enhancement of H I absorption associated with the z = 3.1 large-scale proto-cluster and characteristic structures with
AGNs sculptured over Gpc scale in the SSA22 field”, T. Hayashino, et al, MNRAS 484,5868-5887 (2019).
84. ”Foreground effect on the J-factor estimation of ultra-faint dwarf spheroidal galaxies ”, Koji Ichikawa, Shun-ichi
Horigome, Miho N. Ishigaki, Shigeki Matsumoto, Masahiro Ibe, Hajime Sugai, Kohei Hayashi, MNRAS 479 (2018)
no.1, 64-74 .
85. ”Domain wall and isocurvature perturbation problems in a supersymmetric axion model”, Masahiro Kawasaki and Eisuke
Sonomoto, Phys. Rev. D 97, 083507.
86. ”Cogenesis of LIGO Primordial Black Holes and Dark Matter”, Fuminori Hasegawa and Masahiro Kawasaki,, Phys.
Rev. D 98, 043514.
87. ”Primordial black holes for the LIGO events in the axion-like curvaton model”, Kenta Ando, Keisuke Inomata, Masahiro
Kawasaki, Kyohei Mukaida and Tsutomu T. Yanagida,, Phys. Rev. D 97, 123512 (2018).
88. ”Oscillons from Pure Natural Inflation”, Jeong-Pyong Hong, Masahiro Kawasaki and Masahito Yamazaki, Phys. Rev. D
98, 043531.
89. ”Cosmologically allowed regions for the axion decay constant Fa ”, Masahiro Kawasaki, Eisuke Sonomoto, Tsutomu T.
Yanagida, Phys.Lett.B, 782(2018)181-184.
90. ”QCD axion dark matter from long-lived domain walls during matter domination”, Keisuke Harigaya and Masahiro
Kawasaki, Phys.Lett.B, 782(2018)1-5.
91. ”Primordial black holes and uncertainties on choice of window function”, Kenta Ando, Keisuke Inomata and Masahiro
Kawasaki,, Phys. Rev.D 97,103528 (2018).

9992. ”Decay Rate of Electroweak Vacuum in the Standard Model and Beyond”, S. Chigusa, T. Moroi and Y. Shoji, Phys. Rev.
D 97, 116012.
93. ”Inflaton fragmentation in E-models of cosmological α -attractors”, Fuminori Hasegawa, Jeong-Pyong Hong, Phys.Rev.
D97 (2018) , 083514.
94. ”Gauged Peccei-Quinn Symmetry - A Case of Simultaneous Breaking of SUSY and PQ Symmetry”, Hajime Fukuda,
Masahiro Ibe, Motoo Suzuki, Tsutomu T. Yanagida,, JHEP(2018) 2018: 128.
95. ”Composite Asymmetric Dark Matter with a Dark Photon Portal”, Masahiro Ibe, Ayuki Kamada, Shin Kobayashi, Wakutaka Nakano, JHEP(2018) 2018: 203.
96. ”Formation of primordial black holes as dark matter or LIGO black hole binaries in an axion-like curvaton model”, Kenta
Ando, Masahiro Kawasaki, Hiromasa Nakatsuka, Phys. Rev. D 98, 083508.
97. ”B − L as a Gauged Peccei-Quinn Symmetry”, Masahiro Ibe, Motoo Suzuki, Tsutomu T. Yanagida, JHEP(2018) 2018:49.
98. ”Big Bang Nucleosynthesis Constraint on Baryonic Isocurvature Perturbations”, Keisuke Inomata, Masahiro Kawasaki,
Alexander Kusenko, Louis Yang, JCAP12(2018)003.
99. ”Primordial Black Holes from Affleck-Dine Mechanism”,
JCAP01(2019)027.

Fuminori Hasegawa and Masahiro Kawasaki,

100. ”Long-term dynamics of cosmological axion strings”, Masahiro Kawasaki, Toyokazu Sekiguchi, Masahide Yamaguchi
and Jun’ichi Yokoyama, PTEP 2018, 091E01(2018).
101. ”The swampland conjecture and the Higgs expectation value”, Koichi Hamaguchi, Masahiro Ibe, Takeo Moroi,,
JHEP(2018) 2018: 23.
102. ”Primordial Black Holes and the String Swampland”, Masahiro Kawasaki and Volodymyr Takhistov, Phys. Rev. D 98,
123514 (2018).
103. ”Exploring compensated isocurvature perturbations with CMB spectral distortion anisotropies”, Taku Haga, Keisuke
Inomata, Atsuhisa Ota, Andrea Ravenni, JCAP 1808 (2018) no.08, 036.
104. ”Circular polarization of the cosmic microwave background from vector and tensor perturbations”, Keisuke Inomata,Marc Kamionkowski, Phys.Rev. D99 (2019) no.4, 043501.
105. ”Gravitational waves induced by scalar perturbations as probes of the small-scale primordial spectrum”, Keisuke Inomata, Tomohiro Nakama, Phys.Rev. D99 (2019) no.4, 043511.
106. ”Dark halo structure in the Carina dwarf spheroidal galaxy: joint analysis of multiple stellar components”, Kohei
Hayashi, Michele Fabrizio, Ewa L. Lokas, Giuseppe Bono, Matteo Monelli, Massimo Dall’Ora and Peter B. Stetson,
MNRAS 481,1, 50-261.
107. ”Effects of mass models on dynamical mass estimate: the case of ultradiffuse galaxy NGC 1052-DF2”, Kohei Hayashi
and Shigeki Inoue, MNRAS481,L59-L63.
108. ”Hunting for Statistical Anisotropy in Tensor Modes with B-mode Observations”, T.Hiramatsu, S.Yokoyama, T.Fujita,
I.Obata, Physical Review D98, 083522.
109. ”Footprint of Two-Form Field: Statistical Anisotropy in Primordial Gravitational Waves”, I.Obata, T.Fujita, PhysicalReview D99, 023513.
110. ”Optical Ring Cavity Search for Axion Dark Matter”, I.Obata, T.Fujita, Y.Michimura, Physical Review Letter 121,
161301.
111. ”Ultraviolet Completion of a Composite Asymmetric Dark Matter Model with a Dark Photon Portal ”, Masahiro Ibe,
Ayuki Kamada, Shin Kobayashi, Wakutaka Nakano, JHEP 1903 (2019) 173 .

(b) Conference Papers (Proceedings)
1. ”Development of three silicon photomultiplier detector modules for the MAGIC telescopes for a performance comparison to PMTs”, Hahn, A.; Dettlaff, A.; Fink, D.; Mazin, D.; Mirzoyan, R.; Teshima, M., 8th International Conference on
New Developments in Photodetection (NDIP), JUL, 2017.

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2. ”VHE observations of binary systems performed with the MAGIC telescopes”, Lopez-Oramas, Alicia; Blanch, Oscar; deOna Wilhelmi, Emma; Fernandez-Barral, Alba; Hadasch, Daniela; Moretti, Elena; Munar-Adrover, Pere; Maria Paredes,
Josep; Ribo, Marc; Torres, Diego F.; Bordas, Pol; Brun, Francois; Casares, Jorge; Zanin, Roberta, 6th Conference on
High Energy Phenomena in Relativistic Outflows (HEPRO), SEP 11-15, 2017.
3. ”Simulation of the Large Size Telescope drive system”, Piel, Quentin; Brunetti, Laurent; Eder, Josef; Fiasson, Armand;
Guizzo, Gian Paolo; Monteiro, Inocencio; Noda, Koji; Schweizer, Thomas, Conference on High Energy, Optical, andInfrared Detectors for Astronomy VIII, JUN 10-13, 2018.
4. ”The e-ASTROGAM gamma-ray space observatory for the multimessenger astronomy of the 2030s”, Tatischeff, V.;
Hayashida, M.et al., Conference on Space Telescopes and Instrumentation - Ultraviolet to Gamma Ray, JUN 10-15,
2018.
5. ”Energy Spectrum of Ultra-High-Energy Cosmic Rays Measured by The Telescope Array”, Yoshiki Tsunesada et al.,The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
6. ”Telescope Array anisotropy summary”, Sergey Troitsky on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
7. ”Auger at the Telescope Array: toward a direct cross-calibration of surface-detector stations ”, Sean Quinn on behalf of
the Pierre Auger collaboration and on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray
Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
8. ”Declination Dependence of the Telescope Array Surface Detector Spectrum”, Dmitri Ivanov, The 35th International
Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
9. ”The Cosmic Ray Spectrum above 0.1 EeV measured by the Telescope Array and TALE Fluorescence Detectors”,
JiHee Kim on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017),
BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
10. ”Interpretation of the energy spectrum observed with the Telescope Array detectors”, Eiji Kido on behalf of the Telescope
Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20
JUL 2017.
11. ”Study of muons from ultra-high energy cosmic ray air showers measured with the Telescope Array experiment”, Ryuji
Takeishi on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017),
BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
12. ”Anisotropy search in Energy distribution in Northern hemisphere using Telescope Array Surface Detector data”,
Toshiyuki Nonaka on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference(ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
13. ”A systematic uncertainty on the energy scale of the Telescope Array fluorescence detectors”, Toshihiro Fujii on behalfof the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN,
KOREA, 12-20 JUL 2017.
14. ”Hybrid Measurement of the Energy Spectrum and Composition of Ultra-High Energy Cosmic Rays by the Telescope
Array”, Daisuke Ikeda on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference(ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
15. ”Telescope Array measurement of UHECR composition from stereoscopic fluorescence detection”, Douglas Bergman
and T. Stroman, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL
2017.
16. ”Telescope Array Composition Summary”, William Hanlon on behalf of the Telescope Array collaboration, The 35th
International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
17. ”Report of the Telescope Array- Pierre Auger Observatory Working Group on Energy Spectrum ”, Dmitri Ivanov onbehalf of the Pierre Auger collaboration and on behalf of the Telescope Array collaboration, The 35th International
Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
18. ”Testing the agreement between the Xmax distributions measured by the Pierre Auger and Telescope Array Observations”, Vitor de Souza on behalf of the Pierre Auger collaboration and on behalf of the Telescope Array collaboration,
The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.

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19. ”Telescope Array search for EeV photons and neutrinos”, Grigory Rubtsov on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
20. ”Composition Studies with the Telescope Array surface detector”, Yana Zhezher on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
21. ”All-sky search for correlations in the arrival directions of astrophysical neutrino candidates and ultrahigh-energy cosmic
rays”, I.Al Samarai on behalf of IceCube Collaboration, on behalf of the Pierre Collaboration and on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA,
12-20 JUL 2017.
22. ”Highlights from the Telescope Array”, John N. Matthew on behalf of the Telescope Array collaboration, The 35th
International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
23. ”The TA×4 experiment”, Eiji Kido on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray
Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
24. ”The Telescope Array Low-energy Extension”, S. Udo on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
25. ”TA fluorescence detector calibration by UV LED with an unmanned aerial vehicle”, Y. Tameda on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA,
12-20 JUL 2017.
26. ”Telescope Array Lightning Location System”, T. Okuda on behalf of the Telescope Array collaboration, The 35th
International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
27. ”Evidence of Intermediate-Scale Energy Spectrum Anisotropy in the Northern Hemisphere from Telescope Array”, P.J.Lundquist on behalf of the Telescope Array collaboration, The 35th International Cosmic Ray Conference(ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
28. ”Cosmic Rays Energy Spectrum from PeV to EeV energies measured by the TALE detector”, T. AbuZayyad on behalfof the Telescope Array collaboration, The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN,
KOREA, 12-20 JUL 2017.
29. ”Multi-Messenger Signatures of PeV-ZeV Cosmic Ray Sources”, Meszaros, Peter; Murase, Kohta; Asano, Katsuaki;
Senno, Nicholas; Xiao, Di, 2nd Conference on Cosmic Ray Origin - Beyond the Standard Models (CRBTSM), SEP
18-24, 2016.
30. ”Development status of the simultaneous two-color near-infrared multi-object spectrograph SWIMS for the TAO 6.5m
telescope”, Konishi, Masahiro; Motohara, Kentaro; Takahashi, Hidenori; Kato, Natsuko; Terao, Yasunori; Ohashi,
Hirofumi; Kono, Yukihiro; Kushibiki, Kosuke; Kodama, Tadayuki; Hayashi, Masao; Tanaka, Ichi; Tadaki, Ken-ichi;
Toshikawa, Jun; Koyama, Yusei; Shimakawa, Rhythm; Suzuki, Tomoko; Tateuchi, Ken; Kitagawa, Yutaro; Kobayakawa,
Yutaka; Todo, Soya; Aoki, Tsutomu; Doi, Mamoru; Hatsukade, Bunyo; Kamizuka, Takafumi; Kohno, Kotaro; Minezaki,
Takeo; Miyata, Takashi; Morokuma, Tomoki; Sako, Shigeyuki; Soyano, Takao; Tanabc, Toshihiko; Tanaka, Masuo;
Tarusawa, Ken’ichi; Tamura, Yoichi; Koshida, Shintaro; Ohsawa, Ryou; Uchiyama, Masahito; Mori, Tomohiro; Yamaguchi, Jumpei; Yoshida, Yutaka; Yoshii, Yuzuru, Conference on Ground-Based and Airborne Instrumentation for
Astronomy VII, JUN 10-14, 2018.
31. ”Interplanetary Coronal Mass Ejection and the Sun’s Shadow Observed by the Tibet Air Shower Array,” M. Amenomori
et al., The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
32. ”Solar magnetic field strength and the ”Sun’s Shadow” ,” M. Amenomori et al., The 35th International Cosmic Ray
Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
33. ”Measurement of high energy cosmic rays by the new Tibet hybrid experiment,” M. Amenomori et al., The 35th International Cosmic Ray Conference (ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
34. ”The Tibet AS+MD Project; status report 2017,” M. Amenomori et al., The 35th International Cosmic Ray Conference(ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.
35. ”The overview of the ALPACA Experiment,” A. Asaba et al., The 35th International Cosmic Ray Conference(ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.

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36. ”ALPAQUITA Array in the ALPACA Project,” A. Asaba et al., The 35th International Cosmic Ray Conference(ICRC2017), BEXCO, BUSAN, KOREA, 12-20 JUL 2017.

D. Doctoral Theses
1. Primordial black hole formation in Affleck-Dine baryogenesis,
Fuminori HASEGAWA,
Ph.D Thesis, Mar. 2019
2. Dark Matter search with high-energy gamma-ray observations,
Nagisa HIROSHIMA,
Ph.D Thesis, Mar. 2019
3. Realistic Construction of Axion Model with Gauged Peccei-Quinn Symmetry,
Motoo SUZUKI,
Ph.D Thesis, Mar. 2019
4. Subaru Census of Early Galaxies in the Hierarchical Structure Formation of the Universe,
Yuichi HARIKANE,
Ph.D Thesis, Mar. 2019
5. Demographics of the Cold Universe with ALMA: From Inter-Stellar and Circum-Galactic to Cosmic Structures,
Seiji FUJIMOTO,
Ph.D Thesis, Mar. 2019
6. New parameter estimation method being free from the bias depending on sky region for Gravitational wave from compact
binary coalescence,
Kenji ONO,
Ph.D Thesis, Mar. 2019
7. Theoretical Study on the Radial Profile of Pulsar Wind Nebulae,
Wataru ISHIZAKI,Ph.D Thesis, Mar. 2019
8. Search for Neutrinos associated with Gamma-ray Bursts in Super-Kamiokande,
Asato ORII,
Ph.D Thesis, Mar. 2019
9. Study of Emissions of Gamma ray Bursts above 10GeV with the Standard and Newly Recovered Data of the Fermi-LAT,
Mitsunari TAKAHASHI,
Ph.D Thesis, Sep. 2018
10. Srerile neutrino oscillation studies with the T2K far detector Super-Kamiokande,
Ka Ming TSUI,
Ph.D Thesis, Dec. 2018
11. Development of input optics for the gravitational wave detector KAGRA,
Masayuki NAKANO,
Ph.D Thesis, Dec. 2018

E. Public Relations(a) ICRR NewsICRR News is a quarterly publication written in Japanese about scientific and educational activities at ICRR. It includes:
1. reports on scientific activities of ICRR staff members and those conducted at one or more of its facilities,
2. reports of international conferences on topics relevant to ICRR’s research activities,

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3. reports on topics discussed at ICRR’s Committees,
4. list of publications published by ICRR [ICRR-Report],
5. list of seminars held at ICRR,
6. announcements, and7. other items of relevance.
Below lists the main topics in the issues published in FY 2018:
No.102 (2018 Summer)• Features: Super-Kamiokande’s refurbishment work
• Features: Observation of Ultra-High Energy Cosmic Ray using air fluorescence detectors• Reports: 18th ICRR × IPMU Public Lecture• Press Release: ALMA finds Oxygen 13.28 Billion Light-Years Away
• Event: Sicence Cafe ”Cafe KAGRA”• Staff Reassignment• ICRR Seminar
No.103 (2018 Autumn)• Features: Telescope Array Experiment; studying Ultra-High Energy Comic Rays to confirm their source, and reveal
ultra-high energy phenomena• Reports: Open Campus 2018
• Reports: Inauguration for CTA LST-1• Reports: 19th ICRR × IPMU Public Lecture• Topics: Workshop of cloud chamber at Matsudo International Science Art Festival• Awards• Topics: Release of second version of Super-Kamiokande Jigsaw Puzzle• Topics: Reprodction of first version of Super-Kamiokande Jigsaw Puzzle• Staff Reassignment• ICRR Seminar
No.104 (2019 Winter & Spring)• Features: Focus on the riddles of the large-scale structures and galaxy formation of the universe 13 billion years ago• Reports: Construction of TA extension (TA×4) started in mid winter• Topics: Super-Kamiokande restarted its operation after 6 months’ upgrade work
• Topics: The first Meeting of Hyper-Kamiokande Experiment Financial Forum• Topics: CTA LST-1 Records its First Light• Topics: Final Agreements Signed for CTA’s Southern Hemisphere Site in Chile• Topics: Award

104• Topics: ICRR acknowledged as International Joint Usage/Research Center and start calling Applications• Topics: CTA LST-1 awarded El Suplemento Premios Tecnologia Siglo XXI (21 Century Technology Award)• Information: Staff Reassignment• Information: ICRR Seminar(b) Public Lectures• ”Public Lecuture,” Apr. 15, 2018, Kyushu University, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecuture,” Apr. 27, 2018, Matsumoto University, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecuture,” May. 2, 2018, Chubu University, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecuture,” May. 13, 2018, Higashimatsuyama-City, Saitama, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecture in 20th International Symposium on Very High Energy Cosmic Ray Interations,” May. 23, 2018, Nagoya
University, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecuture,” Jun. 23, 2018, Toho University, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecuture,” Jul. 21, 2018, Kochi-City, Kochi, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Shinao Kizaki Kaki Daigaku,” Aug. 2, 2018, Omachi-City, Nagano, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecture in JPS Autumn Meeting 2018,” Sep. 16, 2018, Matumoto-City, Nagano, Takaaki Kajita (ICRR, The
University of Tokyo).
• ”Public Lecuture,” Sep. 2. 2018, Toyama-City, Toyama, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Kaisei Festival,” Sep. 22, 2018, Arakawa-Ku, Tokyo, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecture,” Sep. 29, 2018, Yasuda Auditorium, The University of Tokyo, Tokyo, Takaaki Kajita (ICRR, The
University of Tokyo).
• ”Public Lecture,” Oct. 21, 2018, Izumo Science Center, Shimane, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecuture,” Nov. 3, 2018, Koshigaya-City, Saitama, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecture,” Nov. 21, 2018, Mitaka-City, Tokyo, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Nagoya University Lecture,” Nov. 23, 2018, Nagoya University, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Fukui Science Festa,” Feb. 11, 2019, Fukui-City, Fukui, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Saturday Program,” Feb. 23, 2019, Tokai Junior and Junior High School, Takaaki Kajita (ICRR, The University ofTokyo).
• ”Global Peace Dialogue in Okinawa,” Mar. 8, 2019, Okinawa Convention Center, Okinawa, Takaaki Kajita (ICRR, The
University of Tokyo).
• ”The 18th ICRR × IPMU Public Lecture,” Apr. 14, 2018, Kashiwa-City, Chiba, Masayuki Nakahata (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Public Lecture for the America-Japan Society in Toyama,” Jun. 27, 2018, ANA Crowne Plaza, Toyama-City, Toyama,
Masayuki Nakahata (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Open College in Hida 2018,” Jul. 14, 2018, Hida Earth Wisdom Center, Masato Shiozawa (Kamioka Observatory,
ICRR, The University of Tokyo).
• ”Geo Space Adventure,” Jul. 14, 2018, Kamioka Observatory, Gifu, Kamioka Observatory, ICRR, The University ofTokyo.
• ”Yume-no-Tamago-Juku,” Aug. 6, 2018, Hida-City, Gifu, Jun Kameda (Kamioka Observatory, ICRR, The University ofTokyo).

105• ”Hirameki Tokimeki Science,” Aug. 9, 2018, Kamioka Observatory, Gifu, Masayuki Nakahata, Yoshinari Hayato(Kamioka Observatory, ICRR, The University of Tokyo).
• ”Public Lecture,” Aug. 26, 2018, Toyama International Conference Center, Toyama, Masayuki Nakahata (Kamioka
Observatory, ICRR, The University of Tokyo).
• ”Hyogo Prefectural Kakogawa Higashi High School,” Aug. 29, 2018, Kashiwa Campus, Chiba, Masaki Yamashita(Kamioka Observatory, ICRR, The University of Tokyo).
• ”VACUUM2018,” Sep. 5, 2018, Pacifico Yokohama, Kanagawa, Masayuki Nakahata (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Science Café in Gifu,” Oct. 13, 2018, Gifu Shinbun sha, Gifu, Masayuki Nakahata (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Public Lecture for Elementary School attached to University of Toyama,” Nov. 12, 2018, Toyama-City, Toyama, Motoyasu Ikeda (Kamioka Observatory, ICRR, The University of Tokyo).
• ”NHK Culture Center,” Dec. 9, 2018, NHK Culture Center Yokohama Landmark, Kanagawa, Masayuki Nakahata(Kamioka Observatory, ICRR, The University of Tokyo).
• ”Public Lecture,” Jan. 12, 2019, Hida-city, Gifu, Masato Shiozawa (Kamioka Observatory, ICRR, The University ofTokyo).
• ”Public Lecture for Kamioka Junior High School,” Feb. 22, 2019, Kamioka Junior High School, Hida-City, Gifu,
Motoyasu Ikeda (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Akita High School,” Aug. 2, 2018, ICRR, The University of Tokyo, Chiba, Takayuki Saito, Yuki Iwamura, Yoshiki
Ohtani (ICRR, The University of Tokyo).
• ”Science Café,” Mar. 21, 2019, Tamarokuto Science Center, Tokyo, Takayuki Saito (ICRR, The University of Tokyo).
• ”Miyagi Prefecture Sendaidaiichi High School,” Jul. 5, 2018, ICRR, The University of Tokyo, Chiba, Toshiyuki Nonaka(ICRR, The University of Tokyo).
• ”The 19th ICRR × IPMU Public Lecture,” Nov. 11, 2018, Yasuda Auditorium, The University of Tokyo, Takashi Sako
(ICRR, The University of Tokyo).
• ”Musashi High school,” Nov. 7, 2018, Musashi High School, Tokyo, Yoshiaki Ono (ICRR, The University of Tokyo).
• ”Science Café, ” Dec. 8, 2018, Tamarokuto Science Center, Tokyo, Ken Mawatari (ICRR, The University of Tokyo).
• ”Public Lecture,” Jun. 20, 2018, Chiyoda-Ku, Tokyo, Shinji Miyoki (KAGRA Observatory, ICRR, The University ofTokyo).
• ”The University of Tokyo’s EMP Club Network,” Jun. 22, 2018, KAGRA Observatory, Gifu, Shinji Miyoki (KAGRA
Observatory, ICRR, The University of Tokyo).
• ”Yoshiki High School,” Jul. 12, 2018, Hida-City, Gifu, Shinji Miyoki (KAGRA Observatory, ICRR, The University ofTokyo).
• ”Public Lecture,” Jul. 14, 2018, Gifu-City, Gifu, Osamu Miyakawa (KAGRA Observatory, ICRR, The University ofTokyo).
• ”Public Lecture,” Jul. 21, 2018, Sanuki-City, Kagawa, Osamu Miyakawa (KAGRA Observatory, ICRR, The Universityof Tokyo).
• ”Hida-Kamioka High School,” Jul. 25, 2018, Hida-City, Gifu, Takaaki Yokozawa (KAGRA Observatory, ICRR, The
University of Tokyo).
• ”Toyama High School,” Jul. 26, 2018, KAGRA Observatory, Gifu, Takahiro Yamamoto (KAGRA Observatory, ICRR,The University of Tokyo).
• ”Momoyama High School,” Jul. 27, 2018, KAGRA Observatory, Gifu, Chihiro Kosakai (KAGRA Collaborator)(Gravitational Wave Project Office, NAOJ).

106• ”Nanao High School,” Aug. 2, 2018, KAGRA Observatory, Gifu, Takafumi Ushiba (KAGRA Observatory, ICRR, The
University of Tokyo).
• ”Suwa Seiryo High School, Matsumoto Fukashi High School,” Aug. 3, 2018, KAGRA Observatory, Gifu, Naoko Ohishi(KAGRA Collaborator)(Gravitational Wave Project Office, NAOJ).
• ”Seisho High School,” Aug. 3, 2018, KAGRA Observatory, Gifu, Yoichi Aso (KAGRA Collaborator)(Gravitational
Wave Project Office, NAOJ).
• ”Public Lecture,” Aug. 5, 2018, Kiso Observatory, Tokyo University, Osamu Miyakawa (KAGRA Observatory, ICRR,The University of Tokyo).
• Yoshiki High School,” Aug. 6-7, 2018, Takayama-city, Gifu, Kohei Yamamoto, Ryohei Kozu (KAGRA Observatory,
ICRR, The University of Tokyo).
• ”Sapporo Nishi High School,” Aug. 7, 2018, KAGRA Observatory, Gifu, Kazuhiro Yamamoto (Visiting Associate
Professor, KAGRA Observatory, ICRR, The University of Tokyo).
• ”Science Café, Café KAGRA,” Aug. 11, 2018, Hida-City, Gifu, Masatake Ohashi (KAGRA Observatory, ICRR, The
University of Tokyo).
• ”Senior High School Attached to Kyoto University of Education,” Aug. 23, 2018, KAGRA Observatory, Gifu, Shinji
Miyoki (KAGRA Observatory, ICRR, The University of Tokyo).
• ”Dutch Vacuum Society,” Sep. 4, 2018, KAGRA Observatory, Gifu, Yoichi Aso (KAGRA Collaborator)(Gravitational
Wave Project Office, NAOJ).
• ”Public Lecture,” Sep. 6, 2018, Pacifico Yokohama, Kanagawa, Yoshio Saito (KAGRA Observatory, ICRR, The University of Tokyo).
• ”Faculty of Science, Toho University,” Sep. 7, 2018, KAGRA Observatory, Gifu, Yoshihisa Obayashi (KAGRA Observatory, ICRR, The University of Tokyo).
• ”KAGRA Tour for Donators to KAGRA Project,” Sep. 8, 2018, KAGRA Observatory, Gifu, KAGRA Observatory,
ICRR, The University of Tokyo.
• ”Faculty of Science, Toyama University,” Sep. 14, 2018, KAGRA Observatory, Gifu, Kazuhiro Yamamoto (Visiting
Associate Professor, KAGRA Observatory, ICRR, The University of Tokyo).
• ”Public Lecture,” Oct. 6, 2018, Saitama Prefectual Kawagoe High School, Saitama, Hideyuki Tagoshi (KAGRA Observatory, ICRR, The University of Tokyo).
• ”High Energy Accelerator Research Organization,” Oct. 9, 2018, KAGRA Observatory, Gifu, Osamu Miyakawa (KAGRA Observatory, ICRR, The University of Tokyo).
• ”Japan Stainless Steel Association,” Oct. 12, 2018, KAGRA Observatory, Gifu, Yoshio Saito (KAGRA Observatory,
ICRR, The University of Tokyo).
• ”Public Lecture,” Oct. 27, 2018, The Nagasaki Institute of Applied Science, Nagasaki, Masatake Ohashi (KAGRA
Observatory, ICRR, The University of Tokyo).
• ”Public Lecture,” Nov. 1, 2018, Hachikoku Elementary School, Saitama, Hideyuki Tagoshi (KAGRA Observatory,
ICRR, The University of Tokyo).
• ”Public Lecture,” Nov. 1, 2018, Rihga Royal Hotel Tokyo, Shinji Miyoki (KAGRA Observatory, ICRR, The Universityof Tokyo).
• ”KAGRA Tour,” Nov. 17, 2018, KAGRA Observatory, Gifu, KAGRA Observatory, ICRR, The University of Tokyo.
• ”Japan Environmental Education Forum,” Dec. 13, 2018, KAGRA Observatory, Gifu, Yoshio Saito (KAGRA Observatory, ICRR, The University of Tokyo).
• ”Asahi Culture Center Yokohama,” Jan. 19, 2019, ASAHI Culture center, Yokohama school, Shinji Miyoki (KAGRA
Observatory, ICRR, The University of Tokyo).
• ”Yume-no-Tamago-Juku,” Mar. 24, 2019, Hida-City, Gifu, Hirotaka Yuzurihara (KAGRA Observatory, ICRR, The
University of Tokyo).

107• ”NHK Culture Center Shinjuku,” Aug. 25, 2018, Shinjuku-ku, Tokyo, Masahiro Kawasaki (ICRR, The University ofTokyo).

(c) Visitors
Kashiwa Campus (Total: 35 groups, 2,336 people)• Junior High and High schools: 5 groups• Universities and Graduate schools: 4 groups• Researchers: 0 group• Inspections: 6 groups• Press: 19 groups• Others: 1 group
KAMIOKA Observatory (Total: 322 groups, 2,769 people)• Junior High and High schools: 25 groups• Universities and Graduate schools: 25 groups• Researchers: 71 groups• Inspections: 47 groups• Press: 56 groups• Others: 98 groups
KAGRA Observatory (Total: 130 groups, 1,622 people)• Elementary Junior High and High schools: 15 groups• Universities and Graduate schools: 18 groups• Researchers: 12 groups• Inspections: 24 groups• Press: 14 groups• Others: 47 groups

108

F.

Inter-University Research Activities

Numbers of Researchers
Number ofApplications

Number of
Adoptions

Number of
Researchers

Facility Usage
Kamioka Observatory
Akeno Observatory
Norikura Observatory
Low-Level Radioisotope Measurement Facility
Cryogenic Laser Interferometer in Kashiwa
Laboratorial Facility in Kashiwa
Computer Facility in Kashiwa
Conference Facility in Kashiwa
Over Sea Facilities
Annual Sums

40
4
8
4
15
3
16
7
26
123

40
4
8
4
15
3
16
7
26
123

1,107
175
95
29
508
31
388
225
392
2,950

Joint Research
Neutrino and Astroparticle ResearchHigh Energy Cosmic Ray Research
Astrophysics and Gravity Research
Research Center for Cosmic neutrinos
Annual Sums

41
54
20
8
123

41
54
20
8
123

1,118
1,181
597
54
2,950

Research Project Titles
1. Astroparticle physics using the Super-Kamiokande detector
2. Study of atmospheric neutrino flux and neutrino oscillations
3. Study of simulation for atmospheric neutrino
4. Studying the Neutrino Mass Hierarchy With Atmospheric Neutrinos
5. Study of flavor identification of atmospheric and beam neutrinos
6. Study of solar neutrino energy spectrum
7. Precise measurement of Day/Night effect for 8 B solar neutrinos
8. Study for Supernova monitor
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
16. Study to improve sensitivity of neutrino oscillation measurement in T2K expriment
17. Joint Oscillation Analysis With the T2K and Super-Kamiokande Experiments

109

18. Energy calibration for Super-Kamiokande
19. Research and development of computer simulation of Super-Kamiokande detector
20. Development of low concentration radon detection system
21. R&D of Megaton scale water Cherenkov Detector Hyper-Kamiokande
22. Development of the Large Aperture Photodetector for a next-generation neutrino detector
23. Development of software for the next generation neutrino detector
24. A Search for Dark Matter using Liquid Xenon Detector
25. Detector structure study for future direct dark matter search experiment
26. Study on neutrino physics using liquid xenon
27. Radon emanation measurement from material using dark matter search experiment(2).
28. A study on scattering processes of scintillation photons in liquid xenon
29. Study on surface background removal in the dark matter search
30. Research and development for XENONnT and search for dark matter
31. Simulation study and data analysis for dark matter search in XENONnT
32. RI measurement of the PMT by HPGe detector
33. Study of double beta decay of 48Ca
34. Direction-sensitive dark matter search
35. Study for lowering backgrounds of radioisotopes in large volume detectors
36. Studies on the background evaluation using laser spectroscopy analysis
37. Development of a radioactivity assay system for underground experiments
38. Dark Matter Search with double-phase Argon detector
39. Integration of crustal activity observation around the Atotsugawa fault
40. Strain, tilt, seismic measurement in Kamioka-mine
41. Searches for neutrinoless double beta decay and dark matter with high-pressure Xenon gas detector
42. Multi-Color Imager for Transients, Survey and Monstrous Explosions
43. Observation of Galactic Cosmic Ray Intensities using Large Area Muon Telescopes
44. Observation of solar neutrons in solar cycle 24
45. Space weather observation using muon hodoscope at Mt. Norikura
46. Observation of cosmogenic nuclides concentrations at Mt. Norikura
47. Study of secondary cosmic rays from Thundercloud at Mt. Norikura
48. Development of high energy proton irradiation technique for devices used in spaceship49. Investigation of alpine plants on Mt. Norikura
50. Effect of forest fragmentation on the belowground microorganisms
51. Symbiosis betweem Pinus pumila and Nucifraga caryocatactes on Mt.Norikura
52. CTA Project
53. CTA-Japan Physics Research

110

54. Development of Focal Plane Instrumemnts for the CTA Large Sized Telescope
55. Development of the readout system for the CTA large sized telescopes
56. Installation and commissioning of the first Large Size Telescope of CTA in La Palma, Canary Islands, Spain
57. Development and commissioning of the onsite data analysis for CTA North in La Palma, Spain
58. Development of camera for CTA small-sized telescopes
59. CTA Monte Carlo Simulation
60. Localization of very high energy gamma-ray emission region in an active galactic nuclei
61. The study on simultaneous observations of gamma ray bursts by CTA and LEAP
62. Early phase observations with CTA Large Sized Telescopes
63. Study of High Energy Gamma-ray Objects with the MAGIC telescope
64. R&D for a Small Atmospheric Cherenkov Telescope in Akeno Observatory
65. Study of Extremely-high Energy Cosmic Rays by Telescope Array
66. Observing ultrahigh-energy cosmic rays with new fluorescence detectors at Telescope Array site
67. Timing and position calibration of surface detectors of TA×4 and TALE experiment
68. Research and development of the surface detectors for the TALE experiment
69. Study of radio detection of highest energy cosmic rays
70. Research and development of a Fresnel lens air fluorescence telescope for the next generation UHECR observation
71. The observation of abnormal shower event with lightning by TA surface particle detector
72. Development of solar power system and detector protection system for the new-type fluorescent detector
73. Calibration of fluorescence detector responce and optical system with standard light source mounted on UAV
74. Development and analysis of night cloud observation by CCD camera for automatic observation of air fluorescence
detector
75. Observation of airshower fluorescence light at the TA FD site by using an Imaging UV telescope
76. Development of new surface detector for observation of ultra high energy cosmic ray at Telescope Array site
77. Development of an advanced Compton camera using SOI pixel semiconductor
78. Design study of a Compton camera for study of cosmic rays
79. Experimental Study of High-energy Cosmic Rays in the Tibet ASγ experiment
80. Study of High Energetic Radiation from Thundercloud in the Altiplano81. Sidereal daily variation of ∼10TeV galactic cosmic ray intensity observed by the Tibet air shower array
82. Study of the composition of cosmic-rays at the Knee
83. A study on variation of interplanetary magnetic field with the cosmic-ray shadow by the sun
84. Air shower observation for high-energy gamma ray and cosmic ray detections at the Chacaltaya Cosmic Ray Observatory
85. Study on High Energy Cosmic Ray Sources by Observation in Spac with CALET
86. Observation with Ahsra
87. Integration of the optical fiber trigger system for Ashra
88. Cosmic-ray chemical composition study using air shower muons

111

89. Study of solar magnetic fields using the cosmic-ray Sun’s shadow observed at the southern hemishpere
90. Cosmic ray interactions in the knee and the highest energy regions
91. Study of Fast Moving Dark Matters and Meteoroids using High Sensitivity CMOS Camera System
92. Comparative study of astrophysical particle acceleration processes
93. The extreme Universe viewed in very-high-energy gamma rays 201894. Development of a new code for cosmic-ray air shower simulation
95. YMAP symposium 2018 (Basic Part)
96. Research of Large-scale Gravitational wave Telescope (VIII)
97. Development of High Performance Cryogenic Mirror Control System
98. Research on cryogenic payload for KAGRA
99. Construction of KAGRA data transfer and storage system (4)100. R&D for the intensity stabilization of the laser system in KAGRA
101. Technical development for effects on high optical power for bKAGRA mode cleaner II
102. Precise geophysical observation at the Kamioka underground site and modeling of crustal activities
103. Development of precision profiler for mirrors of LCGT interferometer 8104. Development of optical cavity for ultranarrow stable lasers105. Installation of the output mode-cleaner for KAGRA
106. Study of Gravitational-wave by cryogenic laser interferometer CLIO in KAMIOKA Mine(II)107. Study for improving a curing time of silicate bonding by controlling gas environment
108. Development of ultra-low loss coating for the KAGRA sapphire mirror-3
109. Numerical Simulation of Electro-Magnetic Wave Propagation in Gravitational wave Detector VI
110. Control and automatic operation for KAGRA
111. Data analysis of KAGRA detector (IV)112. Development of a high performance sapphire mirror suspension
113. Development of KAGRA detector characterization
114. Cosmic Reionization and Galaxy Formation Probed with Large Optical Near-Infrared Telescope
115. Evolution of the universe and particle physics
116. Detection of time variations for cosmogenic nucleid Be-7
117. Evaluation of the erupted radioactivities into the environment
118. Frontier of the planetary material science
119. Time profile of radioactive Cs concentration and its aerosol size distribution in local area
120. Continuous Measuerment of Underground Laboratory Environment
121. Precise calculation of the atmospheric neutrino flux
122. Energy Determination of the LPM showers
123. Neutrino Workshop
124. CRC workshop for future plans in cosmic ray research

112

G. List of Committee Members(a) Board of Councillors
KAJITA, Takaaki
NAKAHATA, Masayuki
SAGAWA, Hiroyuki
OHASHI, Masatake
TAKEDA, Hiroyuki
KOSEKI, Toshihiko
TOKUSHUKU, Katsuo
AOKI, Shinya
TSUNETA, Saku
TORII, Shoji
ITOW, Yoshitaka
TANAKA, Takahiro
MITSUDA, Kazuhisa
ASAI, Shoji
KUSANO, Kanya

ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
Graduate School of Science, The University of Tokyo
The University of Tokyo
Institute of Particle and Nuclear Studies, KEK
Yukawa Institute for Theoretical Physics, Kyoto University
National Astronomical Observatory of Japan
Faculty of Science and Engineering, Waseda University
Institute for Space-Earth Environmental Research, Nagoya University
Yukawa Institute for Theoretical Physics, Kyoto University
Institute of Space and Astronautical Science
International Center for Elementary Particle Physics, The University of Tokyo
Institute for Space-Earth Environmental Research, Nagoya University(b) Advisory Committee
KAJITA, Takaaki
KITANO, Ryuichiro
SUZUKI, Hideyuki
KUZE, Masahiro
AOI, Nori
YOKOYAMA, Masashi
YOSHIDA, Atsumasa
NISHIJIMA, Kyoshi
OGIO, Shoichi
MORI, Masaki
KANDA, Nobuyuki
KAWASAKI, Masahiro
NAKAHATA, Masayuki
SHIOZAWA, Masato
TESHIMA, Masahiro
OHASHI, Masatake
SAGAWA, Hiroyuki
TAKITA, Masato

ICRR, The University of Tokyo
KEK Theory CenterTokyo University of Science
School of Science, Tokyo Institute of Technology
Research Center for Nuclear Physics, Osaka University
Graduate School of Science, The University of Tokyo
Aoyama Gakuin University
Tokai University
Graduate School of Science, Osaka City University
Ritsumeikan University
Graduate School of Science, Osaka City University
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo(c) Inter-University Research Advisory Committee
MUNAKATA, Kazuoki
ITOW, Yoshitaka
OGIO, Shoichi
TAJIMA, Hiroyasu
HIBINO, Kinya
KUBO, Hidetoshi
SATO, Shuichi
TOKANAI, Fuyuki
TAKEUCHI, Yasuo
NAKAHATA, Masayuki
TAKITA, Masato
UCHIYAMA, Takashi
YOSHIKOSHI, Takanori
SAKO, Takashi

Shinshu University
Institute for Space-Earth Environmental Research, Nagoya University
Graduate School of Science, Osaka City University
Institute for Space-Earth Environmental Research, Nagoya University
Kanagawa University
Kyoto University
Hosei University
Yamagata University
Graduate School of Science, Kobe University
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo
ICRR, The University of Tokyo

113

H. List of Personnel
Director

KAJITA, Takaaki,

Vice-Director

KAWASAKI, Masahiro,

NAKAHATA, Masayuki,

Kamioka Observatory (Neutrino and Astroparticle Division)
Scientific Staff
ABE, Ko,
BRONNER, Christophe,
HIRAIDE, Katsuki,
ICHIMURA, Koichi,
KAMEDA, Jun,
KATAOKA, Yosuke,
KISHIMOTO, Yasuhiro,
KOBAYASHI, Kazuyoshi,
MIURA, Makoto,
MORIYAMA, Shigetaka,
NAKAJIMA, Yasuhiro,
NAKAYAMA, Shoei,
SHIOZAWA, Masato,
TAKEDA, Atsushi,
YAMASHITA, Masaki,
YANO, Takatomi,
Administrative Staff
SHIMIZU, Akihiko,Public Relations Staff
TAKENAGA, Yumiko,
Technical Staff
HIGASHI, Tetsuji,
NOZAWA, Noriyuki,
Research Fellow
KOBAYASHI, Masatoshi,
McCAULEY, Neil Kevin,
NAKANO, Yuki,
SATO, Kazufumi,
YANG, Byeongsu,
Secretary
DOI, Kyoko,
FUJIMURA, Yuko,
Research Center for Cosmic Neutrinos (Neutrino and Astroparticle Division)
Scientific Staff
KAJITA, Takaaki,
MASUDA, Kimiaki,
OKUMURA, Kimihiro,
TOKANAI, Fuyuki,
Technical Staff
SHINOHARA, Masanobu,
Research Fellow
ZIEMBICKI, Marcin,
Secretary
KITSUGI, Atsuko,
MASHIMA, Chieko,
High Energy Cosmic Ray Division
Scientific Staff
ASANO, Katsuaki,
KAWATA, Kazumasa,
MARTINEZ, Manel,
NONAKA, Toshiyuki,
SAITO, Takayuki,
TAKEDA, Masahiro,
YAMAMOTO, Tokonatsu,
Technical Staff
AOKI, Toshifumi,
Research Fellow
FUJII, Toshihiro,
Secretary
IDOMURA, Takako,

ENOMOTO, Ryoji,
KIDO, Eiji,
MAZIN, Daniel Mihajlovic,
OHISHI, Michiko,
SAKO, Takashi,
TAKITA, Masato,
YOSHIDA, Tatsuo,
INOME, Yusuke,
NAKAMURA, Yoshiaki,
SHIRAGA, Ryoko,

HAYATO, Yoshinari,
IKEDA, Motoyasu,
KATO, Yo,
MARTI MAGRO, Lluis,
NAKAHATA, Masayuki,
SEKIYA, Hiroyuki,
TANAKA, Hidekazu,

ONOUE, Tatsuya,
MINAKATA, Hisakazu,
TASAKA, Shigeki,
KAMIKAWATO, Rie,NISHIMURA, Yasuhiro,

HADASCH, Daniela,
KUBO, Hidetoshi,
NODA, Koji,
OHNISHI, Munehiro,
SASAKI, Makoto,
TESHIMA, Masahiro,
YOSHIKOSHI, Takanori,
SEKINO, Koichi,
SAKO, Takashi,
SUGAHARA, Midori,

Akeno Observatory (High Energy Cosmic Ray Division)
Scientific Staff
SAGAWA, Hiroyuki,
Technical Staff
KOBAYASHI, Ryoichi,
Norikura Observatory (High Energy Cosmic Ray Division)
Technical Staff
AWAI, Kyosuke,
HAYAKAWA, Hideaki,
OKAZAKI, Nao,SHIMODAIRA, Hideaki,
USHIMARU, Tsukasa,

IMANISHI, Hidenori,
TOMURA, Tomonobu,

114

Astrophysics and Gravity Division
Scientific Staff
IBE, Masahiro,
OUCHI, Masami,
Research Fellow
HAYASHI, Kohei,
TOSHIKAWA, Jun,
KAGRA Observatory (Astrophysics and Gravity Division)
Scientific Staff
BALLMER, Stefan Werner,
KIMURA, Nobuhiro,
MIYAKAWA, Osamu,
SAITO, Yoshio,
TROZZO, Lucia,
YAMAMOTO, Kazuhiro,
Administrative Staff
OKINAKA, Mihoko,
Technical Staff
FURUTA, Kiyoshi,
OBAYASHI, Yoshihisa,
TAKAHASHI, Masahiro,
Research Fellow
LEE, Hyung Won,
PENA ARELLANO, Fabian,
YAMAMOTO, Takahiro,
Secretary
CHIDA, Ai,
KIKUCHI, Rie,
Graduate Students
DoctorMasterAKUTSU, Ryosuke,
HARIKANE, Yuichi,
HIROSHIMA, Nagisa,
ISHIZAKI, Wataru,
MIYAMOTO, Takahiro,
NAGANO, Koji,
ONO, Kenji,
SONODA, Yutaro,
TANAKA, Hiroki,
ZHANG, Haibin,
ANDO, Kenta,
CHEN, Yu-chin,
HILMI, Miftahul,
IMAIZUMI, Shuichi,
KATO, Sei,
KIRII, Shin,
KOZU, Ryohei,
MURAI, Kai,OHTANI,Yoshiki,
OTANI, Francis,
SATO, Yuta,
SUZUKI, Takumi,
TANAKA, Taiki,
YADA, Kohei,

Administrative Division
Administrative Staff
AKAIDA, Yohei,
FUKUHARA, Nana,
KONDO, Hitomi,
NAMATAME, Kaneo,
SAITO, Akiko,
YAMAGUCHI, Akiko,Research Administrator
SATO, Ritsuko,
Public Relations Staff
FUKUDA, Hironobu,

KAWASAKI, Masahiro,

ONO, Yoshiaki,

NAGAI, Ryo,

OBATA, Ippei,

HIROSE, Eiichi,
KOKEYAMA, Keiko,
MIYOKI, Shinji,
TAGOSHI, Hideyuki,
UCHIYAMA, Takashi,
YOSHII, Yuzuru,
TAKAYAMA, Kyoichi,
KAMIIZUMI, Masahiro,
OHOKA, Hideyuki,
TAMORI, Yukio,
NARIKAWA, Tatsuya,
SURESH, Jishnu,
YOKOZAWA, Takaaki,
HARA, Yayoi,
MAEDA, Yukari,

KAWAGUCHI, Kyohei,
MIO, Norikatsu,
OHASHI, Masatake,
TOMARU, Takayuki,
USHIBA, Takafumi,
NAKADA, Kazuo,
SHIMODE, Katsuhiko,
OSHINO, Shoichi
SUZUKI, Toshikazu,
YUZURIHARA, Hirotaka,
IWAMATSU, Miho,

FUJIMOTO, Seiji,
HASEGAWA, Fuminori,
INADA, Tomohiro,
IWAMURA, Yuki,
MIYO, Koseki,
NAKANO, Masayuki,
ORII, Asato,
SUGAHARA, Yuma,
TO, Sho,

FUKAMI, Satoshi,
HASEGAWA, Kunihiko,
INOMATA, Keisuke,
KOJIMA, Takashi,
MUKAE, Shiro,
NAKANO, Wakutaka,
SHIN, Heungsu,
SUZUKI, Motoo,
TSUI, Kaming,

ARAI, Koya,
FUKUNAGA, Masashi,
HSIEH, Bin-Hua,
ITO, Ryohei,
KATO, Nobuyuki,
KOBAYASHI, Shin,
KUMON, Taku,
NAKATSUKA, Hiromasa,
OKADA, Takeshi,
PEDERSEN, Juan William,
SHIMODATE, Karin,
TAKENAKA, Akira,
WANG, Ru,
YAMADA, Tomohiro,

ARAI, Yuya,
HIGUCHI, Ryo,
IIDA, Kento,
KAKUMA, Ryota,
KIKUCHIHARA, Shotaro,
KOBAYASHI, Yukiho,
MOCHIZUKI, Toshiki,
OGAKI, Wataru,
OKAMOTO, Kohei,
SAKURAI, Shunsuke,
SONOMOTO, Eisuke,
TANAKA, Kenta,
XIA, Junjie,
YAMAMOTO, Kohei,

AKIYAMA, Makiko,
HIRAGA, Takuya,
MARUMORI, Yasuko,
OHURA, Kiichi,
SATO, Yu,

BOLENS, Yukari,
KOBAYASHI, Toyoki,
NAKAJIMA, Kazumi,
OKANO, Yuka,
TAKAMICHI, Ryo,

ITO, Yoriko,

NAKAMURA, Makio,