INSTITUTEFORCOSMIC RAY RESEARCH
THE UNIVERSITY OF TOKYOANNUAL REPORT(APRIL 2021 – MARCH 2022)

Editorial Board
NODA, Koji
OHASHI, Masatake
TAKENAGA, Yumiko
NAKAMURA, Makio
ITO, Yoriko©Institute for 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
Preface Research Divisions..................................................................................

1

Neutrino and Astroparticle Division2

High Energy Cosmic Ray Division32

Astrophysics and Gravity Division69

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

86

Norikura Observatory

87

Akeno Observatory

95

Kamioka Observatory

99

Kagra Observatory

100

Research Center for Cosmic Neutrinos

101

High Energy Astrophysics Facility in Canarias

102............................................................

104.............................................................................

109.........................................................................

109

Appendix A.

ICRR Workshops and Ceremonies

Appendix B.

ICRR Seminars

Appendix C.

List of Publications(a) Papers Published in Journals(b) Conference Papers
Appendix D.

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

119

Appendix E.

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

119(a) ICRR News(b) Public Lectures(c) Visitors...........................................................

124.................................................................

130

Appendix F.

Inter-University Research Activities

Appendix G.

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

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

131

PREFACE
This report summarizes the scientific activities of the Institute for Cosmic Ray Research (ICRR) of The University of Tokyo in the Japanese FY (JFY) 2021.
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 Observatory and KAGRA Observatory(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 three major experimental facilities outsideof Japan. They are located in Utah in USA, Yangbajing in Tibet, China and the La Palma island in Spain.
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 ICRR are the results of the collaborative efforts by many researchers from various institutions. In order to produce outstanding results, it is very
important to carry out an experiment by an international collaboration composed of top-level researchers all over
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 institutions of the “International Joint Usage/Research Center”
program by Ministry of Education, Culture, Sports, Science and Technology -Japan (MEXT) in November 2018.
In JFY 2021, our inter-university research activities have been largely affected by COVID-19. In many countries, overseas travels have been strictly restricted. Even the domestic travels within Japan have been limited. Even
during the pandemic, ICRR has carried out various researches as described in this report.
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.

Masayuki Nakahata,
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.

Online public lecture co-organizedwith the Institute for the Physics
and Mathematics of the Universe(Kavli IPMU).

Organization

6/$&.,+*("+3(75&.*8".&,#%/(9/5/".#:(-,;,5,*+
2*".3(*4()*$+#,%*.5!"#$%&'()*$+#,%

9/5/".#:(-,;,5,*+<,=:(>+/.='()*5?,#(9"'(9/5/".#:(-,;,5,*+75&.*8:'5,#5("+3(@.";,&'(9/5/".#:(-,;,5,*+
F"?,*G"(BE5/.;"&*.'
F7@97(BE5/.;"&*.'-,./#&*.

<,=:(>+/.='(75&.*8:'5,#5(4"#,%,&'(,+()"+".,"5(

9/5/".#:(!"#,%,&,/5

9/5/".#:()/+&/.(4*.()*5?,#(6/$&.,+*5
6*.,G$."(BE5/.;"&*.'
7G/+*(BE5/.;"&*.'
0,#/1-,./#&*.

73?,+,5&."&,*+(-,;,5,*+
9/5/".#:(A$88*.&(BC#/
D$E%,#(9/%"&,*+5(BC#/
H+&/.1I+,;/.5,&'(9/5/".#:(73;,5*.'()*??,&&//

73;,5*.'()*??,&&//

H+&/.1I+,;/.5,&'(9/5/".#:(D.*=."?(A/%/#&,*+()*??,&&//

Number of Staff Members (As of May 1, 2021)
Scientific
Staff

Technical
Staff

Research
Fellows

23
20
17
0
60

6
12
8
0
26

0
9
13
0
22

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

Administrators
andSecretaries
19
3
9
21
52

Total

48
44
47
21
160

FY 2014-2021 Budget
Personnel
expenses
Nonpersonnel
expenses
Total

2014

2015

2016

2017

2018

2019

2020

2021

706 000

684 000

683 000

779 000

823 000

872 000

898 000

898 000

1 282 000

1 595 000

1 288 000

1 514 000

1 729 000

1 611 000

1 173 000

2 078 000

1 988 000

2 279 000

1 971 000

2 293 000

2 552 000

2 483 000

2 071 000

2,976,000(in 1 000 yen)

1

RESEARCH DIVISIONS
Neutrino and Astroparticle Division
Overview
Super-Kamiokande
T2K Experiment
XMASS Experiment
XENON ExperimentHyper-KamiokandeHigh Energy Cosmic Ray Division
Overview
Cherenkov Cosmic Gamma-Ray Group
TA: Telescope Array Experiment
Tibet ASγ Experiment
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. T2K later reported the exclusion of the conservation of
CP symmetry in neutrino oscillations at more than 95% CL.
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 neu-trinos 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
the water circulation system had been conducted in 2018 and
0.01% gadolinium has been dissolved in the detector water inthe summer 2020.
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 project has
officially started by receiving its first Japanese funding in the
beginning of 2020. Its construction schedule includes 5 yearsof the cavern excavation, 2 years of instrumentation, and datataking commencement in 2027. In parallel, the J-PARC beam
delivered to the detector will be upgraded from 0.5 to 1.3 MW
over the same time period.

SUPER-KAMIOKANDE[Spokesperson : Masayuki Nakahata(Kamioka Observatory, ICRR, The University of Tokyo)]
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.

3

Events / daySuper-Kamiokande has been observing atmospheric neutrinos
since 1996 and has accordingly made several important measurements with them, including the discovery of neutrino oscillations [1].
After the first phase of Gd loading in the summer of 2020,
SK has been operating its sixth run period, SK6, and has been
accumulating atmospheric neutrino data ever since. Figure 1
shows the atmospheric neutrino data taken with Gd-loaded
water (SK6) in comparison to data taken with pure water(SK5). There is no large charge in the detector’s ability toobserve atmospheric neutrinos and the SK6 data are judged
to be of similarly high quality as those in previous pure-water
periods.

0.16 Multi-GeV 1-ring -like (FCFV)
SK5
SK6
0.12
0.08
0.04
0.00

1.0

0.5

0.0
0.5
cos zenith

1.0

Fig. 1. Comparison of the atmospheric neutrino event rate in SK5 andSK6 for the multi-GeV µ-like sample.

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 the event
rate and, to some extent, alter the spectral shape of SubGeV electron-like data due to the νµ ↔ νe oscillations
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 (UPMU) topologies. Fully-contained
events are characterized by a primary interaction vertex that is
located inside 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 iswithin the fiducial volume, they are characterized by having
at least one charged particle escaping the inner detector anddepositing light in the outer detector. In most cases the escaping particle is a muon. Upward-going muons originate fromhigh 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 toreach the detector and be identified. The FC sample is separated into electron-like and muon-like (µ-like) subsamples byapplying a particle identification algorithm to the most energetic Cherenkov ring of each event. Since PC and upwardgoing events are predominantly produced by muon neutrinos,
no particle identification is applied. Though SK cannot distinguish on an event-by-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 to separate events into νe -like and ν̄e -like subsamples.
Since the start of SK4 SK has had the ability to identify
neutrons, albeit with a limited efficiency of 25%. In 2020 atmospheric neutrino events from this data set were separated
into neutrino-like and antineutrino-like samples taking advantage of the fact that the former are expected to produce fewer
neutrons on average than the latter. Further, a novel event selection based on a boosted decision tree was introduced to improve the classification of multi-ring events in all data periods.
Using these new event selections all SK data until the end of
the SK4 period (2018) were analyzed to update the analysis
presented in [2, 3]. With an expected increase in hierarchy
sensitivity of ∼ 15% this new analysis found a preference forthe normal mass ordering of of between 71.4% and 90.3%
over the range of oscillation parameters allowed at 90% C.L..
This result is somewhat weaker than that of the publication,
whose preference was 81.9% to 96.7%. Similarly, the updated analysis prefers the first octant of sin2 θ23 at roughly 1σ

4and has a weak preference for a non-zero CP-violating phase.
These results are summarized in Table 1 and in Figure 2.
A search for flavor changing neutral current and lepton
universality-violating interactions using atmospheric neutrino
data has also been performed by incorporating their effects on
the predicted oscillations. In particular, these exotic interactions can be represented as additional potentials arising fromexotic interactions with environmental quarks:∗∗εee εeµ
εeτ
√
∗ 
. (1)
Hmatter = VMSW + 2GF N f (⃗r) εµe εµ µ εµτετe ετ µ εττHere the εi j represent additional non-standard interactions(NSI), N f is the position-dependent fermion density, and
VMSW is the standard matter potential used to probe the mass
hierarchy above. To avoid the computational complexity offitting all of these parameters simultaneously, the analysis isbroken into two parts. The first probes new interactions inthe µ − τ sector, allowing for NSI effects to appear atop of
the normal atmospheric disappearance. In the second part thee − τ parameters, which modify the appearance of electrontype neutrinos, are studied. Note that this analysis is an update
and upgrade of that in Ref. [4], that studied these phenomenausing SK data up until 2005. The present analysis updates all
aspects of the analysis, using the full data set up to 2016 andthe event selection found in Ref. [3].
Neither of the NSI analyses found evidence of oscillations
beyond the standard paradigm. At the 90% C.L. the parameters in the µ − τ sector are constrained as−4.5 × 10−2 <εµτ−5.1 × 10−2 < εµ µ − εττεeτ
.
1 + εee

Bibliography[1] Y. Fukuda et al. [Super-Kamiokande Collaboration],
Phys. Rev. Lett. 81, 1562 (1998).
[2] R. Wendell et al. [Super-Kamiokande Collaboration],
Phys. Rev. D 81, 092004 (2010).
[3] K. Abe et al. [Super-Kamiokande Collaboration], Phys.
Rev. D 97, no. 7, 072001 (2018).
[4] G. Mitsuka et al. [Super-Kamiokande Collaboration],
Phys. Rev. D 85, 113008 (2011).

< 1.9 × 10−2

(2)[5] R. Abbott et al. Phys. Rev. X 11, 021053 (2021)< 1.4 × 10−1 .

(3)Solar Neutrinos
Introduction
Solar neutrinos constitute by far the largest component of
neutrino flux on the Earth among those produced from natural sources. Most of solar neutrinos are produced by protonproton fusion, p + p → d + e+ + νe , and its subsequent fusion
reactions (pp-chain)1 . They are categorized into the pp, pep,
7 Be, 8 B and hep neutrinos. Among these, Super-Kamiokande
is sensitive to 8 B and hep neutrinos, which have relatively
higher energy that extends up to ∼20 MeV.
Past observations of solar neutrino flux by SuperKamiokande (SK) [1] and the Sudbury Neutrino Observatory(SNO) [2] led to the discovery of solar neutrino flavor conversion. Our current interest for solar neutrino measurementswith the SK detector [3] is to make a precision test of theneutrino flavor conversion through the Mikheyev-SmirnovWolfenstein (MSW) effect [4, 5]. The MSW effect leads toa resonant conversion of the solar neutrinos within the Sun
and results in an about 30% level of the survival probability
above ∼5 MeV. The survival probability of lower energy neutrinos are described by the 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 is often called as theConstraints on these parameters are shown in the left panel of
Figure 3. The situation in the eτ sector is complicated by theinability of the SK data set to simultaneously determine thethree relevant parameters. Instead the three are related asεττ =oration (LVC), Super-Kamiokande performed follow-up observations of the triggers listed therein. The search was performed in time intervals of 1000 s around each trigger andfound 8 events, 6 FC and 2 UPMU events, in coincidence.
For this search the expected background was 7.13 events, indicating no statistically significant excess. Given the large distances (∼Mpc) to the black hole or neutron star mergers that
produce these triggers, the neutrino signal, if any, is expected
to be weak. On the other hand, as the origin of the gravitational wave signal can be somewhat localized by LVC comparisons of the arriving neutrino direction with each trigger’s
skymap were used to calculate a p-value for coincident event.
Even with this additional directional information, all neutrino
events were found to be consistent with the background expectation. Table 2 summarizes the SK coincident observation
and Figure 4 shows the skymap for the event with the smallest
p-value, a FC event arriving just before trigger S190602aq.

(4)Here, constraints are extracted in the εττ and εeτ plane for aseries of fixed values of εee . An example constraint is shownin the right panel of the figure. In both sectors the fit is slightly
improved by allowing non-zero values of the NSI parameters.
Searches for Neutrinos Associated with Gravitational Waves
Since the direct observation of gravitational waves in the
LIGO and subsequently LIGO and VIRGO detectors, there
has been considerable effort to observe coincident signals withother forms of radiation. In addition to electromagnetic signals already observed in, for instance, neutron star-neutronstar merger events, astrophysical objects creating gravitational
waves may also emit MeV (or higher) neutrinos. At present,no such coincident observation of neutrinos and gravitational
waves has been achieved.
Based on a preliminary version of the GWTC-2 [5] catalog
of gravitational waves detected from the LIGO-Virgo Collab-*1 with a subdominant (∼1%) contribution from the CNO cycle

20

20

15

15

∆ χ2

∆ χ2

5

10

10

99%

5

99%

5

95%
90%

95%
90%

68%

0
0.001

68%

0.002

0.003

0.004

0
0.2

0.005

0.4

0.6

0.8

sin2 θ23| ∆ m232 | , | ∆ m231 | eV2
20

20
Inverted Hierarchy, 2020
Normal Hierarchy, 2020

15

∆ χ2

∆ χ2

15

10

10

99%

99%

5

5
95%
90%

95%
90%

68%

68%

0
0

2

4

0

6

0

0.02

δCP

0.04

0.06

sin2 θ13

Fig. 2. The ∆χ 2 as a function of ∆m223,31 , sin2 θ23 , and δcp . Fits to the normal (blue) and inverted (orange) hierarchy hypotheses are shown in
the top two and bottom left plots. These are the results of a fit with sin2 θ13 constrained to the preferred value from reactor measurements.
Relaxing this constraint, results in a measurement of this parameter and in shown in the bottom right figure. With the extra degree of
freedom the hierarchy significance (roughly the distance between the two lines) is reduced relative to the other plots.

εττεττ-εµµεee = 0.0
Normal Hierarchy

0.4

1Normal Hierarchy
68% C.L.

68% C.L.

0.5

90% C.L.

90% C.L.
99% C.L.

99% C.L.

0.2

0
0

−0.5

−0.2

−1
−1.5

−0.4
−0.06

−0.04

−0.02

0

0.02

0.04

εµτ

0.06

−1

−0.5

0

0.5

1

1.5

εeτ

Fig. 3. Constraints on NSI parameters in the µ − τ sector (left) and in the e − τ sector (right). These calculations assume standard PMNS
oscillations at the best fit of the the 2018 SK analysis [3].

6Fit
SK onlysin2 θ23
0.44+0.05
−0.02
+0.09
0.45−0.03

Hierarchy
NH
IH|∆m232,31 | [×10−3 eV2 ]+0.11
2.40−0.12+0.09
2.40−0.32

δCP
+0.88
4.36−1.39+0.88
4.54−1.32

Table 1. Best fit oscillation parameters obtained by the three flavor oscillation analysis. Fits are conducted for both the normal (NH) and inverted(IH) hierarchy assumptions The fit was performed assuming sin2 θ13 =0.0218, which is taken from PDG.

Name
S190426c
S190513bm
S190602aq
S190728q
S190814bv
S190924h
S191213g
S200316bj

Alert UTC
2019-04-26 15:21:55
2019-05-13 20:54:28
2019-06-02 17:59:27
2019-07-28 06:45:10
2019-08-14 21:10:39
2019-09-24 02:18:46
2019-12-13 04:34:08
2020-03-16 21:57:56

Obs.events (∆t)
UPMU (+279 s)
FC (−183 s)
FC (−287 s)
FC (+103 s)
FC (+249 s)
FC (+412 s)
FC (+289 s)
UPMU (−366 s)pΛ (%)
100.0
5.7
2.8
11.7
48.1
48.1
15.4
52.3

pχ 2 (%)
51.0
15.5
1.6
26.4
57.9
76.5
11.7
99.7

Table 2. Summary of atmospheric neutrino events found within 1000s of a gravitational wave trigger in the GWTC-2 catalog. Here ∆t shows
the time of the neutrino event to the trigger and the p-values describing the compatibility with the background hypothesis incorporating
directional information pΛ and additionally time information pχ 2 .
S190602aq∘@∘2019-06-02 17:59:27.089355
60 ∘

30 ∘

360 ∘

300 ∘

240 ∘

180 ∘

120 ∘

0∘

60 ∘

−30 ∘

SK FC
GW (90% C.L)
1σ ang.unc.

time used for this analysis is 5,805 days, with 2970 days fromthe SK-IV phase.
In addition to the standard neutrino oscillation analysis,
we report the results of searching for solar electron antineutrinos due to spin-flavor precession in the Sun [12]. Finally, we briefly report the status of solar neutrino observationin the SK detector after loading the Gadolinium [13].

−60 ∘

0

GW probability

0.000712058

Fig. 4. Skymap showing the gravitational wave probability distribution
from trigger S190602aq overlaid with the FC event found within
1000 s of its trigger time. The cross denotes the arrival directionof the neutrino and the circle indicates the expected directional
resolution.

“up-turn” of the solar neutrino spectrum. Current experimental data is not enough to directly demonstrate this “up-turn”
because of the experimental sensitivity as well as the statics.
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 important not only totest the MSW effect but also to test several alternative theoretical models, such as sterile neutrinos [6, 7], mass-varying
neutrinos [8], non-standard interactions [9, 10] and so on.
The matter effect can also be tested with the matter in theinterior of the Earth. Electron flavor neutrinos are regenerated due to the matter effect of the Earth, making the neutrino
flux in night is larger than that in day by about a few % level
depending on the neutrino oscillation parameters.
In this article, we report results of solar neutrino measurements using the data taken through the end the fourth SK datataking phase (SK-IV) in May 2018, with an analysis method
improved from our previous publication [11]. The total live-

Analysis
For precisely measuring the energy spectrum of recoil
electrons, the energy scale should be reduced and the removalof background events is essential. For that purpose, the detector simulation was improved with more accurate modeling of
the PMT timing response and the non-uniformity of the water
quality. For example, the energy reconstruction method was
improved with a correction for the PMT gain drift and with a
new correction of the energy scale non-uniformity. The correction for the gain drift was first introduced in 2017 and thenrefined in this analysis with an improved correction for thedark noise contribution. Furthermore, the correction for the
PMT angular response was improved with more realistic simulation of the PMT response and arrangement. By introducing those improvements, the standard variation of the energy
scale variation was reduced down to ±0.5%, which is threetimes smaller than that previously estimated.
In addition to the energy scale improvements, the rejection method for the spallation backgrounds is also improvedby tagging neutrons in the hadronic shower induced by cosmic muons. By applying this cut to the data sample, the signal efficiency improved about 12% while keeping the removal
efficiency of muon spallation events about 90% [14].
8 B solar neutrino flux measurement

The SK detector observes solar neutrinos via elastic scatterings with electrons in pure water. The direction of a re-

0.52

250
SK data(Stat. only)

0.5

Systematic error
SK combined flux

0.48

200

Sun spot number

150

0.46
0.44

Sun spot number

coiled electron is highly correlated with the direction of the incident neutrino. Figure 5 shows the distribution of cos θSun forsolar neutrino candidates observed during the SK-IV phase.
With the 2970 days of data in SK-IV, more than 60,000
events are observed over the background. Adding the solar neutrino events observed in other phases, the total number of the solar neutrino events is more than 100,000. Basedon this data, the 8 B solar neutrino flux is determined to be(2.35 ± 0.04) × 106 /cm2 /sec assuming a pure electron neutrino flavor content. The ratio between the SK result and the
SNO NC current flux (5.25 × 106 /cm2 /sec) [15] is found tobe 0.447 ± 0.008.

Data/MC(Unoscillated)

7

100

0.42
50

0.4
0.38 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 0
Date

Fig. 6. 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
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 thesunspot number provided from [16].

Fig. 5. The solar angle distribution in SK-IV. The horizontal axis shows
the cosine of the solar angle cos θSun and the vertical axis shows
the number of the observed events. The black points shows theobserved data, and the red (blue) histogram shows the best-fit(background) shape.

The solar activity cycle is the 11 years periodic change ofsunspots releasing the magnetic flux at the surface of the Sun.
The number of the sunspots strongly correlated with the solar activity cycle. The solar neutrino data set which is used
for this analysis spans more than 21 years and covers about
two solar activity cycles. Figure 6 shows the SK yearly flux
measured throughout the different phases of SK together withthe corresponding sunspot number (Source: WDC–SILSO,Royal Observatory of Belgium, Brussels [16]). The χ 2 between observed data and no time dependence hypothesis iscalculated with the total experimental uncertainties as χ 2 =23.25/22 d.o.f., which corresponds to a probability of 38.8%.
The SK solar rate measurements are fully consistent with aconstant solar neutrino flux emitted by the Sun.
Energy spectrum analysis
Analysis of solar neutrino energy spectrum was done using all the data from SK-I to SK-IV [1, 17, 18, 11]. The
data quality for SK-IV was significantly improved thanks to
the new front-end electronics for SK-IV [19], the upgradedwater circulation system and the upgraded calibration methods [20]. Owing to these upgrades, SK has achieved the lowest background (induced by radioisotopes in pure water, especially 214 Bi) among all the SK phases [21]. The energy threshold in SK-IV have been lowered to 3.49 MeV in recoil electron kinetic energy (SK-I: 4.49 MeV, SK-III: 3.99 MeV) andthis enabled SK to measure the solar neutrino energy spectrum with higher sensitivity. In addition, in May 2015, thetrigger threshold was changed from 34 observed PMT signals within 200 nsec to 31 hits [22, 23]. Because of thislower threshold, the detection efficiency between 3.49 MeV
and 3.99 MeV was improved from ∼86% to ∼100%. This
improvement led to further reduction of the statistical uncertainty below 4.99 MeV in SK-IV.
Figure 7 shows the energy spectra obtained from SK-I, -II,-III and -IV, overlaid with the best-fits with generic polynomial and exponential functions, and the predictions assuming
the current oscillation parameters described in the next section. Figure 8 shows the combined energy spectrum from
SK-I to SK-IV with the predictions. Note that all SK thephases are combined without regard to energy resolution orsystematic uncertainties in Figure 8, but those uncertainties
are taken into account in the χ 2 calculation between the data
and the prediction. Comparing χ 2 between the data (black)and the predictions (green or blue), the energy spectrum of
SK is consistent within ∼1σ with the MSW up-turn with boththe SK+SNO best fit parameters (green in Fig. 10) and the
KamLAND best-fit parameters (blue in Fig. 10). The dataalso slightly disfavors the flat oscillation probability by ∼1σsignificance.
Day-night asymmetry
The day-night asymmetry of the solar neutrino flux for thefull SK-IV data set was extracted with the same method described in Ref. [24]. Here, the day-night asymmetry is definedas,Φday − Φnight
AD/N =,
0.5(Φday + Φnight )where Φday(night) is the observed solar neutrino flux during
day(night). Figure 9 shows the result of a day-night asymmetry amplitude fit as a function of ∆m221 for the solar neutrino sample at 3.5 < Ekin < 19.5 MeV. The fitted day-night
asymmetry, AFit
D/N , assuming the SK+SNO best fit oscillation

8
Day/Night Variation Amplitude

0.6
0.5

0.01

0.7

ADN

Data/MC (unoscillated)

SK II Spectrum

Data/MC (unoscillated)

SK I Spectrum

0.7

0.6

−0.01

0

−0.02

0.5

0.4

0.4

0.3

0.3

0.2

0.2

−0.03
−0.04
−0.05
−0.06

6

8

10 12

14

16

E

kin

18

8

in MeV

10

12

14

16

18

E

kin

SK IV Spectrum

0.7

0.7

0.6

0.5

0.4

0.4

0.3

0.3

0.2

0.2

4

6

8

10 12 14 16 18
E

kin

in MeV468

10 12 14 16 18
E

kin

in MeV

Data/MC (unoscillated)

Fig. 7. SK-I, II, III and IV recoil electron spectra divided by the unoscillated expectation. The green (blue) line represents the best–
fit to SK data using the oscillation parameters from the fit to SK
and SNO (KamLAND) data. The orange (black) line is the best-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.

SK I/II/III/IV LMA Spectrum

0.6

−0.08
0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

×10
0.18
∆m221

−3

Fig. 9. Result of the day-night asymmetry amplitude fit as a functionof the input ∆m221 value. The black points show the data fit resultswith the statistical uncertainties, while the red curve show the expectation.

0.6

0.5

−0.07

−0.09

Data/MC (unoscillated)

Data/MC (unoscillated)

SK III Spectrum

in MeV

0.58
0.56

Oscillation parameter extraction
The oscillation parameters were extracted using the resultsfrom the solar neutrino measurements at SK and SNO [15],as well as the reactor antineutrino measurement by KamLAND [25, 26]. Figure 10 shows the allowed parameter region from the SK+SNO data as well as the KamLAND data.
SK significantly contributes to the measurement of the solar angle θ12 . From the SK+SNO data, the mixing angle isdetermined to be sin2 θ12 = 0.306 ± 0.014 and the mass dif+1.21
× 10−5 eV2 asference is determined to be ∆m221 = 6.11−0.68
shown in Fig. 10. The SK+SNO fit results favors a lower∆m221 value compared to the KamLAND best fit value of+0.19∆m221 = 7.54−0.18
× 10−5 eV2 . Currently, the SK+SNO data
disfavors the KamLAND best fit value at ∼1.4σ , while it was∼2σ in the previous analysis. Adding the KamLAND result, the oscillation parameters are determined to be sin2 θ12 =+0.013−5 eV2 .
0.306−0.012
and ∆m221 = 7.51+0.19−0.18 × 10

0.54
0.52
0.5
0.48
0.46
0.44
0.42
0.4

4

6

8

10

12

14

16

E

18
in MeVkin

Fig. 8. The energy spectrum combining SK-I through SK-IV as a function of the recoil electron energy. The red points show the ratio ofthe data to the expected flux using a non-oscillated 8 B solar neutrino spectrum. The green (blue) curve shows the expected energy
spectrum assuming the MSW effect inputting oscillation parameters of SK and SNO (KamLAND). The orange (black) line is thebest-fit to SK data of a general exponential or quadratic (cubic) Pee
survival probability.

parameters (green in Fig. 10) was obtained as,
AFit
D/N = (−2.1 ± 1.1(stat.))% (3.5 < E < 19.5 MeV).

Solar neutrino observation after Gd-loading
As reported in the later section, the Gadolinium wasloaded into the Super-Kamiokande’s pure water in 2020 summer [13] to enhance detector’s sensitivity to supernova relicneutrinos by tagging neutrons originated from the inverse beta
decay of electron anti-neutrino interactions.
In order to validate the detector performance, calibration
data were taken after loading Gadolinium and compared withthe results in pure water phase. For example, Figure 11 shows
the reconstructed electron kinetic energy with the DT calibration device [27] before and after the Gd loading. The differences of the peak position and resolution between SK-V (purewater) and SK-VI (Gd concentration 0.011%) are less than±1%. Hence, the detector’s energy scale is enough to continue to observe the solar neutrinos in the MeV region.
Figure 12 shows the typical distributions of cos θSun before
and after the Gd-loading. The peak of solar neutrinos aroundcos θSun = 1 is clearly observed even through we added the Gd
into pure water. Figure 12 also demonstrates that no additional
background exist in the energy range above 6.49 MeV after
the Gd-loading. The background events below that level is under investigation since the detection efficiency for such events

[event/day]

2

9

8
6
4 2σ
2

4
3.5

SK5-all vs. SK6-all
Energy: 6.49-19.49 MeV
SK-IV (2970 days)
SK-V Run: 080539-082915 (380.062 day)
SK-VI Run: 085000-087073 (561.553 day)

3
2.5

1σ

-5
2
2
in 10 eV
21

∆χ

3σ

5
4.5

sin2(Θ12)=0.316+0.034-0.026sin2(Θ12)=0.306±0.014
sin2(Θ12)=0.306+0.013-0.012-52
sin2(Θ13)=0.0219±0.0014∆m221=(7.54+0.19-0.18) 10 eV-52∆m221=(6.11+1.21-0.68) 10 eV-52∆m221=(7.51+0.19
)
10
eV
-0.18

2
1.5

15

1

∆m

0.5
0−1

10

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

cosθSun

5

1σ

0.1

0.2

0.3

0.4

2σ

3σ

0.5 2 4 6 822sin (θ)∆χ

Entry

Fig. 10. The allowed contours for ∆m221 vs. sin2 θ12 from solar neutrino
data at SK and SNO (green solid line). The allowed contour from
KamLAND is also shown in blue. The combined allowed region isshown in red.
5000
Deuterium tritium (DT) calibration
Calibration data
SK-V (pure water)
SK-VI (Gd 0.011%)

4500
4000
3500
3000
2500
2000
1500
1000
500
0
0

2

4

6

8
10
12
14
Reconstructed kinetic energy [MeV]Fig. 11. The comparison of energy distribution between SK-V (purewater) and SK-VI (Gd concentration 0.011%). The calibration data
was taken at the center position of the SK detector.

is different from that evaluated in pure water phase mentioned
in the previous section.
Solar electron anti-neutrinos due to spin-flavor precession
in the Sun
Due to a very low production rate of electron antineutrinos (ν̄e ) via nuclear fusion in the Sun, a flux of solarν̄e is basically unexpected. An appearance of ν̄e in solar neutrino flux opens a new window for the new physics beyond thestandard model. In particular, a spin-flavor precession process
is expected to convert an electron neutrino into an electron
anti-neutrino (νe → ν̄e ) when neutrino has a finite magnetic
moment [28, 29].
In order to search for solar-ν̄e in the energy range of 8 B solar neutrinos, we required the delayed time coincidence technique to tag the neutron originated from the inverse beta decay

Fig. 12. The comparison of the cosine theta distribution between 6.49
and 19.49 MeV before and after the Gd-loading. The phases of
SK-IV (red) and -V (blue) were operated by filling pure water while
SK-VI (light green) was operated by adding Gd into the detector’s
pure water.

of solar-ν̄e . For this purpose, the data reduction, which previously used for the supernova relic neutrino searches, was optimized by lowering the energy threshold down to 7.49 MeVof the produced positron kinetic energy. Figure 13 shows theselected events and the predicted spectrum of kinetic energy
for solar-ν̄e events in an assumption of the conversion probability Pνe →ν̄e = 10−4 . In this analysis, the number of solar-ν̄eevents is derived after the fitting with the signal and background spectra. The observed numbers of events for the fourenergy bins are compared to the best-fit signal and background
predictions. The amplitude of the signal is a free parameter. The signal and backgrounds have a known spectral shapewhich is included in the fit. Therefore, the upper limit of theconversion probability is evaluated in this study.
Figure 14 shows the ∆ χ 2 as a function of the conversion probability Pνe →ν̄e . As a result, the upper limit is determined to be Pνe →ν̄e < 4.7 × 10−4 at 90% C.L., which corresponding to 36 events of solar-ν̄e signal. This limit is a factor of 17 more stringent than the SK-I sensitivity [30]. The
neutrino magnetic moment derived from the Pνe →ν̄e probability in the spin-flavour precession model is calculated as µ ≤1.7 × 10−9 µB (10 kG/BT ) at 90% C.L., i.e. µ ≤ 3 × 10−8 µB
and µ ≤ 2 × 10−12 µB at 90% C.L. in the assumption of BT ∼600 G [31] and ∼ 7 MG [32], respectively.
Summary and outlook
In summary, Super-Kamiokande has precisely measured
the 8 B solar neutrino flux, its time variation and recoil electron
spectrum. Using 5,805 days of data, more than 100,000 solar neutrino interactions were extracted over the background.
No significant correlation between the observed solar neutrino flux and the sunspot number was found with more than
21 years of continuous observation of the solar neutrino flux.
The measured energy spectrum of 8 B solar neutrino is consistent within ∼1σ with the MSW up-turn with both the solar best-fit parameters and the KamLAND best-fit parameters.
The previously existed ∼2σ tension between the solar and
KamLAND best fit ∆m221 values was reduced to ∼1.4σ inthis analysis. Combining the solar neutrino oscillation anal-

1045!"##"$%&'(%")$

!"#"

40

$%"&#'(!)*+!%&",+%-%.#//0$#%1'2'3',$-35

0#1'/23%(*&+4 5678

tron detection capability with Gd can be used to further reducethe cosmic-ray spallation backgrounds. We aim to further improve our solar neutrino measurements with additional data
and analysis improvements in the SK-Gd era.

0#1'/23%(*&+4 .'.95678

Bibliography

25

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20;*<.":+=/':"(+?!> @A(%!*&#*'.+'B+!!!"#!! " #$$%

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30

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10 12 14 16 18 20 22 24 26 28!"#$%"&'$#$()*'+,$-.
Fig. 13. Fit result in the kinetic energy range of 7.5–15.5 MeV. The
black dots are data. The green, magenta, blue, yellow, andred histograms show best-fit predictions for reactor antineutrino
events, 9 Li decay events, atmospheric neutrino’s NCQE interactions and non-NCQE interactions, and accidental coincidences,respectively. The cyan dashed line is solar antineutrino signal
events in an assumption of 10−4 of a neutrino-to-antineutrino conversion probability.

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Fig. 14. Relation between ∆ χ 2 and conversion probability of neutrinos

to antineutrinos. The upper limits on Pνe →ν̄e for ∆ χ 2 = 1.0, 2.3,
2.7, and 4.6 correspond to 3.5 × 10−4 , 4.7 × 10−4 , 5.0 × 10−4 , and
6.0 × 10−4 , respectively.

yses by SK, SNO and KamLAND, the oscillation param−5 eV2 andeters are determined as ∆m221 = 7.51+0.19−0.18 × 10
+0.0132
sin θ12 = 0.306−0.012 .
The Super-Kamiokande detector was recently upgradedby dissolving Gadolinium (Gd), and started a new phase ofobservation as SK-Gd. In order not to cause significant harmto the solar neutrino observation, ultra-pure Gd2 (SO4 )3 powder with extremely low radio-impurity was newly developed
and dissolved into the SK detector. Furthermore, the calibration data after the Gd-loading has been taken Enhanced neu-

[19] H. Nishino et al., Nucl. Instrum. Meth. A 610, 710(2009).
[20] K. Abe et al., Nucl. Instrum. Meth. A 737, 253 (2014).
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11[29] L. B. Okun et al., Sov. J. Nucl. Phys. 44, 440 (1986).
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×10

7000
70
6000
60
5000

Solar Flare on Nov. 4th 2003

50

Derivative of Soft X-ray

4000

Soft X-ray

40

Neutrino event time3000

(19:42:26 Nov. 4 2003)

30

Search window

2000

20
The photon detector on GOES was saturated

1000

10
0

0
11/04
19:40

11/04
19:50

11/04
20:00

Time (UTC)

Fig. 15. The time of observed neutrino event for the solar flare on
November 4th, 2003. The black vertical line shows the time ofneutrino event in the SK detector. The red (green) plot shows thederivative of the light curves (original light curve) recorded by the
GOES satellite. The shaded region shows the search windows
determined by using the derivative of soft X-ray according to themethod developed by [3]. In the case of the solar flare on November 4th, 2003, the instrument on the GOES satellite saturated dueto the high intensity of soft X-rays. That resulted in the satellite notrecording data for more than 15 minutes from 19:45 to 20:00.

Neutrino fluence [cm -2 flare-1]

Searches for neutrinos associated with powerful solar flares
Solar flares are the largest explosive events that occur
around the surface of the Sun. This phenomenon is caused by
the reconnection of magnetic field lines above sun spots andproduces electromagnetic radiation from radio to γ-rays. Solar flares sometime occur associated with coronal mass ejections (CMEs), which are eruptions of the atmospheric plasma
into interplanetary space. The frequency of these explosive
events is strongly correlated with the activity of the Sun.
Neutrinos are only produced by accelerated protons above
300 MeV [1, 2], which can generate pions (π ± and π 0 ) by interacting with dense plasma in the lower solar atmosphere during solar flares. The generated π ± produce neutrinos in theirdecay chain. Neutrinos associated with solar flares (solar-flare
neutrinos) provide information on particle acceleration mechanisms during the impulsive phase of solar flares.
We searched using the SK detector for neutrinos from solar flares that occurred during solar cycles 23 and 24, including the largest solar flare (X28.0) on November 4th, 2003. Inorder to minimize the background rate we searched for neutrino interactions within narrow time windows coincident with
γ-rays and soft X-rays recorded by satellites [3]. In addition,
we performed the first attempt to search for solar-flare neutrinos from solar flares on the invisible side of the Sun by using
the emission time of coronal mass ejections (CMEs).
By selecting twenty powerful solar flares above X5.0 on
the visible side and eight CMEs whose emission speed exceeds 2000 km s−1 on the invisible side from 1996 to 2018,
we found two (six) neutrino events coincident with solar flares
occurring on the visible (invisible) side of the Sun, with a typical background rate of 0.10 (0.62) event flare−1 in the energy
range from 100 MeV to 10 GeV. Figure 15 shows an example
of observed neutrino event from the largest solar flare occurring on November 4th, 2002 (X28.0) at the visible side of the
Sun together with the light curve of soft X-ray (as well as
its time derivative) recorded by GOES satellite. The eventon November 4th 2003 was observed during the impulsive
phase of the solar flare, where particle acceleration is expected
to be active. Furthermore, Ref. [4] reported that relativistic neutrons associated with this solar flare were observed by
the neutron monitors on the ground at 19:45 (UTC), which
is about 3 minutes after the detection of the neutrino candidate in SK. This simultaneous observation also indicates that
hadrons (ions) were accelerated to more than 1 GeV during
this solar flare.
Although two (six) events observed from solar flares inthe visible (invisible) side of the Sun, no significant solar-flare

80 ×103Soft X-ray [Count]

Derivative of Soft X-ray [Count/s][28] A. Cisneros, Astrophys. Space. Sci 10, 87 (1971).

3

1015
Solar flare on the visible side
Homestake (ν e)Kamiokande
SNO
Borexino (ν e)Kamland ES+IBD (ν e)
SK Upper Limit (This work)

1014
1013
1012
1011
1010
109
108
107
0

20

40

60

80

100
Neutrino energy [MeV]Fig. 16. The upper limit of neutrino fluence from the data taken by
SK-I, II, III, and IV (red thick-solid) together with the other experimental results. The orange contour shows the allowed parameterregion from the Homestake experiment [5]. Black long-dashed–dotted, blue dotted, green thin-solid, and pink dashed lines show
the upper limits from Kamiokande [6], SNO [7], Brexino [8], andKamLAND [9] experiments, respectively.

neutrino signal above the estimated background rate was observed. As a result we set the following upper limit on neutrino fluence at the Earth Φ < 1.1 × 106 cm−2 at the 90% confidence level for the largest solar flare on November 4th, 2003.
The resulting fluence limits allow us to constrain some of thetheoretical models for solar-flare neutrino emission.
The Homestake experiment reported an excess of neutrino
interactions when energetic solar flares occurred [5]. This observation suggested a possible correlation between solar flares
and the neutrino capture rate on 37 Cl.
Based on the observed event below 100 MeV in the SK
detector, we calculated the upper limit of neutrino fluence associated from solar flares. Figure 16 shows the upper limitsof neutrino fluence obtained by the SK detector below about
100 MeV together with the experimental limits by other underground neutrino experiments [5, 6, 7, 8, 9].

12Comparing to other experimental limits of neutrino fluence, the SK limit is improved by at least an order of magnitude in the energy region from 20 to 110 MeV. The SK limit
excludes the allowed parameter region which was favored by
the Homestake experiment and gives a strong constraint forneutrino fluence from powerful solar flares.
As prospect, the solar activity is expected to reach at its
maximum around 2026 in solar cycle 25 and to have a powerful flare. The SK-Gd experiment has a high sensitivity to
the electron anti-neutrino because of the existence of Gadolinium in the water tank and this improvement is also helpful todistinct anti-electron-neutrinos from electron neutrinos fromsolar flares in the next solar cycle.

There are 5 selections to extract each K 0 decay mode, twofor KS0 decay (KS0 → 2π 0 , KS0 → π + π − ), with following seriesof criteria A and B, respectively, and three for KL0 decay (KL0 →π ± l∓ ν where l is electron or muon, KL0 → 3π 0 , and KL0 →π + π − π 0 ) with criteria C.
The following cuts are applied to select KS0 → 2π 0 , mode.
A-1 Events should pass FCFV and Evis selection.
A-2 The number of rings should be three, four or five.
A-3 There must be one non-showering ring corresponds tothe primary muon.
A-4 There must be one Michel electron.

Bibliography[1] R. Ramaty, B. Kozlovsky, and R.E. Lingenfelter, Spa.
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33, 239–282 (1995).
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[4] K. Watanabe et al., Astrophys. J., 636, 1135 (2006).

A-5 The reconstructed momentum of non-showering ringshould be 150 < Pµ < 400 MeV /c.
A-6 The reconstructed invariant mass of showering rings
should be 400 < MK < 600 MeV /c2 .
A-7 The reconstructed total momentum should be Ptot <300 MeV /c.

[5] R. Davis, Prog. Part. Nucl. Phys., 23, 13 (1994).

A-8 The reconstructed total invariant mass should be 800 <Mtot < 1050 MeV /c2 .

[6] K. Hirata et al., Phys. Rev. Lett., 61, 2653 (1998).

A-9 There should be no tagged neutrons.

[7] B. Aharmim et al., Astropart. Phys., 55, 1 (2014).

To select KS0 → π + π − mode, the following criteria is used.

[8] M. Agostini et al., Astropart. Phys., 125, 102509 (2021).

B-1 Events should pass FCFV and Evis selection.

[9] S. Abe et al., Astrophys. J., 924, 103 (2022).

B-2 The number of rings should be three.

Search for nucleon decay
Proton decays and bound neutron decays (nucleon decays
in general) are the most dramatic predictions of Grand Unified Theories (GUTs) in which three fundamental forces of
elementary particles are unified into a single force. SuperKamiokande (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 sofar.
A proton decay into one charged lepton and one neutral pion (p → e+ π 0 , p → µ + π 0 ) which baryon number ischanged by 1, is one of the popular decay modes which mostof GUT models predict and have relatively large detection efficiency. However, the predicted proton life time by minimal
SU(5) has been rejected by SK results [1]. On the other hand,the dominant decay mode in ome SUSY GUTs have a different generation of quarks appears in the final state due to thecontribution of the color triplet Higgs boson as p → ν̄K + andp → µ + K 0 . In past, the results of p → µ + K 0 with SK-I SK-III data was published [2]. We updated the results with
SK-IV data resulting 0.37 Mton· year in total exposure. Analysis was also updated for the new data: (1) an improved event
reconstruction was applied [3] ; (2) the selection criteria were
optimized separately for each KL0 decay mode (the same selection criteria were applied to all KS0 decay modes in the previous analysis); (3) the number of tagged neutrons was used tosuppress the background.

B-3 All rings should be non-showering.
B-4 The number of Michel electrons should be one or two.
B-5 The reconstructed invariant mass of the second andthird energetic non-showering rings should be 450 <MK < 550 MeV /c2 .
B-6 The reconstructed total momentum should be Ptot <300 MeV /c.
B-7 The reconstructed total invariant mass should be 800 <Mtot < 1050 MeV /c2 .
KL0 has longer lifetime than KS0 and another selection is
utilized.
C-1 Events should pass FCFV and Evis selection.
C-2 Total observed photoelectrons (p.e.) should be 500 <Qtot < 8000 p.e.
C-3 The number of rings should be 2-3 for KL0 → π ± l∓ ν,
4-6 for KL0 → 3π 0 , and 3-4 for KL0 → π + π − π 0 .
C-4 The number of showering rings should be 0-1 for KL0 →π ± l∓ ν, , the number of non-showering rings should be
1 for KL0 → 3π 0 , and the number of showering rings
should be 2 for KL0 → π + π − π 0 .

13

C-5 The number of Michel electrons should be 2-3 for KL0 →π ± l∓ ν, 1 for KL0 → 3π 0 , and 2-3 for KL0 → π + π − π 0 .
C-6 The reconstructed muon momentum should be 260 <Pµ < 410 MeV /c.
C-7 The reconstructed vertex separation should be 1.5 m <vsep .
C-8 There should be no tagged neutrons.
Fiducial volume (FV) is defined 2 m away from the innertank wall and total visible energy (Evis ) is requred to be morethan 30 MeV to reject low energy BG. Figure 17 shows muon
momentum after selection A-4, Kaon invariant mass after
selection A-5 and number of neutrons after selection A-8. A
peak around 150 MeV/c2 in Kaon invariant mass distribution
is due to neutral pion either from the atmospheric neutrino
interaction or decay of Kaon from proton decay.
KL0 has relatively longer lifetime and the secondary particles have another vertex separated from the primary one.
The improved event reconstruction tool can find multiple
vertices in a event. However, it is difficult to reconstruct all
secondary particles from KL0 and we can not apply invariant
mass cut used in other selections. Instead, a distance between
the primary and secondary vertices is used to distinguish thesignal events from the background. Figure 18 shows vertex
separation distributions after applying all cuts except the cut
on itself. The peak positions are different between the signal
and atmospheric neutrino MC.
The results of these selections are summarized in Table
3. It contains the signal efficiency, the number of background
events and the number of candidate events for each selection.
Systematic uncertainties are also shown in the signal efficiencies and the background rates.
As a result, no candidates are remained for the final samples for all K 0 decays and the proton lifetime limit for p →µ + K 0 is obtained as 3.6 × 1033 years with 0.37 Mton · year
exposure which is two times longer than the previous paper.

Bibliography[1] K. Abe et al. [Super-Kamiokande Collaboration], Phys.
Rev. D 102, 112022 (2020).
[2] C. Regis et al. [Super-Kamiokande Collaboration],
Phys. Rev. D, 85, 112001 (2012).
[3] M. Jiang et al. [Super-Kamiokande Collaboration],
Prog. Theor. Exp. Phys. 2019, 053F01 (2019).
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 becauseof the large backgrounds that constrain our search. The observation of SRNs in general or neutrinos from distant supernovae in particular, would give us some information about theuniverse, for example the core collapse rate from SRNs, andabout the neutrino itself too, for example its lifetime. The

main signal from SRN is expected to be the inverse beta decay
reaction (ν̄e + p → e+ + n), in which a positron and a neutron
are produced, the delayed coincidence between the positroninteraction and the neutron capture should allow to reduce thebackground affecting this analysis.
Before Gd loading in 2020, the SK detector had a poor
neutron detection efficiency. It was then proposed to add 0.2%
of gadolinium (Gd) sulfate by mass into SK in order to achievea high neutron detection efficiency. 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 cascadeof 8 MeV, neutrons could be easily detected at SK (in space,vertices within tens of cm and in time, with the neutron capture delayed about 30 µsec).
EGADS (Evaluating Gadolinium’s Action on Detector
Systems) project was funded in 2009. The main motivationof EGADS was 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.19, to mimic theconditions at SK. The selective water filtration system filtersout water impurities while keeping the Gd in the water.
EGADS has been renamed as Employing Gadolinium
to Autonomously Detect Supernovas after the installationof the new QBee electronics in 2017, following the SuperKamiokande collaboration approbation of the SK Gd loading.
Water system operations
From January 2010 to July 2011 we circulated pure water
through the 200-ton tank and proved that our water system isstable and achieves a high water quality. In 2013, from February 6th to April 20th, the 200-ton tank has been step wise
loaded with Gd sulfate until the final 0.2% concentration wasreached. Two values are monitored at three different depths:
the Cerenkov 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 photo-multipliers 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 20
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 results of these analysis
were published in [1].

14
80

100

0
0

200

400

60

40

20

0
0

600

Number of events

Number of events

Number of events

1
200

200

400

600

800 1000

K0 invariant mass (MeV/c2)

Muon momentum (MeV/c)

0.5

0

0

2

4

6

Number of neutronsFig. 17. Reconstructed muon momentum after selection A-4 (left), Kaon invar iant mass after selection A-5 (center) and number of tagged
neutrons after sele ction A-8 (right). The data (black dots) are compared with the atmospheric neutrino MC (red) normalized by the livetime,
and the signal MC (blue) normalized to the atmospheric neutrino MC.

Table 3. Summary of the p → µ + K 0 search. Uncertainties are quadratic sum of statistical and systematic errors.

Search mode
Efficiency (%) Background (events)
KS0 → 2π 09.7 ± 1.0
0.31 ± 0.14
KS0 → π + π −4.98 ± 0.54
0.8 ± 0.2
KL0 → π ± l ∓ ν
0.91 ± 0.17
1.0 ± 0.3
KL0 → 3π 00.36 ± 0.06
0.12 ± 0.06
KL0 → π + π − π 00.18 ± 0.04
0.16 ± 0.07
SK-IV combined (199 kton·years)
SK-I+SK-II+SK-III+SK-IV combined (372 kton·years)

Candidates (events)
0
0
0
0
0

Lower limit (1033 years)
2.7
1.4
0.2
0.1
0.05
4.6
3.6

±

0
±
10 K L→π l ν

Number of events

50

K L→3π02
1
04

K L→π+π-π0

2
0

− 500

0

500

Vertex separation (cm)Fig. 18. Reconstructed vertex separation distribution in KL0 → π ± l ∓ ν(top), KL0 → 3π 0 (center) and KL0 → π + π − π 0 (bottom) selections. All
cuts except vertex separation are applied. The data (black dots)
are compared with the atmospheric neutrino MC (red) normalized
by the livetime, and the signal MC (blue) normalized to the atmospheric neutrino MC.

In order to test the Gd-loading in SK, 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 pre-treatment 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 system

Fig. 19. In the new cavern, the 200-ton tank (a) with currently 240
photo-multipliers installed, the Gd pre-mixing and pre-treatment
15-ton tank (b), the selective filtration system (d), Gd removal
resins (c) for test and a device to measure the water transparency(e) have been installed.

have been tested in order to determine what is needed to ensure high water transparency in SK without band-pass system.
Figure 21 shows the time variation of LL15 since this loading until May 2020. From March 2018 to June 2018 a slow
but continuous decrease of the water transparency has been
observed ith the fast re-circulation system only. From June2018 to August 2018 the use of the band-pass system allowed
to recover high water transparency. There-after the fast-recirculation system only has been used.
In May 2019, the water system was stopped for about one

15

Fig. 20. Cerenkov 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.

week due to electrical damage following a power-cut. This
led to a reduction of the water transparency. After restarting,
the water transparency was recovered. On September 2019the water flow was modified so that only half of the flow passthrough the special cation exchange resin. The water transparency remained stable and high. Finally, in November 2020,
in order to simulate the future additional Gd loading in SuperKamiokande, EGADS was loaded from 0.01% to 0.03% Gdconcentration.
Ion exchange resin tests
Impurities in water Cerenkov detector’s water can be removed using anion and cation resins as a standard method.
In case of Gd-loaded water, standard cation exchange
resins are expected to also remove Gd. Hence, a special cationexchange resin has been developed and installed in EGADS,
in addition to the usual anion exchange resin. The installation was done in January-February 2019 (in January, the new
cation exchange resin was used alone, leading a water transparency drop). As showed in Figure 21, we observed 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 stresses the importance of the ionization lamps for thewater transparency.
In 2020, we learned the AmberJet 4400 anion exchange
resin will be discontinued soon by its manufacturer. In orderto replace it we started to test several other resin substitutecandidates. In May 2021, we started to test AmberJet 4002
which allowed to quite a high water transparency. The initial
drops following the installation of this new resin were due toa too short resin flushing period.

From November 2021, we learned the AmberJet 1020(Gd) special cation resin was also going to be discontinued byits manufacturer. A substitute candidate, Amberlite IR120B(Gd), was installed in the water system in December 2021.
Preliminary results shows acceptable results, though not as
good as the AmberJet 1020 (Gd).
DAQ operations
In June 2017, EGADS electronics have been upgradedfrom the ATM electronics to SK’s QBee electronics, theDAQ 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 detectand process 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 enable us to instantly and autonomously
detect a galactic SN and report to local experts and neutrino
and astronomy community.

Bibliography[1] Marti, Ll. et al., NIM-A 959, 163549 (2020)

16

Fig. 21. Cerenkov light left at 15 m for Gd loaded water in EGADS since December 2018. 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.

SK-Gd
In SK-Gd project, the sensitivity of anti-electron neutrino
observation has been improved significantly by detecting neutrons with gadolinium (Gd) (specifically, gadolinium sulfate
octahydrate (Gd2 (SO4 )3 · 8H2 O )). Already in 2020, 0.01%
Gd (about 13 tons in total weight of Gd2 (SO4 )3 · 8H2 O )has been introduced, achieving about 50% neutrino capture
efficiency [1]. As a next step, we plan to increase Gd to
0.03% during June - July 2022 to achieve 75% neutron capture efficiency. The plan aims to dissolve twice as much
Gd2 (SO4 )3 · 8H2 O (the total weight of Gd2 (SO4 )3 · 8H2 O for
this time is about 26 tons) in a month as it did in 2020. In
order to achieve it, the dissolving system was improved in thisfiscal year while continuing the physics observation. In parallel, the sensitivities of Super-K at the next Gd concentration
have been estimated. In this report, the sensitivities regarding
the observations of the supernova neutrinos will be explained,
which are main physics targets of SK-Gd.
Supernova Relic Neutrinos
Neutrinos flying like background radiation originatingfrom numerous supernova explosions that have occurred since
the beginning of the universe are called “supernova relic neutrinos (SRN)”. Observation of SRN will be a key to describe
an average picture of a supernova explosion, but it has notbeen discovered yet. The energy spectrum of anti-electron
neutrinos on the earth is shown in Fig.22. The SRN is the
expected to be the main component of the anti-electron neutrino that reaches the earth in the energy range of 10-30 MeV
while reactor neutrinos dominate in 10 MeV or less, and atmo-

spheric neutrinos dominate in 30 MeV or more. After the next
loading in 2022, the neutron capture efficiency by Gd will beenhanced to 75% which increases the detection efficiency forsignals by about 50% compared to the current Gd concentration. Furthermore, it is expected that the accuracy of the atmospheric neutrino component, which is a main background of
SRN, will be improved by using the results of the T2K experiment and the new results from nuclear experiments. Fig.23
shows the sensitivity for SRN. In SK-Gd, it is possible tosearch for SRN with much higher sensitivity than before ina short period of time. By 2022, the sensitivity is expected to
exceed the results of the pure water period with about 16 years
observation, and in the next 5 years until 2027, the sensitivity will be expanded to the region predicted by most models.
Then, we are aiming for the world’s first observation of SRN.
Supernova Burst Neutrinos
One of the most important observable in the supernova
neutrino burst observation by Super-K is the direction of a supernova. Since neutrino bursts can escape from supernovae
faster than light, they are expected to reach Earth minutes tohours earlier than light. Therefore, an announcement of thedirection of the supernova makes it possible to direct astronomical telescopes to the supernova before the light from thesupernova actually arrives. Super-K is the only detector thatcan make such an announcement by itself. SK-Gd also improves the directional accuracy of a supernova. The directionof the observed particles and the direction of the neutrinos
differ depending on the neutrino reaction. In the inverse beta
decay reaction (IBD), the angular distribution is almost uni-

17Fig. 22. Expected energy spectrum of SRN.

form, and information about the direction of the supernova
is lost. On the other hand, in a electron scattering, the scattered electron is scattered in the direction of the neutrino, so
this information can be used to determine the direction of thesupernova. Since electron scattering events account for only
about 5% of the total, the excess caused by them is not clearlyvisible without separating IBDs and electron scatterings. In
SK-Gd, since it is possible to distinguish electron scattering
events from IBD events, the directional accuracy can get better. Fig.24 shows directional resolution of a supernova as afunction of the distance to the supernova. Here, the efficiency
identified as the IBD is assumed to be 68% (neutron capture
efficiency by Gd is 75%, selection efficiency by data analysis is 90%). As shown in the figure, the directional resolution(68% C.L.) for a 10 kpc supernova is 4 to 5 degree with purewater, but it is about 3 degree with SK-Gd.
Pre-Supernova Neutrinos
In SK-Gd, Super-K can also detect neutrino signals thatcome before the collapse of iron core. Massive stars have aprocess of burning silicon (Si) just before gravitational collapse and a supernova burst. At that time, low-energy antielectron neutrinos called pre-supernova neutrinos are emitted,
and they can be detected via IBD with Gd neutron capture signals. Fig.25 shows the number of reactions that occur in the
SK tank for a supernova at a distance of 200 pc (corresponding to the distance to Betelgeuse). Depending on the mass ofthe star and the model, supernova bursts can be predicted 8hours before the explosion with a significance of 3σ . Fig.26
shows the detection efficiency of pre-SN neutrinos as a function of the distance to the supernova. As shown in the figure,it is possible to observe pre-SN neutrinos with a 100% probability up to around 500 pc, and there are about 20 candidates
for supernovae including Betelgeuse.

Fig. 23. Expected sensitivity of SRN detection compared to SRN
model predictions.

Fig. 24. Directional resolution for pointing a supernova as functions
for the distance to the supernova.The blue line shows the expected
improvement after the next Gd loading in 2022.

Bibliography[1] K. Abe et al., Nucl. Instrum. Meth. A, 1027, 166248(2022), arXiv:2109.00360

T2K EXPERIMENT[Spokesperson : Atsuko Ichikawa] Tohoku University
The Tokai to Kamioka (T2K) experiment [1] is a long18

Fig. 25. Expected number of integrated events before the core collapse. The solid lines are the cases with the normal mass ordering, while the dashed lines are the cases with the inverted massordering.

Fig. 26. Expected detection efficiency of pre-SN neutrinos. The efficiency is a probability to observe pre-SN signals more than 3σsignificance. Blue shows the case with a progenitor mass of 15
solar mass, while red shows the case with a progenitor mass of
20 solar mass. In both cases, the neutrino mass ordering is assumed to be the normal mass ordering.

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 ordering 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 ±500µs 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

19

Entries

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 interactions 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 11th run. This 11th run corresponds
to the first data taking with gadolinium dissolved in SuperK water following the upgrade of the detector for the SK-Gd
project. The data from this last run have not yet been used
for physics analyses, but their quality was checked and confirmed the proper functioning of the experiment with this new
setting. Additionally, evidence for the signal corresponding
to the capture of neutrons on gadolinium was seen in those
data: an excess of low energy events is seen after the trigger.
The number of events in this excess follows an exponential
decrease in time, with a time constant compatible with the expected capture time on gadolinium (Fig 27).

40

χ2 / ndf
p0
p1

35

27.29 / 2136.25 ± 16.23
71.71 ± 25.05

Table 4. T2K data taking periods and integrated numbers of protons
on target (POT) used in the far detector analysis. The numbers forrun 11 are still under preparation and have not been included.

Run
Period
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Run 8
Run 9
Run 10
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
Oct.2019 - Feb.2020
Jan.2010 - Feb.2020

×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
4.73
–
19.66 16.35

The details of the data that have been used to date in the far
detector analysis can be found in table4. The run 11 data correspond to an additional 1.78×1020 POT (before quality cut
at the far detector) collected in March and April 2021. Overthe whole period, the event rates and the beam direction were
found to be consistent with the expectations and stable by themeasurements of the muon monitor and the on-axis near detector. In particular, the beam direction remained stable wellwithin the ±1mrad target. During run 10 and 11, stable operation at more than 500 kW was achieved.
A long shutdown of the experiment is currently on-going,
for upgrades of both the accelerator used to produce the neutrino beam, and of the off-axis near detector. The experimentplans to restart data taking at the beginning of 2023, with increased beam power, and a new set of detectors to replace theπ 0 detector part of the current ND280. Those new detectors
will give improved ability to study neutrino interactions, witha larger efficiency for high angle events, lower threshold todetect protons, and the ability to reconstruct neutron energy.
A selection of results published during FY2021 is presented
below.

30
25
20
15
10
5
0
−500 −400 −300 −200 −1000

100 200 300 400 500
dt0 [µs]

Fig. 27. Number of low energy events seen in run 11 as a functionof time since beam trigger (dt0 ). An exponential fit is performed:n(t) = p0 × e−p1/t + cte, and the time constant obtained is compatible with expected capture time at the current Gd concentration(115µs).

Improved constraints on neutrino mixing from theT2K experiment with 3.13×1021 protons on target
T2K presented in [9] the details of the analysis used to
study neutrino oscillations with the run 1 to 9 data, which led
to the exclusion of the conservation of CP symmetry with 2σstatistical significance, as reported earlier in [8]. This new paper provides a fuller description of the method and a broader
range of results and tests of the validity of the analysis.
This analysis uses a SK dataset collected up to the end of
May 2018. This corresponds to an exposure of 14.94 ×1020
protons on target (POT) in neutrino mode and 16.35 × ×1021
in antineutrino mode, the same dataset as used to report indications of CP violation in [8]. Compared to the previous update [10] this is a nominal increase of 1% in neutrino mode,but 116% in antineutrino mode, which is particularly of interest for indications of ν̄e appearance, and an update to thissearch is reported in the paper. In parallel with the statistical

20

increase, our event selection has been refined: event reconstruction is now based on an algorithm that matches the pattern of light observed in SK directly [11]. This makes use of
more information about the event, providing better discrimination between event categories and improving the resolutionof the lepton momentum and vertex location. As a result, thefiducial volume can also be expanded, roughly equivalent to a
20% increase in statistics for the νe samples.
A large fraction of the analysis development focuses on
the interaction model, which incorporates constraints froma number of new external datasets and theoretical improvements. The dominant charged-current quasielastic (CCQE)
models have been updated in various respects, including thehandling of weak charge screening in nuclei; the handlingof nucleon removal energy and its effect on lepton kinematics; and additional freedom allowed in the kinematic dependence of interactions involving correlated nucleon pairs(2p2h). Modeling of (and uncertainties assigned to) subdominant processes have also been improved, including coherent
scattering and neutral-current interactions.
The neutrino oscillation probability depends upon the energy of the neutrino and its path length from creation to interaction. In long-baseline accelerator-based neutrino experiments the neutrino path length is fixed. All neutrino experiments use models to link the observed final states back to theinitial neutrino energy. There is no set of models that describe
the world’s neutrino data but there are a number of models
that are in comparable agreement to the world’s data. However, these models map true neutrino energy to reconstructed
neutrino energy in different ways. The choice of interaction
model therefore affects the neutrino energy distribution thatexperiments infer from their observed neutrino events, which
in turn can affect their measurement of the neutrino oscillation parameters. In this analysis a comprehensive set ofneutrino interaction models have been tested using simulated
data studies (procedure described in [12]) to quantify their effect on the T2K oscillation result. In all cases the observed
bias on sin2 θ23 and δCP was insignificant compared to existing systematic uncertainty on the parameter and so no additional uncertainty was introduced. Non-negligible bias wasobserved for ∆m232 . The quadrature sum of the observed
biases, 4.1×10−5 eV2 c−4 , was added as an additional uncertainty on ∆m232 .
Measurements of the parameters sin2 θ23 , ∆m232 and δCP
were performed using different statistical approaches, both
frequentist and Bayesian (with analysis of sensitivity to prior).
The global best fit was found to be for the normal ordering,and the data show a preference for the upper octant. Bayesianposterior probabilities were computed to quantify those preferences, and found to be 0.889 for the normal ordering and0.795 for the upper octant. The obtained 90% confidence
regions for (sin2 θ23 , δCP ) are displayed on Fig 28. The 2σ
CL intervals for δCP constructed using the Feldman-Cousins
unified approach do not include the CP-conserving values of
0 and π, meaning that the conservation of CP symmetry inneutrino oscillations is excluded at the 2σ level. Additional
checks on the validity of this results were performed, and thisexclusion was found to be robust with respect to the effects
checked.

Fig. 28. The observed constant ∆χ 2 90% confidence regions ofsin2 θ23 and δCP with normal and inverted mass orderings and withand without the constraint on sin2 (2θ13 ) from reactor experiments.
Normal and inverted mass ordering contours are independent.
∆χ 2 values are calculated independently for the functions with andwithout the reactor constraint.

A search for ν̄e appearance was performed by evaluating the significance of the ν̄µ → ν̄e oscillation under the assumption of two different hypotheses, corresponding to noν̄e appearance, and to ν̄e appearance consistent with our current knowledge of the PMNS mixing parameters. The analysis is performed by multiplying the ν̄µ → ν̄e PMNS
 oscillation probability
byafactor,
β
,
i.e.
P
ν̄
→
ν̄
µ
e = β ×
PPMNS ν̄µ → ν̄ . The parameter β is set to either 0 or 1 to select a null hypothesis for two independent tests: when β = 0,
the null hypothesis under consideration is that there is no ν̄eappearance, while for β = 1 the null hypothesis is that ν̄e appearance occurs according to the current best knowledge of
the PMNS parameters. For each hypothesis, p-values are produced from two analyses: rate-only and rate+shape.
The expected and observed test statistic distributions produced are shown in Fig. 29. The hypothesis of no ν̄e appearance (β = 0) is excluded at the 1.9σ and 2.4σ levels, respectively using the rate-only and rate+shape analyses. Ourdata are consistent with the PMNS ν̄e appearance hypothesis(β = 1), with p-values of 0.32 and 0.30 for the rate-only andrate+shape analyses, respectively (corresponding to a 1σ exclusion).
First T2K measurement of transverse kinematic
imbalance in the muon-neutrino charged-current
single-π + + production channel containing at least
one proton
To achieve the designed sensitivity of future long baseline experiments, nuclear effects have to be modelled accurately and consistently amongst all interaction channels. Experimental studies probing nuclear effects in carbon, throughthe measurement of transverse kinematic imbal ance (TKI)
in CC interactions, have been performed in T2K [13] andMINERνA [14]. TKI explores the lepton-hadron correlations
on the plane that is transverse to the initial neutrino direction
and helps precisely identify intranuclear dynamics, or the absence thereof, in neutrino-nucleus interactions. These measurements focused either on final-state topologies without any

Fraction of pseudo-experiments

21
0.16

β=0
β=1
T2K Data

0.14
0.12

δ αT in T2K results is in strong contrast to MINERνA results,
indicating a possible energy dependence of hadronic FSI.

0.1
0.08
0.06
0.04
0.02
0
0

5 10 15 20 25 30 35 −40
20 −10

Nevents

0

10

20

30

40

∆ χ2 = χ2(β=0) - χ2(β=1)

Fig. 29. Distributions of the observed rate-only (left) and rate+shape(right) test statistics compared to the value for the data. Here
Nevents denotes the number of observed events in the antineutrino
mode single-ring e-like sample.

pions, or final-state topologies with at least one neutral pion,
while none with one charged pion has been performed. Thesestudies suggest that modeling nuclear effects with Fermi gas
initial state models is insufficient, but more data are neededto draw solid conclusions. Positively charged pion production
from electron neutrinos is one of the signal channels measured
at the T2K far detector and employed in the oscillation analyses. Studying the same interaction channel in CC muon neutrino interactions can provide a better understanding of thecommon underlying nuclear effects and pion production mechanics. Furthermore, it will provide valuable information
towards the future inclusion of such sample among the oneused in T2K oscillation analysis. In [15], T2K described its
first measurement of the νµ cross section on hydrocarbon as afunction of TKI variables in CC production of exactly one π +and no other mesons, and at least one proton.
The standard set of three TKI variables, δ pT T , pN andδ αT are used. These observables are designed to characterize
the nuclear effects that are most relevant to oscillation experiments: the initial nuclear state, such as the Fermi motion of
initial state nucleon and the nucleon removal energy, and thefinal state interactions (FSI) of outgoing hadrons. The firstobservable δ pT T is the double-transverse momentum imbalance, The second observablepN is the initial nucleon momentum and the third observable δ αT is the transverse boostingangle which quantifies whether the hadronic system is accelerated or decelerated by nuclear effects.
An extensive comparison of the extracted results to stateof-the-art neutrino interaction models shows a slight preference for GiBUU (Fig. 30), which uses a more realistic nuclear ground state to handle all interaction channels consistently. Our results are statistically limited and a large partof the model separation power comes from normalization differences. In general the simple Fermi gas models (RFG and
2 /ndof ¿ 2,
LFG) show a large disagreement in pN with χtotwhich indicates a mismodeling of the nucleon Fermi motion.
The similar data-MC comparison to the MINERνA CCπ 0 results [16] seems to confirm that the mismodeling is generalin pion production channels. While tight phase space restrictions limit our sensitivity to FSI modeling, the relatively flat

Fig. 30. Measured differential cross sections per nucleon as a function of δ pT T (top), pN (middle), and δ αT (bottom), together withpredictions from NEUT, GENIE and GiBUU. In the tails of δ pT T
and pN (beyond the magenta lines), the cross sections are scaled
by a factor of 5 for better visualization. The legend also shows the2 used to quantified the agreement between the measurement
χtot
and the predictions.

Future analyses will aim to unfold cross sections in multiple TKI variables simultaneously and obtain their correlations
which can then be used to separate effects due to the initial
nuclear state and FSI. The upcoming ND280 upgrade is going
to expand the measurable phase space, especially in the low
energy and high angle regions. Thus the ND280 upgrade is
expected to increase our statistics and model sensitivity significantly.

22

Bibliography[1] K. Abe et al. (T2K Collaboration), Nucl. Instrum. Meth.
A 659, 106 (2011).
[2] D. Beavis, A. Carroll, I. Chiang, et al., Long Baseline
Neutrino Oscillation Experiment at the AGS (Proposal
E889), 1995. Physics Design Report, BNL 52459.
[3] K. Abe et al. (T2K Collaboration), Phys. Rev. Lett. 112,061802 (2014).
[4] T. Bhlen et al., Nucl. Data Sheets 120, 211 (2014).
[5] N. Abgrall et al. (NA61/SHINE Collaboration), Eur.
Phys. J. C 76, 84 (2016).
[6] C. Zeitnitz and T. A. Gabriel, Proceedings of International Conference on Calorimetry in High Energy
Physics (World Scientific, Corpus Christi, Texas, 1992),
ISBN 9789810213039, pp. 394-404.
[7] Y. Hayato, Acta Phys. Pol. B 40, 2477 (2009).
[8] K. Abe et al. (T2K Collaboration), Nature, 2020,
580(7803), pp. 339–344[9] K. Abe et al. (T2K Collaboration), Phys. Rev. D 103,
112008 (2021)[10] K. Abe et al. (T2K Collaboration), Phys. Rev. Lett. 121,
171802 (2018)[11] M. Jiang et al. (Super-Kamiokande Collaboration), Prog.
Theor. Exp. Phys. 2019, 053F01 (2019)[12] K. Abe et al. (T2K Collaboration), Phys. Rev. D 96,
092006 (2017); 98, 019902(E) (2018)[13] K. Abe et al. (T2K Collaboration), Phys. Rev. D 98,
032003 (2018)[14] X. Lu et al. (MINERvA Collaboration), Phys. Rev. Lett.
121, 022504 (2018).
[15] K. Abe et al. (T2K Collaboration), Phys. Rev. D 103,
112009 (2021)[16] D. Coplowe et al. (MINERvA Collaboration), Phys. Rev.
D 102, 072007 (2020)

XMASS EXPERIMENT[Spokesperson : Shigetaka Moriyama]
Kamioka Observatory, ICRR, the University of Tokyo
Introduction
The XMASS project was 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, scalability of the size of the detector mass, easy purification to reduce internal radioactive backgrounds (BGs), and shielding
ability against radiation from outside the detector due to ahigh atomic number (Z = 54) [2]. A breakthrough technology was developed to purify liquid xenon from the radioactive contaminant of 85 Kr found in commercial gas xenon [3].
This makes liquid xenon detectors suitable for rare event
searches. The detector containing ∼830 kg of LXe, was constructed in September 2010. Commissioning data was takenfrom December 2010 to May 2012. Based on commissioning data results were published on DM searches [4, 5, 6], solar axions [7], and two-neutrino double electron capture on
124 Xe [8]. We also studied the possibility of detecting galactic supernova neutrinos via coherent elastic neutrino-nucleus
scattering [9].
Analyzing the commissioning data we found that a majority of events at low energy originated from radioactive contamination in the aluminum seal of the photomultiplier tube
(PMT) window. In order to minimize the impact of this BG,
the detector was refurbished. The contaminated seals werecovered with copper rings and plates to stop scintillation lights
and radiation caused by this contamination to reach the innerdetector volume. The PMT windows were cleaned with nitric acid, and copper parts were electropolished in order toremove possible surface contamination. After a year of detector refurbishment, data-taking resumed in November 2013
with background significantly reduced and continued for morethan five years. We completed data taking in February 2019.
Results were published from searches for annual modulationfrom dark matter [10, 11], solar Kaluza-Klein axions [12],two-neutrino double electron capture on 124 Xe [13], dark
matter through elastic-scattering [14], hidden photons [15],inelastic-scattering off 129 Xe [16], sub-GeV dark matter [17],exotic neutrino-electron interactions [18], and for events coincident with the arrival of gravitational waves [19].
In the following sections, we briefly introduce the
XMASS-I detector and report the latest physics results fromXMASS data collected after the refurbishment.
The XMASS-I detector
XMASS-I was a single-phase LXe scintillator detector located underground (2700 m water equivalent) at the Kamioka
Observatory [20]. Fig. 31 shows a schematic drawing of the
XMASS-I detector. It contained ∼830 kg of LXe in its active region. The active volume is viewed by 630 hexagonal

23
Electronics
hut

Water tank

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

~1.2m

72 50cm PMTs (veto)Fig. 31. Schematic drawing of the XMASS-I detector.

and 12 cylindrical Hamamatsu R10789 PMTs arranged on
an 80 cm diameter pentakis-dodecahedron support structure.
These PMTs were developed to achieve low background requirements [21]. The largest contributions to the reduction ofradioactivity came from the PMTs’ stem and its dynode support. The glass stem was exchanged for a the Kovar alloy
one, and the ceramic support was changed to a quartz one.
The R10789 is the first model of Hamamatsu Photonics K. K.
that adopted these materials for low background purposes andprovided a groundbreaking step for further reductions of radioactivity in PMTs. A total photocathode coverage of morethan 62% of the inner surface was achieved in XMASS. The
almost spherical array of PMTs was supported in a doublewalled vessel made of oxygen-free high conductivity copper [22]. The waveforms of each PMT were recorded withCAEN V1751 waveform digitizers with a 1 GHz sampling
rate and 10 bit resolution. The detector was calibrated regularly with a 57 Co source inserted along its central vertical axis
and an external 60 Co source. From the data taken with the
57 Co source at the center of the detector volume, the photoelectron (PE) yield was determined to be ∼14 PE/keV. Two
different energy scales were used: keVee represents an electron equivalent energy, and keVnr denotes the nuclear recoil
energy. The scintillation decay time constant was investigatedin liquid xenon with the XMASS detector. The results weresummarized in [23] and [24].
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. This water tank was equipped with 72 Hamamatsu R3600 20-inch PMTs to provide both an active muon
veto and a passive shield against external BGs. XMASS-I was
the first direct detection dark matter experiment equipped withsuch an active water Cherenkov shield. The LXe and water
Cherenkov detectors were called the Inner Detector (ID) andthe Outer Detector (OD), respectively.
It is also worth noting that there were some important
technical studies related to pulse-shape discrimination between nuclear recoils and electron recoils [25], radon removal using charcoal filters [26], scintillation performanceof liquid xenon at room temperature [27], a 220 Rn calibration source [28], and 3-inch low background photomultipliertubes [29], though we did not use them for XMASS-I.

Search for event burst in XMASS-I associated withgravitational-wave events [19]In 2015, the gravitational-wave (GW) signal from a binary black-hole merger was first detected by the Advanced
LIGO experiment. During LIGO/Virgo’s observing periods O1 (September 2015–January 2016) and O2 (November
2016–August 2017), 10 binary black-hole mergers and a binary neutron-star merger were observed. Moreover, electromagnetic counterparts were detected for the first time, associated with the GW event from the binary neutron-star merger
named GW170817. Follow-up searches for neutrino events
associated with these GW events were also conducted by large
neutrino detectors all over the world. The neutrino follow-up
searches are of interest because there are some theoretical predictions of emission of neutrinos with energies of a few tensof MeV, and even higher-energy neutrinos are expected frombinary neutron-star mergers. The XMASS-I detector accumulated data with a stable condition continuing from November
2013 until February 2019, resulting in an entire data set witha total live time of 4.4 years.
We use the full 832 kg of xenon as an active target in thisparticular analysis. Events with four or more hits in the innerdetector without an associated outer-detector trigger were selected. We then applied four furhter selection cuts that mostly
remove obvious backgrounds. The detection efficiency after
those cuts is properly accounted for. The data are divided
into four NPE ranges: <450 (referred to as Low-E), 450–4500(Middle-E), 4500–45000 (High-E), and >45000 (Very HighE, or V. H. E. hereinafter), corresponding to energy ranges
of approximately <30, 30–300, 300–3500, and >3500 keVee ,respectively.
The event burst search was conducted in a time window
between −400 and +10, 000 s around each GW event. Thissearch window is motivated by two arguments. The time window within ±400 s from each GW event is considered in
the search for neutrinos. In addition, the extended time window up to 10,000 s is considered in the model-independent
search because for massive axion-like particles, for instance,
the travel time would depend on their mass.
To search for event bursts without assuming any particularburst model, the number of events in a sliding time window
is scanned for each energy range. The coincidence time window is slid at a step of 0.01 s and various width of the window(twidth ) are tested: 0.02, 0.04, 0.1, 0.2, 0.4, 1, 2, 4, and 10 s.
As the result of the model-independent search, no coincidence
time window with a global significance of more than 3σ wasfound in the time range between −400 and +10, 000 s fromthe GW170817 event. The same analysis is performed forother 10 GW events classified as binary black hole mergers.
Among them, a burst candidate with a small probability was
found in the High-E energy range for the GW151012 event.
After considering the look-elsewhere effect of the 4 energy
ranges, the global significance of this burst candidate identified in association with GW151012 is 3.0σ . Since we performthe analysis separately on the 11 GW events, there is an additional look-elsewhere effect. The significance of finding sucha burst candidate in any of the 11 GW events is 2.1σ . By investigating energy and vertex distributions, no significant deviation from the background distributions is found.

24

-2

Neutrino fluence (cm )

600

1013

-2

Neutrino fluence (cm )For the GW170817 event, we also derive constraints onneutrino fluence for the sum of all neutrino flavors via CEvNS
under the assumption of two types of neutrino energy distributions: a Fermi-Dirac spectrum with average neutrino energy
of 20 MeV and mono-energetic neutrinos in the range between
14 and 100 MeV. The simulated neutrino events are found toconcentrate at low energy, and hence the Low-E sample (below 450 PE) is used to derive constraints on neutrino fluence.
The detection efficiency crosses 50% at 4.5 PE which corresponds to 3.8 keVnr . No significant event burst is observed in
the Low-E sample, the 90% confidence level (CL) upper limit
on neutrino fluence is calculated. Figure 32 shows the 90%
CL upper limits on neutrino fluence for the Fermi-Dirac spectrum as a function of the coincidence time width twidth . Figure 33 shows our 90% CL upper limits on fluence for monoenergetic neutrinos as a function of neutrino energy between
14 and 100 MeV. Limits obtained by Super-Kamiokande arealso shown. The XMASS limit is comparable to the νµ,τ andν̄µ,τ limits of Super-Kamiokande.

1012
1011

1010
109
108
107
106
0

20

40

60
80
100
Neutrino energy (MeV)

Fig. 33. 90% CL upper limits on mono-energetic neutrino fluence for
GW170817 as a function of neutrino energy. The black solid line
shows the upper limit from the on-time window, and the blue band
represents the range of limits from the sliding window within the±400 s search window. Limits obtained by Super-Kamiokande forν̄e (green solid), νe (magenta dashed), νµ,τ (red dotted), and ν̄µ,τ
(blue dash-dotted) are also shown.

×109[7] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 724(2013) 46.

500
400

[8] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 759(2016) 64.

300[9] K. Abe et al. (XMASS Collaboration), Astropart. Phys.
89 (2017) 51.

200
100
0

10-1

[10] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 759(2016) 272.
1

twidth (s)10

Fig. 32. 90% CL upper limits on neutrino fluence for GW170817 assuming the Fermi-Dirac spectrum with ⟨Eν ⟩=20 MeV as a functionof the coincidence time width twidth . The black solid line shows
the upper limit from the on-time window centered at t = tGW witha width twidth , and the green band represents the range of limits
from the sliding window with width twidth within the ±400 s searchwindow. Note that twidth is scanned discretely at 0.02, 0.04, 0.1,
0.2, 0.4, 1, 2, 4, and 10 s.

Bibliography[11] K. Abe et al. (XMASS Collaboration), Phys. Rev. D 97(2018) 102006.
[12] N. Oka et al. (XMASS Collaboration), Prog. Theor. Exp.
Phys. 2017 (2017) 103C01.
[13] K. Abe et al. (XMASS Collaboration), Prog. Theor. Exp.
Phys. 2018 (2018) 053D03.
[14] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 789(2019) 45.
[15] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 787(2018) 153.

[1] Y. Suzuki et al., hep-ph/0008296.
[2] K. Abe et al. (XMASS Collaboration), Astropart. Phys.
35 (2012) 609.
[3] K. Abe et al. (XMASS Collaboration), Astropart. Phys.
31 (2009) 290.
[4] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 719(2013) 78.
[5] H. Uchida et al. (XMASS Collaboration), Prog. Theor.
Exp. Phys. (2014) 063C01.
[6] K. Abe et al. (XMASS Collaboration), Phys. Rev. Lett.
113 (2014) 121301.

[16] T. Suzuki et al. (XMASS Collaboration), Astropart.
Phys. 110 (2019) 1.
[17] M. Kobayashi et al. (XMASS Collaboration), Phys. Lett.
B 795 (2019) 308.
[18] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 809(2020) 135741.
[19] K. Abe et al. (XMASS Collaboration), Astropart. Phys.
129 (2021) 102568.
[20] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 716 (2013) 78.

25[21] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 922 (2019) 171.
[22] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 884 (2018) 157.
[23] H. Takiya et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 834 (2016) 192.
[24] K. Abe et al. (XMASS Collaboration), JINST 13 (2018)
P12032.
[25] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 659 (2011) 161.
[26] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 661 (2012) 50.
[27] K. Abe et al. (XMASS Collaboration), Nucl. Instrum.
Meth. A 594 (2008) 148.
[28] M. Kobayashi, M. Yamashita, A. Takeda, K. Kishimoto,
S. Moriyama, Journal of Physics: Conference Series 718(2016) 042069 (TAUP2015).
[29] K. Abe et al. (XMASS Collaboration), JINST 15 (2020)
P09027.

XENON EXPERIMENTXENON ICRR group[Principal investigator : Shigetaka Moriyama]
Kamioka Observatory, ICRR, the University of Tokyo
Introduction
The XENONnT experiment [1] is the most sensitive direct dark matter search experiment conducted at LNGS in
Italy. The XENON collaboration includes about 180 collaborators from 27 institutions in 12 countries. From Japan,
the University of Tokyo (ICRR and Kavli IPMU), Nagoya
University, and Kobe University participate since 2017. The
XENON group uses liquid xenon and employs a time projection chamber (TPC), characterized by its ability to discriminate between nuclear and electron recoil. The Japanese groupis making a significant contribution toward the discovery of
DM particles contributing a purification technology for liquid xenon developed in XMASS and the neutron detection
technology developed for Super-Kamiokande. The XENON
collaboration completed XENONnT detector construction in2020 and started data taking in 2021. Japan also participatedin the data analysis of the predecessor XENON1T experiment,from which the XENON collaboration reported a significant
3.3σ excess of electron recoil events over the background in2020 [2]. Since this could be interpreted as a signal of axions produced in the Sun, the ICRR members, who have also
searched for solar axions in XMASS [3], contributed to interpreting the data. In the XENONnT experiment, where datataking is ongoing, the background is expected to be further
reduced, and its large sensible volume is expected to soon
provide new insight into that excess seen in XENON1T. The
contributions of the ICRR group are described below.

Fig. 34. Principle of operation of liquid xenon time projection chamber(TPC). The interaction of dark matter produces scintillation light
S1, and electrons drifted by an electric field give a delayed signal
S2; the observed pattern of S2 can identify the planar location of
the interaction, and the time difference between S1 and S2 provides depth information. The ratio of S1 to S2 also holds information about the type of particle that caused the interaction. Figure
from the XENON collaboration.

26

83
5 Liquid purification in XENONnTFigure 5.5: Electron lifetime trend since the beginning of XENONnT purification. The first grey period corresponds
to liquid filter bypass mode. The red period corresponds to Q5 and the yellow period corresponds to St707. The grey

Fig. 35. Lifetimes of electrons in liquid xenon compared with different purification methods. It is measured with a purity monitor developed byperiod in January 2021 is a maintenance and liquid circulation was completely stopped as well as gas purification.
ICRR. Gray, red, and yellow areas indicate purification by gas and getter, liquid and Q5, and liquid and St707, respectively. For the first time
The wedashedthe expected
electronlifetime
trend liquids.
in gas purification
only scenario.
in the world,
wereline
ableshowsto establish
a methodfor directly
purifyingSt707 is expected
to be used for long-term operation since itemits almost no radon compared to Q5. Since the XENONnT detector is 1.5 m high, it is expected to have a lifetime of more than 3 ms,
which was sufficient to obtain a good performance. Figure from Ref. [5].

108(magenta) andXe (orange)). Field correction is applied. Purity monitor uncertainty band is shown in blue.

Liquid xenon purification and monitoring
Purifying liquid xenon is an essential technique for observing interactions of dark matter particles with the liquid
xenon target. As shown in Figure 34, the signal from a liquid
xenon TPC consists of a scintillation light S1 signal when recoil occurs, and an S2 signal obtained when an applied electricfield extracts the generated ionization electrons. If S2/S1 islarge, it is classified as an electron recoil (ER) event, whereas
if S2/S1 is small, it is classified as a nuclear recoil (NR) event,such as from a WIMP dark matter event candidate. For suchdiscrimination to perform well, ionized electrons must be extracted efficiently. In particular, impurities in liquid xenon that
trap electrons before they can be extracted will reduce the S2
signal and adversely affect the discrimination.
Our research was to develop a technique to reduce theseelectron-capturing impurities. A typical example of impurities
that capture electrons is oxygen. It is important to note, how-ever, that the addition of new contaminants such as radon need
to be negligible compared with our target values, 1 µBq/kg in
the case of radon, when these impurities are removed. In previous experiments, such as the XENON1T experiment [2], purification was performed by evaporating liquid xenon into thegas phase, passing it through a device called a getter that canremove oxygen and other impurities, and then liquefy it again.
In the XENONnT experiment, on the other hand, the amountof liquid xenon was about five times larger than in XENON1T,
and it was thought that the required performance could not be
obtained because the purification speed would not be sufficient with the gas purification. Therefore, purification in theliquid phase was planned and conducted. The R&D was conducted in collaboration with Columbia University. The filter
material Q5, which had been used in liquid argon, was a candidate, but it was known to emit radon gas, which is harmful
to the search for dark matter. The Japanese group proposed to

6 Electron lifetime from TPC data

83m monitor (blue222
218
129m
Fig.
36. Electron
measuredby theXENONnTfrom
detector(dots) andby the(purityof the measureFigure
6.11: lifetime
Electronlifetime
evolution
varioussourcesKr (black), band
Rnshowing(red), uncertainty
Po (green),Xement) during science
131mrun data taking. Figure from Ref. [5].

XENONnT Status
March-May 2020:- commissioning
PMTs used
ed electrodes
use the getter material St707,
which is normallyfor gas in vacuum- commissioning
cryogenic
systeminpurification in high temperature,also for the
purification27

the liquid phase since this material
emits a negligible amountJune-July 2020:of radon gas. With the help- oftheColombia
group, ofthenVeto
ma- and Calibration
preparation
for installation
terial was tested and confirmed
tobeusefulfor
liquidxenon
installationofnVeto
electronics
in DAQ roomof the Rn distillation
column ug- Installation
purification [4]. Since Q5 hasa high performancein removing impurities, it was proposed to use for reducing impurities
August-September 2020:in the early stages of the experiment.
Then
St707
used tonVeto
and wasCalibration- installation
continually remove impurities
slowlyemittedfromthedetec- started filling cryostat with LXe
tor components. This is because the half-life of radon is 3.8
October-November
2020: a certain
days, so even if Q5 emits radon,
it will decay within
8.6 t of LXeperiod and have no negative- completed
impact. Infillingthisofway,
the St707- Muon Veto restoration
was adopted for liquid xenon
purification
duringXENONnT
December 2020
data taking.
- Started filling Water Tank with DeMi-water
Figure 35 shows the performance during the commissionJanuary
2021 ->wasnowperformed using phase. At different times,
purification
full commissioning of TPC, nVeto, MV- starteding gas alone (gray), purificationusing
Q5 (red), and purifi- Rn column integration to the overall cryo system
cation using St707 (yellow).- The
vertical axis iscommissioning,
the electron commissioning… commissioning,
lifetime, and since the goal wastoachievemore
than
3 ms, we plantFig. 37. Overview of the XENONnT detector. The white part is the- preparation for the Gd purification
ki Yamashita, Kavli IPMU, UTokyo
Masaki
Yamashita
neutron veto detector. The liquid
xenon
TPC is at 7the center, surcan see that both Q5 and St707 were sufficient to achieve thatrounded by the white reflective material, and 120 photomultipliergoal after optimizing amount and shape of the filter. During
tubes are used to detect the Cherenkov lights produced by gamma
the actual science run, Fig. 36 shows electron lifetime duringrays when neutrons are captured by gadolinium or other nuclei andscience data taking. As can be seen, we confirmed purity fromused to identify the neutrons. The water tank is 10 m in diameterand 10 m high, and 0.2% of gadolinium will be added to the entire
electronegative contaminants well beyond the goal of 3 ms. It
system. Figure from the XENON collaboration.
will be applied to larger detectors in the future. These results
were summarized in a doctoral dissertation from the University of Tokyo [5].
obtained in Japan and delivered to LNGS [7]. In 2021, the installation of a gadolinium-loaded water purification plant was
Neutron background tagging utilizing SK-Gd techcompleted. The Super-Kamiokande collaboration also helped
nologyus to design the water plant for our application. At present, thePurifying liquid xenon, discussed in the previous section, plant operates with pure water. The gadolinium loading into
is an essential technique for maximizing the dark matter sig- the water in the tank will be done after the commissioning ofnal. On the other hand, background reduction is critical in rare the water plant.
event searches, and there are two primary background sources
important in XENONnT: one is radon in liquid xenon, and the Data analysis for XENON1T
other is neutrons emitted from instrument components. The
The XENON1T experiment, the predecessor of theJapanese group is contributing to the realization and operation XENONnT experiment, found no WIMP signal and gave theof a neutron veto detector that tags these neutrons and reduces world’s best limit on WIMPs. The WIMP search used events
the background by event basis, using the gadolinium-doped classified as NR events, as described in the introduction secwater Cherenkov detector technology developed at Kamioka tion. However, so-called ER events, which were discarded
for Super-Kamiokande [6]. Figure 37 shows a full view of the as radioactive background events in that search, are still at
XENONnT detector, which is entirely housed in a water tank the world’s lowest level in this energy range. In XMASS,
with a white reflective material optically isolating the neutron the lowest background was at around 60 keV at a level ofveto immediately surrounding the liquid xenon TPC at its cen- about 0.4 × 10−3 events/kg/keV/day [8], but in the larger
ter. The neutron veto is equipped with 120 PMTs. By adding XENON1T, it was only 0.2 × 10−3 events/kg/keV/day around
0.2% of Gd to the water in the whole tank, neutrons emitted three keV. This can be utilized to search for new physics sigfrom the detector components are expected to be captured by nals that could not be searched for before. Here, we searched
Gd with a high probability if they reach the neutron veto after for phenomena produced by axions that may be produced inrecoiling from xenon nuclei in the TPC. After capturing the the sun, a magnetic moment of neutrinos from the sun, andneutron on Gd, we expect PMTs in the neutron veto detec- absorption of bosonic types of dark matter by xenon atoms.
tor to observe Cherenkov light caused by gamma rays emitted Analysis of the data showed a statistical excess of 3.3 σ overfrom the Gd nucleus that captured the neutron. This enables the expected background at 2-7 keV, as shown in Figure 38.
us to tag neutrons that cause background for the WIMP search At this stage, it was pointed out that liquid xenon may containand thus maximize the experiment’s sensitivity. XENONnT a radioactive impurity, tritium, which was included in the data
aims to tag approximately 87% of such neutron events that are analysis. The results showed a significance of 3.4σ for theotherwise indistinguishable from dark matter events. During solar axion hypothesis, 3.2σ for the tritium hypothesis, andthe construction period, the neutron veto detector was com- 3.2σ for solar neutrinos with a magnetic moment [2]. The difpleted. With the help of the Super-Kamiokande collaboration, ference in these significance levels depends on how well theabout 1.7 tons of the purest grade of gadolinium sulfate was expected energy spectrum matches the observed data. In theC.L.) waseff
e gan . This28

projection (Fig. 8 bottom). Also shown in Fig. 8 areconstraints from other axion searches [84,85,112–116] aswell as predicted values from the benchmark QCD models
DFSZ and KSVZ.
(b)

HYPER-KAMIOKANDE[Co-Spokespersons: Masato Shiozawa1 , Francesca Di
Lodovico2 ]1: Kamioka Observatory, ICRR, The University of Tokyo
2: Department of Physics King’s College London(d)Fig. 38. Energy spectra of electron recoil events obtained withXENON1T. The black dots are data, the gray line is the background
event, and the red line is the background event plus the spectrum
expected from the axion produced in the Sun. Figure from Ref. [2].

search for solar axions, in addition to the coupling of axions toelectrons and photons, the coupling to nucleons [9] was additionally considered based on our ICRR suggestion. By fitting
all three constants independently, information of the details of
the axion models can be obtained. There are many papers,
more than a hundred, that were published in response to ourpaper. The observed excess is expected to be confirmed or
he null and alternative hypotheses in each scenario are denoted by gray (solid) andrejectedbymagnetic
the XENONnT
soon.
axion (b), andneutrino
moment experiment(c) searches, very
the null
hypothesis is theis B0 plus the respective signal. Contributions from selected components in each
Panel (d) shows the best fits for an additional statistical test on the solar axion
ent is included
in both null and alternative hypotheses. This tritium component
Bibliography
best-fit rate is negligible in the alternative hypothesis, which is illustrated by the[1] E. Aprile et al. (XENON collaboration), JCAP 11 (2020)
031.

[2] E. Aprile et al. (XENON collaboration), Phys. Rev. D
072004-13102, 072004 (2020).
[3] K. Abe et al. (XMASS collaboration), Phys. Lett. B 724(2013) 46.

Introduction
The Hyper-Kamiokande (Hyper-K or HK) project [1]is the world-leading international scientific research project
hosted by the university of Tokyo and High Energy Accelerator Research Organization (KEK) consisting of a next generation underground water Cherenkov detector and upgraded
Japan Proton Accelerator Research Complex (J-PARC) neutrino beam. The supplementary budget for FY2019 including
the first-year construction budget for Hyper-K project was approved by the Japanese Diet on January 2020 and the Hyper-K
project has officially started.
Figure 39 shows a schematic drawing of the Hyper-K
cylindrical detector. The detector is filled with 0.260 million metric tons of ultra pure water, which serves an order of magnitude larger fiducial mass of 0.188 million metric tons than Super-K, equipped with newly developed highsensitivity photosensors, and a high-intensity neutrino beam
produced by an upgraded J-PARC accelerator facility. It provides an enormous potential to discover leptonic charge-parity(CP) violation by observing neutrino and anti-neutrino beams
from J-PARC, investigate the Grand Unified Theory by exploring proton decay, and determine the neutrino mass ordering by observing atmospheric neutrinos combining beam data.
Hyper-K will also have far better capabilities to observe solar
neutrinos and neutrinos from other astronomical sources thanthose of predecessor experiments.

[4] G. Plante et al., arXiv: 2205.07336.
[5] N. Kato, doctoral dissertation, The University of Tokyo,
2021.
[6] L. Marti, et al. (Super-Kamikande collaboration), Nucl.
Instrum. Meth. A, 959, 163549 (2020); K. Abe et al.
(Super-Kamiokande collaboration), Nucl. Instr. Meth.
A, 1027 (2022) 166248.
[7] E. Aplile et al. (XENON collaboration), arXiv:
2112.05629.
[8] K. Abe et al. (XMASS Collaboration), Phys. Lett. B 787(2018) 153.
[9] S. Moriyama, Phys. Rev. Lett. 95 (1995) 3222.

Fig. 39. Schematic view of the Hyper-K water tank.

The Hyper-K international collaboration consists of about
500 researchers from 99 institutes in 20 countries. The detector technology has been developed based upon the successful
Super-K experiment and feasibility studies have been completed by international group. The Hyper-K operation plans
to start in 2027.

29

Hyper-K will have capability to measure the magnitudeof the CP violation with high precision, which could explain
the baryon asymmetry in the Universe. Figure 40 shows an
expected significance of the CP violation discovery by ten
years operation. With expected systematic error improvement,
Hyper-K covers the 60% of the parameter space with 5σ ormore as red dotted line shows in the figure. More than 8σ
significance will be expected if δCP = −π/2 as suggested by
T2K [2] and NOνA [3] results.

Fig. 41. The p→e+ π 0 discovery reach in proton lifetime with 3σ significance as a function of year. It shows Hyper-K (HK) planing tostart in 2027, superimposed with the ongoing Super-K (SK) andplanned DUNE experiments. The DUNE project assumes 10 kton
operation from 2026, toward full 40 kton by increasing 10 kton every year [4].

Fig. 40. Significance of the leptonic CP violation discovery with a
10-year observation in Hyper-K (HK) as a function of the unknown
CP phase assuming the normal neutrino mass hierarchy. Red dotted line shows the result with expected systematic error improvement, blue dotted line shows the result with achieved systematic
error by T2K 2018 and black line shows the result with only statistical error.

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. High sensitive Hyper-K PMTs result in strong background (atmospheric
neutrinos) reduction and p→e+ π 0 search in Hyper-K is expected to be “background-free” particularly in the free-protonenhanced signal region. Figure 41 shows the 3σ discovery potential for the p→e+ π 0 mode as a function of 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.
Construction status
Figure 42 is schematic drawing of Hyper-K tunnels andthe cavern. The excavation of the tunnel toward the Hyper-K
cavern site was started from 6th May, 2021 after the completion of the entrance yard construction. Without any serious
trouble, the first part of the tunnel, 1873.5m access tunnel
excavation was finished at February as scheduled. Though
the excavation started lower speed, excavation speed was improved gradually owing to hard and stable bedrock status andtiny amount of spring water. We achieved another significant
milestone towards the realization of the Hyper-Kamiokande
project. Photos in Fig. 43 shows the start of access tunnel excavation and the groundbreaking ceremony held on 28th May

Fig. 42. Overview of the Hyper-Kamiokande excavation. The firstpart of tunnel is called as the access tunnel (red line). The blackline after the access tunnel is the approach tunnel required for thecavern excavation.

at the tunnel entrance. The excavated tunnel is shown in the
top pannel of Fig. 44. The bottom panel of the figure wastaken at 25th Feb. at the end of the access tunnel.
The second part of the tunnel, the approach tunnel excavation started from March. After the approach tunnel, the excavation of Hyper-K cavern is scheduled to start from October
2022. Fig. 45 shows first branch of the approach tunnels.
20-inch PMT, Delivery and inspection
Newly developed photomultiplier tubes R12860 by Hamamatsu Photonics K.K is shown in Fig. 46. The total detection
efficiency of R12860 for single photon is twice higher thanthat of the Super-K PMT owing to 1.4 times higher quantum
efficiency (about 32%) and improved photoelectron collection
efficiency. 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. Pressure tolerance of PMT was also improved
up to the 125 m water depth, so that new PMT can be used
under about 70 m depth of the detector water.
Since the start of mass production at October 2020, delivery of 3̃00 R12860 for every month is ongoing. Total delivered
number reached about 3300 by the end of FY2021 without any
delay from the schedule. At the facility where delivered PMTs

30

Fig. 43. Photos at the start of the access tunnel excavation. The excavation started from 6th May, 2021(top). Hyper-K ground breaking
ceremony held on 28th May at the tunnel entrance(bottom). As
nine areas including Tokyo and Osaka were under the COVID-19
state of emergency, about half of the 50 participants attended online.

Fig. 44. The excavated access tunnel(top). Edge of the access tunnel at 25th Feb. 2022(bottom). The approach tunnel excavation
started beyond this point).

have been stored, inspections of the PMTs are also ongoing.
The inspections are consist of three items, visual inspection
for bulb and waterproof quality, signal check for signal property and noise rate, and long term stability check. Optimized
setup of those inspections are shown in Fig. 47, top pannel isthe signal check setup, middle is the eye inspection and bottom is the long term stability check setup. After all the first
700 PMTs were inspected, 10% sample has been inspected
every month.

Bibliography[1] “Hyper-Kamiokande Design Report,”
arXiv: 1805.04163.
[2] K. Abe et al. [T2K Collaboration],
Nature 580, 339-344 (2020).

Fig. 45. The first branch of the approach tunnels. One goes upward
to the cavern dome and the other goes down to the bottom of thecavern.

06/30101400.html.

[3] P. Adamson et al. [NOνA Collaboration],
Phys. Rev. Lett. 118, 231801.

[7] F. An et al. [JUNO Collaboration],
J. Phys. G 43, no. 3, 030401 (2016).

[4] R. Acciarri et al. [The DUNE Collaboration],
arXiv:1601.05471 [physics.ins-det].

[8] Yu-Feng LI, Presentation at ”Workshop on Supernova at
Hyper-Kamiokande” (2017).

[5] H. Sekiya [Super-K Collaboration],
PoS(ICHEP2016) 982 (2016).
[6] ICRR News (2016),
http://www.icrr.u-tokyo.ac.jp/2016/

31

Fig. 46. New 50 cm photomultiplier tube with a box-and-line dynode(R12860, Hamamatsu Photonics K.K.).

Fig. 47. Inspections of PMTs.

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. High Energy Astrophysics
Facility in Canarias was established as a base for the projectin April, 2019.
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 partially constructed by March of 2019.
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 Technol-

33Fig. 1. Artist view of the CTA observatory. CTA consists of three types of telescopes, Large Size Telescopes (23 m diameter), Mid Size
Telescopes (12 m) and Small Size Telescopes (4 m), and covers the broad energy band from 20 GeV to 300 TeV. [Credit: Gabriel Pérez
Diaz (IAC)/Marc-André Besel (CTAO)/ESO/ N. Risinger (skysurvey.org)]ogy, Kumamoto University, Kumamoto, Japan; Departmentof Physics, Kyoto University, Kyoto, Japan; Department ofApplied 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; Astrophysical Big Bang laboratory,
RIKEN, Wako, Japan; Department of Physics, Rikkyo University, Tokyo, Japan; Department of Physics, Saitama University, Saitama, Japan; Department of Physics, Tokai University, Kanagawa, Japan; Faculty of Integrated Arts and Sciences, The University of Tokushima, Tokushima, Japan; 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.
CTA Project
During the past years, Very High Energy (VHE) gammaray astronomy has made spectacular progress and has established itself as a vital branch of astrophysics. To advance thisfield 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 will attainan 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 the origin of
the highest energy cosmic rays in our Galaxy and beyond, thephysics of energetic particle generation in neutron stars andblack 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 thenature of space and time.
VHE gamma rays from 100 GeV to 10 TeV can be observed with ground-based current IACTs. The history of
VHE gamma ray astronomy began with the discovery of VHE
gamma rays from 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 experiences
gained from the current IACTs. The project is presently inits 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 (1 mCrab∼ 10−14 erg s−1 cm−2 ), together with a much broader energy
coverage from 20 GeV up to 300 TeV, 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
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

34

onto supermassive black holes, the long-standing puzzle of theorigin 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 dedicatedobserving strategies, CTA will also elucidate many aspects ofthe mysterious nature of gamma ray bursts (GRBs), the mostenergetic explosions in the Universe. Detailed studies of both
AGNs and GRBs can also reveal the origin of the highest energy cosmic rays in the Universe, probe the cosmic historyof star formation including the very first stars, and 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 very much feasible, thanks to the positive experiences gained from the current
IACTs.
The CTA-Japan consortium [1] is contributing particularly
to the construction of the Large Size Telescopes (LSTs, Fig.2)
and is involved in their development. The LST covers thelow energy domain from 20 GeV to 3 TeV and is especially
important for studies of high redshift AGNs and GRBs. The
diameter and area of the mirror are 23 m and 400 m2 , respectively, in order to achieve the lowest possible energy thresholdof 20 GeV. All optical elements and detectors require highspecifications, such as high reflectivity, high collection efficiency, high quantum efficiency, ultra-fast digitization of signals, and so on. For this purpose, CTA-Japan has developed
high quantum efficiency photomultipliers and ultrafast readout electronics (Fig. 3), and high precision segmented mirrors(Fig. 4). On the strength of their experiences gained from theconstruction of the MAGIC telescope, Max-Planck-Institutefor 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 in 20 seconds) to promptly observe GRBs. The first LST(LST-1) has been built in the CTA North, La Palma, Spain, in
2018, and now in the engineering run. Three more LSTs will
be built until 2023, then four more LSTs in the CTA South are
expected to be built in the ESO site in Paranal, Chile.
The results of the commissioning of the LST-1 camera has
been presented in the international conference [10]. The noiselevel, the charge calibration precision, the trigger rates, theDAQ performance and operation stability were well studied
characterized there. Fig.6 shows one of those results, showing
the stability of noise level as a function of time and the nightsky background intensity.
The Critical Design Review of LST-1 was held by CTAO
engineers and external experts from high energy physics. The
LST-1 has been regularly operated since January 2020, andhas already shown an excellent performance as expected,
even in a single telescope mode, through the observation of
Crab Nebula, Crab Pulsar, and nearby Active Galactic Nuclei(AGN). These results certify LST-1 achieved the design performance expected by Monte Carlo simulations. The phase
diagram of the Crab Pulsar observation is shown in Fig. 7,
which certifies the low threshold energy of LST-1 and also theaccuracy of the recorded event timings. In July 2021, the LST
collaboration issued the first telegram of reporting the detection of the AGN BL Lac with LST-1 [11]. The location of theFig. 2. Large Size Telescope (23 m 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.

LST array in the CTA North has an overlap with MAGIC telescopes, which will allow us to perform the cross-calibration
between LST and MAGIC telescopes and to operate the threetelescopes together in the early phase of the construction.
On 19th September, 2021, a volcanic eruption initiated in
Cumbre Vieja (El Paso) more than 20 km away in a straightline from the Roque de los Muchachos Observatory (ORM)
where LST-1 is located. The LST collaboration decided tostop the LST-1 operation. The eruption continued until 25th
December, 2021. After the inspection of LST-1 and the recovery works, an observation with LST-1 restarted on 21th
January, 2022.
In addition to the LST activities, CTA-Japan members arealso contributing to the overall CTA project in the Analysis

35
HV ON, shutter open, Park Out, High Gain, pixel 0
std. dev. of pedestal (relative)

1.04
1.02
1.00
0.98
0.96
2020-09-13
23:00:00

2020-09-13
23:30:00

2020-09-14
00:00:00

2020-09-14
01:30:00

2020-09-14
02:00:00

pixel 0500
400pedestal std. dev.

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 60-mm thick aluminum honeycomb sandwiched by 3-mm thin glass on both sides. A surface
protection coat consisting of the materials SiO2 and HFO2 is applied to enhance the reflectivity and to elongate the lifetime.

2020-09-14 2020-09-14
00:30:00
01:00:00
time (UTC)300
98.8 x pow(x, 0.5) -20.5
run 2645 (shutter closed)
run 2681 (shutter open)
run 2618 (shutter open)
run 2619 (shutter open)
1NSB (1.6 uA)
10NSB (16 uA)
5
10
15
20
25
30
normalized anode_current [uA]

200
100
0

0

Fig. 6. (Top) Time evolution of the noise for a single pixel for threehours. Each point represents ∼ 50000 events (∼ 12 seconds) anderror bars show standard deviation of the noise for this single pixel.
(Bottom) Noise level during multiple observation runs as a functionof the anode current. Orange and red dotted lines show dark nightsky background (NSB) and 10 times higher than dark NSB level,
respectively.

Fig. 5. The Large Size Telescope prototype (LST-1) constructed at
Observatory de Roque de los Muchachos is now in the engineering run. The diameter of dish and mirror surface area are 23 m
and 400 m2 .

and Simulations Working Group (ASWG). The performanceof the planned CTA layout is estimated by simulations andthe resulting IRFs (Instrument Response Functions) are public at [12]. Since the most important factor in determining the
gamma-ray sensitivity of an IACT array is the rate of misidentified cosmic-ray proton backgrounds, the accuracy of the description of hadronic showers affects the estimation of the
gamma-ray sensitivity. By testing recent four hadronic interaction models (QGSJET-II-03, QGSJET-II-04, EPOS-LHC,and SIBYLL2.3c) in the CTA simulation, variations in background rates of up to a factor 2 with respect to QGSJET-II-03
are observed between the models, which leads to ∼30% differences in the estimated gamma-ray sensitivity in the 1 - 30
TeV region (50-hour case, for a point source). The results also
show that IACTs have a significant capability in the verification of hadronic interaction models [13].
The Cherenkov Cosmic Gamma Ray group is also operating the MAGIC Telescopes [14] on La Palma, Canary Islands(See the next subsection). This facility is used not only for

Fig. 7. Phase diagram of Crab Pulsar as measured by LST-1. The
pulsar is known to emit pulses of gamma rays during phases P1
and P2. The energy threshold in this observation is estimatedabout 40-50 GeV.

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,

36

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.

Fig. 8. MAGIC Stereo System with two Cherenkov telescopes of 17
m diameters, so far achieved the threshold energy of 25 GeV withthe sum trigger. It locates near the mountain top of the Roque delos Muchachos on the Canary Island of La Palma. Two telescopes
are located with the distance of 85 m.

[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] Commissioning of the camera of the first Large Size
Telescope of the Cherenkov Telescope Array, Proc. ofICRC2021 at Berlin Germany (Online), T. Saito et al.,arXiv:2108.01920.
[11] Detection of very-high-energy gamma-ray emission
from BL Lac with the LST-1, ATEL #14783.
[12] CTA Observatory website (CTAO performance)
https://www.cta-observatory.org/
science/ctao-performance/[13] Effect of the uncertainty in the hadronic interaction models on the estimation of the sensitivity of the Cherenkov
Telescope Array, M. Ohishi et al., J. Phys. G: Nucl. Part.
Phys. 48 075201 (2021)[14] MAGIC Collaboration website:
mppmu.mpg.de/.

http://magic.

MAGIC
The MAGIC Collaboration has built in 2004 a first large
atmospheric imaging Cherenkov telescope, MAGIC-I, with amirror surface of 236 m2 and equipped with photomultipliertubes of optimal efficiency. In 2009, a second telescope of essentially the same characteristics was added; MAGIC-II was
installed at a distance of 85 m from MAGIC-I. With the accentof these instruments on large mirror surface and best light collection, cosmic gamma-rays at an energy threshold lower thanany existing or planned terrestrial gamma-ray telescope havebecome accessible. So far achieved has been a threshold of 25
GeV. The Japanese group has joined the MAGIC collaboration since 2010, and contributed to the operation, observations
and data analyses. The MAGIC telescopes are upgraded withnew cameras, electronics and partially new mirrors in 2012,

Fig. 9. 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) [5].

and are now operated with an unprecedented sensitivity by aninternational collaboration of about 165 scientists from 24 institutes and consortia from 12 countries.
The recent highlights from MAGIC are, 1) The first detection of the Gamma Ray Burst GRB 190114C with the groundbased gamma-ray telescope MAGIC [1], [2], [3], 2) Observation of sub-TeV gamma rays from the IceCube 170922A[4], [5] (Fig. 9), 3) the successful observation of pulsed
gamma ray signal from the Crab pulsar up to TeV regime[6], 4) the discoveries of the most distant blazers 3S 0218+35
with the redshift of 0.944 and PKS 1441+25 with the redshiftof 0.939 [7], 5) the observation of the very fast flare of 1min time scale from the blazar inside Perseus cluster, IC310[8], 6) the conclusions of a 15 years joint observation campaign of the gamma-ray binary HESS J0632+057 together
with the Cherenkov telescope experiments H.E.S.S. and VERITAS [14]. These results brought new questions on the pulsar emission mechanism, the EBL energy density, and gamma
ray emission mechanism from the supermassive blackholes orvicinity of them.

37

Gamma Ray Bursts
In January 2019, the MAGIC telescopes, for the first time,observed the TeV Gamma-Ray Burst GRB 190114C with theredshift of 0.42 [1],[2], [3]. The observation shows the 100
times higher flux of gamma rays than the Crab Nebula at T0+ 80 sec, and the resultant statistical significance of gammarays was more than 50 sigma. The highest-energy photon
reaches above 1 TeV, and the observed spectrum after the EBLde-absorption did not show any roll-off feature in the energy
spectrum [2]. This observation will require the new gammaray emission mechanism in the GRB to explain the TeV emission, possibly the inverse Compton emission. The light curveshows the power-law decay t −1.6 , which is surprisingly parallel to the X-ray lightcurve measured by XRT [2]. It is worth
mentioning that the energetics of the TeV emission is comparable with that of the X-ray emission. The two bump structurein the spectral energy distribution can be fit consistently with
the Synchroton Self-Compton model as shown in Fig. 10 [3].
In 2020 MAGIC published a followup paper, a constraint on
Lorentz invariance violation using the data of the GRB [9].

Broadband Multi-wavelength Properties of M87 during
the 2017 Event Horizon Telescope Campaign
In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology andsize are consistent with theoretical expectations for a weakly
accreting supermassive black hole of mass ∼ 6.5 × 109 M⊙ .
The EHTC also partnered with several international facilities
in space and on the ground, to arrange an extensive, quasisimultaneous multi-wavelength campaign [13]. MAGIC participated in the campaign, together with other major TeV instruments such as H.E.S.S. and VERITAS. During the campaign M87 was captured in a historically low state, and thecore flux dominates over HST-1 at high energies, making it
possible to combine core flux constraints with the more spatially precise very long baseline interferometry data.

Fig. 11. Simultaneous multi-wavelength spectrum of M87 during theEHT campaign in 2017 [13].

Fig. 10. Modelling of the broadband spectra in the time intervals
68-110 s and 110-180 s for GRB 190114C [3].

In 2021 an important paper was published about a hintof detection from GRB 160821B. The GRB is a short GRB
at z = 0.162, which is the closest GRB observed by MAGIC(before 190829A), and it has a kilonova emission reported in
2019. After informed of the report, we have revised the modeling of the TeV flux suggested by MAGIC data, revealingthat the Synchrotron Self-Compton model does not work wellfor this GRB [10]. In ICRC 2021 we have shown other two
important results about long GRBs; a hint of detection froma low-luminosity GRB 201015A [11], and a clear detection
from the farthest VHE GRB 201216C at z = 1.1 [12]. Bothcontributions have been led by Japanese colleagues in MAGIC
Collaboration. These results show an importance of continuing GRB observations to increase the number of detections
both for long and short GRB types.

We present the most complete simultaneous multiwavelength spectrum of the active nucleus to date in Figure 11. Applying two heuristic, isotropic leptonic single-zone
models to provide insight into the basic source properties, weconclude that a structured jet is necessary to explain M87’s
spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ rays can only be produced in the inner jets
(inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons
and secondaries cannot yet be excluded.
The gamma-ray binary HESS J0632+057
The three Cherenkov telescope experiments H.E.S.S.,MAGIC and VERITAS carried out the deepest study of the
gamma-ray binary HESS J0632+057, comprising a total of
450 h of data spanning almost 15 years [14]. This multi-year
campaign is embedded in a multi-wavelength context, which
includes X-ray (Swift-XRT, Chandra, XMM-Newton, NuStarand Suzaku) and optical Hα observations obtained using the
HIDES (High-Dispersion Echelle Spectrograph), deployed on
a 188 cm telescope at Okayama Astrophysical Observatory.
For the first time, the orbital period at TeV energies was determined, yielding a value of 316.7 ± 4.4 days. This solution is38

in agreement with the 317.3 ± 0.7-day period derived fromthe latest Swift-XRT X-ray data set. The ratio of gammaray to X-ray flux underlines the equality or even dominanceof the gamma-ray energy range for the emission of HESS
J0632+057. In the phase-folded light curve (Fig. 12), twowell-differentiated peaks are visible, a dip phase and a broader
plateau phase. The VHE SEDs for all of these phases (except the dip phase, where only upper limits could be derived)
are generally characterized as power-laws, showing no variability in the spectral slope within statistical errors. Only thespectrum measured with VERITAS during the phases 0.2–0.4
favors a power-law with exponential cutoff at 1.75 TeV. The
strong correlation between X-rays and gamma rays suggest
a common origin of the radiation, indicating the existence ofa single population of particles. An indication for an X-ray
source partially not related to the gamma-ray emission was
however found. The lack of correlation between Hα and Xray or gamma-rays might point towards a negligible role ofthe disk of the Be star in the modulation of the non-thermal
emission, but is possibly an effect of the fast variability of Hαcompared with the sparse overlap of the datasets at different
energies. Two outbursts during orbits 9 (2011, January) and17 (2018, January) revealed enhanced gamma-ray and X-ray
emission comparable to the highest flux ever observed fromthis binary. Furthermore, a flux decay of roughly 20 days orless was detected for two orbits. Contemporaneous Hα datataken on 2018, January 25 indicate that the size of the circumstellar disk had increased during those days, suggesting thatthe decretion disk was larger and its structure had changed.
The determination of the orbital geometry of the system isof utmost importance and requires a coordinated multi-year
optical campaign. Finally, the wealth of data presented for
HESS J0632+057 awaits theoretical modelling taking consistently all aspects of the spectral and temporal measurements
into account.
1e 12
VERITAS
HESS
MAGIC

Flux E>350 GeV (1/(cm2s))

86Bibliography[1] The first detection of a GRB at sub-TeV energies;
MAGIC detects the GRB190114C., ATEL # 12390.
[2] Teraelectronvolt emission from the γ-ray burst GRB
190114C, Nature 575, 455 (2019).
[3] Observation of inverse Compton emission from a long
γ-ray burst; MAGIC collaboration et al. Nature 575, 459(2019).
[4] First-time detection of VHE gamma rays by MAGIC
from a direction consistent with the recent EHE neutrino
event IceCube-170922A, ATel #10817.
[5] Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A, Science 12 July 2018, Science.eaat1378.
[6] 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.
[7] Discovery of Very High Energy Gamma-Ray Emission
from the distant FSRQ PKS 1441+25 with the MAGIC
telescopes, ATEL # 7416.
[8] Black hole lightning due to particle acceleration at subhorizon scales, the MAGIC collaboration, Science 346(2014) 1080-1084.
[9] Bounds on Lorentz Invariance Violation from MAGIC
Observation of GRB 190114C, the MAGIC collaboration, PRL 125 (2020) 021301.
[10] MAGIC Observations of the Nearby Short Gamma-Ray
Burst GRB 160821B, the MAGIC collaboration, ApJ
908 (2021) 90.
[11] Observation of a relatively low luminosity long duration
GRB 201015A by the MAGIC telescopes, Y. Suda et al.,PoS ICRC2021 (2021) 797.
[12] Very-high-energy gamma-ray emission from GRB
201216C detected by MAGIC, S. Fukami et al., PoS
ICRC2021 (2021) 788.

4[13] Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign, J.C.
Algaba et al. (the Event Horizon Telescope Collaboration, the Fermi Large Area Telescope Collaboration,
H.E.S.S. Collaboration, MAGIC Collaboration, VERITAS Collaboration, EAVN Collaboration), The Astrophysical Journal Letters, L11 (2021) 911.

2
0
2
0.0

0.2

0.4

0.6

orbital phase

0.8

1.0

Fig. 12. Gamma-ray (>350 GeV) lightcurve as a function of the orbital
phase, assuming an orbital period of 317.3 days (MJD0 =54857.0)).
Vertical error bars indicate statistical uncertainties [14].

[14] Observation of the Gamma-Ray Binary HESS
J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes, the MAGIC + H.E.S.S. + VERITAS
collaborations, ApJ 923 (2021) 241A.

39

TA: Telescope Array Experiment
Spokespersons: S. Ogio1,2 , C.C.H. Jui3
1 : Graduate School of Science, Osaka City University
2 : ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582
3 : Dept. of Physics and Astronomy, University of Utah

Collaborating Institutions:Fig. 13. Akeno atmospheric Cherenkov telescope of 3 m diameter,
located in the Akeno Observatory.

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 13 (Akeno telescope, hereafter) is currently the only atmospheric Cherenkov telescope
located in Japan [1]. We have developed an R & D imaging
camera system with the Akeno telescope since 2009, the purpose 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.
Utilizing some central PMTs of the above system, we havealso observed the optical Crab pulsar in Akeno. The purpose of this study is to investigate the efficient accelerationsite of the Crab Nebula in a microscopic way with simultaneous observations of radio and optical pulses passing throughthe nebula plasma. We performed the first observations of the
optical Crab pulsar in January 2020 after developing a dedicated pulse counting system, which were followed by further
observations in 2021 and 2022 with some improvements inthe system. From about 10 hr data taken in 2021, we founda marginal signal which coincides with the Crab main pulse
phase, at a 3σ level. The 2022 data taken with further improvements are still under analysis.

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).

Chubu Univ., Kasugai, Japan; ERI, Univ. of Tokyo, Tokyo,
Japan; Hiroshima City Univ., Hiroshima, Japan; Hanyang
Univ., Seoul, Korea; ICRR, Univ. of Tokyo, Kashiwa, Japan;
INR, Moscow, Russia; CEICO, Inst. of Phys, Czech Acad. ofSci., Prague, Czech Republic; Kanagawa Univ., Yokohama,Japan; KEK/IPNS, Tsukuba, Japan; Kochi Univ., Kochi,
Japan; Kyoto Univ., Kyoto, Japan; Loyola Univ., Chicago,
USA; Moscow M.V. Lomonosov State University, Moscow,
Russia; NICT, Tokyo, Japan; Osaka Electro-Comm. Univ.,
Neyagawa, Japan; Osaka City Univ., Osaka, Japan; RIKEN,
Wako, Japan; Ritsumeikan Univ., Kusatsu, Japan; Saitama
Univ., Saitama, Japan; Shibaura IT, Tokyo, 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 Nova Gorica, Nova
Gorica, Slovenia; Univ. of Utah, Salt Lake City, USA; Univ.
of Yamanashi, Kofu, Japan; Yonsei Univ., Seoul, Korea
1. 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 140 researchers from USA, Russia,
Korea, Belgium, Czech, Slovenia 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 of Salt Lake City in Utah in USA. The SDs were
deployed on a square grid with 1.2-km spacing, and the SD
array covers an area of approximately 700 km2 . Each SD has
two layers of 1.2-cm-thick scintillator with an area of 3 m2 .
The full operation of SDs started in March 2008. The duty
cycle is approximately 95% on average. One northern FD station at the Middle Drum (MD) site uses 14 refurbished HiRes
telescopes. Two southern FD stations at the Black Rock Mesa
(BRM) and Long Ridge (LR) sites were built newly each with
12 telescopes. The MD FD views 3◦ - 31◦ and the BRM andLR FDs view 3◦ - 33◦ above horizon. All three FD stations
started the observation in November 2007, and have duty cycles of approximately 10%.
TA found evidence for intermediate-scale anisotropy ofarrival directions of cosmic rays with greater than 5.7×1019
eV of energy. With enhanced statistics, it is expected to observe the structure of the hotspot along with other possible ex-40cesses and point sources along with the correlations with extreme phenomena in the nearby universe. TA proposed TAx4
project to quadruple the effective area of the TA SD aperture
including the existing TA SD array by installing additional
500 newly designed SDs on a square grid with wider, 2.08km spacing between each. The layout of TAx4 is shown in
Fig. 14.
The design of SD for the TAx4 experiment is identical tothe design of SD for the TA Low energy Extension(TALE)
experiment, which will be described in detail later. The basic
design is common to TA , but the number and arrangementof wavelength shifter fibers (WLSF), the performance of thephotomultiplier tubes (PMTs), and the wireless LAN module and used protocols are different. In the TAx4 and TALE,
we use a PMT with significantly improved quantum efficiency
and surface uniformity. This has allowed us to reduce the total
length of WLFSs by 67% while maintaining the performanceof the SD as a whole. A new wireless LAN module is used
for the TAx4 and TALE SDs because the one used in TA wasout of production.
Detector installation for TAx4 began in 2019, and 257 SDs
were deployed in February and March 2019. By this installation, the total coverage area with SDs including the original
TA SD array has been increased by a factor of 2.5. The new
SD array started the data acquisition in April 2019. Since the
new SD array is divided into two lobes, two FD stations are
required to overlook each lobe in order to increase the numberof hybrid events for the measurement of Xmax and to confirm
the energy scale. These FD stations consist of refurbished
HiRes telescopes: four telescopes at the MD site and eighttelescopes at the BR site. The TAx4 FD station at the northernsite was constructed and started the stable observation in June
2018. The TAx4 FDs at the southern site were constructed andstarted the observation in 2019. The recent status of TAx4 wasreported in [1]. Fig. 15 show the footprint, the event display
on the FD and the time fit using hybrid reconstruction for atypical TAx4 hybrid event.
The TA Low energy Extension (TALE) enables detailed
studies of the energy spectrum and composition from ∼1016.5
eV upwards. The main target of TALE is to clarify the expected transition from galactic to extra-galactic cosmic rays
and the comparison of the data with Monte Carlo (MC) simulation that takes into account the results of the latest LHC experiments. The TALE detector is located north of the TA site
(MD site). This FD station consists of 10 refurbished HiRes
telescopes. The TALE FD views 31◦ - 59◦ in elevation angle.
The TALE detectors are located at the northern corner of theTA detectors. The TALE FD operation was commenced in
the spring of 2013. The TALE SD array consists of 80 SDs(40 SDs with 400-meter spacing and 40 SDs with 600-meterspacing). The TALE SD array was completed in February
2018 and is in stable operation. The mode energy of TALE
hybrid events is 1016.8 eV. There is a plan to install additional
54 SDs with 200-meter spacing near the TALE FD station toobserve even lower-energy cosmic rays, for which the modeenergy is 1015.5 eV.
The Non-Imaging CHErenkov (NICHE) array is planned
to have graded spacings, ranging from 100 meters to 400 meters. The NICHE Prototype Array consists of 14 counters eachFig. 14. The layout of the proposed TAx4. The array of 507 SDs(red filled circles on the left) is the original TA SD array. There arethree TA FD stations (MD to the north, LR to the west, and BRM
to the east of the TA SD array) with black cross symbols. The array of surface detectors (green) to the north of the TA SD arrayis the TALE SD array. The additional TAx4 500 surface detectors(blue) are located in separate lobes on the northeast and southeast sides of the TA SD array. The 257 large blue filled circles
denote deployed SDs, 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 the northeast and southeast as denoted each by theblue frame of the fan. Purple diamond symbols denote TAx4 communications towers.

with a 3-inch PMT that collects non-imaging air-Cherenkov
light, which started stable observation in May 2018 [2]. The
array is situated 600-1000 m from the TALE FD. NICHE andTALE work together as a hybrid detector for cosmic rays withenergies in the range between 1015 eV and 1018.5 eV.
TA, TAx4 and TALE SD arrays continued regular operations during recent two years even if various activities ceased
due to the spread of COVID-19. On the other hand, FD observations that require on-site operations had been suspended.
However, in June 2020, TA FDs and TAx4 FDs at the MC site
resumed operations. In addition, the operation of TAx4 FDs
at the BRM site and TALE FDs resumed in July 2020 and January 2021, respectively. The TA FDs at the BRM and the LRsite are not operational as of the end of JFY2021.
2. Publications in refereed journals
The results from the TA collaboration published in refereed journal papers in JFY 2021 are reported below.

41

strikes. These variations lasted for several tens of minutes,
and the footprints on the ground were 6 to 24 km in diameterand moved in the same direction as the thunderstorms.
Technical report on the TAx4 surface detectors
The design and the expected performance based on simulations of the TAx4 SDs are reported [4]. The status of SD
construction, the results of calibration, quality checks and thedata acquisition system are also explained in the paper.
3. Other physics results
Update of UHECR spectrum measured by the TA SDs
We presented an updated TA combined energy spectrum
produced using an 11 year TA SD spectrum above 1018.2
eV and the 22 months TALE FD monocular spectrum below
1018.2 eV as shown in Fig. 16 [5].

Fig. 16. The TA combined energy spectrum (black points) composedof 11 years of the TA SD data (blue squares) and 22 months ofTALE monocular data (red circles). The combined TA spectrum
exhibits the knee feature at about 1015.5 eV, the low energy ankle
feature at 1016.22±0.02 eV, the second knee at 1017.04±0.04 eV, theankle at 1018.69±0.01 eV, and the suppression at 1019.81±0.03 eV.
Fig. 15. A typical TAx4 hybrid event. (top) The footprint of an observed air-shower event.The horizontal and vertical axes denote
the SD position IDs in the east-west and north-south directions,
respectively. Circle centers correspond to the SD positions. Circle
areas are proportional to the logarithmic sizes of signals. Colors
denote arrival times at the detectors. (middle) FD event display ofthe event. (bottom) Time verse angle plot using hybrid reconstruction. The red data point is taken from the timing of the closest SD
to the shower core.

Observation of variations in cosmic ray single count rates
during thunderstorms
We have published the first surface detector observationsof the effect of thunderstorms on the development of cosmic
ray single count rate intensity over a 700 square kilometerarea [3]. In this study, we studied the variation of cosmic rayintensity (single count rate) with thunderstorm clouds using
surface detectors and found average levels of 0.5 to 1% anda maximum of 2%. These observations also showed both excesses and deficits. In addition, we found variations that correlated not only with thunderstorms, but also with lightning

In 2020, a new feature called ”shoulder” or ”instep” wasfirst observed by the Pierre Auger Observatory (PAO) [6].
Most of PAO’s observational field of view is concentrated in
the southern hemisphere. It is interesting to see if this newfeature also exists in the Northern Hemisphere. Following
PAO’s report, we combined HiRes I monocular, TA BRM-LR
monocular, and TA SD measurements to search for the newfeature in the Northern Hemisphere.
The result are shown in Fig. 17. The three spectra were
fit into a thrice broken power law. We found the ”shoulder”
feature at 1019.25±0.03 eV, and the statistical significance of thefeature is estimated to be 5.3 standard deviations.
UHECR spectrum measured by the TAx4 SDs
The TAx4 SD array began operation in 2019 as an air
shower array of 257 SDs and has been collecting data steadily
since then. In JFY2021 we reported an observed cosmic rayenergy distribution based on TAx4 SD data obtained from
November 2019 to October 2020, as well as a preliminary cosmic ray energy spectrum based on the energy distribution. (42
E3 J [ eV-2 m-2 sr-1 s-1 ]BR/LR Mono Combined (2020)

1024

18.5

19

19.5

20

20.5

1024

21

18

18.5

19

19.5

log (E/eV)

20

20.5

21

log (E/eV)

10

10

(a)

is much lower than for simultaneous SD array and FD observation events, i.e., events in which both trigger conditions are
satisfied. In ICRC2021, we reported a preliminary result of
the cosmic ray energy spectrum and the depth of maximum
shower development based on hybrid trigger event data obtained from October 2010 to September 2014 [8]. Fig. 19 is apreliminary mean Xmax as a function of primary energy basedon four years of observation data from TA hybrid comparing
with the pure proton and iron MC predictions.

PoS(ICRC2021)341

E3 J [ eV-2 m-2 sr-1 s-1 ]HiRes-I (2008)(b)

E3 J [ eV-2 m-2 sr-1 s-1 ]TA SD (2019) outside of BR / LR Obs. Period

1024

18

18.5

19

19.5

20

20.5

21

log (E/eV)
10(c)Figure 3: Joint fit (red line) to the HiRes and TA spectra, which are measured mostly in the Northern
Fig. 17. Joint fit (red line) to (a) HiRes I monocular, (b) TA BRM–LR
Hemisphere. The spectrum shoulder feature is seen at 1019.25 eV in (a) HiRes I monocular, (b) TA Black
monocular,(c) TA SD
Rock (BR)– Long Ridgeand(LR) monocular,
andspectra.
(c) TA surface detector (SD) spectra. The significance of theshoulder is found by comparing the number of events expected in the absence of the feature (blue line) andthe number of events reported by the experiments. The number of events between 1019.25 and 1019.85 eV,
expected in the absence of the shoulder, is 1269.3, and the number of events measured by HiRes and TA
in that energy range is 1086. The chance probability of this being a Poisson fluctuation is 7.4 ⇥ 10 8 , or,equivalently, 5.3 standard deviations.

see Fig. 18) The data analysis method used for this was almost identical to that of the TA experiment, and the same energy correction factor for the energy scale of 1/1.27, which
was determined in the TA SD array, was used [7].

4Fig. 18. (left) Energy distribution measured by the TAx4 SD array.
(right) A preliminary energy spectrum measured by the TAx4 SD
array together with the TA SD and the PAO SD spectra.

Averaged Xmax measured by TA hybrid
The TA experiment is designed to allow simultaneous observations of UHECR air showers with FDs and SDs, socalled hybrid observations. Hybrid analysis using both FD
and SD data is generally known to provide better reconstruction accuracy of shower geometry than FD or SD analysis
alone. Since hybrid analysis is valid even when informationfrom only one SD is added, the TA SD array implements a
so-called hybrid trigger function, in which the FD trigger issent to the SD array and the SD array data is collected bythis external trigger. The energy threshold for hybrid events

Fig. 19. The preliminary mean Xmax (black) as a function of primaryenergy based on four years of observation data from TA hybrid
comparing with the pure proton (red) and iron (blue) MC predictions.

Cosmic ray composition study with TA SD data
Mass composition is one of the characteristics of cosmic rays that is not only related to the propagation of UHECRs, but also to the cosmic ray acceleration mechanism at thesources and source population. It can be measured from extensive air showers observed on the Earth. The most widelyused method is FD observation to derive the depth of theshower maximum, Xmax, as a composition-sensitive observable. However, FD observations are limited in their statistics by small duty cycles. Therefore, we are conducting study
on the determination of mass composition using only SD array observations. Specifically, we extracted some observables
sensitive to mass composition from the SD data and used amachine learning method, called Boosted Decision Tree, todetermine the mass composition. We used 12 years of datafrom the TA SD array to determine the energy change in the
average of the natural logarithm of the mass number, < ln A >.
The results are shown in Fig. 20 [9].
Update on the large-scale cosmic-ray anisotropy
Following the significant detection of a large-scaleanisotropy in the arrival direction distribution of UHECRs
above 8 EeV in the southern sky by PAO [10], the TA experiment also investigated and reported large-scale anisotropy
in the northern sky using 11 years of TA SD observation
data [11]. In FY2021, we reported the updated results withmore data [12].
The right ascension distribution and the sky map of theresidual intensity are shown in Fig. 21. The energy thresholdis 8.8 EeV, which was determined a priori to account for the
10% systematic difference in energy scale from the PAO. The
PAO result has an amplitude of 4.7% and a phase of 100 degrees, and the updated results of the TA experiment agree withthis within errors. However, it should be noted that the results

43Fig. 20. < ln A > values for 12-year dataset derived with QGSJET II-04(orange) compared with the TA MD hybrid (upper, green) results
and with the Pierre Auger Observatory SD delta results (lower,
blue and red).

Fig. 21. (a) Residual intensities of UHECRs above 8.8 EeV observedin 12 years of TA data. The black curve represents the fitted dipole
to the TA and the red dashed line represents the dipole reported
by PAO. (b) Sky map of residual intensities in the equatorial coordinate. The arrival direction is oversampled by a cylindrical function of radius 45 degrees. The galactic plane (G.P.) and the supergalactic plane (S.G.P.) are indicated by thick and thin dottedlines, respectively. The galactic center (G.C.) is shown as an open
square.

4. Other activitiesof the TA experiment are also consistent with an isotropic distribution due to the small statistical significance; the upper
limit of the amplitude at 99% CL is 7.6%.
Update on the Hotspot and a new excess
Based on 11 years of observations with the TA SD array,we find a new excess in the arrival direction distribution of
UHECRs above 1019.4 eV, with a local significance of 4 σ forthe results of the 20 degree radius oversampling analysis. The
maximum excess appears at the position (RA, dec) = (17.4◦ ,36.0◦ ,) and it appears in the region of the Perseus-Pisces supercluster. The chance probability of finding such an excess
in the direction of the Perseus-Pisces supercluster is estimatedto be 3.5 σ . The result is shown in Fig. 22(a) [13].
In 2014, we reported indications of a cosmic ray arrival
direction above 57 EeV concentrated in the direction of Ursa
Major, or rather a hot spot [14]. We have now updated ouranalysis of this hot spot based on the latest 11 years of observations. The number of events above 57 EeV observed was
179. Of these, 40 events came from within a 25-degree radius
centered at (RA, dec) = (144◦ , 40.5◦ ), which is 5.1 σ significant compared to the expected value of 14.6 from an isotropic
distribution as shown in Fig. 22(b) [13]. The post trial significance is 3.2 σ , and also we conclude that the increase rate of
the events inside the hotspot circle is consistent with a constant within ±1 σ fluctuation.

Other research results and activities reported at
ICRC2021, JPS meetings and/or other conferences arelisted and introduced below.
• Search for UHECR sources considering the deflection
by the galactic magnetic field [15]• Monocular energy spectrum using the TAx4 FDs [16]• Energy spectrum and composition in the second knee
region measured with the TALE hybrid detector [17]• Energy spectrum and the shower maxima measured
with the NICHE detectors [18]• Anisotropy search in UHECR spectrum [19]• Measurement of the p-air cross section with TA BRM,
LR, SD array in hybrid mode [20]• Performance evaluations of the TALE SD array [21]• Measurement of optical properties of atmospheric fluorescence telescope using UAV-mounted light source• Joint analysis of the energy spectrum measured at PAO
and TA [22]44

in FY2022.

Fig. 23. The layout of TALE infill SDs. Purple squares denote theplanned locations of 54 SDs for the infill array. The array controlledwith a host PC at the TALE FD station indicated with the filled
circle.

Fig. 22. Sky maps in equatorial coordinates. The color scheme indicates the Li-Ma significance and shows the excess (red) or deficit(blue) of events compared to isotropy. The positions of maximum
excesses are marked with the black diamonds. An intermediate
angular scale of 20◦ -radius circles was used for oversampling analysis for different energy thresholds. The energy threshold for each
map is (a) 1019.4 eV and (b) 57 EeV. Note that the right ascensionof 0◦ is at the center of the sky map.

• Joint correlation study of directional correlation between the arrival direction of UHECRS and nearby
galaxies [23]• Joint analysis of the large scale anisotropy measured at
PAO and TA [24]5. Future Plans
TALE infill SD array
We are planning to lower the energy threshold of the
TALE hybrid observations by installing additional SDs athigher densities between the TALE SD array and the TALE
FD station. 54 SDs will be placed within 2 km of the TALE
FD station with 100 m or 200 m intervals. The SDs will be
deployed to cover 0.32 km2 . A map of the planed installationsite is shown in Fig. 23. From this arrangement the expectedmode energy of the infill SD array is 1015.3 eV, and 1015.8 eV
for the infill hybrid observation.
Although we have already obtained permission to use all
installation points and have completed preparation of hardware, the transportation of equipment from Japan and installation work in the field has been suspended due to the spreadof COVID-19. The installation is scheduled to be completed

Research and development of next-generation detectors
On site research and development activities with the prototypes of the next-generation UHECR detectors at the TA BRM
site such as EUSO-TA [25] for the JEM-EUSO mission [26]in space, and FAST [27] and CRAFFT [28] as low-cost widearea ground detectors were suspended in 2021 due to COVID19. However, activities in data analysis and instrument developments have been maintained, and each group has reported
its results.
Discussions are also underway for the next-generation observatories, to measure the properties of the highest-energy
cosmic rays with unprecedented accuracy in the future suchas TA2 and GCOS (The Global Cosmic Ray Observatory).
In May 2021, he first international workshop of GCOS was
held [29].

Bibliography[1] R. Abbasi et al., “Current status and prospects of surface
detector of the TAx4 experiment”, PoS(ICRC2021), 203,
2021.
[2] Y. Omura et al., “TA experiment 381: jNICHE - Energy
spectrum and composition analysis of cosmic rays measured with the NICHE detectors at the TA site”, 2022 Annual JPS Meeting.
[3] R.U. Abbsi et al., “Observation of Variations in Cosmic
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Rev. D 105, 062002, 2022.
[4] R.U. Abbsi et al., “Surface detectors of the TAx4 experiment”, NIM A 1019, 165726, 2021.
[5] D. Ivanov et al., “Recent measurement of the Telescope
Array energy spectrum and observation of the shoulder feature in the Northern Hemisphere”, PoS(ICRC2021), 341,
2021.

45

[6] A. Aab et al., “Features of the Energy Spectrum of Cosmic Rays above 2.5 × 1018 eV Using the Pierre Auger Observatory”, Phys. Rev. Lett., 125, 121106, 2020.
[7] H. Jeong et al., “Reconstruction of Air Shower Events
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[8] H. Shin et al., “The measurements of the cosmic rayenergy spectrum and the depth of maximum shower development of Telescope Array Hybrid trigger events”,
PoS(ICRC2021), 305, 2021.
[9] Y. Zhezher et al., “Cosmic-ray mass composition with the
TA SD 12-year data”, PoS(ICRC2021), 300, 2021.
[10] A. Aab et al., “Observation of a Large-scale Anisotropy
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[11] R.U. Abbasi et al., “Search for Large-scale Anisotropy
on Arrival Directions of Ultra-high-energy Cosmic Rays
Observed with the Telescope Array Experiment”, Astrophys. J. Lett, 898, L28, 2020.
[12] T. Fujii et al., “Update on the large-scale cosmic-ray
anisotropy search at the highest energies by the Telescope
Array Experiment”, PoS(ICRC2021), 291, 2021.
[13] J. Kim et al., “Hotspot Update, and a new Excess of
Events on the Sky Seen by the Telescope Array Experiment ”, PoS(ICRC2021), 328, 2021; R.U. Abbasi et al.,”Indications of a Cosmic Ray Source in the Perseus-PiscesSupercluster”, arXiv:2110.14827, 2021
[14] R.U. Abbasi et al., “Indications of intermediate-scaleanisotropy of cosmic rays with energy greater than 57 EeV
in the northern sky measured with the surface detector of
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L21, 2014.
[15] R. Higuchi et al., “Effects of Galactic magnetic fieldon the UHECR anisotropy studies”, PoS(ICRC2021), 358,
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thesis (U. Tokyo), 2022
[16] M. Potts et al., “Monocular Energy Spectrum using
the TAx4 Fluorescence Detector”, PoS(ICRC2021), 343,
2021.
[17] K. Fujita et al., “Cosmic Ray Composition in the Second
Knee Region as Measured by the TALE Hybrid Detector”,
PoS(ICRC2021), 353, 2021.
[18] Y. Omura et al., “Energy spectrum and the shower maxima of cosmic rays above the knee region measured with
the NICHE detectors at the TA site”, PoS(ICRC2021), 329,
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[19] T. Nonaka et al., “Anisotropy search in the Ultra HighEnergy Cosmic Ray Spectrum in the Northern Hemisphereusing latest data obtained with Telescope Array surface detector”, PoS(ICRC2021), 344, 2021.

[20] R. Abbasi et al., “Measurement of the Proton-Air Cross
Section with Telescope Arrays Black Rock, Long Ridge,
and Surface Array in Hybrid Mode”, PoS(ICRC2021), 395,
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[21] K. Sato et al., “Cosmic ray energy spectrum in the
2nd knee region measured by the TALE-SD array”,
PoS(ICRC2021), 362, 2021.
[22] Y. Tsunesada et al., “Joint analysis of the energy
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PoS(ICRC2021), 240, 2021.
[26] M. Bertaina “An overview of the JEM-EUSO program
and results”, PoS(ICRC2021), 406, 2021.
[27] T. Fujii, “Latest results of ultra-high-energy cosmic ray measurements with prototypes of the Fluorescence detector Array of Single-pixel Telescopes (FAST)”,
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CRAFFT project for the next generation UHECR observationy”, PoS(ICRC2021), 408, 2021.
[29] GCOS
workshop,May
https://agenda.astro.ru.nl/event/18/

17-21,

2021;

Tibet ASγ Experiment[Spokesperson: M. Takita]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba 277-8582

1.

Experiment

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 square

46

Tibet III Air Shower Array (2003)

South

Displacement (degrees)

36,900 m2

North

Energy (TeV) / Z
0.1

2.94 4.20 6.46 11.4

21.6

45.4

0
−0.1
−0.2
−0.3
−0.4

Data
MC

−0.5

Data Best fit

−0.6
−0.7
10

100

1000

Σρ FT

15 m

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

789 detectors

Fig. 1. Schematic view of Tibet III.

grid 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 (approxi-

mately 0.5 degrees in diameter) in cosmic rays. The deficit
map of cosmic rays around the Moon demonstrates the angular resolution to be around 0.9◦ at a few TeV for the Tibet III
array. The pointing error is estimated to be better than ∼0.01◦ ,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.
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.
After the success of the prototype underground muon detector, we added a large underground muon detector (MD) array and the experimental configuration was changed startingfrom February 2014. The current array consists of 597 plastic scintillation detectors with an area of 0.5 m2 as shown bysmall dots in Fig. 4, covering an area of 65,700 m2 . The Tibet MD array (3,400 m2 in total area) consists of 64 waterCherenkov-type detectors located at 2.4 m underground of
the AS array as shown by open squares in Fig. 4. Each detector is a waterproof concrete cell filled up with water of
1.5 m in depth, 7.35 m × 7.35 m in area, equipped with a 20inch-diameter downward-facing photomultiplier tube (PMT)47

Energy (TeV) / Z
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

150

100
100

1000

Σρ FT
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.

on the ceiling. The inner walls and floor have a white Tyvek
sheet lining to efficiently gather catoptric water Cherenkov
light. The electromagnetic component is shielded by the soil
overburden which corresponds to ∼19 radiation lengths. Theenergy threshold for muons is approximately 1 GeV. A photon induced air shower has much less muons compared witha cosmic-ray induced one. The Tibet MD array enables us
to significantly discriminate a cosmic-ray background eventfrom a photon signal by means of counting the number ofmuons in an air shower.

Physics Results

50
50
0

0

-50

-100

-50

-150
-100
-200
-150
-150

(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 incosmic rays.

We will introduce a part of the results obtained in thisfiscal year[2],[3].
We successfully detected gamma-rays in the 100 TeV region from Cygnus OB1 and OB2 regions.
A photon induced air shower contains much less muonscompared with a cosmic-ray induced air shower. Therefore,

-100

-50

0

50

100

150

Relative position [m]

(b)

102

10

1

10-1
1

Our current research theme is classified into 4 categories:

Relative timing [ns]100

Relative position [m]

−0.7
10

2.

150

(a)

Particle density [ particles/m 2 ]West

Displacement (degrees)

East

0.1

the MD array enables us to distinguish a photon signal eventfrom a cosmic-ray induce background event. Using the AS
and MD arrays, we analyzed the air shower data and muon
data during 719 live days from February 2014 to May 2017.
Figure 4 (a) and (b) show the event display and the lateral distribution, respectively, of a typical photon-like event
observed with E = 251 TeV from the Crab nebula[4]. The
total uncertainty of the energy ∆E is defined as the quadratic
sum of the absolute energy-scale error (12%) [1] and the energy resolution estimated by using MC events with the same
θ and core distance from the array center (R). The ∆E of the
251 TeV photon-like event is estimated to be +45−43 TeV.

102
10Distance from the shower axis [ m ]103

Fig. 4. From Ref.[4], references therein. (a) An event display of theobserved photon-like AS of energy 251 TeV. The size and color ofeach circle represent the logarithmic particle density and the relative timing in each detector, respectively. The arrow head anddirection indicate the AS core location and incident direction, respectively. Dots and open squares denote scintillation detectors
and underground MDs, respectively. The enclosed area by thedashed line indicates the fiducial area of the AS array. (b) Lateral distribution of the photon-like shower event in panel (a). The
solid circles and curve show the experimental data and fitting result by the Nishimura-Kamata-Greisen (NKG) function to the data
recorded by detectors more than 10 m apart from the AS axis. The
dashed curve and open circles are an extrapolation of the NKG
function fitting and the unused data within 10 m from the AS axis.

We reconstruct the arrival direction of an air shower bymeans of the relative timing information recorded at eachscintillation detector. The color scale and circle size in Fig. 4(a) stand for the relative timing (t) and the number of particle
density (ρ) in an air shower. In the first place, we estimate theair shower core location weighted by ρ. The arrow head position and direction in the figure indicate the estimated core position and azimuthal direction. The t’s in the air shower front

48

are fitted by a conical shape, and its cone angle is optimizedby the MC simulation depending on the air shower size. The
angular resolutions (50% containment) are estimated to be approximately 0.5◦ and 0.2◦ for 10 TeV and 100 TeV photon,
respectively.
The secondary particles in an air shower deposit energy,
which is proportional to ρ, in a scintillator. The ρ at eachdetecto r is defined as the PMT output charge divided by thesingle particle peak which is monitored every 20 minutes tocorrect temperature dependence of each detector gain. Theenergy of each air shower is reconstructed using the lateral
distribution of ρ, above 10 TeV. Figure 4 (b) shows an example of the lateral distribution of ρ. As an energy estimator, we
use S50[5], which is defined as ρ at a distance of 50 m fromthe air shower axis in the best-fit Nishimura-Kamata-Greisen(NKG) function. The energy resolutions with S50, which depend on air shower core location and zenith angle, are roughly
estimated to be 40% at 10 TeV and 20% at 100 TeV. On theother hand, the energy below 10 TeV was estimated directlyfrom Σρ corresponding to the sum of of the particle density
measured by each scintillatoin detector, as the number of hit
detectors is too low to fit S50. The absolute energy scale uncertainty is estimated to be 12 by the westward displacementof the Moon shadow position.
The electromagnetic cascade in the air shower is strongly
suppressed by the bremsstrahlung process in the overburden,while muons and a part of hadronic components penetrate into
the underground MD array. The number of detected particles
at an MD (Nµ ) is defined as the output charge divided by thesingle muon peak which is monitored every 20 minutes. The
sum of detected particles in all MDs (ΣNµ ) is used as a parameter to separate between photon and cosmic-ray induced
air showers.
An event trigger is issued at any 4-fold coincidence with
detectors recording more than 0.6 particles within the area
enclosed by the dashed lines in Fig. 4. At air shower energies smaller than 10 TeV, we employ the same selection criteria and estimate the event energy as in the previous work[1],except for the MD selection criterion. At air shower energies greater than 10 TeV, we employ the following event
selections: (1) zenith angle of arrival direction (θ ) shouldbe θ < 40◦ ; (2) number of available detectors for the air
shower reconstruction should be ≥ 16; (3) among 6 detectors recorded with the largest ρ’s, 5 should be contained in
the fiducial area enclosed by the dashed lines in Fig. 4; (4)log(S50) should be > −1.2; (5) age parameter (s) in the bestfit NKG function should be 0.3 < s < 1.3. (6) ΣNµ should beΣNµ < 0.15(Σρ)2.8 or ΣNµ < 3.6 × 10−4 (Σρ)1.4 , where Σρ isthe sum of particle density measured by the AS array. The MD
cut is optimized for photon detection by our MC simulation.
To estimate the background contribution from cosmic
rays, we adopt the Equi-Zenith Angle method which is thesame as in our previous works [1]. The background counts
are estimated from the number of events averaged over 20 offsource bins of the same size as the on-source bin located at theequi-zenith angle.
√ The radius of the on/off-source bin is set to
Rsw (Σρ) = 6.9/ Σρ (deg.). In order to efficiently extract signals in the higher energy region at low background level, thelower limit of Rsw is set to 0.5◦ , which corresponds to ∼ 90%

containment of photons above 100 TeV.
To generate air shower events in the atmosphere, Employed are the CORSIKA code v7.4000 with EPOS-LHC for
the high-energy hadronic interaction model and FLUKA codev2011.2b for the low-energy hadronic interaction model. The
very-high-energy photons at the top of atmosphere are inputted, assuming the power-law spectrum with a differential
index −2.9 above 0.3 TeV. The generated secondary particles
in an air shower are fed into the detector response simulation for the AS array developed based on the GEANT4 code
v4.10.00. The MC simulation for the MD array considering
the overburden has been developed based on the GEANT4
code. The electromagnetic and hadronic cascades by the secondary particles in the overburden, as well as the ray tracing of
Cherenkov lights emitted in the water cells are simulated by
the GEANT4 code. The number of photoelectrons detectedin the 20-inch PMT is converted to Nµ referring to the single muon peakin each cell. The muonq cut is determined tomaximize the figure of merit NγMC /DATA , where
NγMC + NCRDATA denote the expected number of photons by
NγMC and NCR
the MC and the number of background events in the data after the muon cut. The photon-like events are defined as theremaining events after the muon cut.
Figure 5 (b) shows a significance map above 10 TeV in
the direction of Cygnus OB1 obtained by the Tibet ASγ experiment. The centroid of gamma-ray emissions is estimatedat (R.A., Dec.) =(304.99◦ ± 0.11◦ 36.84◦ ± 0.08◦ ) with the
pre-trial (post-trial) detection significance of 6.7σ (6.2σ ). We
name this source TASG J2019+368. The centroid of TASG
J2019+368 is consistent with that reported by HAWC [20]within the 1σ level, and by VERITAS [11] within the 2σ
level. The pulsar PSR J2021+3651, located 0.23◦ east of
the TASG J2019+368 centroid, has a nebula extending westwards from the pulsar, PWN G75.2+0.1, which is coincident
with the location of TASG J2019+368. Figure 6 (b) shows
the angular distribution of the events observed above 10 TeV.
The experimental data can be fitted with a Gaussian function
with a source extension of σEXT = 0.28◦ ± 0.07◦ above 10
TeV, consistent with the extension reported by VERITAS [11](HAWC [19]) at the 2.1σ (0.3σ ) level. The χ 2 /ndf of the fitting is 49.1/38. Figure 7 (b) shows the differential gamma-ray
energy spectrum of TASG J2019+368, which is in good agreement with the HAWC spectrum and makes a reasonable connection with the VERITAS spectrum in 2014. The measured
spectrum is expressed either as dF/dE = N0 (E/40 TeV)−Γ
with N0 = (10.6 ± 1.3) × 10−16 TeV−1 cm−2 s−1 and Γ =2.70 ± 0.13(χ 2 /ndf = 10.4/5), or assuming an exponential
cutoff as dF/dE = N0 (E/40TeV)−Γ exp(E/Ecut ) with N0 =(3.6 ± 2.0) × 10−15 TeV−1 cm−2 s−1 , Γ = 1.6 ± 0.5 and Ecut =44 ± 21 TeV (χ 2 /ndf = 3.0/4).

Figure 5 (a) shows a detection significance map aroundthe gamma-ray source detected by the Tibet ASγ experimentwith photon energies above 10 TeV in the direction of Cygnus
OB2. The sky is gridded in 0.1◦ × 0.1◦ pixels and the significance value of each pixel calculated according to [27] issmoothed by a circular search window of radius, Rw centeredat the pixel. Assuming a symmetrical two-dimensional Gaus-

49

Fig. 5. From Ref.[2], references therein. The gamma-ray emission
source detected above 10 TeV in the directions of Cygnus OB2 for(a) and Cygnus OB1 for (b), respectively. The figure (a) is a significance map around the source, smoothed by search windows.
The point spread function (PSF) is shown in the inset figure. The
red-filled star with a position error circle is the centroid of TASG
J2032+414 in the Cygnus OB2 region obtained by the Tibet ASγ
experiment, while the magenta open cross is the centroid of VER
J2031+415, and the blue asterisk is that of HAWC J2031+415.
The green-filled diamonds show Fermi-LAT sources. The blue
open triangle indicates the centroid of MAGIC J2031+4134. The
green-filled diamond coincident with our gamma-ray emission centroid is the pulsar PSR J2032+4127. The sky-blue contours indicate 1420 MHz radio emissions provided by the Canadian Galactic Plane Survey, and the pink contours indicate 24 µm infraredemissions by the Cygnus-X Spitzer Legacy Survey [30, 31]. The
figure (b) is a significance map around the source. The red-filledstar with a position error circle is the centroid of TASG J2019+368
in the Cygnus OB1 region obtained by the Tibet ASγ experiment.
The white open circles are NuSTAR X-ray sources [32], and thegray-filled inverted triangles are Wolf-Rayet stars [33]. The green–
filled diamond located at 0.23◦ east of our emission centroid isthe pulsar PSR J2021+3651. The magenta open cross located at(R.A., Dec.) = (303.99◦ , 37.21◦ ) is another VERITAS source VER
J2016+371 [11]s, which is not detected significantly in this work.

sian distribution for the gamma-ray excess, we fit the events
within the 4◦ ×4◦ region around the source using the unbinned
maximum likelihood method. The centroid of gamma-ray
emissions detected at the pre-trial (post-trial) detection significance of 5.3σ (4.7σ ) above 10 TeV is estimated at (R.A.,
Dec.) = (308.04◦ ± 0.08◦ , 41.46◦ ± 0.06◦ ). We name thisFig. 6. From Ref.[2], references therein. The figure (a) shows the distribution of the number of events observed with photon energies
above 10 TeV as a function of the square of the opening angle
between the estimated arrival direction and the centroid of TASG
J2032+414 in the Cygnus OB2 region, while the figure (b) shows
the distribution of the number of events observed with photon energies above 10 TeV as a function of the square of the opening
angle between the estimated arrival direction and the centroid of
TASG J2019+368 in the Cygnus OB1 region. The red-filled circles
are the experimental data, with the best fit Gaussian function indicated by the solid line. The blue histogram is the distribution of
events expected by the MC simulation assuming a point-like gamma-ray source.

source TASG J2032+414. The location of TASG J2032+414
is in good agreement with that of the pulsar PSR J2032+4127
and consistent with that of HAWC J2031+415 [28] at the 1.7σ
level, while it appears to deviate from that of TeV J2032+4130
reported in [29] at the 2.8σ level.
Figure 6 (a) shows the distribution of the number of events
observed with photon energies above 10 TeV as a function of
the square of the opening angle φ between the estimated arrival direction and the centroid of TASG J2032+414. To estimate a possible source extension, we perform the χ 2 fitting
2
2of the data with the function A exp[−φ 2 /2(σPSF+ σEXT)] +NB G where A and σEXT are two fitting parameters andσPSF = 0.36◦ and NBG = 224.5 are the point spread function (PSF) of our instrument above 10 TeV and the numberof background events estimated from the background cosmicray data, we get σEXT = 0.00◦ ± 0.14◦ , which is consistent
with that obtained from the maximum likelihood fitting described above. The χ 2 /ndf of the fitting is 33.8/38. Witha large error of 0.14◦ , the σEXT above 10 TeV does notindicate whether TASG J2032+414 is extended or not even50

Fig. 7. From Ref.[2], references therein. The figure (a) shows
the differential gamma-ray energy spectra of TASG J2032+414
in the Cygnus OB2 region with 95 % C.L. upper limits measured by this work (red filled squares/arrows). The blue-filled circles/arrows (sky-blue open circles) show the gamma-ray spectrum reported by VERITAS in 2018 (2014) [10, 11], the grayfilled triangle by HAWC [16], and the dark-green filled circles by
ARGO [14]. Additionally, the gold-filled diamonds are reported byFermi-LAT [11], the green open squares by MAGIC [9], and thepink pentagons/arrow by HEGRA [7]. The figure (b) shows thedifferential gamma-ray energy spectra of TASG J2019+368 in the
Cygnus OB1 region with 95% C.L. upper limits measured by thiswork (red-filled squares/arrows). The blue-filled circles(sky-blueopen circles) show the gamma-ray spectrum reported by VERITAS in 2018 (2014) [11, 21], the gray open triangles/arrow by
HAWC [16, 19], and dark-green filled arrow by ARGO [14].

if it is consistent with the previous measurements at multiTeV energies by IACTs, ARGO and HAWC within the 2σ
level [7, 9, 11, 14, 28, 29]. Figure 7 (a) shows the differential energy spectrum of TASG J2032+414. Although
there is a discrepancy in flux at multi-TeV energies, our flux
data points above 10 TeV are consistent with the previous
measurements by IACTs if the spill-over of gamma-ray signals outside their integration radius is taken into account.
Our spectrum from 10 TeV to 120 TeV can be expressedby a simple power-law as dF/dE = N0 (E/40 TeV)−Γ whereN0 = (4.13 ± 0.83) × 10−16 TeV−1 cm−2 s−1 is the differential
gamma-ray flux at 40 TeV and Γ = 3.12 ± 0.21 is the spectral
index (χ 2 /ndf = 1.6/4).
For more details on the gamma rays in the 100 TeV region
from Cygnus OB1 and OB2, see [2].
Figure 8 shows the distribution of sub-PeV gamma-raylike events in the equatorial coordinates. To single out diffuse
gamma-ray signals from an overwhelming amount of cosmicray background events, we employ a tighter muon cut than in
the point-like source analysis. With the tighter muon cut, wehave succeeded in reducing the cosmic-ray background events
down to approximately 10−6 above 398 TeV. As a result, 38
gamma-ray-like events survive after the cut above 398 TeV,
and 23 gamma-ray-like events are observed along the galactic disk within |b| < 10◦ against low (2.73) cosmic-ray background events [3] estimated by real cosmic-ray data.
The high galactic-latitude events (|b| > 20◦ ) are assumed
to be the cosmic-ray background events in this analysis. The
highest-energy event among the 23 events along the galactic
plane has unprecedentedly as high as 957 TeV, nearly 1 PeV.
Surprisingly, the observed gamma rays above 398 TeV do notcome from any known TeV gamma-ray objects as is shown in
Fig 9, but are widely spread over the galactic disk [3]. These
spatially spread gamma rays are thought to have been produced by the interaction of cosmic-ray protons with the interstellar gas in our galaxy. High-energy electrons would interact
with low-energy photons filled in our galaxy, and also produce
ultra-high-energy (UHE) gamma rays (> 100 TeV). However,UHE gamma rays of electron origin should be generated very
close to a source and confined nearby it typically within 1
degree, as the electrons lose their energy rapidly and cannot
travel far from their origin. From Fig. 9, the electron origin
is rejected. Furthermore, the measured fluxes in the UHE region [3] are in reasonable agreement with a recent model [35]based on the hadronic cosmic-ray interactions, where UHE
diffuse galactic gamma rays of electron origin is estimated tobe negligible compared to those of cosmic-ray origin.
This is the first experimental evidence that cosmic rays
were/are accelerated to PeV energies in our galaxy. This
also gives conclusive evidence that cosmic-ray PeVatrons existed/exist in the past and/or present galaxy. The existence ofPeVatrons in our galaxy has been a matter of discussion for
sixty years since the discovery of cosmic-ray Knee. For the
first time, the existence is verified by the Tibet ASγ experiment. This work is also the first experimental proof of theoretical models that cosmic rays accelerated up to the “Knee”
energy region are trapped by the magnetic field in our galaxy,forming a pool of cosmic rays.
In addition, four events out of the 23 gamma-ray-likeevents located within |b| < 10◦ above 398 TeV concentratein the Cygnus Cocoon region (around l = 80◦ , b = +1◦ in
Fig. 8), which is a very promising candidate for a PeVatron [15, 36, 28]. This work provides further strong evidence
that the Cygnus Cocoon is a Pevatron.

51

Number of events

101

100

10-1

10-2

Isotropic MC
Diffuse Model MC
Data >398TeV
0.1

1
10
Distance to the closest TeV source [deg.]

Fig. 9. Figure from [3]. The distribution of the angular distance between the arrival direction of each observed gamma-ray like eventwith E > 398 TeV and the direction of its closest known TeV source
listed in the TeV gamma-ray catalog [34]. The red solid circles
show the observed data, while the dashed and solid histograms
display the MC results expected from the isotropic event distribution and the diffuse gamma-ray model [35], respectively, to be observed with our geometrical exposure.

Fig. 8. Figure from [3]. he arrival direction of each gamma-ray likeevent observed with (a) 100 < E < 158 TeV, (b) 158 < E < 398 TeV,
and (c) 398 < E < 1000 TeV, respectively, in the equatorial coordinate. The blue solid circles show arrival directions of gamma-ray
like events observed by the Tibet AS+MD array. The area of eachcircle is proportional to the measured energy of each event. The
red plus marks show directions of the known galactic TeV sources(including the unidentified sources) listed in the TeV gamma-ray
catalog [34]. The solid curve indicates the galactic plane, while theshaded areas indicate the sky-regions outside the Field of View of
the Tibet AS+MD array.

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 Research Plans with MD(1) Chemical composition of primary cosmic rays making theknee in the all-particle energy spectrum

Fig. 10. YAC2 set up at Yangbajing.

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
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. 10, 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[37] is very effective to reduce cosmic-ray background. We added a large (∼3,400 m2 ) underground water

52

Cherenkov muon detector (Tibet-MD) array under the present
Tibet air shower array (Tibet AS ∼65,700 m2 ). By Tibet AS+ MD, we aim at background-free detection of celestial pointsource 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 northernsky. The measurement of cut off energies in the energy spectra of such gamma rays in the 100 TeV region may contributesignificantly to understanding of the cosmic- ray acceleration
limit at the acceleration sites. Above 100 TeV, the angularresolution of Tibet AS with 2-steradian wide field of view is0.2◦ and the hadron rejection power of Tibet MD is typically
1/1000 for a point-like source.
Development of Monte Carlo simulation is under way forcomparison with real data. Various analysis tools are also extensively being developed.
As is described in the text, we succeeded in the first detection of photons with energies above 100 TeV from Crabin the world, opening a new energy (sub-PeV) window in astronomy. We have also succeeded in detection of gamma raysin the 100 TeV region from G106.3+2.7, Cygnus OB1 andOB2 regions as well as sub-PeV diffuse gamma rays from thegalactic disk. Analyses are under way in search of other subPeV gamma-ray sources.

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[4] “First Detection of Photons with Energies beyond 100
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53

The Tibet ASγ Collaboration
M. Amenomori1 , S. Asano2 , Y. W. Bao3 , X. J. Bi4 , D. Chen5 ,T. L. Chen6 , W. Y. Chen4 , Xu Chen4 , Y. Chen3 , Cirennima6 ,S. W. Cui7 , Danzengluobu6 , L. K. Ding4 , J. H. Fang4,8 ,K. Fang4 , C. F. Feng9 , Zhaoyang Feng4 , Z. Y. Feng10 ,Qi Gao6 , A. Gomi11 , Q. B. Gou4 , Y. Q. Guo4 , Y. Y. Guo4 ,H. H. He4 , Z. T. He7 , K. Hibino12 , N. Hotta13 , Haibing Hu6 ,H. B. Hu4 , K. Y. Hu4,8 , J. Huang4 , H. Y. Jia10 , L. Jiang4 ,P. Jiang5 , H. B. Jin5 , K. Kasahara14 , Y. Katayose11 , C. Kato2 ,S. Kato15 , T. Kawashima15 , K. Kawata15 , M. Kozai16 ,D. Kurashige11 , Labaciren6 , G. M. Le17 , A. F. Li18,9,4 ,H. J. Li6 , W. J. Li4,10 , Y. Li5 , Y. H. Lin4,8 , B. Liu19 , C. Liu4 ,J. S. Liu4 , L. Y. Liu5 , M. Y. Liu6 , W. Liu4 , X. L. Liu5 ,Y.-Q. Lou20,21,22 , H. Lu4 , X. R. Meng6 , Y. Meng4,8 ,K. Munakata2 , K. Nagaya11 , Y. Nakamura15 , Y. Nakazawa23 ,H. Nanjo1 , C. C. Ning6 , M. Nishizawa24 , M. Ohnishi15 ,S. Okukawa11 , S. Ozawa25 , L. Qian5 , X. Qian5 , X. L. Qian26 ,X. B. Qu27 , T. Saito28 , Y. Sakakibara11 , M. Sakata29 ,T. Sako15 , T. K. Sako15 , J. Shao4,9 , M. Shibata11 ,A. Shiomi23 , H. Sugimoto30 , W. Takano12 , M. Takita15 ,Y. H. Tan4 , N. Tateyama12 , S. Torii31 , H. Tsuchiya32 ,S. Udo12 , H. Wang4 , Y. P. Wang6 , Wangdui6 , H. R. Wu4 ,Q. Wu6 , J. L. Xu5 , L. Xue9 , Z. Yang4 , Y. Q. Yao5 ,J. Yin5 , Y. Yokoe15 , N. P. Yu5 , A. F. Yuan6 , L. M. Zhai5 ,C. P. Zhang5 , H. M. Zhang4 , J. L. Zhang4 , X. Zhang3 ,X. Y. Zhang9 , Y. Zhang4 , Yi Zhang33 , Ying Zhang4 ,S. P. Zhao4 , Zhaxisangzhu6 and X. X. Zhou10
1 Departmentof Physics, Hirosaki University, Hirosaki
036-8561, Japan.
2 Department of Physics, Shinshu University, Matsumoto
390-8621, Japan.
3 School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China.
4 Key Laboratory of Particle Astrophysics, Institute of HighEnergy Physics, Chinese Academy of Sciences, Beijing
100049, China.
5 National Astronomical Observatories, Chinese Academy of
Sciences, Beijing 100012, China.
6 Department of Mathematics and Physics, Tibet University,
Lhasa 850000, China.
7 Department of Physics, Hebei Normal University, Shijiazhuang 050016, China.
8 University of Chinese Academy of Sciences, Beijing
100049, China.
9 Institute of Frontier and Interdisciplinary Science and
Key Laboratory of Particle Physics and Particle Irradiation(MOE), Shandong University, Qingdao 266237, China.
10 Institute of Modern Physics, SouthWest Jiaotong University, Chengdu 610031, China.
11 Faculty of Engineering, Yokohama National University,
Yokohama 240-8501, Japan.
12 Faculty of Engineering, Kanagawa University, Yokohama
221-8686, Japan.
13 Faculty of Education, Utsunomiya University, Utsunomiya
321-8505, Japan.
14 Faculty of Systems Engineering, Shibaura Institute ofTechnology, Omiya 330-8570, Japan.
15 Institute for Cosmic Ray Research, University of Tokyo,
Kashiwa 277-8582, Japan.
16 Institute of Space and Astronautical Science, Japan
Aerospace Exploration Agency (ISAS/JAXA), Sagamihara
252-5210, Japan.
17 National Center for Space Weather, China Meteorological
Administration, Beijing 100081, China.
18 School of Information Science and Engineering, Shandong
Agriculture University, Taian 271018, China.
19 Department of Astronomy, School of Physical Sciences,
University of Science and Technology of China, Hefei
230026, China.
20 Department of Physics and Tsinghua Centre for Astrophysics (THCA), Tsinghua University, Beijing 100084,
China.
21 Tsinghua University-National Astronomical Observatoriesof China (NAOC) Joint Research Center for Astrophysics,
Tsinghua University, Beijing 100084, China.
22 Department of Astronomy, Tsinghua University, Beijing
100084, China.
23 College of Industrial Technology, Nihon University,
Narashino 275-8576, Japan.
24 National Institute of Informatics, Tokyo 101-8430, Japan.
25 National Institute of Information and Communications
Technology, Tokyo 184-8795, Japan.
26 Department of Mechanical and Electrical Engineering,
Shangdong Management University, Jinan 250357, China.
27 College of Science, China University of Petroleum, Qingdao 266555, China.
28 Tokyo Metropolitan College of Industrial Technology,
Tokyo 116-8523, Japan.
29 Department of Physics, Konan University, Kobe 658-8501,
Japan.
30 Shonan Institute of Technology, Fujisawa 251-8511, Japan.
31 Research Institute for Science and Engineering, Waseda
University, Tokyo 162-0044, Japan.
32 Japan Atomic Energy Agency, Tokai-mura 319-1195,
Japan.
33 Key Laboratory of Dark Matter and Space Astronomy,
Purple Mountain Observatory, Chinese Academy of Sciences,
Nanjing 210034, China.
(The Tibet ASγ Collaboration as of 2021)

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

Cosmic rays are supposed to be accelerated up to theknee energy (PeV) region at supernova remnants (SNRs),
star formation regions and PWNe in our galaxy. Therefore,
we naturally expect gamma rays at 100 TeV energies, which
originate in π 0 decays produced by the accelerated cosmic

54rays interacting with matter surrounding the acceleration sites.
The gamma-ray emission of electron origin might be highly
suppressed above 10 TeV due to rapid decrease of inverseCompton cross section by the Klein-Nishina effect as well
as synchrotron radiation energy loss in the strong magnetic
field around the acceleration sites. The detection and spectral measurement of gamma rays in the 100 TeV region fromtheir 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 modelof cosmic rays. Furthermore, diffuse gamma rays from theFermi bubbles recently reported by 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 existenceof PeVatron[2], from which we expect to detect gamma rays
at 100 TeV energies. In 2021, the Tibet ASγ experiment succeeded in the first detection of sub-PeV diffuse gamma raysfrom the glactic disk. The natural interpretation is that the
sub-PeV gamma rays are of cosmic ray origin. Therefore, thisis the first experimental evidence that PeVatrons exist in ourgalaxy. However, there has been no agreed-upon PeVatron in
the northern sky so far. Thus, a wide field-of-view gammaray imaging at 100 TeV energies in the southern sky, where
the HESS sources[3], the Fermi bubbles and the galactic center are located within field of view, will be a key experiment.

5.

Experiment

The ALPACA[4],[5] (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 constructed.
We plan to set up a 3,700 m2 underground (approximately one to a few meters) muon detector array (MD) andan 83,000 m2 air shower array (AS). MD of water Cherenkov
type is composed of eight pools with each pool (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

Fig. 1. From Ref.[4]. 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.

in the Moon as well as by some of the bright stable TeV
gamma ray sources in the southern sky.

6.

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. 2.
The AS + MD in the southern hemisphere will be aunique/complementary experiment to on-going experiments(FERMI, CALET, DAMPE, HESS, VERITAS, MAGIC, Tibet
ASγ, HAWC, LHAASO) and future projects (CTA, SWGO)
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, or not expected to start
operation in the near future (within 5 to 10 years). Thus, the
new energy window in the 100 TeV region observing gamma
rays with wide field-of-view will be opened first in the southern sky by the ALPACA experiment. We expect to detectestablished more than a dozen of sources, i.e., young SNRs
(SN1006, RX J1713.7-3946, RX J0852.0-4622), Pulsar Wind
Nebulae, the galactic center, etc) in the 100 TeV region, some

55
10

3
[GeV]

[TeV]

netic field structure in the heliosphere. The ALPACA experiment gives complementary data in the TeV region to thosefrom IceCube above a few tens of TeV.
Furthermore, measurement of the Sun shadow in cosmic
rays above the TeV energy region in the southern hemispherealso helps understand the modeling of the magnetic fields between the Sun and the Earth, complementary to the observations in the northern hemisphere.

[PeV]

Integral Flux x E (eV cm−2 s −1 )

Past

10

2

Present
FutureCrab flux
Tibet AS

10

1
EGRET

10

CASA−MIAMAGIC
VERITAS
H.E.S.S.
HAWC

Milagro

0

FERMI

10

10

−1

−2

10

ALPACA

8

10

9

10

10

10

11

7.

CTA

5 sigma or 10 events
50 hours or 1 year (Wide FoV instrument)
12

10
10
Energy (eV)

13

10

14

10

15

10

16

Fig. 2. Sensitivity of ALPACA to high-energy gamma-ray point source.
Experimental data points are from HEGRA. The ALPACA sensitivity is evaluated from Ref.[6].

of which may be cosmic-ray PeVatron candidates. Furthermore, our wide field-of-view sensitivity to diffuse gamma
rays allows us to study extremely diffuse gamma-ray sources
which are difficult to detect by IACTs. The diffuse gamma
rays from the Fermi bubbles recently reported by the FermiLAT group may be clearly detected, if they extend up to the
100 TeV region. Similarly, detection of diffuse gamma rays
above 100 TeV from extended region from the galactic centeris promising, where the gamma-ray energy spectrum strongly
suggests existence of PeVatron. The sub-PeV diffuse gamma
rays along the galactic disk in the southern sky are an interesting target to understand the PeV cosmic-ray propagation in
our galaxy. Detection and spectral measurement of gamma
rays in the 100 TeV region from these celestial sources, together with multi-wavelength (radio, X-ray, gamma-ray) observations, are key points enabling us to discriminate between
the two processes (cosmic-ray/electron origins), to locate theacceleration site of cosmic rays and to examine the standard
acceleration and propagation model of cosmic rays. In astronomical point of view, we will open the ultra-high-energy(above 100 TeV) window in gamma-ray astronomy in thesouthern sky. Besides, gamma-ray emission from near-by extragalactic sources, e.g. M87, Cen A, gamma rays of dark
matter origin, those from the Sun disk recently obseved byFermi[9] may be interesting subjects.
We also aim at measuring energy spectra of proton, helium and iron components separately around the knee energy
region with the new AS + MD. The standard cosmic-ray acceleration model at SNR predicts the knee energy of each nucleus
component being proportional to Z (atomic number). We candiscriminate proton and iron components by MD, as an ironnucleus produces approximately 2 times more muons than aproton with the same energy. Thus, the cosmic-ray acceleration scenario (SNR shock acceleration) will be verified by observing the linearly Z(atomic number)-dependent knee(=bent)
positions of proton, helium, iron components around the knee
energy region.
Precise cosmic-ray anisotropy measurement at siderealtime frame in the TeV energy region in the southern sky provides unique data for the community to understand the mag-

ALPAQUITA

As a proto-type experiment, the ALPAQUITA[7] air
shower array which is ∼25 % of the ALPACA air shower array in area is being constructed at the experimental site, Chacaltaya Plateau. The containers including materials and equipments necessary to construct the ALPAQUITA air shower array arrived at La Paz, Bolivia in early 2019. The main infrastructures including electricity, experimental hut, fence, cableguide drains are ready in 2021. Due to COVID19 problems,
the construction of ALPAQUITA air shower array was delayed. The ALPAQUITA air shower array is under construction now, as is seen in Fig. 3. Due to COVID19 problems,
We aim at completing construction of the ALPAQUITA air
shower array under which we will add a ∼900-m2 muon detector array in 2022.

Fig. 3. ALPAQUITA air shower array under construction.

The ALPAQUITA sensitivity to gamma rays from a point
source is estimated by a Monte Carlo simulation[7], as isshown in Fig. 4. Even with the ALPAQUI TA air shower array with a 900 m2 MD, we expect to detect several bright 100
TeV gamma-ray sources.

8.

ALPACA(half)

The budget which affords to construct approximately halfof ALPACA was funded in 2020. We replanned to set up ALPACA(half), a 3,700 m2 underground (approximately one to afew meters) muon detector array (MD) and an 83,000 m2 air
shower array (AS), shown in Fig. 5. MD of water Cherenkov
type is composed of four pools with each pool (approximately
1 m deep) containing sixteen 56 m2 unit detectors. AS is
made up of 200 1 m2 plastic scintillation counters at 21 m

56

Fig. 4. From [7]. Sensitivity of the ALPAQUITA air shower array witha 900 m2 MD (black curve) to point-like gamma-ray sources. The
measured energy spectra of the H.E.S.S. sources are represented
by solid curves, while the dashed curves represent the extrapolations of the measured spectra.

spacing. The performance of ALPACA(half)[8] for cosmic
gamma rays above 100 TeV is expected to be similar to thatof ALPACA.
We will finish contruction of ALPACA(half) in 2023.

Bibliography
Papers and references[1] C. Lunardini et al., PRD, 92, 021301-1-5 (2015).
[2] A. Abramoswski et al., Nature, 531, 476 (2016).
[3] H. Abdalla et al., A&A 612, A1 (2018).
[4] M. Takita for the ALPACA Collaboration, THE EUROPEAN PHYSICAL JOURNAL, 145, 01002-1-3, (2017).
[5] F. Aceves de la Cruz et al. (The ALPACA Collaboration), Proc. of ICRC2021, POS(ICRC2021)733, Online,
July 12-23, (2021).
[6] T. K. Sako et al., Astroparticle Physics, 32, 177 (2009).
[7] S. Kato et al (The ALPACA Collaboration), Experimental Astronomy, 52, 85-107 (2021).
[8] F. Aceves de la Cruz et al. (The ALPACA Collaboration), Proc. of ICRC2021, POS(ICRC2021)899, Online,
July 12-23, (2021).
[9] C. Y. Kenny et al., arXiv:1508.06276v1.
The ALPACA Collaboration, as of 2021
F. Aceves de la Cruz1 , S. Asano2 , C. A. H. Condori3 ,E. de la Fuente1,4 , J. L. Garcia-Luna1 , A. Gomi5 , K. Hibino6 ,N. Hotta7 , A. Jimenez-Meza8 , Y. Katayose9 , C. Kato2 ,S. Kato10 , T. Kawashima10 , K. Kawata10 , T. Koi11 ,H. Kojima12 , D. Kurashige5 , J. Lozoya13 , R. Mayta14,15 ,P. Miranda3 , K. Munakata2 , K. Nagaya5 , Y. Nakamura10 ,Y. Nakazawa16 , C. Nina3 , M. Nishizawa17 , S. Ogio14,15 ,Fig. 5. Schematic view of the ALPACA(half) experiment. The small
black squares indicate 200 1 m2 plastic scintillation detectors at
21m spacing, forming an air shower array with 83,000 m2 inarea.The black large rectangles indicate four underground muon
detector pools, each of which contains sixteen 56 m2 muon detector units. The total area of the underground muon detector array is
3,700 m2 . The area enclosed in the lower-left octagon represents
ALPAQUITA.

M. Ohnishi10 , S. Okukawa5 , F. Orozco4 , A. Oshima11 ,M. Raljevich3 , H. Rivera3 , T. Saito18 , Y. Sakakibara5 ,T. Sako10 , T. K. Sako10 , S. Shibata12 , A. Shiomi16 ,M. Subieta3 , N. Tajima19 , W. Takano6 , M. Takita10 ,Y. Tameda20 , K. Tanaka21 , R. Ticona3 , I. ToledanoJuarez22,23 , H. Tsuchiya24 , Y. Tsunesada14,15 , S. Udo6 ,K. Yamazaki11 and Y. Yokoe10
1 Departamento de Fı́sica, CUCEI, Universidad de Guadalajara, Guadalajara, Jalisc o, Mexico.
2 Department of Physics, Shinshu University, Matsumoto,
Nagano 390-8621, Japan.
3 Instituto de Investigaciones Fı́sicas, Universidad Mayor de
San Andrés, La Paz 8635, Bolivia.
4 Doctorado en Tecnologı́as de la Información, CUCEA,
Universidad de Guadalaj ara, Zapopan, Jalisco, Mexico.
5 Graduate School of Engineering Science, Yokohama National University, Yokohama, Ka nagawa 240-8501, Japan.
6 Faculty of Engineering, Kanagawa University, Yokohama,
Kanagawa 221-8686, Japan.
7 Utsunomiya University, Utsunomiya, Tochigi 321-8505,
Japan.
8 Departamento de Tecnologı́ias de la Información, CUCEA,
Universidad de Guad alajara, Zapopan, Jalisco, Mexico.

57
9 Facultyof Engineering, Yokohama National University,
Yokohama, Kanagawa 240-8501, Japan.
10 Institute for Cosmic Ray Research, University of Tokyo,
Kashiwa, Chiba 277-8582 , Japan.
11 College of Engineering, Chubu University, Kasugai, Aichi
487-8501, Japan.
12 Chubu Innovative Astronomical Observatory, Chubu University, Kasugai, Aichi 487 -8501, Japan.
13 Departamento de Ciencias de la Información y Desarrollo
Tecnológico, Cu tonala, Universidad de Guadalajara, Tonalá,
Jalisco, Mexico.
14 Graduate School of Science, Osaka City University, Osaka,
Osaka 558-8585, Japan .
15 Nambu Yoichiro Institute for Theoretical and Experimental
Physics, Osaka City U niversity, Osaka, Osaka 558-8585,
Japan.
16 College of Industrial Technology, Nihon University,
Narashino, Chiba 275-8576, Japan.
17 National Institute of Informatics, Chiyoda, Tokyo 1018430, Japan.
18 Tokyo Metropolitan College of Industrial Technology,
Arakawa, Tokyo 116-8523, J apan.
19 RIKEN, Wako, Saitama 351-0198, Japan.
20 Faculty of Engineering, Osaka Electro-Communication
University, Neyagawa, Osaka 572-8530, Japan.
21 Graduate School of Information Sciences, Hiroshima City
University, Hiroshima, Hiroshima 731-3194, Japan.
22 Doctorado en Ciencias Fı́sicas, CUCEI, Universidad de
Guadalajara, Guadalaja ra, Jalisco, Mexico.
23 Maestrı́a en Ciencia de Datos, Departamento de Métodos
Cuantitativos, CU CEA, Universidad de Guadalajara, Zapopan, Jalisco, Mexico.
24 Japan Atomic Energy Agency, Tokai-mura, Ibaraki 3191195, Japan.

waves. Gravitational-wave astronomy is also our target. We
have been probing the formation history of massive stars fromthe early universe to the present time with the observation results by gravitational-wave detectors.
Research Topic 1: Multi-Messenger Signals from theComa Cluster
Galaxy clusters are considered to be gigantic reservoirsof cosmic rays (CRs). Some of the clusters are found withextended radio emission, which provides evidence for the existence of magnetic fields and CR electrons in the intra-cluster
medium. The mechanism of radio halo (RH) emission is still
under debate, and it has been believed that turbulent reacceleration plays an important role. We study the reacceleration of
CR protons and electrons in detail by numerically solving the
Fokker–Planck equation, and show how radio and gamma-ray
observations can be used to constrain CR distributions and resulting high-energy emission for the Coma cluster. We take
into account the radial diffusion of CRs and follow the time
evolution of their one-dimensional distribution, by which weinvestigate the radial profile of the CR injection that is consistent with the observed RH surface brightness. We find that therequired injection profile is nontrivial, depending on whether
CR electrons have a primary or secondary origin. Although
the secondary CR electron scenario predicts larger gammaray and neutrino fluxes, it is in tension with the observed RH
spectrum for hard injection indexes, α < 2.45. This tension
is relaxed if the turbulent diffusion of CRs is much less efficient than the fiducial model, or the reacceleration is more
efficient for lower-energy CRs. In both the secondary and primary scenario, we find that galaxy clusters can make a sizable
contribution to the all-sky neutrino intensity if the CR energy
spectrum is nearly flat.

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 studies of violent astrophysical phenomena, in which
nonthermal cosmic-ray particles are being accelerated. Targets of the group’s study include high energy astrophysicalobjects such as supernova remnants/explosions, pulsars, 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 for compact binary mergers. Our research carries themulti-messenger astronomy forward, which probes astronomical phenomena through collaborating observations of electromagnetic waves, cosmic rays, neutrinos, and gravitational

Fig. 1. Nonthermal electromagnetic and all-flavor neutrino spectra for
the Coma cluster. From left to right, synchrotron, inverse-Compton, and π 0 gamma emission are shown in black curves. The red
curves show neutrino fluxes. The “loud” and “quiet” states areshown in the thick solid and thick dashed curves, respectively. The
loud-state spectra for different injection spectral index α are also
shown: α = 2.1 (thin solid), 2.2 (thin dotted–dashed), and 2.45
(thin dotted).

Research Topic 2: Interpretation for CALET Electron
and Positron Spectrum
Focusing on the electron and positron spectrum measured
with the Calorimetric Electron Telescope (CALET), which

58

shows characteristic structures, we calculate the flux contributions of cosmic rays that have escaped from randomly appearing supernova remnants. We adopt a Monte Carlo method
to take into account the stochastic nature of the appearanceof nearby sources. We find that without a complicated energy dependence of the diffusion coefficient, simple powerlaw diffusion coefficients can produce spectra similar to the
CALET spectrum, even with a dispersion in the injection index. The positron component measured with AMS-02 is consistent with a bump-like structure around 300 GeV in the
CALET spectrum. One to three nearby supernovae can contribute up to a few tens of percent of the CALET flux at 2–4
TeV, while ten or more unknown and distant (>500 pc) supernovae account for the remaining several tens of percent of theflux. The CALET spectrum, showing a sharp drop at ∼ 1 TeV,
allows for a contribution of cosmic rays from an extraordinary
event that occurred ∼ 400 kyr ago. This type of event releases
electrons/positrons with a total energy more than 10 times theaverage energy for usual supernovae, and its occurrence rate
is lower than one three-hundredth of the usual supernova rate.

Fig. 2. An example of electron and positron spectra with additional
components consistent with the positron spectrum measured by
AMS-02. The upper spectral data is the CALET data. The redline is the sum of the electron–positron pair component (double
the positron flux denoted by the blue solid line) and the supernova
H line),which is a realizationelectron component (the black solidof the same Monte Carlo simulations. The positron component' r(the blue solid line) is the sum of the two components denoted bydashed lines. The blue dashed line is a positron component andthe magenta dashed line is calculated from an electron–positronpair
H ejection
 that0 occurred 420 kyr ago at a distance of 1 kpc.

Research Topic 3: Collisionless Relativistic Jet
After the discovery of relativistic jets in early 20th century,
the formation and acceleration mechanism of relativistic jets
are not still understood. Recent observation of M87, which isone of the best laboratories to study jets, discovered the tripleridge structure in its jet. The triple-ridge structure can be interpreted as the observational feature of the spine-sheath structure of the jet, which is composed of the fast-flowing spine andthe slow-flowing sheath. The spine structure is, especially, notreproduced by relativistic magnetohydrodynamic simulations
in which the macroscopic physics can be solved. This indicates that the kinetic plasma (i.e., microscopic) effects may be
important for the spine jet structure formation.
'r



)

Fig. 3. The electron number density (top) and the magnetic fieldstrength (bottom) in the early (left) and later (rihgt) stages of theparticle-in-cell simulation. The simulation domain is on the transverse plane of the jets. The number density of the electrons accelerated by MI-driven-MRs increases with time in the jet center.
The physical quantities in the figures are displayed with an arbitrary unit.

We, therefore, studied the kinetic plasma dynamics incollisionless relativistic jets with velocity shear, by carryingout particlein- cell simulations in the transverse plane of ajet. It was discovered that intermittent magnetic reconnections (MRs) are driven by mushroom instability (MI), which
is an important kinetic-scale plasma instability in the plasma
shear flows with relativistic bulk speed. We referred to thissequence of kinetic plasma phenomena as “MI-driven MR.”
The MI-driven MRs intermittently occur with moving the location of the reconnection points from the vicinity of the initial velocity-shear surface toward the center of the jet. As
a consequence, the number density of high-energy electrons
that are accelerated by MI-driven MRs increases with time in
the region inside the initial velocity-shear surface with the accompanying generation and subsequent amplification of magnetic fields by MI. The maximum Lorentz factor of electrons
increases with initial bulk Lorentz factor of the jet. The MIdriven MR might be related to the formation of the spine structure in relativistic jets.
Research Topic 4: Radio Observation of Sgr A* and Theoretical Interpretation
EAVN (East Asia VLBI Network) has been developed toobserve the highly resolved radio images of nearby active
galactic nuclei by using VLBI (Very Long Baseline Interferometry) techniques. We firstly observed the relatively nearby
region of the central black hole (≲ 103 rg , where rg is the gravitation radius of the black hole) in Sgr A* at the wavelength
1.3cm and 7mm. In lower frequency bands including 1.3cm
and 7mm, the effects of the scattering caused by the interstellar plasma can drastically make the images of Sgr A* drastically blurred due to the effects of the diffraction and refraction. By using the state-of-the-art models of the scattering

59

screen of the interstellar plasma, the intrinsic images of Sgr
A* is successfully obtained. The morphology of the intrinsic
images are almost circular rather than elliptic, whose brightregion extends down at least upto several 100 rg .

Fig. 4. The theoretical images of Sgr A* at 1.3cm assuming viewing angle i = 20◦ (left) and 60◦ (right). The top and bottom panels display the model with thermal electron only and thermal plus
nonthermal electrons, respectively. The effects of the synchrotron
emission via nonthermal electrons significantly enlarge the size ofthe bright region and consistent with the observed data.

To interpret the results, we have calculated the images andspectra of the Sgr A* accretion flow by using a general relativistic radiative transfer code RAIKOU. A toy model so called
Keplerian shell model is assumed for the accreting plasma
onto Sgr A*. We have found that the circular mophology of
the images can be reproduced when the viewing angle, whichis the angle between the rotation axis of the accretion flow
and the line of sight, is ≲ 30◦ –40◦ , i.e., the neary face-on
view. The observed large image sizes, importantly, requires
the accelerated electrons. This may give us the important informations for the origin of the cosmic rays in the region near
the Sgr A*.
Research Topic 5: Radio jets bent by magnetic fields
Galaxy clusters are known to harbor magnetic fields, thenature of which remains an open question. Radio observationsof clusters can be used to study the distributed populations ofcosmic-ray electrons and the magnetic field of the intracluster
medium. However, the measurement of the magnetic fields
from radio intensity is inherently uncertain owing to the many
assumptions. In this study, we focus on the second-brightest
radio galaxy, MRC 0600-399, in the merging galaxy cluster
Abell 3376. We first time discovered the evidence of the interaction between a radio jet and an intracluster magnetic field
through the unprecedented combination of brightness sensitivity, dynamic range, and angular resolution of a new 1.28
GHz MeerKAT continuum image (left panel of figure 5). The
jets from galaxy MRC 0600-399 bend at where the cluster
plasma property has changed, and the collimated jet further
extends over 100 kpc from the bend point. In addition to this,
we identified the diffuse radio emissions elongated not only tothe bending direction but also to the opposite direction. These
are unusual features in the typical bent jets. To understandthe main mechanism of the origin of the unusual bent jet, wehave performed magnetohydrodynamic simulations of the interaction between the jet and the intracluster field. We foundthat the motion of the jet across the ordered magnetic fields
surrounding the cluster is suppressed due to the magnetic fieldtension. Then, the jet changes direction along with the ordered
field. The overall morphology of the bent jet bears remarkablesimilarities with the simulations (right panel of figure 5). Our
results provided new physical insight into the dynamics of radio jets, but also a novel tool for the measurement of the intracluster magnetic field from high-quality radio observations.

Fig. 5. The observed image of MRC 0600-399 (left) and the simulation of the interaction between galactic jets and magnetic
fields calculated by ATERUI II (right).(Credit: [Left] Chibueze,
Sakemi, Ohmura et al. (2021) Modification from Nature Fig. 1(b). [Right] Takumi Ohmura, Mami Machida, Hirotaka Nakayama,
4D2U Project, NAOJ)

Research Topic 6: Neutron star merger ejecta and Kilonova Emission
We predicted the kilonova lightcurve models in various
setups based on the latest numerical relativity simulations andradiative transfer simulations, and discussed the property ofthe progenitor employing our kilonova lightcurve models.
We studied the long-term evolution of ejecta formed ina binary neutron star (BNS) merger that results in a longlived remnant NS by performing a hydrodynamics simulation with the outflow data of a numerical relativity simulation as the initial condition. At the homologously expanding
phase, the total ejecta mass reaches ≈ 0.1 M⊙ with an average velocity of ≈ 0.1 c and lanthanide fraction of ≈ 0.005.
We further perform the radiative-transfer simulation employing the obtained ejecta profile. We find that, contrary to anaive expectation from the large ejecta mass and low lanthanide fraction, the optical emission is not as bright as thatin GW170817/AT2017gfo, while the infrared emission canbe brighter. This light curve property is attributed to preferential diffusion of photons toward the equatorial direction
due to the prolate ejecta morphology, large opacity contribution of Zr, Y, and lanthanides, and low specific heating rate of
the ejecta. Our results suggest that these light curve features
could be used as an indicator for the presence of a long-lived
remnant NS. We also found that the bright optical emission
broadly consistent with GW170817/AT2017gfo is realized forthe case that the high-velocity ejecta components in the polar
region are suppressed. These results suggest that the remnant
in GW170817/AT2017gfo is unlikely to be a long-lived NS,

60

but might have collapsed to a black hole within O(0.1) s.

BBH merger rate density[Gpc 3 yr 1 M 1]

103

101
100
10 1
10 2
10 3

Fig. 6. The left top and bottom panels denote the rest-mass densityand electron fraction profile of neutron star merger ejecta, respectively, obtained by the long-term hydrodynamics simulations using
the outflow data of a numerical relativity simulation as the initialcondition. The right top and bottom panels denote the bolometricand broad-band kilonova light curves calculated by performing radiative-transfer simulations for obtained ejecta profiles.

GWTC-2Pop I/II BBH (Belczynski et al. 2020)
Pop I/II BBH (Salpeter like)
Pop III BBH (Kinugawa et al. 2020)

102

0

20

40

80

100

Fig. 7. BBH merger rate density as a function of primary mass (M1 ).
The blue line and the light blue region are the population distribution and 90% credible interval of the power law + peak model by
GWTC-2, respectively. The green dashed line is the Pop I/II BBH
merger rate density. The gray line is the Salpeter like power law
model (M1−2.35 ) for Pop I/II BBHs which is normalized by GWTC-2
rate at M1 = 10M⊙ . The red line shows the Pop III BBH merger
rate density.

observations with that derived theoretically.
(A) a = 31475R⦿

Research Topic 7: Gravitational Wave Astronomy
We perform the binary evolution of zero-metal (population
III or Pop III) stars by using population synthesis simulations.
We find that Pop III binaries tend to be binary black holes(BBHs) with chirp mass Mchirp ∼ 30 M⊙ and they can merge
at present day due to long merger time. These Pop III binaries might contribute to some part of the massive BBH gravitational wave (GW) sources detected by aLIGO/aVIRGO(Fig.7). The probability number distribution function of binary black hole mergers observed by LIGO/Virgo O3a has
double peaks as a function of chirp mass Mchirp , total mass
Mtotal , primary black hole mass M1 , and secondary one M2 ,respectively. The larger chirp mass peak is at Mchirp ∼ 30M⊙ .
For initial mass functions of Pop III in the form of f (M) ∝M −α , population synthesis numerical simulations with 0 ≤α ≤ 1.5 are consistent with O3a data for Mchirp ≳ 20M⊙ .
We also calculate a binary population synthesis calculation incorporating very massive Pop III stars up to 1500 M⊙
and investigate the nature of BBH mergers. Above the pairinstability mass gap, we find that the typical primary BH massis 135-340 M⊙ . The maximum primary BH mass is as massive
as 686 M⊙ (Fig. 8). The BBHs with both of their components
above the mass gap have low effective inspiral spin ∼ 0. So
far, no conclusive BBH merger beyond the mass gap has been
detected, and the upper limit on the merger rate density is obtained. If the initial mass function of very massive Pop III stars
is simply expressed as ζm (m) ∝ m−α (single power law), we
find that α ≳ 2.8 is needed in order for the merger rate density
not to exceed the upper limit. In the future, the gravitational
wave detectors such as Einstein Telescope and Pre-DECIGO
will observe BBH mergers at high redshift. We suggest that
we may be able to impose a stringent limit on the Pop III IMF
by comparing the merger rate density obtained from future60

M1 [M ]MS
1220M⦿

686M⦿

360M⦿

360M⦿

BH
219M⦿

double common envelope
CHeB

ShHeB

a = 314R⦿

BH

MS(B) a = 26086R⦿

1220M⦿

(E)

(C)

(D)

a = 314R⦿

BH
686M⦿

nHe
219M⦿

Fig. 8. Evolutionary channel leading to the BBH with the most massive primary BH. ’MS’, ’CHeB’, ’ShHeB’ and ’nHe’ stand for themain sequence phase, the core helium burning phase, shell helium burning phase and naked helium star, respectively.

Research Topic 8: Observational Study of Giant Radio
PulsesGiant radio pulses (GRPs) are sporadic bursts emitted bysome pulsars that last a few microseconds and are hundreds
to thousands of times brighter than regular pulses from thesesources. The only GRP-associated emission outside of radio
wavelengths is from the Crab Pulsar, where optical emission
is enhanced by a few percentage points during GRPs. We
observed the Crab Pulsar simultaneously at x-ray and radio
wavelengths, finding enhancement of the x-ray emission by
3.8 ± 0.7% (a 5.4σ detection) coinciding with GRPs. Thisimplies that the total emitted energy from GRPs is tens to hundreds of times higher than previously known.
Research Topic 9: 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 spectra612017 Event Horizon Telescope Campaign”, Astrophys.
J. Lett., 911, L11(43pp) (2021).
4. Takaaki Kitaki, Shin Mineshige, Ken Ohsuga, Tomohisa Kawashima “The Origins and Impact of Outflow
from Super-Eddington Flow”, Pub. Astron. Soc. J., 73,
450-466 (2021).
5. R. Narayan, D. C. M. Palumbo, M. D. Johnson, et al.
“The Polarized Image of a Synchrotron-Emitting Ringof Gas Orbiting a Black Hole”, Astrophys. J., 912,
35(26pp) (2021).
Fig. 9. X-ray and optical pulse profiles of the Crab Pulsar comparedwith GRPs. Black and red circle symbols connected with solidlines show the x-ray profiles without and with GRP association,
respectively, with error bars indicating the 1σ statistical uncertainties (error bars of the black circle and gray triangle points are too
small to be visible). The blue histogram shows the GRP occurrence distribution. The faint dashed lines (black and red triangle
symbols) show the optical profiles without and with GRP association, respectively, normalized by an arbitrary scaling.

of electrons, gamma-rays and nuclei.

E2.6 x Flux [m-2 s-1 sr-1 (GeV/n)1.6]

5

4

3

CALET
Systematic uncertainties
Sanriku
ATIC 02 (2003)
CRN-spacelab2
HEAO3-C2
H.E.S.S. (2004-2006)

Statistical uncertainties
Total (stat.+syst.) uncertainties
TRACER
NUCLEON (KLEM - 2019)
CREAM-II
AMS-02

Iron

2

1

0

10 2

10

10 3

Kinetic Energy per nucleon [GeV/n]

Fig. 10. Iron spectrum measured by CALET (red points).

The energy spectra of iron in cosmic-rays obtained withCALET is shown in Figure 10. The measurement is based on
observations carried out from January 2016 to May 2020. The
observed differential spectrum is consistent within the errors
with previous experiments. In the region from 50 GeV/n to
2 TeV/n our present data are compatible with a single powerlaw with spectral index -2.60 ± 0.03.

Bibliography
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2016”, Astron. Astrophys., 648, A23(22pp) (2021).
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6. James O. Chibueze, Haruka Sakemi, Takumi Ohmura,
Mami Machida, Hiroki Akamatsu, Takuya Akahori, Hiroyuki Nakanishi, Viral Parekh, Ruby van Rooyen, andTsutomu T. Takeuchi “Jets from MRC 0600-399 Bent
by Magnetic Fields in the Cluster Abell 3376”, Nature,593, 47-50 (2021).
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in Kilonova Spectra”, Astrophys. J., 913, 26(12pp)(2021).
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Masaomi Tanaka, Shinya Wanajo “A Low-Mass Binary
Neutron Star: Long-Term Ejecta Evolution and Kilonovae with Weak Blue Emission”, Astrophys. J., 913,
100(24pp) (2021).
9. Tomoya Kinugawa, Takashi Nakamura, Hiroyuki
Nakano “Gravitational Waves from Population III Binary Black Holes are Consistent with LIGO/Virgo O3a
Data for the Chirp Mass Larger than ∼ 20M⊙ ”, Mon.
Not. Roy. Astro. Soc., 504, L28-L33 (2021).
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Soc., 504, 1427-1451 (2021).
11. O. Adriani, Y. Akaike, K. Asano, et al. “Measurementof the Iron Spectrum in Cosmic Rays from 10 GeV/n
to 2.0 TeV/n with the Calorimetric Electron Telescopeon the International Space Station”, Phys. Rev. Lett.,
126, 241101(7pp) (2021).
12. Kotaro Hijikawa, Ataru Tanikawa, Tomoya Kinugawa,Takashi Yoshida, Hideyuki Umeda “On the Population III Binary Black Hole Mergers beyond the PairInstability Mass Gap”, Mon. Not. Roy. Astro. Soc.,
505, L69-L73 (2021).
13. Ataru Tanikawa, Tomoya Kinugawa, Takashi Yoshida,
Kotaro Hijikawa, Hideyuki Umeda “Population III Binary Black Holes: Effects of Convective Overshooting
on Formation of GW190521”, Mon. Not. Roy. Astro.
Soc., 505, 2170-2176 (2021).

62

14. Yuh Tsunetoe, Shin Mineshige, Ken Ohsuga, Tomohisa
Kawashima, Kazunori Akiyama “Polarization Imagesof Accretion Flow around Supermassive Black Holes:
Imprints of Toroidal Field Structure”, Pub. Astron. Soc.
J., 73, 912-928 (2021).

25. V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, et al. “Multiwavelength Study of the Gravitationally Lensed Blazar QSO
B0218+357 between 2016 and 2020”, Mon. Not. Roy.
Astro. Soc., 510, 2344-2362 (2022).

15. V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet
Engels, M. Artero, K. Asano, et al. “First Detectionof VHE Gamma-Ray Emission from TXS 1515–273,
Study of its X-ray Variability and Spectral Energy Distribution”, Mon. Not. Roy. Astro. Soc., 507, 15281545 (2021).

26. V. A. Acciari, T. Aniello, S. Ansoldi, L. A. Antonelli,
A. Arbet Engels, M. Artero, K. Asano, et al. “Investigating the Blazar TXS 0506+056 through Sharp Multiwavelength Eyes during 2017–2019”, Astrophys. J.,
927, 197(15pp) (2022).

16. M. Janssen, H. Falcke, M. Kadler, et al. “Event Horizon
Telescope Observations of the Jet Launching and Collimation in Centaurus A”, Nature Astron., 5, 1017-1028(2021).
17. V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, et al. “Investigation of theCorrelation Patterns and the Compton Dominance Variability of Mrk 421 in 2017”, Astron. Astrophys., 655,
A89(36pp) (2021).
18. Kosuke Nishiwaki, Katsuaki Asano, Kohta Murase“Particle Reacceleration by Turbulence and Radio
Constraints on Multimessenger High-Energy Emission
from the Coma Cluster”, Astrophys. J., 922, 190(23pp)(2021).
19. V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, et al. “Search forVery High-Energy Emission from the Millisecond Pulsar PSR J0218+4232”, Astrophys. J., 922, 251(14pp)(2021).
20. C. B. Adams, W. Benbow, A. Brill, et al. “Observationof the Gamma-Ray Binary HESS J0632+057 with the
H.E.S.S., MAGIC, and VERITAS Telescopes”, Astrophys. J., 923, 241(30pp) (2021).
21. K. Satapathy, D. Psaltis, F. Ozel, et al. “The Variabilityof the Black-Hole Image in M87 at the Dynamical Time
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22. Katsuaki Asano, Yoichi Asaoka, Yosui Akaike, Norita
Kawanaka, Kazunori Kohri, Holger M. Motz, Toshio
Terasawa “Monte Carlo Study of Electron and Positron
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23. Ataru Tanikawa, Takashi Yoshida, Tomoya Kinugawa,
Alessandro A. Trani, Takashi Hosokawa, Hajime Susa,
Kazuyuki Omukai “Merger Rate Density of Binary
Black Holes through Isolated Population I, II , III and
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24. I. Cho, G.-Y. Zhao, T. Kawashima, et al. “The Intrinsic Structure of Sagittarius A∗ at 1.3 cm and 7 mm”,
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27. Tomohisa Kawashima, Seiji Ishiguro, Toseo Moritaka, Ritoku Horiuchi, Kohji Tomisaka “MushroomInstability-Driven Magnetic Reconnections in Collisionless Relativistic Jets”, Astrophys. J., 928, 62(10pp)(2022).
28. V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, et al. “Combined
Searches for Dark Matter in Dwarf Spheroidal Galaxies
Observed with the MAGIC Telescopes, Including New
Data from Coma Berenices and Draco”, Phys. Dark
Uni., 35, 100912(10pp) (2022).

Other Activities
Ashra NTA
[Spokesperson : M. Sasaki]
Collaboration list:
ICRR, The University of Tokyo, Chiba, Japan: M. Sasaki,
T. Aoki; Physics Department, Toho University, Chiba, Japan:
S. Ogawa; Academia Sinica, Taipei, Taiwan: S. Haino;
Institute for Interdisciplinary Science, Okayama University,
Okayama, Japan: K. Yoshimura; IPNS, KEK, Ibaraki, Japan:
M. Tanaka; et al.
VHE Physics and Astronomy
Very-high energy (VHE) physics is the development of ahigher energy frontier that is complementary to HEP using
accelerators to investigate interactions in space caused by fundamental particles and to study the structure and fundamental
interactions of elementary particles. Probing for VHE elementary particles will also enable the discovery of VHE celestial objects and the elucidation of their phenomena. In VHE
objects, 1st- and 2nd-generation neutrinos should be emittedwith photons from the process by accelerated protons, but theyremain unidentified. Comparison of VHE neutrino and photon flux spectra from the same object will also allow for fundamental particle physics and cosmological investigations. VHE
neutrinos and photons can also probe super heavy dark matterand dark energy.
Neutrinos and photons are well-known subatomic particles from the Standard Model and travel straight in magnetic
fields, making them powerful probes for ultra-high-energy

63

particle physics astronomy (VHEPA). Neutrino oscillations
cause neutrino fluxes to homogenize between generations during propagation. Tau neutrino observations are also tau appearance experiments. VHE tau neutrinos skim the earth, areconverted to tau, and after decaying in air become an upward
air shower at a shallow elevation angle, emitting Cherenkov
light and fluorescence. The NTA unit is a unique wide-angle,
high-precision optical system with a light-branching trigger
imaging system. This detection scheme is particularly powerful for the simultaneous observation of Cherenkov light andfluorescence from VHE tau and photons.
NTA for VHE Tau and Photon probes
NTA will radially observe air-shower light in a vast
amount of night-time air on the mountain within more thanπ sr field of view covered with 1 minute square pixels [1].
Earth-skimming or mountain-hitting tau neutrino is converted
to tau in the rock, and tau decays make upward air shower.
The upward tau shower signal is very clear and there are almost no background events. Downward coming photon showers are observed in both Cherenkov and fluorescence light.
The Cherenkov light coming from a large zenith angle earns
a large effective detection area. The lookout layout allows
Cherenkov and fluorescence observations over a wide energy
range. Due to the narrow Cherenkov cone of the tau shower,
distance is needed to gain effective area. But fluorescence isuseful for observing near showers coming from behind. The
Xmax is at a distance of roughly 1km from the detector, andeven 1-PeV class taus can be detected as shown here. This isa good advantage of look-out layout on the mountain.
The geo-propagation of tau by GEANT and ALLM simulations revealed emergent tau remembers the tau neutrino direction above 1 PeV with much better than 1 minute of accuracy. We also confirmed shower reconstruction with CORSIKApsimulation, resulting in the point-back resolution of
0.1◦ / Esh /PeV where Esh is the shower energy. The emergent tau energy is deformed from the parent tau neutrino energy due to the inelastic nature of the charge current interaction and energy loss in the rock. But it still allows for a
relatively good energy reconstruction of ντ with the energy
around 1 PeV. We confirmed energy resolution of nu-tau with
ANIS simulation, resulting in 20 or 40 % around 1 PeV depending on the elevation angle.
Regarding the NTA’s view of the southern sky, NTA canalways cover the Galactic bulge of the night sky (Figure 11
upper). The NTA detector unit’s field of view center should bealigned with this arc, considering a high multi-eye observation
ratio.
NTA Galactic bulge monitor with viewing radius of 20
degrees has a good advantage of including 12 % of total
mass of the dark matter halo, which corresponds to mass of
3×1056 GeV assuming the Einasto profile. The effective detection area of the surface detector has a geometrical problemof the COS-dependent at large zenith angle. These plots are
the accumulated sums of annual galactic center observation
hours per altitude angle. In the NTA case, galactic center is in
the moonless night sky for 980 hours in a year. In the case of adetector at 16.4 degrees south latitude and a 45 degrees zenith
angle cut, the observation time must be less than 2300 hours

Fig. 11. Superimposed on the image of the Milky Way galaxy, theposition and orbit of the galactic center is represented by a yellow
cross and red arc (top). The contour map of Hawaii Island is input
to our MC simulation (bottom). The radius of the wheel represents
the distance to Xmax of the 1PeV shower coming toward NTA at
80 degrees zenith. Four stations will be placed on the mountainside. One station is responsible for 210 degree azimuth range
indicated by four fans.

Fig. 12. The basic design is a 1.5-fold scaled-up version of the
Ashra-1 light collector, and four of these collectors are stacked
together to form one NTA detector unit with the same field of view.

per year even if 100 % duty cycle. The factor is roughly 2.3.
NTA proto-project, Ashra-1, was placed in 2008 at thenorth side (Figure 11 bottom). It allows for binocular andtrinocular observation, resulting in a high multi-eye observation rate. The basic design is a 1.5-fold scaled-up version of
the Ashra-1 light collector, and four of these collectors arestacked together to form one NTA detector unit with the same
field of view (Figure 12). The effective pupil diameter is 3m, providing good light gathering power. It can also facilitate

64[9] P. Gorham,et al.,arXiv:1902.04005.
[10] A. M. Otte,Phys. Rev. D99, 083012 (2019).
[11] M. Sasaki,PoS (ICRC2019) 1003.

γI GroupγI Project
Fig. 13. A comparison of diffuse neutrino sensitivity from various VHE
neutrino detectors. NTA has the best sensitivity from a few PeV
to 100 PeV [2]-[11].

background muon rejection, transport and construction.
NTA Diffuse Neutrino Sensitivity
Figure 13 shows a comparison of diffuse neutrino sensitivity from various VHE neutrino detectors. NTA has the best
sensitivity from a few PeV to 100 PeV. Trinity observes Earthskimming neu-tau as well as NTA but only with Cherenkov
light. The sensitivity of its higher energy part is determinedby the large zenith angle shower, which is consistent with
NTA. The NTA sensitivity at lower energy is superior to thatof Trinity due to the tau shower fluorescence observed near
the mountain surface as mentioned earlier.
Conclusions
This is the era of synergy between HEP and VHEP. Theenergy front around the unitarity bound interesting e.g. superheavy dark matter. Combined detection of PeV tau and photons will open up more comprehensive studies of VHEP and
VHEA. NTA is most sensitive to PeV tau (ν) and photon, withgood coverage and accuracy, which will take over importantresults from TeV and pass more developed results to EeV.

Bibliography[1] NTA LOI (look-in layout array), M. Sasaki and G. W.S. Hou,ArXiv:1408.6244 (2014). NTA look-out layout summit array, M. Sasaki, PoS (ICRC2017) 941.
M. Sasaki, ICHEP2022.
[2] J. Álvarez-Muñiz,et al.,Mechanics and Astronomy 63,
219501 (2019).
[3] J. F. Krizmanic,et al.,UHECR2018 (Paris).
[4] C. R. Persichilli,Thesis,Univ. California (2018).
[5] P. Allison,et al.,Phys. Rev. D93, 082003 (2016).
[6] R. A. Batista,et al.,J. Cosmol. Astropart. Phys., 01, 002,
(2019).
[7] M. Aartsen,et al.,Phys. Rev. D98,062003 (2018).
[8] A. Aab,et al.,arXiv:1708.06592.

γI Consortium[Spokesperson : R.Enomoto]Collaboration list:
ICRR, The University of Tokyo, Chiba, Japan: R. Enomoto;
National Institute of Technology, Sendai College, Miyagi,
Japan: M. Kagaya; College of Science, Ibaraki University, Ibaraki, Japan: H. Katagiri, R. Kimijima, K. Harada,
K. Yoshida; Faculty of Medical Engineering and Technology, Kitasato University, Kanagawa, Japan: H. Muraishi, S.
Ishikawa; IPNS, High Energy Accelerator Research Organization, Ibaraki, Japan: M. M. Tanaka; National Cancer Center Hospital East, Chiba, Japan: D. Kano, T. Watanabe
Overview
We have conducted a feasibility study of a gamma-ray imager specializing in de-excitation lines with energies in therange of 2–10 MeV that is capable of searching for low-energy
cosmic rays around 1 GeV. The imager is an electron-tracking
Compton camera using scintillation fibers. The principal investigator of this project is H. Katagiri of Ibaraki University. First, we developed a small prototype camera in 2018
and a realistic simulation model based on the Geant4 simulation toolkit. Using the simulation model and environmental cosmic-ray muon data, we developed a method for reconstructing tracks of charged particles and an energy calibration
method. We also designed and developed a large prototype
camera which allows us to investigate the multiple scattering
events of gamma rays, and evaluated the performance of a
CMOS camera on the gamma-ray imager. Note that we applied for a patent for reconstructing the energy and directionof gamma rays, which was found when designing the large
camera. In this report, we first show the background and purpose of the research. Then, we describe the concept and design of the gamma-ray camera and finally report the current
status.
1 Background and purpose
The question of where and how cosmic rays in our galaxy
are accelerated is a long-standing important issue in astroparticle physics. The major part of Galactic cosmic rays isthought to be accelerated in blast waves of supernova remnants (SNRs), but until now it has not been completely solved.
To study such hypothesis, it is important to observe the
gamma rays produced by the interaction of cosmic rays in
SNRs with the surrounding material. To date, gamma rays

65

in TeV energies from SNRs have been detected by imaging
atmospheric Cherenkov telescopes (e.g., [1], [2], [3], [4], [5]),and GeV gamma rays from several tens of SNRs have been
discovered by the Large Area Telescope (LAT) on board the
Fermi Gamma-ray Space Telescope (Fermi) (e.g., [6]). Recently, the detection of the gamma-ray emission from the SNR
G106.3+2.7 above 100 TeV was reported by Tibet ASγ Collaboration [7]. These observations have indicated particle acceleration in GeV to PeV energies in SNRs in our Galaxy.
However, some of the fundamental problems remain in the
research of Galactic cosmic-ray. The followings are some of
the key questions. (1) Are high-energy particles in SNRs really cosmic-ray nuclei? Gamma rays have been detected inmany SNRs, but for most SNRs, we cannot firmly rule out
the possibility that they come from cosmic-ray electrons. Currently, one of the most direct evidences of cosmic-ray nuclei inan SNR is the detection of gamma rays from decays of π 0 particles generated by the interaction between cosmic-ray nuclei
and interstellar material by the LAT instrument on board the
Fermi observatory (e.g., [8]). A spectrum of π 0 -decay gamma
rays has a characteristic broad peak around 70 MeV (Figure 14), but since the lower detection threshold of the LAT
instrument is around 60 MeV, such peak shapes have not beenclearly investigated. In addition, since the effective area of

Meneguzzi&Reeves (1975)
10MeV

π0

de-excitation
lines

E(MeV)
1

10

100

Fig. 14. Predicted gamma-ray emissivity from the interaction of Galactic cosmic rays with the interstellar gas [9].

the LAT decreases sharply below 100 MeV, the search for π 0
decay gamma rays is restricted to only very bright gammaray-emitting SNRs. (2) Is the cosmic-ray energy budget of the
Milky way mainly explained by SNRs? At present, gamma-

ray observations have revealed that particle acceleration operates in many SNRs, thus the cosmic-ray energy budget seems
to be satisfied given that the particles accelerated in the SNRs
are mainly cosmic-ray nuclei. However, it should be notedthat π 0 -decay gamma rays are not generated unless protons
have a momentum of above ∼ 1 GeV/c, and they are not sensitive to low-energy cosmic rays below 1 GeV. They can be
responsible for a substantial part of the energy density of cosmic rays based on the measurements beyond the heliopause
by Voyager 1 [10], which are thought to be free of solar modulation, but currently, there is no direct probe for low-energy
cosmic rays more distant from the Earth.
The observation of de-excitation line gamma rays can be asmoking gun to solve the questions (1) and (2) in the cosmicray research. Such lines with the energy of several hundred
keV to about 10 MeV seen in Figure 14 are produced when
low-energy cosmic rays and the interstellar matter interact.
However, it is difficult to identify those below 2 MeV because
the spectrum in these energy ranges is contaminated by nuclear gamma-ray lines from radioactive decay of unstable isotopes produced in massive star or supernova nucleosynthesis.
Therefore, we will develop a gamma-ray imager specializing
in de-excitation lines in 2–10 MeV energies that is capable ofsearching for low-energy cosmic rays around 1 GeV and verify its performance experimentally. The proposed gamma-ray
imager in this report is an electron-tracking Compton camera
using scintillation fibers.
2 Concept and design of the electron-tracking Compton
camera using scintillation fibers
The purpose of this study is to develop a gamma-ray camera specializing in de-excitation lines in 2–10 MeV energies
and to verify its performance experimentally. In this subsection, we describe the concept and design of our electrontracking Compton camera using scintillation fibers.
A Compton camera utilizes the Compton scattering process, which is the dominant gamma-ray interaction in 2–
10 MeV energies. It allows us to measure the distribution of
gamma rays in these energies with a wide field of view without
using any heavy shielding or collimator. The Imaging Compton Telescope (COMPTEL) on board the Compton Gammaray Observatory (CGRO [11]) was the first major applicationof a Compton camera for MeV gamma-ray astronomical observations in the 1990s. However, no observational evidence
for the de-excitation lines from an SNR was found, and thepredicted flux is close to the level of the COMPTEL sensitivity [12]. Measuring the track of a recoil electron produced byCompton scattering is one of the powerful methods to reduce a
substantial amount of the background and improve the detection sensitivity dramatically. The left figure in Figure 15 illustrates a method used in searching for a radiation source using a
conventional Compton camera. The directions of gamma rays
are determined by Compton kinematics from the energies deposited in the scatterer and absorber. With this method, thedirection of a gamma ray can be limited to a ring shape (theleft figure in Figure 16) but cannot be determined uniquely.
Thus, the background increases due to the contamination fromother radiation sources. If a detector with high positional resolution is adopted as a scatterer, the direction of a gamma ray

66

Fig. 15. Schematic diagram of Compton reconstruction: (left) without
the electron-track information and (right) with the electron-track information.

Fig. 17. Schematic view of the measurement system of a prototype
camera.

However, both the cost and the difficulty in manufacturing increase with increasing number of readout channels; therefore
we adopted 1 mm fiber in this feasibility study. The crosssectional shape of the available commercial fibers is roundor square. We adopted square fibers because they can havehigh light collection efficiency from their geometry. With apositional accuracy of 1 mm, it is possible to detect the direction of a recoil electron even for low-energy gamma rays (e.g.,
1.3 MeV) that can be easily used in a laboratory when a scattering angle is relatively large.

Fig. 16. Schematic explanation of Compton reconstruction: (left) without the electron-track information, (right) with the electron-track information.

can be uniquely determined event by event by capturing thedirection of a recoil electron as shown in the right figures in
Figure 15 and Figure 16 (electron-tracking Compton camera)
respectively. We are conducting the feasibility study of anelectron-tracking Compton camera using scintillation fibers.
Scintillation fibers can achieve a large fiducial volume inexpensively with positional resolution of a few hundreds of µm
although the energy resolution is limited. The schematics ofthe measurement system is shown in Figure 17. The systemof the camera under development is described in detail below.
Multilayer fiber plates
A single fiber plate is composed of many fibers, and eachplate is stacked and rotated 90◦ with respect to the one above
or below to obtain a three-dimensional position. The track
length of a recoil electron with respect to a 4.4 MeV gamma
ray, which is expected to come from astronomical objects withthe highest intensity among de-excitation gamma-ray lines, isabout 5 mm at a scattering angle of 15◦ . The smaller diameter of the fiber we adopt, the more accurate tracking is possible, enabling us to reconstruct gamma-ray events with a small
recoil energy, which corresponds to a small scattering angle.

Photodetector and readout system
When the diameter of the fiber is 1 mm, it is necessary
to realize the readout of 32 × 64 = 2048 fibers equippedwith signal amplification to capture faint lights from thesefibers. To achieve that, we will build a system that combines
an image intensifier and a high-speed readout CMOS camera.
Since the diameter of an effective area of the available image
intensifier is 17 mm, which is smaller than the fiber plates,
fiber optics tapers are used to guide the light from the fiber
plates. Photodetectors such as a multianode photomultipliertube (MAPMT) and a multi-pixel photon counter (MPPC) array are equipped to the other end of the fiber plates to generatea trigger signal for the CMOS camera and to provide a crude
spatial distribution of the energy depositions of the fiber plates
complementary to the image intensifier.
3 Current status
Development of calibration and analysis methods using
the first small prototype camera
To develop the camera mentioned above, first we developed the small prototype camera in 2018 with a limited budget to conduct the pilot studies such as reconstruction of tracks
of charged particles and energy calibration. The first camera
consists of 8 × 8 fibers without an optical taper (Figure 18)
where the size and shape of the fibers are 1 mm square. Scintillation photons produced in the fibers are directly read out
by a 64-channel MPPC array.
Due to its small size, the range of the scattering angle thatcan be verified is limited, and impossible to verify multiple
scattering events. However, since the system is simple and67

Fig. 18. Photograph of the first prototype camera.

small, this detector is appropriate for developing calibration
and analysis methods. First, we started to build a Compton
camera system by combining detectors that serve as absorbers.
The absorbers are GAGG(Ce) scintillators with the size of
1 cm cube, which have a short decay time of scintillation light,
leading to precise coincidence between the scatterer and absorber. Also, they have high light yield and are expected tohave a good energy resolution. We adopted GAGG(Ce) witha shorter decay time of 50 ns than typical ones. We developed an offline analysis program to derive the energy resolution of the GAGG(Ce) scintillators by measuring the energy
spectrum of 662 keV gamma rays from 137 Cs.
Additionally, to develop an analysis algorithm for actual
data, we constructed a realistic simulation model with theGeant4 simulation toolkit [13]. We have developed a simple track reconstruction algorithm based on cosmic-ray muon
events (Figure 19). We also developed a method of energy
calibration of the fiber plates by simulating cosmic-ray muonsand comparing their spectrum of energy deposited in the fiber
plates with actual measurements (Figure 20).
Detailed design study and development of the second
prototype camera
First, we examined candidate scintillation fibers in detail.
The fiber is 1 mm square, and we selected BCF-20 (Saint
Gobain) that emits green light that matches the wavelength
response of the GaAsP photocathode of an image intensifier
used. Furthermore, the reflective surface of the fiber has a
two-layer structure known as multi-clad, which has a highlight collection efficiency. Using our developed Geant4 particle simulation and the large-scale computer system of theInstitute for Cosmic Ray Research (ICRR) at the University of Tokyo, we evaluated the size of the detector within
the limited budget where practical measurements are possible
with a faint radioactive source in the laboratory. The size is
32 mm × 32 mm × 64 mm with the detection efficiency toverify multiple scattering events. Finally, we made the second
large prototype camera (Figure 21), where only the lower halfof the multilayer fiber plates was manufactured due to the limited budget, and the upper half will be added this year. Also,
we applied for a patent for reconstructing the energy and direction of gamma rays, which was found when designing thelarge camera.
We also developed a signal polarity inversion board forFig. 19. Images of a simulated cosmic-ray muon event. Top: Distribution of energy deposited by the fibers as seen from the X-layer
direction. Note that there are blanks for Y layers because the
X and Y layers overlap alternately. Bottom: Reconstruction of amuon track based on the positions of the hit fibers. The solid line
represents the reconstructed direction of the track, whereas thegreen arrow represents the true incident position and direction of
the muon.

the EASIROC signal readout module used to read out the signals generated by MAPMTs. Since the EASIROC ASIC chip
responds only to positive signal inputs, polarity inversion isrequired to read out negative MAPMT signals. We selecteda chip transformer with less signal loss and distortion, anddeveloped a multi-channel signal inversion board. We investigated the distortion of the waveform and frequency responsewith respect to the input signal, and confirmed that it is acceptable to the use in the camera.
The evaluations of the performance of the CMOS camera
such as noise level for each pixel, dynamic range, and delay
time due to an external trigger for the image intensifier wereconducted and we confirmed that it can be used in the feasibility studies.
Acknowledgments
This study was supported by JSPS KAKENHI Grant
Numbers JP19H01919, JP22K03647, a Grant-in-Aid “Research Booster” from Ibaraki University, and a Grant-in-Aid
from Takahashi Industrial and Economic Research Foundation. This work was partially carried out by the joint research
program of the Institute for Cosmic Ray Research (ICRR), theUniversity of Tokyo. We would like to thank the Open source
consortium of instrumentation (Open-It), Japan for its supportand Enago (www.enago.jp) for its English language review.
We thank T. Yoshida and S. Yanagita at Ibaraki University for
useful comments. We also thank M. Kobayashi, Z. Jiang and68[3] Detection of Gamma Rays around 1 TeV from
RX J0852.0-4622 by CANGAROO-II, Katagiri, H.,
Enomoto, R., Ksenofontov, L. T., et al. , 619, L163
(2005)[4] Observation of VHE γ-rays from Cassiopeia A with the
MAGIC telescope, Albert, J., Aliu, E., Anderhub, H., etal., Astronomy and Astrophysics, 474, 937 (2007)[5] Population study of Galactic supernova remnants at very
high γ-ray energies with H.E.S.S., H. E. S. S. Collaboration, Abdalla, H., Abramowski, A., et al. 2018, Astronomy & Astrophysics, 612, A3. doi:10.1051/00046361/201732125[6] The First Fermi LAT Supernova Remnant Catalog,
Acero, F., Ackermann, M., Ajello, M., et al. 2016,
Astrophysical Journal Supplement Series, 224, 8.
doi:10.3847/0067-0049/224/1/8[7] Potential PeVatron supernova remnant G106.3+2.7 seenin the highest-energy gamma rays, Tibet ASγ Collaboration, Amenomori, M., Bao, Y. W., et al. 2021, Nature
Astronomy, 5, 460. doi:10.1038/s41550-020-01294-9

Fig. 20. Energy spectra of cosmic-ray muons: (top) simulation, (bottom) actual measurement. The actual measurement can include
signals other than cosmic-ray muons.

[8] Detection of the Characteristic Pion-Decay Signature in
Supernova Remnants, Ackermann, M., Ajello, M., Allafort, A., et al. 2013, Science, 339, 807[9] Nuclear gamma ray production by cosmic rays,
Meneguzzi, M. & Reeves, H., 1975, A&A, 40, 91[10] Galactic Cosmic Rays in the Local Interstellar Medium:
Voyager 1 Observations and Model Results, Cummings,
A. C., Stone, E. C., Heikkila, B. C., et al. 2016, Astrophysical Journal, 831, 18[11] Instrument Description and Performance of the Imaging
Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory, Schoenfelder, V., Aarts,
H., Bennett, K., et al. 1993, Astrophysical Journal Supplement, 86, 657. doi:10.1086/191794[12] Nuclear de-excitation line spectrum of Cassiopeia A,
Summa, A., Elsässer, D., & Mannheim, K. 2011, Astronomy & Astrophysics, 533, A13. doi:10.1051/00046361/201117267

Fig. 21. Photograph of the second prototype camera.

Y. Kawashima at Ibaraki University for their help.

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remnant RX J1713.7-3946, Enomoto, R., Tanimori, T.,
Naito, T., et al., Nature, 416, 823 (2002)[13] Geant4–a simulation toolkit, Agostinelli, S., Allison, J., Amako, K., et al. 2003, Nuclear Instruments and Methods in Physics Research A, 506, 250.
doi:10.1016/S0168-9002(03)01368-8

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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 waves with researchers of gravitational
wave experiments and theory in Japan. The main items are theconstruction of the large-scale cryogenic interferometer (KAGRA) at Kamioka underground and the operation of CLIO.
For this purpose, KAGRA observatory was established at the
beginning of the fiscal year of 2016 to assist the constructionof KAGRA gravitational wave telescope.
The Observational Cosmology Group studies 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 imager ofHyper Suprime-Cam mounted on the Subaru telescope.
Theory Group conducts both theoretical studies of the
Universe and astroparticle physics.

Fig. 1. Schematic view of the bKAGRA interferometer[3]. Type-A,
Type-B, Type-Bp, and Type-C are the names of vibration isolation
systems 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.
Figures 1 and 2 show a schematic view of the optical layout of the bKAGRA interferometer and the KAGRA vibration isolation systems. Table 1 shows design parameter ofthe bKAGRA interferometer[3]. The bKAGRA interferometer will employ a Resonant Sideband Extraction (RSE) interferometer with a 180-Watt laser, cryogenic Sapphire mirrors,
and several kinds of vibration isolation systems. The bKAGRA interferometer should attain a sensitivity high enoughfor the detection of gravitational waves with the help of theFig. 2. KAGRA vibration isolation systems[3]. KAGRA equips fourkinds of vibration isolation systems as Type-A, Type-B, Type-Bp,
and Type-C.

high power laser and RSE interferometer to reduce the quantum noise, the cryogenic Sapphire mirrors to reduce the thermal noise, and the vibration isolation systems to reduce theseismic noise. Figure 3 shows designed sensitivities of bKAGRA in the cases of Broadband RSE (BRSE) and of Detuned
RSE (DRSE), where the incoherent sum of the fundamental
noise sources is assumed. The observation range for an inspiral and merger of neutron-star binary reaches 135 Mpc in
BRSE and 153 Mpc in DRSE with the same definition of theobservation range as LIGO and Virgo.
The KAGRA observatory signed the three documents between LIGO and VIRGO in order to realize international joint

70
Table 1. The design parameters of the bKAGRA interferometer[3].

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

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[7]. The next international
collaborative observation called O4 will be started from March2023 as of June 17, 2022.
Fig. 3. The designed sensitivity of the bKAGRA interferometer[3]. ”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) cases. 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.

observation on October 4, 2019. The documents are the Memorandum of Agreement between VIRGO, KAGRA, and LIGO
(main part)[4], Memorandum of Agreement between VIRGO,
KAGRA, and LIGO (Attachment A)[5], and Letter of Intent
for KAGRA to Join the O3 Run[6]. On the same day, the
KAGRA observatory held a completion ceremony.
Figure 4 shows the international collaborative observation
scenario[7] as of June 17, 2022. LIGO conducted Observation
1 (O1) from September 12, 2015, to January 19, 2016, andObservation 2 (O2) from November 30, 2016, to August 25,
2017. Virgo joined O2 on August 1, 2017. LIGO and Virgo
started Observation 3 (O3) from April 1, 2019. Initially, O3
was planned to continue until the end of April in 2020, but it
was suspended on March 27, 2020, due to the influence of the
COVID-19[8]. The next international collaborative observation called O4 will be started from March 2023 as of June 17,2022[7].
In FY2019 we started interferometer commissioning
works to reach the sensitivity required to join O3. The
required sensitivity of 1 Mpc was defined as an observation range of neutron star binary coalescences[6]. KAGRA

reached the required sensitivity almost the end of March 2020.
Along with the commissioning works the KAGRA observatory carried out several engineering runs and two Observation runs by the end of April 2020. The first observation run
was carried out only by KAGRA with the observation rangeof 0.5 Mpc from February 25 to March 7, 2020. The second
observation run was carried out with GEO600 from April 7
to 21, 2020. This observation called O3GK is regarded as
an official joint observation with GEO600 by LIGO, VIRGO,
and KAGRA. KAGRA was operated in O3GK with the observation range of almost 0.7 Mpc and duty factor of 53 % has
achieved.
Searching for transient gravitational-wave signals in observation data obtained in O3GK has been performed. Dedicated searches for binary coalescence signals and generic transients associated with gamma-ray burst events observed during O3GK have been also performed. No gravitational wave
events were identified[9]. Details will be shown in the ”Dataanalysis” section.
After the O3GK observation, we restricted activities by
the end of May 2020 to prevent a spread of COVID-19. Activity restrictions have been gradually relaxed from June 2020,
and interferometer upgrades and refurbishment works for O4
are underway accordingly. In FY2021 we continued the upgrade and refurbishment works. Details of the works areshown in the following sections.
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.

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The rapidly progressing status of KAGRA were presented
in many international conferences. Many papers about the
progress of KAGRA were also published [10], [11], [12], [13],[14], [15]. We also presented activities on our web-page.[16]Bibliography[1] ”KAGRA: 2.5 generation interferometric gravitational
wave detector”, KAGRA collaboration, Nature Astronomy, Vol. 3, January 2019, 35-40[14] ”Effects of lightning strokes on underground gravitational waves observatories”, T. Washimi, T. Yokozawa,
M. Nakano, T. Tanaka, K. Kaihotsu, Y. Mori, T. Narita,
JINST 16 P07033 (2021)[15] ”Method for environmental noise estimation via injection tests for ground-based gravitational wave detectors”, Tatsuki Washimi, Takaaki Yokozawa, Taiki
Tanaka, Yosuke Itoh, Jun’ya Kume, and Jun’ichi
Yokoyama, Class. Quantum Grav. 38 125005 (2021)[2] ”Constructionof
KAGRA:
anundergroundgravitational-wave observatory”, KAGRA collaboration, Prog. Theor. Exp. Phys. 2018, 013F01 (2018)[16] http://gwcenter.icrr.u-tokyo.ac.jp/en/[3] ”First cryogenic test operation of underground km-scale
gravitational-wave observatory KAGRA”, KAGRA collaboration, Class. Quantum Grav. 36 165008 (2019)

Input and Output Optics[4] https://gwdoc.icrr.u-tokyo.ac.jp/cgibin/DocDB/ShowDocument?docid=10663[5] https://gwdoc.icrr.u-tokyo.ac.jp/cgibin/DocDB/ShowDocument?docid=10664[6] https://gwdoc.icrr.u-tokyo.ac.jp/cgibin/DocDB/ShowDocument?docid=10813[7] https://www.ligo.caltech.edu/news/ligo20220617[8] https://www.ligo.caltech.edu/news/ligo20200326[9] The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration, ”First joint observation by the underground gravitational-wave detector
KAGRA with GEO 600”, Prog. Theor. Exp. Phys. 2022
063F01(37 pages)[10] ”The Current Status and Future Prospects of KAGRA,
the Large-Scale Cryogenic Gravitational Wave Telescope Built in the Kamioka Underground”, KAGRA collaboration, Galaxies 2022, 10(3), 63[11] ”Application of the Hilbert-Huang transform for analyzing standing-accretion-shock-instability induced gravitational waves in a core-collapse supernova”, M. Takeda,
Y. Hiranuma, N. Kanda ,K. Kotake, T. Kuroda, R.
Negishi, K. Oohara, K. Sakai, Y. Sakai, T. Sawada,
H. Takahashi, S. Tsuchida, Y. Watanabe, T. Yokozawa,
Physical Review D 104, 084063 (2021)[12] ”Local Hurst exponent computation of data from triaxial seismometers monitoring KAGRA”, A. Longo,
S. Bianchi, W. Plastinao, K. Miyo, T. Yokozawa, T.
Washimi, A. Araya, Pure and Applied Geophysics volume 178, pages3461–3470 (2021)[13] ”The KAGRA underground environment and lessons
for the Einstein Telescope”, F. Badaracco, C. D. Rossi,
I. Fiori, J. Harms, K. Miyo, F. Paoletti, T. Tanaka,
T. Washimi, T. Yokozawa, Phys. Rev. D 104, 042006(2021)[Spokesperson : Osamu MIYAKAWA]
ICRR, The Univ. of Tokyo, Hida, Gifu 506-1205

The input and output optics (IOO) of KAGRA consistsof the pre-stabilization laser (PSL), auxiliary locking system (ALS), input optics chain, output optics chain. PSL
includes the intensity stabilization servo (ISS), pre-modecleaner (PMC), and modulation system for the main interferometer. ALS includes the phase-locking system for thegreen beam each for X and Y arms, the fiber system, and thelocking system for the arm cavities. The input optics chain
includes the input mode cleaner (IMC), input Faraday isolator (IFI), and two input mode matching telescopes (MMT).
The output optics chain includes the output mode matchingtelescopes (OMT), output Faraday isolator (OFI), and output
mode cleaner (OMC).
In the fiscal year of 2021, KAGRA spent most of thetime for upgrading the vibration isolation system, so IOO proceeded some developments of laser outside of KAGRA mine,and stabilized each system of IOO without the main interferometer.
For the high power laser, a 60W high power laser is being developed at the University of Toyama. We found dirty
mode shapes around the beam waist. We suspected that modeshapes in master laser were something wrong. We replaced
the master laser and the shapes became a little better, however we realized that the shape of beam depends on alignment
between the master laser and the power amplifier. Intensity
noise, frequency noise are the same with the old master laser,
so we have decided we leave it. Another issue was that wesaw several moving peaks on the intensity noise. Frequency of
peaks changes with time. After some investigations, we concluded that the peaks come from the power supply for LDs.
We were afraid to break our current power supply, we decided
to leave it without any further investigation until spare power
supply and LDs arrive in FY2022. Even we have such peaks,
still the intensity noise level of new laser is better than the current KAGRA laser, so we decided to replace the current laser
to the new high power laser.
All optical components of ISS will be also replaced to thenew design that has two sets of power receiving PDs. This
multiple PD setup accepts higher laser power in future. We
will develop them in air for O4, and they will be placed into a
vacuum chamber for O5.

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PMC is basically stably locked almost of time, and weare preparing remote input beam alignment system using pico
motors.
As for ALS, it was pointed out that the actuator range for
feedback signals was narrow, and we tried to increase the dynamic range to employ two kinds of PZT. One is a short range
PZT for high frequency feedback and another is a long range
PZT for lower frequency. However we noticed that the longrange PZT introduces a huge misalignment to the fiber coupler with a large feedback signal. It must be fixed before O4
starts, or it provides us a narrower feedback range than O3’s
range.
IMC ASC (Alignment Sensing and Control) is one of themost developed one in FY2021 in IOO. We succeeded in feeding back to 3 MC mirrors and one of the PZT on a steering
mirror at the end of PSL table to adjust the angle of input beam
for IMC. All the servo works pretty well and keep the alignment even the temperature in the PSL room changed drastically when HEPA filters switched on/off.
We are preparing the whole IOO system working stably
for O4 that will start in FY2022.
Cryogenic system[Spokesperson : Takafumi USHIBA]

Fig. 5. Schematic view of the cryogenic payload.

ICRR, The Univ. of Tokyo, Hida, Gifu 506-1205

One of the unique features of KAGRA is the use of cryogenic sapphire mirrors in the 3-km arm cavities. The people
working in this group are mainly from ICRR, KEK and University of Toyama. Here, we summarize the activity of members of ICRR in FY 2021.
Hardware update of the cryogenic payload A KAGRA sapphire mirror is suspended by a 9-stage suspension and its bottom 4 stages that include sapphire mirror is called cryogenic
payload. Figure 5 shows the schematic view of the cryogenic
payload.
In the fiscal year 2021, actuators and sensors on the cryogenic payloads were updated to increase the actuation range
and improve local control of the payload, respectively. After these update, suspension functions including room temperature parts are checked one by one to confirm that the suspension is acceptable for the observation. ITMX and ETMX
passed the check and the vacuum evacuation for their suspensions were started. The check for ITMY and ETMY werestarted at the end of fiscal year 2021.
Cryogenic system KAGRA uses 6 cryocoolers for cooling
one sapphire mirrors. Two is used for cooling the cryogenic payload, two is used for cooling two layers of radiation
shields, and the other two is used for cooling duct shields.
In fiscal year 2021, cryocoolers for duct-shield coolingwere replaced for achieving maintenance-free long-term operation and improving cooling performance. All 8 duct-shield
cryocoolers were replaced and duct-shield cryocoolers for
ITMX and ETMX were operated after the vacuum evacuation.
Characterization and local controls of the cryogenic payload To control the angular and logitudinal motion of thesuspensions, characterization of the cryogenic payload was
performed. Suspension mechanical transfer functions were
measured and mechanical resonances below 10 Hz are characterized: Q-values of the resonances are measured.
After the characterization, local damping control filters
were designed and implemented to ITMX and ETMX. Owing to the damping, residual RMS angular motions of ITMX
and ETMX could be reduced below 1 urad, which was enoughsmall for initial alignment of the arm cavity. In addition tothis, a part of characterization for comparing room temperature performance and cryogenic one was performed.
Acknowledgement Mechanical Engineering Center of KEK
makes a large contribution through providing many products
for our research.

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

In preparation for the start of international joint observations of gravitational wave, we have expanded our network.
We have achieved improved fault tolerance by building a separate network from the previous one. We have also confirmedthat this network is capable of 40 Gbps data transfer speed.
We built a separate network from the one we had been using for control, and moved equipment that was not fast enoughto communicate to that network. This dramatically reduced
the number of times the equipment hung up. In addition, we
installed additional optical fiber and network switches in themine to improve the network environment.

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We are creating a new type of I/O chassis. In FY2021 westarted mass production of I/O chassis. In total, we created 10
chassis. This new I/O chassis will be able to hold four PCIe expansion cards and will be connected to the computer via
MTP optical fiber cables.
We have created new LVDT drivers, PCal drivers, 20
whitening filters, 10 HP Coil driver boards, and 10 D-sub
BNC converter for ADC. We also prepared for building several types of D-sub cables, which were becoming scarce in
our inventory.
Detector Charcterization[Spokesperson : Takahiro Yamamoto]
ICRR, The Univ. of Tokyo, Hida, Gifu 506-1205

Data validation for O3GK The goal of Detector Characterization (DetChar) activities is to improve the reliability of gravitational waves detection by serving the data quality information which is one of criteria for selecting data using gravitational waves searches.
Some data quality indicators were provided in the realtime (o(1s) latency) process as reported in the Annual report
2020-2021.
We conducted to validate all provided indicators as an offline process to publish about the data quality information.
There is no major change between online and offline ones.
But more detailed information on equipment malfunctions
was provided. Calibrated strain signal which passes the data
quality check and the data quality information itself are now
available on Gravitational Wave Open Science Center [1].
Preparation for O4 The LIGO-Virgo-KAGRA (LVK) Collaboration has now reached an agreement to begin observations in 2023, and preparations for that observation system
are underway.
DetChar group lead to unify the data sharing about thedetector status and the data quality information in observing
runs.
Especially low-latency event validation for the gravitational wave candidates is one of the most important mission
for DetChar group. We constructed the framework to sharethe detector status in various latencis name as Data Quality
Report (DQR). DQR activity includes the evaluation of thedetector noise status around the time when gravitation wave
candidates occurs. In order to unify the contents of information among LVK, we developed the DQR software with LIGO
and Virgo’s DetChar groups.

Bibliography[1] R. Abbot et al. SoftwareX 13 (2021) 100658.

Environment Monitors[Spokesperson : Takaaki YOKOZAWA ]ICRR, The Univ. of Tokyo, Hida, Gifu 506-1205

To detect the Gravitational Waves(GWs), the distinguishfrom the noise is important. Also, because the amplitude of
the GW is extremely small, small vibration from essential instruments, small sound from outside of the experimental areaand so on can produce noise source contamination that reduces the sensitivity. To evaluate the noise sources, about
100,000 auxiliary channels are recorded by the KAGRA digital system. The main purposes of the physical and environmental monitoring are as followings.
The first is to characterize the underground and cryogenic
environment; KAGRA detector has two unique features, underground and cryogenic environment, and those features willbe essential in the next-generation detectors. We performed
the correlation analysis between GW strain data and monitors. We found several interesting result, one result is thatwhen the strong wind was appeared in the weather station
which is placed in the entrance of KAGRA experimental area,
the infrasound (∼ 0.2 Hz sound) was also appeared inside themountain. The principle of developing the infrasound would
be caused by the vibration of the surface of the mountain. We
will continuously monitor the infrasound by increasing thedetectable microphone. Next interesting topic was measuring the Schumann resonance inside and outside the mountain.
The Schumann resonances are a set of spectrum peaks in theextremely low frequency (ELF) portion of the Earth’s electromagnetic field spectrum. Schumann resonances are global
electromagnetic resonances, generated and excited by lightning discharges in the cavity formed by the Earth’s surface andthe ionosphere. The preliminary result showed that the magnetic field of the frequency of the Schumann resonance inside
the mountain is larger than that of the outside the mountain,
but this reason is under discussing.
The second aim is understanding the interferometer noise
by environmental monitors. We installed various PEMs, including accelerometers, microphones, magnetometers, thermometers, weather station and so on. After the O3GK observation, we performed several analysis using observation data.
First analysis is the lock loss study. To detect the GW using the interferometer, we must control several degree of freedom, but when some external disturbance occurred, those control became unstable and lock loss happened. To understandthe reason of lock loss, we can achieve the longer observation
time. The number of identified lock-loss in the O3GK was 75.
10 of lock loss were caused by the saturation of the control
and photo detector, 48 of lock loss were related by Abnormal
feedback signal, 53 of lock loss were related by the bad interferometer alignment, and 13 of then were the 1 Hz oscillation.
Next step is development of the auto analysis tool of lock loss
study. Second analysis is the offline noise subtraction using
the independent component analysis(ICA). In the O3GK, theacoustic noise limit the sensitivity of KAGRA at middle frequency. By investigating the noise path and by applying theoffline noise subtraction, we can perform the improvement of
the sensitivity. ICA is the method of blind source separation,
this can be used as the non-gaussian noise subtraction methodby using auxiliary channels such as PEM channels. The result showed that by using the microphone data inside the PSL
room, we succeeded to perform the noise subtraction for thewhole term of the O3GK period.

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BNS range (Mpc)

1.1
1.0
0.9
0.8
0.7
0.6
0.5

GEO (median: 1.06 Mpc)

0.4

KAGRA (median: 0.66 Mpc)
0

2

4

6

8

10

12

14

Time (days) from April 7, 2020

Fig. 6. Noise amplitude spectral density of GEO (black) and KAGRA(yellow) during O3GK run.

Data Analysis[Spokesperson : Hideyuki Tagoshi]
ICRR, The Univ. of Tokyo, Kashiwa, Chiba, 277-8582

KAGRA signed a Memorandum of Agreement (MoA) between LIGO and Virgo. The MoA defines the policy of thejoint data analysis and the data sharing. Based on this MoA,
all data analysis activities are now done jointly with LIGO
and Virgo. In LIGO-Virgo-KAGRA (LVK) collaboration,
there is a Data Analysis Council (DAC) which is a committee
which manage data analysis activities in LVK. Under DAC,
there are four working groups regarding gravitational wave
signal search, Compact Binary Coalescence working group(CBC), Burst working group (Burst), Continuous Wave Working group (CW), and Stochastic background working group(Stochastic). In LVK, there is also Operations group which
manages various data related activities. Those include calibration, detector characterization, computing and software,
low latency, open data, run planning and so on. In KAGRA,
Data Analysis Committee manages data analysis and date related activities.
KAGRA performed an observing run from April 7th to
20th, 2020 (UTC) together with GEO 600 in Germany. Originally, this observation was supposed to be done jointly with
LIGO and Virgo at the end of the 3rd observing run of LIGO
and Virgo. However, while KAGRA was preparing the observing run, the LIGO and Virgo detectors were forced toterminate operations on March 27, 2020 (UTC) due to the
COVID-19 pandemic. Fortunately, however, GEO600 in Germany was operating and continuously taking data. Thus, GEO
and KAGRA decided to perform an observing run jointly. In
LIGO-Virgo-KAGRA (LVK) collaboration, this observation
was officially approved as an joint observation under the LVK
framework. This observing run was named O3GK.
In this year, we have continued the analysis of O3GK data.
Figure 6 represents the Noise amplitude spectral density of
GEO (black) and KAGRA (yellow) during O3GK run. Narrow peaks in the spectra are due to such sources as resonancesof the suspension system (violin modes) and harmonics of theelectrical grid frequency (50 Hz for GEO and 60 Hz for KAGRA). Figure 7 represents the detectable distance of the inspiral of binary neutron stars (BNS).
Several gravitational wave event searches were performed.

Fig. 7. BNS inspiral ranges for GEO and KAGRA over the joint run.

GRB Name
200412A
200415A
200418A
200420A

Data Source
Fermi-GBM
Fermi-GBM, IPN
Fermi-GBM
Fermi-GBM

Type
Long
Short
Long
Short

Table 2. GRBs observed during O3GK run
times when both detectors were taking science-quality data.

Those include all-sky searches for binary neutron star coalescences by using the GstLAL pipeline, all-sky searches for unmodeled short duration burst waves with the coherent WaveBurst pipeline, targeted CBC searches associated with shortGamma Ray Bursts (GRBs) by using pyGRB pipeline, andtargeted un-modeled burst searches associated with short andlong GRBs by using X-pipeline. No candidate gravitational
wave events were identified.
The analyzed GRB events are listed in Table 2. Amongthem, a short GRB, GRB 200415A, was subsequently associated with a magnetar giant flare in the nearby galaxy NGC253
at 3.5 Mpc based on its sky position, temporal and spectral
properties and inferred energy. Since the distance is very
short, it is especially interesting to search for gravitational
wave signals associated with this GRB. However, the sensitivity of O3GK observing run was not sensitive to observe sucha distance, and we could not constrain the progenitor of thisGRB.
The lack of detected events in this observing run is expected given the sensitivity of the GEO–KAGRA network at
the time. However, the sensitivity of KAGRA is expected to
improve by more than two orders of magnitude later in thisdecade, becoming comparable to that of the LIGO and Virgo
detectors. Our analyses have demonstrated the ability to incorporate KAGRA data into standard transient search pipelinesthat have been used to detect GW in LIGO and Virgo data.
These results were published in PTEP[1]. The O3GK data
was publicly released at GWOSC site [2].
As one of the main computing resources in KAGRA, themain data server of KAGRA (System A) is located at ICRR
Kashiwa. It has a 2.5PiB data storage. All KAGRA datataken at Kamioka are packed into one file for every 32 seconds, and are transferred continuously to the main data server

75

at Kashiwa. Beside this, low latency data transfer is also doneby packing only the gravitational wave channel and a few
other channels 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). In order to share the low latency data
file among LIGO-Virgo-KAGRA, the low latency data files
are also transferred to a server at LIGO Caltech. On the other
hand, low latency data files of LIGO and Virgo detectors aretransferred from Caltech to the main data server at Kashiwa.
An additional KAGRA main data server (System B) is under construction at ICRR Kashiwa. It has a 4 PiB data storage.
System B will be operational in 2022.

Bibliography[1] The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration, PTEP 2022,
063F01 (2022).
[2] The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration, The O3GK Data
Release, https://doi.org/10.7935/38s2-7g84 (2022).

We present Keck/LRIS follow-up spectroscopy for 13
photometric candidates of extremely metal-poor galaxies(EMPGs) selected by a machine-learning technique applied
to the deep (∼ 26 AB mag) optical and wide-area (∼ 500
deg2 ) Subaru imaging data in the EMPRESS survey. Nineout of the 13 candidates are EMPGs with an oxygen abundance (O/H) less than ∼ 10% solar value (O/H)⊙ , and four
sources are contaminants of moderately metal-rich galaxies orno emission-line objects. Notably, two out of the nine EMPGs
have extremely low stellar masses and oxygen abundances of
5 × 104 –7 × 105 M⊙ and 2%–3% (O/H)⊙ , respectively. With asample of five EMPGs with (Fe/O) measurements, two (three)of which are taken from this study (the literature), we confirm
that two EMPGs with the lowest (O/H) ratios of ∼ 2% (O/H)⊙
show high (Fe/O) ratios of ∼ 0.1, close to the solar abundance
ratio. Comparing galaxy chemical enrichment models, wefind that the two EMPGs cannot be explained by a scenario of
metal-poor gas accretion/episodic star formation history due
to their low (N/O) ratios (Figure 8). We conclude that the two
EMPGs can be reproduced by the inclusion of bright hypernovae and/or hypothetical pair-instability supernovae (SNe)preferentially produced in a metal-poor environment. Thisconclusion implies that primordial galaxies at z ∼ 10 could
have a high abundance of Fe that did not originate from TypeIa SNe with delays and that Fe may not serve as a cosmic
clock for primordial galaxies.

Bibliography[1] Isobe, Y., Ouchi, M., Suzuki, A., Moriya, T., Nakajima, K., Nomoto, K., Rauch, M., Harikane, Y., Kojima,T., Ono, Y., Fujimoto, S., Inoue, A. K. , Kim, J. H.,
Komiyama, Y., Kusakabe, H., Lee, C.-H., Maseda, M.,
Matthee, J., Michel-Dansac, L., Nagao, T., Nanayakkara,T., Nishigaki, M., Onodera, M., Sugahara, Y., Xu, Y.,
2022, The Astrophysical Journal, 925, 111

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

EMPRESS. IV. Extremely Metal-poor Galaxies Including Very Low-mass Primordial Systems with M∗ =104 –105 M⊙ and 2%–3% (O/H): High (Fe/O) Suggestiveof Metal Enrichment by Hypernovae/Pair-instability
Supernovae [1]In collaboration with the members of National Astronomical Observatory of Japan, Kavli Institute for the Physics andMathematics of the Universe, Carnegie Observatories, University of Copenhagen, Waseda University, Subaru Telescope,
SOKENDAI (The Graduate University for Advanced Studies), University of Geneva, NSF’s National Optical-Infrared
Astronomy Research Laboratory, Leiden University, ETH
Zurich, University of Lyon, Ehime University, Swinburne
University of Technology, and The University of Tokyo.

GOLDRUSH. IV. Luminosity Functions and Clustering Revealed with 4,000,000 Galaxies at z ∼ 2–7:
Galaxy-AGN Transition, Star Formation Efficiency,
and Implication for Evolution at z > 10 [2]In collaboration with the members of Shanghai Jiao Tong University, Saint Mary’s University Kitami Institute of Technology, University of Arizona, The University of Tokyo, Laboratoire d’Astrophysique de Marseille, University of Geneva,
University of Copenhagen, NRC Herzberg Astronomy andAstrophysics, National Astronomical Observatories of China,
Waseda University, and Ehime University.
We present new measurements of rest-UV luminosity
functions and angular correlation functions from 4,100,221
galaxies at z ∼ 2–7 identified in the Subaru/Hyper SuprimeCam survey and CFHT Large Area U-band Survey. The obtained luminosity functions at z ∼ 4–7 cover a very wide UV∗ combined with previluminosity range of ∼ 0.002–2000LUV
ous studies, confirming that the dropout luminosity function
is a superposition of the active galactic nucleus (AGN) luminosity function dominant at MUV ≲ −24 mag and the galaxy

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Fig. 8. Fe/O ratio as a function of EW0 (Hβ ) and N/O described in the left and right panels, respectively. The red circles and magenta diamonds
are our EMPGs and those found in the literature. The gray circles indicate local metal-poor galaxies reported in the literature, for which we
only show errors of Fe/O. In the left panel, the cyan, blue, purple, and yellow curves with the shaded regions show the MW, the HN 100%,
the BrHN 20%, and the PISN models, respectively. The black curve indicates the No HN/PISN model. In the right panel, the cyan andgreen curves represent evolution tracks of Fe/O and N/O when first and episodic starbursts occur in a galaxy, respectively. The numbers
accompanied by the curves indicate the ages in the unit of Myr. We draw the evolution tracks with solid and dashed lines before and after
100 Myr, respectively, because all the EMPGs shown in the left panel have ages ≲ 100 Myr. To predict Fe/O and N/O ratios, we use previous
theoretical models. The gray lines show the solar abundances.

luminosity function dominant at MUV ≳ −22 mag, consistent
with galaxy fractions based on 1037 spectroscopically identified sources. Galaxy luminosity functions estimated from thespectroscopic galaxy fractions show the bright-end excess beyond the Schechter function at ≳ 2σ levels, possibly made byinefficient mass quenching, low dust obscuration, and/or hidden AGN activity. By analyzing the correlation functions atz ∼ 2–6 with HOD models, we find a weak redshift evolution(within 0.3 dex) of the ratio of the star formation rate (SFR) to
the dark matter accretion rate, SFR/Ṁh , indicating the almost
constant star formation efficiency at z ∼ 2–6, as suggested byour earlier work at z ∼ 4–7. Meanwhile, the ratio gradually increases with decreasing redshift at z < 5 within 0.3 dex, whichquantitatively reproduces the cosmic SFR density evolution,suggesting that the redshift evolution is primarily driven by
the increase of the halo number density due to the structureformation, and the decrease of the accretion rate due to thecosmic expansion. Extrapolating this calculation to higher
redshifts assuming the constant efficiency suggests a rapid decrease of the SFR density at z > 10 with ∝ 10−0.5(1+z) , which
will be directly tested with the James Webb Space Telescope(Figure 9).

Bibliography[2] Harikane, Y., Ono, Y., Ouchi, M., Liu, C., Sawicki, M.,
Shibuya, T., Behroozi, P. S., He, W., Shimasaku, K.,
Arnouts, S., Coupon, J., Fujimoto, S., Gwyn, S., Huang,J., Inoue, A. K. , Kashikawa, N., Komiyama, Y., Mat-suoka, Y., Willott, C. J., 2022, The Astrophysical Journal
Supplement Series, 259, 20

SILVERRUSH. IX. Lyα Intensity Mapping with Starforming Galaxies at z = 5.7 and 6.6: A Possible Detection of Extended Lyα Emission at ≳ 100 Comoving Kiloparsecs around and beyond the Virial-radius
Scale of Galaxy Dark Matter Halos [3]
In collaboration with the members of Waseda University,
Graduate University for Advanced Studies (SOKENDAI),
University of Geneva, NSF’s National Optical-Infrared Astronomy Research Laboratory, Ehime University, The University of Tokyo, Kitami Institute of Technology, and The Open
University of Japan.
We present results of the cross-correlation Lyα intensity mapping with Subaru/Hyper Suprime-Cam (HSC) ultradeep narrowband images and Lyα emitters (LAEs) at z = 5.7
and 6.6 in a total area of 4 deg2 . Although an overwhelming amount of data quality controls have been performed forthe narrowband images, we further conduct extensive analyses evaluating systematics of large-scale point-spread function
wings, sky subtractions, and unknown errors based on physically uncorrelated signals and sources found in real HSC images and object catalogs, respectively. Removing the systematics, we carefully calculate cross-correlations between Lyα
intensity of the narrowband images and the LAEs. We tentatively identify very diffuse Lyα emission with the ≃ 3σ

77

Fig. 9. Comparison of the cosmic SFR density at z > 7. The red curve with the shade represents the cosmic SFR density calculated in this work
based on the constant star formation efficiency at z > 5, integrated down to the SFR of 0.3M⊙ yr−1 (MUV = −17 mag), as previous studies.
The gray dashed curve shows the extrapolation of the relation of previous work at z > 6. The other curves show predictions from models in
the literature. All results are converted to use the Salpeter IMF.

(≃ 2σ ) significance at ≳ 100 comoving kiloparsecs (ckpc)
far from the LAEs at z = 5.7 (6.6), around and probably
even beyond a virial radius of star-forming galaxies with
Mh ∼ 1011 M⊙ (Figure 10). The diffuse Lyα emission possibly
extends up to 1000 ckpc with the surface brightness of 10−20 –10−19 erg s−1 cm−2 arcsec−2 . We confirm that the small-scale(< 150 ckpc) Lyα radial profiles of LAEs are consistent withthose obtained by recent Multi-Unit Spectroscopic Explorer
observations. Comparisons with numerical simulations suggest that the large-scale (∼ 150–1000 ckpc) Lyα emission
are not explained by unresolved faint neighboring galaxies including satellites, but by a combination of Lyα photons emitted from the central LAE and other unknown sources, suchas cold-gas streams and galactic outflow. We find no evolution in the Lyα radial profiles of our LAEs from z = 5.7 to
6.6, where theoretical models predict a flattening of the profile
slope made by cosmic reionization, albeit with our moderately
large observational errors.

Fig. 10. Cross-correlation function between the LAEs and the Lyα
emission intensity after subtracting the systematics. The filled circles in the left (right) panel represent the weighted mean of theresults in UD-COSMOS and UD-SXDS fields of z = 5.7 (6.6). The
S/Ns of the outer three data points are lower than 1. The black
crosses correspond to the stacked Lyα radial profile at z = 5–6
that is taken from the observational results of MUSE in the literature. The gray dashed lines show the DMH virial radius (rvir ) of
the LAEs in our samples whose DMH masses are estimated to be
Mh ∼ 1011 M⊙.

Bibliography[3] Kakuma, R., Ouchi, M., Harikane, Y., Ono, Y., Inoue, A.
K., Komiyama, Y., Kusakabe, H., Liu, C.-H., Matsuda,Y., Matsuoka, Y., Mawatari, K., Momose, R., Shibuya, T.,
Taniguchi, Y., 2021, The Astrophysical Journal, 916, 22

SILVERRUSH X: Machine Learning-aided Selectionof 9318 LAEs at z = 2.2, 3.3, 4.9, 5.7, 6.6, and 7.0 fromthe HSC SSP and CHORUS Survey Data [4]In collaboration with the members of Kitami Institute of Technology, Waseda University, Ehime University, University ofTsukuba, The University of Tokyo, National Astronomical
Observatory of Japan, The Open University of Japan, University of Copenhagen, Institut Teknologi Bandung, Kindai
University, Kure College, University of Geneva, and NSF’s
National Optical-Infrared Astronomy Research Laboratory.

We present a new catalog of 9318 Lyα emitter (LAE) candidates at z = 2.2, 3.3, 4.9, 5.7, 6.6, and 7.0 that are photometrically selected by the SILVERRUSH program with amachine learning technique from large area (up to 25.0 deg2 )
imaging data with six narrowband filters taken by the Subaru Strategic Program with Hyper Suprime-Cam and a Subaru
intensive program, Cosmic HydrOgen Reionization Unveiled
with Subaru. We construct a convolutional neural networkthat distinguishes between real LAEs and contaminants witha completeness of 94% and a contamination rate of 1%, enabling us to efficiently remove contaminants from the photometrically selected LAE candidates (Figure 11). We confirm
that our LAE catalogs include 177 LAEs that have been spectroscopically identified in our SILVERRUSH programs andprevious studies, ensuring the validity of our machine learning selection. In addition, we find that the object-matching

78

True label

1.0
Positive(Class 1-4)

Negative(Class 5-8)

TP =
0.688

FN =
0.312

FP =
0.005

TN =
0.995

Positive

Negative

True label

Predicted label(S/N>5)

TP =
0.937

FN =
0.063

Negative(Class 5-8)

FP =
0.005

TN =
0.995

Positive

Negative

Positive

0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0

Predicted label

Fig. 11. Confusion matrix. In the top panel, Classes 1–4 (5–8) of thetrue labels are classified as positive (negative). For the predictedlabels, we classify sources as positive if they show a high scoreof being an LAE. The numbers in the cells denote true positives(TP), false negatives (FN), false positives (FP), and true negatives(TN). In the best case scenario, the confusion matrix has non-zeroelements only in its diagonal cells and zero elements in the others.
The bottom panel is the same as the top panel, except that positive
for the true labels corresponds to simulated LAEs with S/N > 5.

rates between our LAE catalogs and our previous results are≃ 80%–100% at bright NB magnitudes of ≲ 24 mag. We also
confirm that the surface number densities of our LAE candidates are consistent with previous results. Our LAE catalogs
will be made public on our project webpage.

Bibliography[4] Ono, Y., Itoh, R., Shibuya, T., Ouchi, M., Harikane, Y.,
Yamanaka, S., Inoue, A. K., Amagasa, T., Miura, D.,
Okura, M., Shimasaku, K., Iwata, I., Taniguchi, Y., Fujimoto, S., Iye, M., Jaelani, A. T., Kashikawa, N., Kikuchihara, S., Kikuta, S., Kobayashi, M. A. R. , Kusakabe, H.,
Lee, C.-H., Liang, Y., Matsuoka, Y., Momose, R., Nagao,T., Nakajima, K., Tadaki, K., 2021, The AstrophysicalJournal, 911, 78

First HETDEX Spectroscopic Determinations of Lyα
and UV Luminosity Functions at z = 2–3: Bridging a
Gap between Faint AGNs and Bright Galaxies [5]In collaboration with the members of The University of Texas
at Austin, The Pennsylvania State University, Max Planck Institute for Extraterrestrial Physics, Ludwig-Maximilians University Munich, The State University of New Jersey, NYC

College of Technology, Flatiron Institute, University of Oxford, University of the Western Cape, and Lawrence Berkeley
National Laboratory.
We present Lyα and ultraviolet (UV)-continuum luminosity functions (LFs) of galaxies and active galactic nuclei(AGNs) at z = 2.0–3.5 determined by the untargeted optical spectroscopic survey of the Hobby-Eberly Telescope Dark
Energy Experiment (HETDEX). We combine deep Subaru
imaging with HETDEX spectra resulting in 11.4 deg2 offiber spectra sky coverage, obtaining 18,320 galaxies spectroscopically identified with Lyα emission, 2126 of whichhost type 1 AGNs showing broad (FWHM > 1000 km s−1 )Lyα emission lines. We derive the Lyα (UV) LF over 2
orders of magnitude covering bright galaxies and AGNs in
logLLyα /[erg s−1 ] = 43.3–45.5 (−27 < MUV < −20) by the
1/Vmax estimator (Figure 12). Our results reveal that thebright-end hump of the Lyα LF is composed of type 1 AGNs.
In conjunction with previous spectroscopic results at the faintend, we measure a slope of the best-fit Schechter function to+0.13
be αSch = −1.70−0.14, which indicates that αSch steepens fromz = 2–3 toward high redshift. Our UV LF agrees well withprevious AGN UV LFs and extends to faint-AGN and brightgalaxy regimes. The number fraction of Lyα-emitting objects∗ ∼ −21 to bright magnitude due to(XLAE ) increases from MUVthe contribution of type 1 AGNs, while previous studies claim∗ , suggestthat XLyα decreases from faint magnitudes to MUV∗ . Coming a valley in the XLyα–magnitude relation at MUVparing our UV LF of type 1 AGNs at z = 2–3 with those atz = 0, we find that the number density of faint (MUV > −21)
type 1 AGNs increases from z ∼ 2 to 0, as opposed to the evolution of bright (MUV < −21) type 1 AGNs, suggesting AGN
downsizing in the rest-frame UV luminosity.

Bibliography[5] Zhang, Y., Ouchi, M., Gebhardt, K., Mentuch C. E., Liu,C., Davis, D., Jeong, D., Farrow, D. J., Finkelstein, S.
L., Gawiser, E., Hill, G. J., Kakuma, R., Acquaviva, V.,
Casey, C. M., Fabricius, M., Hopp, U., Jarvis, M. J., Landriau, M., Mawatari, K., Mukae, S., Ono, Y., Sakai, N.,
Schneider, D. P., 2021, The Astrophysical Journal, 922,
167

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.
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 well

79

Fig. 12. Top: best-fit Lyα LF of our LAE sample with 2 < z < 3.2 (redsquares) and previous results at 2.0 < z < 3.2 (red circles). The red(blue) line shows our best-fit result of model 1 (2) with the measurements of our study and previous work, with the shaded region
corresponding to the 1σ uncertainty (68.27% equal-tailed credibleinterval). The black open downward triangles (dashed line), circles(solid line), and upward triangles (dotted line) represent the binned
Lyα LFs (best Schechter fit) from previous work at z ∼ 2.25, 2.54,
and 3.24, respectively. The gray triangles indicate the previous
results. We also show with gray lines the fitting results in the literature. The dashed, dotted, and solid gray lines indicate their best-fit
Schechter component, power-law component, and overall LF, respectively. Bottom: residuals of models 1 (red) and 2 (blue) fit inunits of the uncertainty in each luminosity bin.

as 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, six GW events were detected by LIGO. Among them is thefirst detection of GW from binary neutron star. The discovery
suggested strongly the existence of kilonova which can produce r-process elements. If they continue to find GW eventsfrom black holes and neutron stars, their origin becomes oneof 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 for-mation 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

Beyond Standard Model• Muon g - 2 in gauge mediation without SUSY CP problem [1]
In collaboration with the members of ICRR, Kavli IPMU
We discuss gauge mediated supersymmetry breaking
models which explain the observed muon anomalous magnetic moment and the Higgs boson mass simultaneously. The
successful explanation requires the messenger sector which
violates the relation motivated by the grand unification theory (GUT). The naive violation of the GUT relation, however,ends up with the CP problem. We propose a model in which
the phases of the gaugino masses are aligned despite the violation of the GUT relation. We also consider a model which
generates the µ-term and the additional Higgs soft masses
squared without causing CP violation. As a result, we finda successful model which explains the muon anomalous magnetic moment and the Higgs boson mass. The model is also
free from the CP, flavor-changing neutral current and the lepton flavor violation problems caused by the subdominant gravity mediation effects.

80
10−2
10−3

K ± decay

1000m`˜L < 500 GeV
700

600

10
sin(θ)µ [GeV]

Disappearing charged track

τ =1
10−5
10

Tri-lepton

10−7

100

1000

Supernova
τ = 10 4• Gauge kinetic mixing and dark topological defects [2]
In collaboration with the members of Rikkyo Univ., ICRR,
Kavli IPMU, TD-Lee Inst., Morpho, Inc.
We discuss how the topological defects in the dark sector affect the Standard Model sector when the dark photon
has a kinetic mixing with the QED photon. In particular, weconsider the dark photon appearing in the successive gauge
symmetry breaking, SU(2) → U(1) → Z2 , where the remaining Z2 is the center of SU(2). In this model, the monopole istrapped into the cosmic strings and forms the so-called beadsolution. As we will discuss, the dark cosmic string induces
the QED magnetic flux inside the dark string through the kinetic mixing. The dark monopole, on the other hand, does not
induce the QED magnetic flux in the U(1) symmetric phase,even in the presence of the kinetic mixing. Finally, we showthat the dark bead solution induces a spherically symmetric
QED magnetic flux through the kinetic mixing. The induced
flux looks like the QED magnetic monopole viewed from a
distance, although QED satisfies the Bianchi identity everywhere, which we call a pseudo magnetic monopole.
• Cosmological constraints on dark scalar [3]In collaboration with the members of ICRR, Kavli IPMU
We discuss cosmological constraints on a dark scalar particle mixing with the Standard Model Higgs boson. We pay
particular attention to the dark scalar production process when
the reheating temperature of the Universe is very low, whichallows us to give a conservative limit on the low-mass scalarparticle. We also study the effect of the self-interaction of
the dark scalars and find this has a significant impact on thecosmological constraints. We obtain the most conservative
cosmological constraint on the dark scalar, which is complementary to accelerator experiments and astrophysical observations.

sec

TR > 100 GeV

TR = 5 MeV

10−8
10−1

M2 [GeV]Fig. 13. The current collider constraints on the Higgsino-Wino system.
The orange lines show the rough upper limits on the left-handed
slepton mass to explain the observed aµ .

sec

−6

Soft di-lepton

LEP

100

−4

100

mφ [MeV]

101

102

Fig. 14. The cosmological constraints on the dark scalar model. The
gray region shows the most conservative constraint, which is excluded even for an extremely low reheating temperature. The blue
region shows the excluded region for the reheating temperature
greater than 100 GeV. The white region is cosmologically consistent region.

Bibliography[1] M. Ibe, S. Kobayashi, Y. Nakayama and S. Shirai,
JHEP 07 (2021), 098 doi:10.1007/JHEP07(2021)098[arXiv:2104.03289 [hep-ph]].
[2] T. Hiramatsu, M. Ibe, M. Suzuki and S. Yamaguchi,
JHEP 12 (2021), 122 doi:10.1007/JHEP12(2021)122[arXiv:2109.12771 [hep-ph]].
[3] M. Ibe, S. Kobayashi, Y. Nakayama and S. Shirai,
JHEP 03 (2022), 198 doi:10.1007/JHEP03(2022)198[arXiv:2112.11096 [hep-ph]].

Dark Matter• Chiral composite asymmetric dark matter [1]In collaboration with the members of Kavli IPMU, ICRR
The asymmetric dark matter (ADM) scenario solves thebaryon-dark matter coincidence problem when the dark matter (DM) mass is of O(1)GeV. Composite ADM models
based on QCD-like strong dynamics are particularly motivated since the strong dynamics naturally provides the DM
mass of O(1)GeV and the large annihilation cross-section simultaneously. In those models, the sub-GeV dark photon often plays an essential role in transferring the excessive entropy
in the dark sector into the visible sector, i.e., the Standard
Model sector. This paper constructs a chiral composite ADM
model where the U(1)D gauge symmetry is embedded into
the chiral flavor symmetry. Due to the dynamical breaking of
the chiral flavor symmetry, the model naturally provides themasses of the dark photon and the dark pions in the sub-GeV
range, both of which play crucial roles for a successful ADM
model.

Bibliography[1] M. Ibe, S. Kobayashi and K. Watanabe, JHEP 07 (2021),
220 doi:10.1007/JHEP07(2021)220 [arXiv:2105.07642

81[hep-ph]].

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

subgroups are broken with the spatial rotation SO(3) symmetry. As specific examples, we construct the stable solutions for
N = 3 and 4. We also numerically solve the gauge field dynamics and confirm that our analytic solutions are complete
and attractor. It is straightforward to apply our procedure to
the other simple Lie groups.

Beyond Standard Model• Gauge field production and Schwinger reheating inrunaway axion inflation [3]• Lepton Asymmetric Universe [1]In collaboration with the members of Univ.
RESCEU, ICRR

of Tokyo,

In collaboration with the members of ICRR, Kavli IPMU
The recent observation of 4 He implies that our universe
has a large lepton asymmetry. We consider the Affleck-Dine(AD) mechanism for lepton number generation. In the AD
mechanism, non-topological solitons called L-balls are produced, and the generated lepton number is confined in them.
The L-balls protect the generated lepton number from being
converted to baryon number through the sphaleron processes.
We study the formation and evolution of the L-balls and findthat the universe with large lepton asymmetry suggested by
the recent 4 He measurement can be realized.

Bibliography[1] M. Kawasaki and K. Murai, [arXiv:2203.09713 [hepph]].

Inflation• Gravitational wave trispectrum in the axion-SU(2)model [1]In a class of (pseudoscalar) inflation, inflationary phase
is followed by a kination phase, where the Universe is dominated by the kinetic energy of the inflaton that runs away ina vanishing scalar potential. In this class of postinflationary
evolution of the Universe, reheating of the Universe cannot
be achieved by the inflaton particle decay, which requires its
coherent oscillation in a quadratic potential. In this study, we
explore the U(1) gauge field production through the ChernSimons coupling between the pseudoscalar inflaton and thegauge field during the kination era and examine the subsequent pair-particle production induced by the amplified gauge
field known as the Schwinger effect, which can lead to reheating of the Universe. We find that with a rough estimate of the
Schwinger effect for the Standard Model hyper U(1) gauge
field and subsequent thermalization of the pair-produced particles, a successful reheating of the Universe can be achieved bytheir eventual domination over the kinetic energy of the inflaton, with some reasonable parameter sets. This can be understood as a concrete realization of the ”Schwinger reheating”.
Constraints from the later-time cosmology are also discussed.

In collaboration with the members of Waseda Univ, RESCEU,
ICRR, Kavli IPMU, Max Planck Institute, NIT Kagawa• Inflation with two-form field: the production of primordial black holes and gravitational waves [4]We study the trispectrum of the gravitational waves (GWs)
generated through the dynamics of an axionic spectator fieldand SU(2) gauge fields during inflation. In non-Abelian gauge
theory, the gauge fields have four-point self-interactions,which induce the tree-level GW trispectrum. We formulate
this type of the GW trispectrum including the non-dynamical
contributions and evaluate it in the equilateral limit as a uniquesignal of this model. We find that the ratio of the GW trispectrum to the cube of the scalar power spectrum can be as largeas O(106 ) in the viable parameter space, which could be captured in the CMB observations.

In collaboration with the members of Waseda Univ.,
RESCEU, ICRR, Max Planck Institute, Leiden Univ.

• SU(N) natural inflation [2]In collaboration with the members of Waseda Univ, RESCEU,
KEK, SOKENDAI, ICRR, Kavli IPMU
We study SU(N) gauge fields that couple to the inflaton
through the Chern-Simons term. We provide a general procedure to construct homogeneous, isotropic, and attractor solutions of the gauge fields during inflation. The gauge fields
develop various VEVs corresponding to different spontaneous
symmetry breaking patterns of SU(N) where embedded SU(2)Antisymmetric tensor field (two-form field) is a ubiquitous component in string theory and generally couples to thescalar sector through its kinetic term. In this paper, we propose a cosmological scenario that the particle production of
two-form field, which is triggered by the background motionof the coupled inflaton field, occurs at the intermediate stageof inflation and generates the sizable amount of primordial
black holes as dark matter after inflation. We also compute thesecondary gravitational waves sourced by the curvature perturbation and show that the resultant power spectra are testablewith the future space-based laser interferometers.
• Universality of linear perturbations in SU(N) natural inflation [5]In collaboration with the members of Waseda U., RESCEU,
ICRR, Kavli IPMU, RIKEN
We prove the universality of predictions for linear perturbations from the entire class of models of inflation driven by a82

pseudo-scalar field coupled to an SU(N) gauge boson, whereSU(2) subgroups in the SU(N) crossed with the backgroundspatial SO(3) spontaneously break into a single SO(3). The
effect of which SU(2) subgroup in SU(N) acquires a VEV
through spontaneous symmetry breaking can be quantified bya single parameter λ , which always appears in combination
with the gauge coupling constant g. In the linear perturbations, as well as the background system, the same dynamics
and predictions as in the chromo-natural inflation hold for its
SU(N) extension by replacing g → gλ . The latter models
thereby draw the same prediction curve on the ns -r plane as
the former at the tree level as long as gλ stays constant during inflation. We briefly discuss possible transitions from one
value of λ to another during inflation and the observational
prospects.
• Gravitational waves detectable in laser interferometersfrom axion-SU(2) inflation [6]In collaboration with the members of Waseda U., RESCEU,
ICRR
Chromo-natural inflation (CNI) is an inflationary model
where an axion coupled with SU(2) gauge fields acts as the
inflaton. In CNI, the gauge fields have nonzero vacuum expectation values (VEVs), which results in the enhancement ofgravitational waves (GWs). The original CNI is ruled out by
the Planck observations due to the overproduction of GWs.
In this work, we consider an inflationary model where thegauge fields acquire nonzero VEVs after the CMB modes exit
the horizon. Moreover, we add to the model another fieldthat dominates the universe and drives inflation after the axion starts to oscillate and the gauge field VEVs vanish. By
performing numerical simulations, we find a parameter space
where the enhanced GWs do not violate the CMB constraints
and can be detected by the future GWs observations such as
LISA, BBO, and ET.

Bibliography[1] T. Fujita, K. Murai, I. Obata and M. Shiraishi, JCAP 01(2022) no.01, 007 doi:10.1088/1475-7516/2022/01/007[arXiv:2109.06457 [astro-ph.CO]].
[2] T. Fujita, K. Mukaida, K. Murai and H. Nakatsuka, Phys. Rev. D 105 (2022) no.10, 103519
doi:10.1103/PhysRevD.105.103519 [arXiv:2110.03228[hep-ph]].
[3] S. Hashiba, K. Kamada and H. Nakatsuka, JCAP 04(2022) no.04, 058 doi:10.1088/1475-7516/2022/04/058[arXiv:2110.10822 [hep-ph]].
[4] T. Fujita, H. Nakatsuka, I. Obata and S. Young,[arXiv:2202.02401 [astro-ph.CO]].
[5] T.
Fujita,
K.
Murai
and
R.
Namba,
Phys. Rev. D 105 (2022) no.10,
103518doi:10.1103/PhysRevD.105.103518 [arXiv:2203.03977[hep-ph]].

[6] T. Fujita, K. Imagawa and K. Murai, [arXiv:2203.15273[astro-ph.CO]].

Axions• Axion dark matter search using arm cavity transmitted
beams of gravitational wave detectors [1]
In collaboration with the members of JAXA, ICRR, Univ.
of Wisconsin-Milwaukee, Waseda Univ., Univ. of Tokyo,
PRESTO, Max Planck Institute
Axion is a promising candidate for ultralight dark matter
which may cause a polarization rotation of laser light. Recently, a new idea of probing the axion dark matter by opticallinear cavities used in the arms of gravitational wave detectors
has been proposed [Phys. Rev. Lett. 123, 111301 (2019)]. In
this article, a realistic scheme of the axion dark matter searchwith the arm cavity transmission ports is revisited. Since photons detected by the transmission ports travel in the cavity
for odd-number of times, the effect of axion dark matter on
their phases is not cancelled out and the sensitivity at lowmass range is significantly improved compared to the searchusing reflection ports. We also take into account the stochastic
nature of the axion field and the availability of the two detection ports in the gravitational wave detectors. The sensitivity
to the axion-photon coupling, gaγ , of the ground-based gravitational wave detector, such as Advanced LIGO, with 1-year
observation is estimated to be gaγ ∼ 3 × 10−12 GeV−1 below
the axion mass of 10−15 eV, which improves upon the limit
achieved by the CERN Axion Solar Telescope.
• Free Streaming Length of Axion-Like Particle After Oscillon/ I-ball Decays [2]
In collaboration with the members of ICRR, Kavli IPMU
Axion-like particles (ALPs) are pseudoscalar bosons predicted by string theory. The ALPs have a shallower potential
than a quadratic one, which induces the instability and canform the solitonic object called oscillon/I-ball. Although the
lifetime of oscillons can be very long for some type of potentials, they finally decay until the present. We perform thenumerical lattice simulations to investigate the decay processof oscillons and evaluate the averaged momentum of ALPs
emitted from the oscillon decay. It is found that, if oscillons
decay in the early universe, the free-streaming length of ALPs
becomes too long to explain the small-scale observations of
the matter power spectrum. We show that oscillons with long
lifetimes can change the density fluctuations on small scales,
which leads to stringent constraints on the ALP mass and theoscillon lifetime.
• Anisotropies in Cosmological 21 cm Background by
Oscillons/I-balls of Ultra-light Axion-like Particle [3]In collaboration with the members of ICRR, Kavli IPMU
Ultra-light axion-like particle (ULAP) with mass m ∼10−22 eV has recently been attracting attention as a possi-

83

Bibliography

1000
100

[1] M. Kawasaki, K. Murai and H. Nakatsuka, JCAP
10 (2021), 025 doi:10.1088/1475-7516/2021/10/025[arXiv:2107.03580 [astro-ph.CO]].

10
1
0.100

Big Bang Nucleosynthesis

0.010
0.001
10-8

10-6

10-4

0.01

Fig. 15. The merger rate distribution of PBHs with the mass 30 M⊙
and the abundance fPBH . The orange line is the observed−3 −1
merger rate of LIGO-VIRGO experiment, R = 23.9+14.3−8.6 Gpc yr .
The different lines represent the significance of the clusteringwith ξ = ξ∗ (r/1Mpc)−2 . The dotted lines represent the condition
rmean < yt (xt ), where PBHs are produced too close to avoid three–body problem.

ble solution to the small-scale crisis. ULAP forms quasistable objects called oscillons/I-balls, which can survive up toa redshift z ∼ 10 and affect the structure formation on a scale∼ O(0.1) Mpc by amplifying the density fluctuations. We
study the effect of oscillons on 21 cm anisotropies caused byneutral hydrogen in minihalos. It is found that this effect canbe observed in a wide mass range by future observations suchas Square Kilometer Array (SKA) if the fraction of ULAP to
the total dark matter density is O(0.01–0.1).

Bibliography[1] K. Nagano, H. Nakatsuka, S. Morisaki, T. Fujita,
Y. Michimura and I. Obata, Phys. Rev. D 104 (2021)
no.6,
062008 doi:10.1103/PhysRevD.104.062008[arXiv:2106.06800 [hep-ph]].
[2] K. Imagawa, M. Kawasaki, K. Murai, H. Nakatsuka andE. Sonomoto, [arXiv:2110.05790 [hep-ph]].
[3] M. Kawasaki, K. Miyazaki, K. Murai, H. Nakatsuka andE. Sonomoto, [arXiv:2112.10464 [astro-ph.CO]].

Primordial Black Holes• Strong clustering of primordial black holes from
Affleck-Dine mechanism [1]
In collaboration with the members of ICRR, Kavli IPMU
Primordial black hole (PBH) is a fascinating candidate forthe origin of binary merger events observed by LIGO-Virgocollaboration. The spatial distribution of PBHs at formation
is an important feature to estimate the merger rate. We investigate the clustering of PBHs formed by Affleck-Dine (AD)baryogenesis, where dense baryon bubbles collapse to form
PBHs. We found that formed PBHs show a strong clustering
due to the stochastic dynamics of the AD field. Including theclustering, we evaluate the merger rate and isocurvature perturbations of PBHs, which show significant deviations fromthose without clustering.

• EMPRESS. VIII. A New Determination of Primordial
He Abundance with Extremely Metal-Poor Galaxies: A
Suggestion of the Lepton Asymmetry and Implications
for the Hubble Tension [1]In collaboration with the members of ICRR, Univ. of Tokyo,
NAOJ, Kavli IPMU, Univ. College London, Waseda Univ.,
SOKENDAI, Osaka Univ., Univ. of Nevada, Univ. of Geneva,
Univ. of Notre Dame, Tohoku Univ., Monash Univ., Kitami
Institute of Technology, Cosmic DAWN Center, Niels Bohr
Institute, Ichinoseki College, Osaka Sangyo Univ., RESCEU,
Carnegie Observatories, Univ. of Hyogo, Nagoya Univ., Univ.
of Tsukuba, Univ. of Arizona
The primordial He abundance YP is a powerful probe ofcosmology. Currently, YP is best determined by observationsof metal-poor galaxies, while there are only a few known local extremely metal-poor (< 0.1Z⊙ ) galaxies (EMPGs) having
reliable He/H measurements with HeIλ 10830 near-infrared(NIR) emission. Here we present deep Subaru NIR spectroscopy and He/H determinations for 10 EMPGs, combining
the existing optical data and the Markov chain Monte Carloalgorithm. Adding the existing 3 EMPGs and 51 moderately
metal-poor (0.1 − 0.4Z⊙ ) galaxies with reliable He/H esti+0.0031
mates, we obtain YP = 0.2379−0.0030
by linear regression inthe (He/H)-(O/H) plane, where our observations increase thenumber of EMPGs from 3 to 13 anchoring He/H of the mostmetal-poor gas in galaxies. Although our YP measurement andprevious measurements are consistent, our result is slightly( 1σ ) smaller due to our EMPGs. Including the existing primordial deuterium DP constraints, we estimate the effective
number of neutrino species to be Ne f f = 2.41+0.19−0.21 showing a¿ 2 σ tension with the Standard Model value (Ne f f = 3.046),
which may be a hint of an asymmetry in electron-neutrino
νe and anti-electron neutrino ν̄e . Allowing the degeneracy
parameter of electron-neutrino ξe to vary as well as Ne f f
and the baryon-to-photon ratio η, we obtain ξe = 0.05+0.03−0.03 ,
+0.33
+0.04
Ne f f = 3.22−0.30, and η × 1010 = 6.13−0.04
from the YP and
DP measurements with a prior of η taken from Planck Collaboration et al. (2020). Our constraints suggest a νe − ν̄easymmetry and allow for a high value of Ne f f within the 1σ
level, which could mitigate the Hubble tension.

Bibliography[1] A. Matsumoto, M. Ouchi, K. Nakajima, M. Kawasaki,
K. Murai, K. Motohara, Y. Harikane, Y. Ono,
K. Kushibiki and S. Koyama, et al. [arXiv:2203.09617[astro-ph.CO]].

84
3. × 10-27

is found that the constraint from the CMB power spectrum iscomparable to that from the extragalactic gamma rays. We
confirm that our constraint mainly comes from the energy injection in early time (z ≳ 100) and hence it is independent of
the uncertainty of minihalo properties in the late time.

2.5 × 10-27
2. × 10-27
1.5 × 10-27
1. × 10-27
5. × 10-28
0

1

10

100

1000

104

Fig. 16. CMB constraints on the annihilation cross section for four decay channels of dark matter. We plot the 95% exclusion contoursof ⟨σ v⟩/mDM (vertical axes) for mass of dark matter (horizontalaxes). We calculate the posterior distribution using two sets ofdata: “Planck only” uses the Planck CMB data for TT-, TE- andEE-modes and “Planck+ext” uses BAO, DES, and the CMB lensing power spectrum data in addition to the Planck CMB data.

CMB• Revisiting CMB constraints on dark matter annihilation[1]
In collaboration with the members of ICRR, Kavli IPMU,
Univ. of Tokyo, KEK
The precision measurements of the cosmic microwave
background power spectrum put a strong constraint on thedark matter annihilation cross section since the electromagnetic energy injection by the dark matter annihilation affects
the ionization history of the universe. In this paper, we update our previous simulation code for calculating the ionization history with the effect of dark matter annihilation by including Helium interactions and improving the precision of
calculations. We give an updated constraint on the annihilation cross section and mass of dark matter using the modified
RECFAST code with the Planck 2018 datasets.
• Constraints on small-scale primordial density fluctuation from cosmic microwave background through dark
matter annihilation [2]In collaboration with the members of ICRR, Kavli IPMU, Tohoku Univ, Univ. of Tokyo
The cosmic microwave background (CMB) observationby the Planck satellite precisely determines primordial curvature fluctuations on larger scales than O(1) Mpc, while thesmall-scale curvature fluctuation is still less constrained. The
constraint on small-scale fluctuations is highly improved if we
assume the standard thermal relic dark matter scenario. Whensmall-scale fluctuations are large enough, dense regions collapse to form small halos even in a redshift z ≳ 103 , whichis called ultracompact minihalos. These minihalos enhance
the annihilation of the dark matter and it is constrained by observations such as extragalactic gamma rays and the CMB.
We revisit the effect of minihalos formed by the small-scaledensity fluctuations and calculate the ionization history modified by the dark matter annihilation. We perform the Markov
Chain Monte Carlo method to constrain the size of smallscale curvature fluctuations by the CMB power spectrum. It• Is cosmic birefringence due to dark energy or dark
matter? A tomographic approach [3]In collaboration with the members of ICRR, Kavli IPMU,
Max Planck Institute
A pseudoscalar ”axionlike” field, φ , may explain the 3σ
hint of cosmic birefringence observed in the EB power spectrum of the cosmic microwave background (CMB) polarization data. Is φ dark energy or dark matter? A tomographic
approach can answer this question. The effective mass ofdark energy field responsible for the accelerated expansion of
the Universe today must be smaller than mφ ≃ 10−33 eV. If
mφ ≳ 10−32 eV, φ starts evolving before the epoch of reionization and we should observe different amounts of birefringencefrom the EB power spectrum at low (l ≲ 10) and high multipoles. Such an observation, which requires a full-sky satellite
mission, would rule out φ being dark energy. If mφ ≳ 10−28
eV, φ starts oscillating during the epoch of recombination,leaving a distinct signature in the EB power spectrum at highmultipoles, which can be measured precisely by ground-based
CMB observations. Our tomographic approach relies on the
shape of the EB power spectrum and is less sensitive to miscalibration of polarization angles.

Bibliography[1] M. Kawasaki, H. Nakatsuka, K. Nakayama
and T. Sekiguchi, JCAP 12 (2021) no.12,015
doi:10.1088/1475-7516/2021/12/015[arXiv:2105.08334 [astro-ph.CO]].
[2] M. Kawasaki, H. Nakatsuka and K. Nakayama, JCAP 03(2022) no.03, 061 doi:10.1088/1475-7516/2022/03/061[arXiv:2110.12620 [astro-ph.CO]].
[3] H. Nakatsuka,
T. Namikawa and E. Komatsu, Phys. Rev. D 105 (2022) no.12, 123509
doi:10.1103/PhysRevD.105.123509 [arXiv:2203.08560[astro-ph.CO]].

Observational Cosmology• Physics of star-formation history and the luminosity
function of galaxies therefrom [1]
In collaboration with the members of ICRR, Kavli IPMU
We show that the star formation history, the reionization
history and the present luminosity function of galaxies are
reproduced in a simple gravitational collapse model withinthe ΛCDM regime to almost a quantitative accuracy, when
the physical conditions, the Jeans criterion and the coolingprocess, are taken into account. Taking a reasonable set ofthe model parameters, the reionisation takes place sharply

85

at around redshift 1 + z ≃ 7.5, and the resulting luminosity function turns off at L ≃ 1010.7 L⊙ , showing the consistency between the star formation history and the reionisationof the Universe. The model gives the total amount of starsΩstar = 0.004 in units of the critical density compared to theobservation 0.0044 with the recycling factor 1.6 included. Inorder to account for the observed star formation rate and thepresent luminosity function, the star formation efficiency is
not halo mass independent but becomes maximum at the halo
mass ≃ 1012 M⊙ and is suppressed for both smaller and larger
mass haloes.

Bibliography[1] M. Fukugita and M. Kawasaki, Mon. Not. Roy. Astron.
Soc. 513 (2022) no.1, 8-14 doi:10.1093/mnras/stac774[arXiv:2111.01389 [astro-ph.GA]].

86

OBSERVATORIES andA RESEARCH CENTER
Location of the Institute and the Observatories

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

Norikuradake, Nyukawa-cho, Takayama-shi, Gifu Prefecture 506-2100+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+81-551-25-2301 / +81-551-25-2303

Kamioka Observatory
Location:
Telephone / Fax:

456 Higashi-mozumi, Kamioka-cho, Hida-shi, Gifu Prefecture 506-1205+81-578-85-2116 / +81-578-85-2121

KAGRA Observatory
Location:
Telephone / Fax:

238 Higashi-mozumi, Kamioka-cho, Hida-shi, Gifu Prefecture 506-1205+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

High Energy Astrophysics Facility in Canarias
Location:

C/Via Lactea, s/n E-38205 La Lagua - Tenerife Espana

87

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-88

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 therelation between thunder and cosmic-ray air shower, westarted an experiment in 2015, mainly using gamma-ray
detectors at Norikura Observatory of Institute for Cosnic
Ray Research, the University of Tokyo. This experimentconsists of gamma-ray detectors, a lightning sensor, anelectric field meter, a weather monitor, and an air shower
array. Gamma-ray detectors using three crystals, NaI, CsI,and BGO respectively, cover the energy range over 3 ordersof magnitude from 70 keV to 120 MeV as a whole.

89

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 expectedduring rainfall. Also, gamma ray detectors using CsI and
BGO for detecting gamma rays of 3 MeV or more did notobserve an increase in gamma rays of 3 MeV or more. Thismeans that gamma rays from radon can be distinguishedfrom contributions from other gamma rays not derived fromradon, and it can be expected that these detectors can operate
normally even when they are installed at an altitude about
4000 m for a certain future project.
Observation period: 30 days from July 19 to September 16,
2016 (excluding the summer season)
No direct lightning strike event occurred during the observation period, but 6 atmospheric electric field fluctuations
exceeding 30 kV/m, which seemed to be an influence ofthunder cloud passage, were observed. In one of the events, it
seems there was a lightning strike in the vicinity.
Observation period: 41 days from July 31 to September 9,
2017
Unfortunately, during this observation period there 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-

90

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 γray 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 µm to 11 µm. The 81.7% of 7 Be was covered withthe aerosol sizes less than 1.1 µm and the 7 Be with the aerosol
sizes above 1.1 µm 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.

91

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

92

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.

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95

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 the Akeno Giant Air Shower Array (AGASA) ofapproximately 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 approximately 100 km2 in the suburban areaof Akeno, outside of the observatory campus. The AGASA
served as the largest air shower array in the world since its
commissioning in 1990 until it stopped data taking in January
2004, when the construction 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

96

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.

simulation. This difference of energy scale obtained by thesurface array and by the fluorescence telescope remains as abasic question in the understanding of the air shower phenomena.

Recent Research Activities
Measurement of UHECRs
Since the AGASA measurement in 1998, the High Resolution Fly’s Eye (HiRes) 5 , the Pierre Auger Observatory (PAO)
6 , and the Telescope Array (TA) 7 measured the energy spectraof 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 plotted in Fig.4 with the energy scale of each experiment adjusted
to a reference energy, which is set halfway between PAO andTA/HiRes.

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 by the ICRR
and university researchers. Fig.5 shows a recent photographof 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 (A), thelarge experimental hall (B), the research building (C) and the lodging facility (D) from the left.

J(E) × E3/1024 [eV2 m-2 s-1 sr-1]

10

In 2020, as countermeasures against COVID-19 infection,
an air conditioner and a heat-exchange ventilator were installed in the large room of the research building, and a heatexchange ventilator was installed in the dining room of thelodging facility along with some other measures.

1

10-1
18

TA

HiRes-I

PAO

HiRes-IIAGASA

Yakutsk18.5

19log (E/eV)

19.5

20

20.5

10

Fig. 4. Compilation of UHECR energy spectra (UHECR2012). Theenergy scales were adjusted as ×1.10 for PAO, ×0.91 for TA and
HiRes, ×0.65 for AGASA and ×0.56 for Yakutsk.

The HiRes, PAO and TA confirmed a 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 statisticallyinsignificant. The estimate of systematic uncertainty of theenergy measurement is approximately 20% for all the experiments, and rescalings for the TA/HiRes and PAO are within
this limit. Rescaling of the surface array energy for AGASA
and Yaktsuk indicates that there exist larger systematic uncertainties than originally estimated by running the air shower
*4
*5
*6
*7
*8

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 installed in the Akeno Observatory in 2003 (Fig. 6). The telescope has an aperture of 50 cm, an FOV of 28’ × 28’ and isequipped with a tricolor CCD camera capable of g′ RC IC -bands
photometry (g’:400∼550 nm, Rc:570∼730 nm, Ic:730∼ 850nm). It is operated remotely from the Tokyo Tech at the
Ookayama Campus via a dedicated line. The operation of thetelescope has continued in FY2021 as described below 9 .
Upon receiving a GRB alert, 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.
Information about GRB events was reported to Gamma-Ray
Burst (GRB) Coordinates Network Circular (GCNC).
In order to perform follow-up visible-light observationsof gravitational wave objects, the MITSuME participated in*9 N. Kawai et al., Research Result Presentations Meeting of the ICRRInter-University Research Program 2021.

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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 scientificthemes. In FY2021, the observation of gravitational wave by
LIGO/Virgo was stopped. Therefore, the MITSuME participated in the follow-up observation of IceCube neutrino events
that was derived from J-GEM.
The MITSuME team also conducted long-term monitoring observations of X-ray binary starts and blazars. The MITSuMe has also participated OISTER (Optical and Infrared
Synergetic Telescopes for Education and Research).

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
prototype tests with atmospheric Cherenkov observations of
gamma rays on the ground 11 . This telescope is the only telescope to observe atmospherice Cherenkov light emitted fromair showers induced by TeV gamma rays in Japan.
In January 2021, the telescope was used to observe the
Crab Pulsar in the visible light region for about ten hours. The
collected data were analyzed in FY2021. The result was used
to improve the signal-to-noise ratio of the Crab Pulsar observation and to study the removal of external noise.

Fig. 6. The dome in which the MITSuME telescope was installed in
Akeno.
Fig. 7. The Cherenkov telescope at the tour of the 40th anniversaryof the Akeno Observatory in 2017.

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
and have been continuously measuring the cosmic ray muonssince 2003. Fig. 2 shows one of the three muon houses (M1,
M5, M8). The mode energy of the primary cosmic rays isapproximately 2.5 GeV corresponding to 2m thick concrete
ceiling of the muon house at the latitude of the Akeno Observatory. The measurement in Akeno is combined with a simultaneous measurement by the GRAPES-3 experiment at Ootyin India, and this telescope aims at measuring the modulation
effects and anisotropy of galactic cosmic rays in the regionof about 100 GeV and detecting trasient phenomena of cosmic rays. It is expected to understand cosmic-ray flow in theuniverse magnetic field and to obtain a clue to the solution ofcosmic-ray propagation. And it is challenging to establish themethod of a new space weather observation by simultaneous
multi-directional observations with high statistical precision.
In FY2021, the data acquisition at M5 was restored in
November and tuned in December. However, almost continuous observations have been made at M5 over the past 3.5 years
except during power outages, etc 10 .
*10 A. Oshima et al., Research Result Presentation Meeting of the ICRRInter-University Research Program 2021.

Dark matter and Interstellar Meteoroid Study (DIMS)
at the Akeno Observatory by F. Kajino (Konan University) et al.
The Dark Matter and Interstellar Meteoroid Study (DIMS)
is a new experiment that aims at searching for macroscopic
dark matter and interstellar meteoroids 121314 . Nuclearites arenuggets of stable Strange Quark Matter (SQM), which areneutral in charge and hypothetical super-heavy macroscopic
particles (macros), and may be important components of thedark matter in our universe. Nuclearites of galactic origins is
expected to have a typical velocity of about 250 km/s in galactic frame. The DIMS team studies the possibility of searching for such fast moving particles using very high-sensitivity
CMOS camera with a wide field of view. In the first stage,
the DIMS system consists of 4 high-sensitivity CMOS camera
stations. and is going to be constructed at the Telescope Arraycosmic-ray experimental site in Utah, USA. Due to COVID19 pandemic, the DIMS team did not transport the DIMS systems to Utah, and one of the systems was installed on the roofof the research building at the Akeno Observatory as shownin Fig. 8. The observed Geminids Meteor Shower taken by*11 M. Ohishi et al., 33rd ICRC, (Rio deJaneiro), 587 (2013).
*12 D. Barghini et al., PoS(ICRC2021)500.
*13 D. Shinto et al., PoS(ICRC2021)502.
*14 F. Kajino et al., PoS(ICRC2021)554

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the DIMS camera at the Akeno Observatory was shown in
Fig. 9. The preliminary results on limiting magnitudes of stars
and the number of meteors as a function of magnitude werereported 15 . Three camera systems are installed at the Kiso
Observatory, Shinshu University and Akeno Observatory forstereoscopic observations.

Fig. 8. The DIMS camera installed on the roof of the research building
at the Akeno Observatory in August 2021.

Fig. 9. Geminids Meteor Shower that was taken by DIMS camera at
the Akeno Observatory on December 13th, 2021. Approximately
500 meteoroids are superimposed on the photograph.

Research and development for the Telescope Arrayobservation in Utah by the TA collaboration and others
All the TA fluorescence imaging cameras and a part of the TA
surface detectors were assembled in the Akeno Observatory
by the TA collaboration team. All the unit mirrors of the TA
fluorescence telescope were tested and the atmospheric monitoring lidar of TA using YAG laser was developed. In JFY
2015, the R&D of the surface detectors were performed for theTAx4 project that aims at quadrupling the TA surface detector
array in Utah. In August in 2016, 2017 and 2018 the TAx4
and TALE (TA Low Energy extension) scintillator counterswere assembled.
The assembly of the surface detectors for the TALE-infill
array 16 that is described in the TA section was scheduled at*15 F. Kajino et al., Research Result Presentation Meeting of the ICRR InterUniversity Research Program 2021.
*16 S. Ogio et al., ”The status of the TALE surface detector array and a TALE
infill project”, PoS(ICRC2021)255.

the Akeno Observatory in 2020, but due to the COVID-19 epidemic, the assembly had to be postponed. The 50 surface detectors for the TALE-infill array were assembled at the Akeno
Observatory from the middle of October to early November in
2021. Figure 10 shows one of the surface detectors that was
being assembled.

Fig. 10. A surface detector for the TALE-infill array that was being
assembled at the Akeno Observatory. The student was aligning
the ends of bundled fibers.

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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 JPARC proton accelerator has started in 2009. In 2011, theexperiment has observed 6 νe appearance events indicating
non-zero θ13 which was as yet determined the last neutrino
oscillation parameter. From 2014, T2K started taking data using an anti- neutrino beam, and confirmed the disappearanceof muon antineutrinos through oscillations. Candidate events
for the appearance of electron anti-neutrinos were also found.
Additionally, T2K published in 2020 an indication that CP
symmetry is not conserved in neutrino oscillations, with theprobability of the observations being the results of a random
fluctuation if CP symmetry was conserved being less than 5%.
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 prototype of the 3 km gravitational wave antenna (KAGRA). Using the low radioactive background environment in
the Kamioka Mine, a dark matter experiment, called XMASS
was conducted in Lab-C. The XMASS group constructed a
800kg liquid xenon detector and data were taken from 2010
to 2019. Searches for dark matter interactions and rare phenomena in liquid xenon have been conducted using the obtained data. The R&D study of a tracking type detector fordark matter detection led by the Kobe University group (theNEWAGE 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 runningin Lab-D. The study to improve the neutrino detection sensitivity by adding gadolinium to Super-Kamiokande (called SKGd project) has been performed at Lab-E. A Low background
Germanium detector and ICP-MS system are equipped inLab-1 and Lab-A to measure extremely low radioactive backgrounds. Lab-G area was excavated in 2015 and the SKGd equipments which include a Gd dissolving system, a pretreatment system and a Gd-water circulation system were installed in 2016. Other preparations for Gd-loading have been
performed including a tank open work to repair the water leakfrom the SK tank in 2018 and the test of the SK-Gd equipments in 2019. In 2020, SK added soluble Gd salt into the water. The next-generation neutrino experiment project, HyperKamiokande, was started in 2020. The construction has beenrunning underground the Kamioka mine, aiming to begin observations in 2027.

G

Fig. 1. Kamioka Underground Observatory.

Fig. 2. The surface buildings in Mozumi.

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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 ”KAGRA”. KAGRA itself has a L-shape tunnel facility, and it is located morethan 200m under Mt.Ikeno-yama. The corner station of the Lshape tunnel is accessible through a 500-m horizontal accesstunnel 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 gravitational waves (GWs) as
one of observatories 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 Expense and Grants-in-Aid for Scientific Research from
Ministry of Education, Culture, Sports, Science and Technology (MEXT).

In the KAGRA project, Institute for Cosmic Ray Researchplays a role of a host promoting institute, and National Astronomical Observatory of Japan (NAOJ) and High Energy
Accelerator Research Organization (KEK) are the co-host institutions, then more than 417 researchers in 85 institutes anduniversities in the world are collaborating for construction anddata analysis of KAGRA.

Fig. 1. Surface Research Building.

Fig. 2. Atotsu Entrance of KAGRA.

The tunnel excavation started in May 2012, and finished in
March 2014. After that, the basic laboratory environment was
prepared until September 2015. A Michelson interferometerwith 3km arm (iKAGRA) was demonstrated in March 2016,
and the first engineering run was performed until May 2016.
In 2019, all the interferometer components had been installed
to complete the KAGRA Observatory that adopts a power recycled Fabry-Perot Michelson type interferometer with theresonant sideband extraction. On February 25th, 2020, KAGRA started its first observation run. After performing thetwo week observation run ”O3GK” with GEO600 on April
2020, KAGRA will be upgraded and prepared for the joint
observation with LIGO and Virgo ”O4” which is expected tostart in March 2023.

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RESEARCH CENTER FOR COSMIC NEUTRINOS
The Research Center for Cosmic Neutrinos (RCCN) wasestablished in April 1999 to conduct research on neutrinos
and related topics. The members of the Center participate in
the Super-Kamiokande experiment, which is the neutrino research facility representing ICRR, maximize our knowledgeof neutrinos by analyzing the observed neutrino data in SuperKamiokande and the long-baseline neutrino oscillation experiment T2K, and explore new avenues for neutrino research bypromoting the exchange of the research ideas between theoretical and experimental physicists. We are also participating in
the Hyper-Kamiokande project, a next-generation large-scale
experiment, and developing the detector elements and preparing the construction.
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 2020. Since 2004, RCCN has
been accepted inter-university programs related to activities
in the low-background underground facility also (Fig 2). In
FY2021, we accepted 12 programs, including both domestic
and international applications.

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

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

Since 2009, ICRR and Kavli Institute for the Physics andMathematics of the Universe (Kavli IPMU) have held two
public lectures a year, one in the spring and one in the summer. The lecture was broadcast in real time and archived on
YouTube, and was viewed approximately 3,800 times in total.
The lecture was titled ”The Mysteries of a Universe Full of
Matter,” and invited Associate Professor Asaoka and Professor Murayama of Kavli IPMU to give lectures on the HyperKamiokande project and neutrinos, respectively. The two lecturers also cross-talked with the participants to answer theirquestions.

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HIGH ENERGY ASTROPHYSICS FACILITY IN
CANARIASHigh Energy Astrophysics Facility in Canarias, established on April 1, 2019 in La Palma, Canary Islands, Spainis the actual base of international collaborations on CTA(Cherenkov Telescope Array) project co-hosted by CTA groupof ICRR.
The Cherenkov Telescope Array (CTA) is the next generation ground-based observatory for gamma-ray astronomy
at very-high energies, participated by more than 1400 researchers from 31 countries. With more than 100 telescopes
located in the northern and southern hemispheres, CTA willbe the world’s largest and most sensitive high-energy gammaray observatory. The CTA-Japan consortium is making a sig-

nificant contribution to the construction of Large-sized Telescopes (LSTs), four proto types of which are planned to construct at northern hemisphere site in La Palma. The first LST
was completed and made the first light at the end of Decemberof 2018.
Establishment of the local corporation assets were neededfor daily activities such as local procurement and employment
to manage and operate the site. High Energy Astrophysics Facility in Canarias was established as a room inside the buildingof CALP (The Centro de Astrofı́sica en La Palma), a branchof IAC (Instituto de Astrofisca de Canarias).

Fig. 1. High Energy Astrophysics Facility in Canarias.

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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)

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

Kashiwa Dark Matter Symposium 2021
Date: Nov.29-Dec.2, 2021
Place: Online
Outline: For the third time, ICRR organized the yearly Kashiwa Dark Matter symposium taking place online from November 29 to December 2, 2021. The symposium covered recent updates from the Dark Matter searches in the different fields ofcollider, direct, and indirect searches and developments in Dark Matter theory and astrophysics. Special focus was drawn tosearch for Primordial Black Holes. The symposium lasted four days, over five hours each day.
The talks and oral discussions were held on Zoom. The coffee breaks and poster session were held using the Gather.Town
tool, with more than 40 participants discussing the posters at the poster session. Additionally, a Slack workspace was available
for offline discussions and the talks were additionally live streamed worldwide on YouTube.
The theory session opened the symposium with a focus on electroweak dark matter. The following Astrophysics Session
focused the implications of observations of dwarf spheroidal and ultrafaint galaxies for Indirect Dark Matter searches. In the
Indirect Detection session, besides high-energy gamma-ray searches, attention was drawn on new search strategies at lower
photon energies from MeV to radio emission. In the Direct Detection session, both latest noble gas and cryogenic detection
efforts were discussed. In the collider session, measurements with the LHC and Belle experiments at KEK were presented.
As in the last years, all talks and posters are publicly available on the webpage (https://kashiwa-darkmatter-symposia.org/),
and also many of the talks are archived on YouTube. Also, a prize was awarded to the best presentation and poster summary
video, which can be watched on YouTube.
Participants: 317 participants from 28 countries worldwide.

Fig. 1. Kashiwa Dark Matter Symposium 2021

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SAZERAC SIPS Early Galaxy Formation Near and Far —Preparing for a Long Journey with JWST —Date: Nov.29-Dec.3, 2021
Place: Online
Outline: The international online conference, SAZERAC SIPS Early Galaxy Formation Near and Far — Preparing for aLong Journey with JWST —, was held in November 29 - December 3 on zoom.
In this international conference, 240 researchers, 170 out of whom joined outside Japan, particpated, and discussed local
dwarf galaxies with both observational and theoretical perspectives to reveal how the local dwarf galaxies help us understandthe early formation of high-redshift galaxies in the JWST era. The discussions included HST/COS surveys targeting therest-frame UV emission of local-dwarf galaxies whose high-redshift counterparts would be studied with JWST with goodsensitivity. Ground-based large-area data, such as those taken by SDSS and Subaru programs, pinpointed premature dwarf
galaxies, and the evolution of such population over cosmic time was also discussed, which builded a bridge between local
galaxies and high-redshift galaxies at the epoch of reionization.
We had a total of 60 Presentations, 44 Talks and 16 Poster/Recorded talks (+lightning talks). There are interaction andposter sessions twice in a day and Q&A sessions that were held via zoom and the conference slack channel, where we encouraged questions from students and young astronomers. The conference is recorded and published in the YouTube channel,
https://www.youtube.com/channel/UCvrk105bi1U7sMgkEUEneSw
Participants: 240 participants from 23 countries worldwide

Fig. 2. Conference group photo106Black Hole Astrophysics with VLBI 2022
Date: Feb. 7-9, 2022
Place: ICRR, The University of Tokyo and Online
Outline: The workshop “Black Hole Astrophysics with VLBI 2022” was held from the 7th to the 9th of February 2022
at ICRR. The workshop focused on the various aspects of the black hole astrophysics from horizon scale to the global scale,motivated by the recent progress of the VLBI observation including the Event Horizon Telescope (EHT).
A Total of 92 people joined this workshop. Attendees are mainly from the institutes in east Asia, while some of theparticipants are from those in US and Europe. Due to the situation of covid-19, the number of persons attending on site waslimited to 13 people and the most of attendees remotely joined the workshop via zoom and slack. The 11 invited talks, 17
contributed talks, and 7 poster talks are given during the workshop.
The workshop brought researchers working on observation and theory/simulations to discuss our present understanding of
the spacetime of black holes, the physics of accretion flow, jets, winds, and the related high energy phenomena around compact
objects. In particular, from the observational side, the EHT observation of the polarization features of M87* and a recent
update of the long-term East Asia VLBI Network (EAVN) observation of the precession feature of M87 jet were sensationallydemonstrated. From the theoretical side, the recent theoretical studies of black hole magnetosphere in the context of the jetformation mechanism are deeply discussed.
Participants: 92 participants.

Fig. 3. Zoom window showing participants in the symposium, onsite in the large Seminar Room, ICRR and from other places

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UTokyo NY Event by ICRR
“Exploring the Universe with Multi-Messengers”
Date: Feb. 12, 2022
Place: Online
Outline: UTokyo NY Event “Exploring the Universe with Multi-Messengers” was held by Institute for Cosmic Ray Research, The University of Tokyo on February 12, 2022 (EDT).
This event was the first symposium organized by ICRR to connect people living in Japan, the USA and the other areasin the world, commemorating the University of Tokyo New York Office (UTokyoNY) after its full renovation. As one of the
UTokyoNY events, ICRR was very happy to be supported by the External Relations Promotion Group, UTokyo and UTokyoNY.
Initially, the event had planned to be held at a real venue in New York with online mode but decided to be operated completely
online from midnight-Japan due to the COVID-19 situation.
At the symposium, Prof. Masato Takita acts as a moderator and researchers from ICRR and other research institutes anduniversities in the USA made presentations on their cutting-edge research topics to the general-public. During the symposium,
four research fields: Cherenkov Telescope Array (CTA), Telescope Array (TA), Tibet/ALPACA Experiment, gravitational wave
observation at LIGO and KAGRA were featured. Researchers from ICRR and the USA in turns made presentations on theirlatest topics and answered questions from the audience (Zoom chat and Sli.do).
Participants: More than 300 people on Zoom and YouTube

Fig. 4. Speakers in the symposium. From upper left: Prof. Takita, Prof. Sagawa, President Masuyama (top), President Spergel, Director Kajita,
Prof. Teshima (upper middle), Prof. Mukherjee, Prof.Farrar, PI Fritschel (lower middle), Prof. Shawhan, Prof. Tagoshi (bottom)

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Workshop ”Synergies at New Frontiers at Gamma-rays,
Neutrinos and Gravitational Waves”
Date: March. 24-25, 2022
Place: ICRR, The University of Tokyo and Online
Outline: The goal of this workshop was to connect scientific communities in Japan working at different wavelengths andmessengers. The participants explored potential synergies between several multi-messenger windows of the highly-energetic
Universe using the following large experiments: the gamma-ray observatory CTA, with its first Large Size Telescope (LST1)
in operation, the cosmic ray detector CALET, the gamma-ray imaging Cherenkov telescopes MAGIC, the gravitational wave
project KAGRA and the neutrino observatories Super-Kamiokande and Hyper-Kamiokande.
The idea for this workshop came from recent discoveries of high-energy cosmic neutrinos and gravitational waves fromastrophysical objects that have led to the new era of multimessenger astrophysics. Observatories and experiments are now morethan ever able to observe the sky in different energy ranges and with different messengers. Each class of messengers – photons,
neutrinos, cosmic-rays and gravitational waves – provides distinct and valuable information of the most violent phenomena in
the Universe. Only with multi-messenger astronomy will we be able to fully unveil the mechanisms at operation in different
galactic and extragalactic sources.
The workshop was originally intended to take place in 2020, however it was delayed to 2022 due to COVID pandemic. It
was finally held in hybrid style: speakers gathered in person at ICRR and the audience connected online via ZOOM. Furthermore, we created a Slack channel for easier communication and discussion. A total of 70 researchers participated in this event,
25 onsite and 45 online.
Participants: 70 participants (25 onsite, 45 online)

Fig. 5. Memorial photo of participants taken at piloti of ICRR during the Symposium

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B. ICRR Seminars
1. May, 27, 2021: Dr. Masato Takita (ICRR), “First observation of sub-PeV galactic diffuse gamma rays by the Tibet
ASgamma experiment”
2. August, 2, 2021: Dr. Eduardo de la Fuente Acosta (Departamento de Fisica, CUCEI, Universidad de Guadalajara, and
ICRR), “PeVatrons and Star Formation Regions”
3. October, 13, 2021: Dr. Yasuo Fukui (Department of Physics, Nagoya University), “Quantification of the hadronic andleptonic gamma-ray components in the TeV gamma ray supernova remnant RX J1713.7-3946”
4. December, 8, 2021: Dr. Moritz Huetten (ICRR), “Searches for Dark Matter with the MAGIC and CTA gamma-ray
telescopes: Latest results and a glimpse into the future”

C. List of Publications(a) Papers Published in Journals
1. ”Search for solar electron anti-neutrinos due to spin-flavor precession in the Sun with Super-Kamiokande-IV”, SuperKamiokande Collaboration, Astroparticle Physics Volume 139, June 2022, 102702, Mar. 2022.
2. ”First Gadolinium Loading to Super-Kamiokande”, The Super-Kamiokande Collaboration, Nuclear Inst. and Methods
in Physics Research, A 1027 (2022) 166248, Dec. 2021.
3. ”Diffuse supernova neutrino background search at Super-Kamiokande”, Super-Kamiokande Collaboration, PHYSICALREVIEW D 104, 122002 (2021), Dec. 2021.
4. ”Precise measurement of the scintillation decay constant of ZnWO4 crystal”, M Shibata, H Sekiya, K Ichimura, Prog.
Theor. Exp. Phys. 2022 013C01, Oct. 2021.
5. ”Search for neutrinos in coincidence with gravitational wave events from the LIGO-Virgo O3a Observing Run with theSuper-Kamiokande detector”, The Super-Kamiokande Collaboration, 2021 ApJ 918 78, Sep. 2021.
6. ”Search for tens of MeV neutrinos associated with gamma-ray bursts in Super-Kamiokande”, Super-Kamiokande Collaboration, Prog. Theor. Exp. Phys. 2021, 103F01, Jul. 2021.
7. ”The NEUT neutrino interaction simulation program library”, Yoshinari Hayato, Luke Pickering, Eur.Phys.J.ST
230(2021)24, 4469-4481, Jun. 2021.
8. ”Measurements of ν̄µ and ν̄µ + νµ charged-current cross-sections without detected pions or protons on water and hydrocarbon at a mean anti-neutrino energy of 0.86 GeV”, T2K Collaboration, PTEP 2021 (2021) 4, 043C01, Dec. 2021.
9. ”Improved constraints on neutrino mixing from the T2K experiment with 3.13 × 1021 protons on target”, T2K Collaboration, Phys.Rev.D 103 (2021) 11, 112008, Dec. 2021.
10. ”First T2K measurement of transverse kinematic imbalance in the muon-neutrino charged-current single-π + production
channel containing at least one proton”, T2K Collaboration, Phys.Rev.D 103 (2021) 11, 112009, May. 2021.
11. ”Simultaneous measurement of the muon neutrino charged-current cross section on oxygen and carbon without pions in
the final state at T2K”, T2K Collaboration, PhysRevD.101.112004, May, 2021.
12. ”T2K measurements of muon neutrino and antineutrino disappearance using 3.13 × 1021 protons on target”, T2K Collaboration, PhysRevD.103.L011101, May, 2021.
13. ”First Measurement of the Charged Current antinumu Double Differential Cross Section on a Water Target without Pionsin the final state”, T2K Collaboration, Phys.Rev.D 102 (2020) 1, 012007, Apr. 2021.
14. ”Supernova Model Discrimination with Hyper-Kamiokande”, Hyper-Kamiokande Collaboratioonn, Astrophys.J. 916(2021) 1, 15, Jul. 2021.

11015. ”Comparison of validation methods of simulations for final state interactions in hadron production experiments”, S.
Dytman, Y. Hayato, R. Raboanary, J. T. Sobczyk, J. Tena-Vidal and N. Vololoniaina, Phys.Rev.D 104 (2021) 5, 053006,
Sep. 2021.
16. ”Scintillation light increase of carbontetrafluoride gas at low temperature”, K. Mizukoshi, T. Maeda, Y. Nakano, S.
Higashino and K. Miuchi, JINST 16 P12033, Dec. 2021.
17. ”Evaluation of the radon adsorption efficiency of activated carbon fiber using tetrafluoromethane”, Y Kotsar, Y
Nakano, Y Takeuchi, K Miuchi, Progress of Theoretical and Experimental Physics, Volume 2022, Issue 2, 023H01,
Feb. 2022.
18. ”History of Solar Neutrino Observations”, Masayuki Nakahata, Progress of Theoretical and Experimental Physics,
ptac039, Mar. 2022.
19. ”Search for event bursts in XMASS-I associated with gravitational-wave events”, K. Abe et al., Astroparticle Physics
Volume 139, June 2022, 102702, May. 2021.
20. ”222 Rn emanation measurements for the XENON1T experiment”, E. Aprile et al., The European Physical Journal C volume 81, Article number: 337 (2021), Apr. 2021.
21. ”Search for inelastic scattering of WIMP dark matter in XENON1T”, E. Aprile et al., Phys. Rev. D 103, 063028, Mar.
2021.
22. ”Search for Coherent Elastic Scattering of Solar 8 B Neutrinos in the XENON1T Dark Matter Experiment (2021), arXiv:2012.02846”, E. Aprile et al., Phys. Rev. Lett. 126, 091301, Mar. 2021.
23. ”Improved quality tests of R11410-21 photomultiplier tubes for the XENONnT experiment ”, V.C. Antochi et al.,2021 JINST 16 P08033, Aug. 2021.
24. ”Characterization of new silicon photomultipliers with low dark noise at low temperature”, K. Ozaki, S. Kazama, M.
Yamashita, Y. Itow and S. Moriyama, 2021 JINST 16 P03014, Mar. 2021.
25. ”Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign”, J.C. Albaga etal. (EH Telescope, MAGIC collaboration), Astrophys.J.Lett. 911 (2021) 1, L11, April.2021.
26. ”Gamma-Ray Bursts at TeV Energies: Observational Status”, Koji Noda and Robert Daniel Parsons, Galaxies 2022,10(1), 7, Jan. 2022.
27. ”Combined searches for dark matter in dwarf spheroidal galaxies observed with the MAGIC telescopes, including new
data from Coma Berenices and Draco”, MAGIC collaboration, Physics of the Dark Universe, Volume 35, article id.
100912., Mar. 2022.
28. ”Investigating the Blazar TXS 0506+056 through Sharp Multiwavelength Eyes During 2017-2019”, MAGIC collaboration, The Astrophysical Journal, Volume 927, Issue 2, id.197, 15 pp., Mar. 2022.
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(b) Conference Papers (Proceedings)
1. ”Latest Results from the XMASS Experiment”, Ichimura, K, 14th Asia-Pacific Physics Conference (APPC), NOV 17-22,
20192. ”COSMOS X as a general purpose air shower simulation tool”, T. Sako, T. Fujii, K. Kasahara, H. Menjo, N. Sakaki, N.
Sakurai, A. Taketa, Y. Tameda and the COSMOS X development team, 37th Intrenational Cosmic Ray Conference, JUL
12-23 2021
3. ”Telescope Array anisotropy summar”, T. Fujii, D. Ivanov, C.C.H. Jui, K. Kawata, J.H. Kim, M.Yu. Kuznetsov, T.
Nonaka, S. Ogio, G.I. Rubtsov, H. Sagawa, G.B. Thomson, P.G. Tinyakov, I. Tkachev and S. Troitsky on behalf of theTelescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
4. ”Mass composition of Telescope Array’s surface detectors events using deep learning”, I. Kharuk, and O. Kalashev onbehalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
5. ”The UHECR dipole and quadrupole in the latest data from the original Auger and TA surface detectors”, P. Tinyakov,
L. Anchordoqui, T. Bister, J. Biteau, L. Caccianiga, R. de Almeida, O. Deligny, A. di Matteo, U. Giaccari, D. Harari,
J. Kim, M. Kuznetsov, I. Maris, G. Rubtsov, S. Troitsky and F. Urban on behalf of the Pierre Auger and the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
6. ”Cosmic ray energy spectrum in the 2nd knee region measured by the TALE-SD array”, K. Sato on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
7. ”Effects of Galactic magnetic field on the UHECR anisotropy studies”, R. Higuchi, T. Sako, K. Kawata, T. Fujii and E.
Kido on behalf of the the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
8. ”Performance and simulation of the surface detector array of the TAx4 experiment”, K. Fujisue on behalf of the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
9. ”Cosmic Ray Composition in the Second Knee Region as Measured by the TALE Hybrid Detector”, K. Fujita on behalfof the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
10. ”Cosmic Ray Energy Spectrum measured by the TALE Fluorescence Detector”, T. AbuZayyad on behalf of the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
11. ”Cosmic Ray Composition between 2 PeV and 2 EeV measured by the TALE Fluorescence Detector”, T. AbuZayyad on
behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
12. ”Insight Into Lightning Initiation via Downward Terrestrial Gamma-ray Flash Observations at Telescope Array”, J.
Remington on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
13. ”Anisotropy search in the Ultra High Energy Cosmic Ray Spectrum in the Northern Hemisphere using latest data obtained with Telescope Array surface detector”, T. Nonaka on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
14. ”Monocular Energy Spectrum using the TAx4 Fluorescence Detector”, M. Potts and C. Jui on behalf of the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
15. ”Recent measurement of the Telescope Array energy spectrum and observation of the shoulder feature in the Northern
Hemisphere”, D. Ivanov, D. Bergman, G. Furlich, R. Gonzalez, G. B. Thomson and Y. Tsunesada on behalf of theTelescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
16. ”Telescope Array 10-Year Monocular Spectrum Measurement”, D. R. Bergman and G. Furlich on behalf of the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
17. ”Telescope Array Combined Fit to Cosmic Ray Spectrum and Composition”, D. R. Bergman on behalf of the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021

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18. ”Joint analysis of the energy spectrum of ultra-high-energy cosmic rays measured at the Pierre Auger Observatory and theTelescope Array”, Y. Tsunesada on behalf of the Pierre Auger and the Telescope Array Collaboration, 37th Intrenational
Cosmic Ray Conference, JUL 12-23 2021
19. ”Analysis of TAx4 hybrid trigger and events”, S. W. Kim on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
20. ”Reconstruction of Air Shower Events Measured by the Surface Detectors of the TAx4 Experiment”, H. Jeong on behalfof the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
21. ”Energy spectrum and the shower maxima of cosmic rays above the knee region measured with the NICHE detectors at
the TA site”, Y. Omura, R. Tsuda, Y. Tsunesada, D. R Bergman and J. F. Krizmanicc on behalf of the Telescope Array
Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
22. ”Hotspot Update, and a new Excess of Events on the Sky Seen by the Telescope Array Experiment”, J. Kim, D. Ivanov,K. Kawata, H. Sagawa and G. Thomson on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray
Conference, JUL 12-23 2021
23. ”UHECR arrival directions in the latest data from the original Auger and TA surface detectors and nearby galaxies”, A.
di Matteo, L. Anchordoqui, T. Bister, J. Biteau, L. Caccianiga, R. de Almeida, O. Deligny, U. Giaccari, D. Harari, J.
Kim, M. Kuznetsov, I. Maris, , G. Rubtsov, P. Tinyakov, S. Troitsky and F. Urban on behalf of the Pierre Auger and theTelescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
24. ”The measurements of the cosmic ray energy spectrum and the depth of maximum shower development of Telescope
Array Hybrid trigger events”, H. Shin on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray
Conference, JUL 12-23 2021
25. ”Cosmic-ray mass composition with the TA SD 12-year data”, Y. Zhezher on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
26. ”Mass composition anisotropy with the Telescope Array Surface Detector data”, Y. Zhezher on behalf of the Telescope
Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
27. ”Measurement of the Proton-Air Cross Section with Telescope Arrays Black Rock, Long Ridge, and Surface Array inHybrid Mode ”, R. Abbasi and W. Hanlon on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic
Ray Conference, JUL 12-23 2021
28. ”UHECR mass composition from anisotropy of their arrival directions with the Telescope Array SD”, M. Kuznetsov
and P. Tinyakov on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23
2021
29. ”Update on the large-scale cosmic-ray anisotropy search at the highest energies by the Telescope Array Experiment”, T.
Fujii on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
30. ”The status of the TALE surface detector array and a TALE infill project”, A. Iwasaki and S. Ogio on behalf of theTelescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
31. ”Telescope Array Surface Detector Energy and Arrival Direction Estimation Using Deep Learning”, O.E. Kalashev, D.
Ivanov, M.Yu. Kuznetsov, G.I. Rubtsov, T. Sako, Y. Tsunesada and Y. V. Zhezher on behalf of the Telescope Array
Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
32. ”The atmospheric transparency of Telescope Array observation site by the CLF”, T. Tomida, T. Nakamura, K. Yamazaki,
J. Matthews on behalf of the Telescope Array Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23
2021
33. ”Current status and prospects of surface detector of the TAx4 experiment”, E. Kido on behalf of the Telescope Array
Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
34. ”Telescope Array Cloud Ranging Test”, T. Okuda on behalf of the Telescope Array Collaboration, 37th Intrenational
Cosmic Ray Conference, JUL 12-23 2021
35. ”Highlights from the Telescope Array experiment”, G. I. Rubtsov on behalf of the Telescope Array Collaboration, 37th
Intrenational Cosmic Ray Conference, JUL 12-23 2021
36. ”A simulation study for one-pion exchange contribution on very forward neutron productions in ATLAS-LHCf common
events”, K. Ohashi, H. Menjo, Y. Itow and T. Sako, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021

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37. ”Status and Prospects of the LHCf and RHICf experiments”, H. Menjo on behalf of the LHCf and RHICf Collaboration,
37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
38. ”LHCf plan for proton-oxygen collisions at LHC”, E. Berti and L. Bonechib on behalf of the LHCf Collaboration, 37th
Intrenational Cosmic Ray Conference, JUL 12-23 2021√
39. ”Very-forward π 0 production cross section in s = 13 TeV measured with the proton-proton collisions at LHCf experiment”, A. Tiberio on behalf of the LHCf Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
40. ”High-redshift gamma-ray burst for unraveling the Dark Ages Mission - HiZ-GUNDAM -”, Yonetoku, D; Mihara, T;
Doi, A; Sakamoto, T; Tsumura, K; Ioka, K; Amaya, Y; Arimoto, M; Enoto, T; Fujii, T; Goto, H; Gunji, S; Hiraga,
J; Ikeda, H; Kawai, N; Kurosawa, S; Li, J; Maeda, Y; Mitsuishi, I; Murakami, T; Nakagawa, Y; Ogino, N; Ohno, M;
Sawano, T; Sei, K; Serino, M; Sugita, S; Tamagawa, T; Tamura, K; Tanaka, T; Tanimori, T; Tashiro, MS; Tomida, H;
Wang, H; Yamaguchi, T; Yamamoto, A; Yamaoka, K; Yamauchi, M; Yatsu, Y; Yoshida, A; Yuhi, D; Akitaya, H; Fukui,
A; Ita, Y; Kaneda, H; Kawabata, K; Kawata, Y; Kurimata, M; Matsumoto, T; Matsuura, S; Miyasaka, A; Motohara,
K; Narita, N; Noda, H; Ohashi, A; Okita, H; Sano, K; Tanaka, M; Urata, Y; Wada, T; Yamaguchi, H; Yanagisawa, K;
Yoshida, M; Asano, K; Inayoshi, K; Inoue, S; Ito, H; Izumiura, H; Kawanaka, N; Kinugawa, T; Kisaka, S; Kiuchi,
K; Kyutoku, K; Matsumoto, J; Mizuta, A; Murase, K; Nagakura, H; Nagataki, S; Nakada, Y; Nakamura, T; Niino, Y;
Suwa, Y; Takahashi, K; Tanaka, T; Toma, K; Totani, T; Yamazaki, R; Yokoyama, J, Conference on Space Telescopes
and Instrumentation - Ultraviolet to Gamma Ray / SPIE Astronomical Telescopes + Instrumentation Conference, DEC
14-18, 2020
41. ”Highlights from Gamma-ray Observation by the Tibet ASγ Experiment”, M. Takita on behalf of the Tibet ASγ Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
42. ”Sensitivity of the Tibet hybrid experiment (Tibet-III + MD) for primary proton spectrum between 30 TeV and a few
hundreds of TeV’s”, D. Kurashige, D. Chen, N. Hotta, J. Huang, Y. Katayose, K. Kawata, M. Ohnishi, T. Saito, T. K.
Sako, M. Shibata and M. Takita, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
43. ”Observation of Ultra-High-Energy Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array”, K.
Kawata on behalf of the Tibet ASγ Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
44. ”Gamma-ray Observation of the Cygnus Region with the Tibet Air Shower Array”, Y. Katayose and T. K. Sako on behalfof the Tibet ASγ Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
45. ”A northern sky survey for ultra-high-energy gamma-ray source using the Tibet air-shower array and muon-detector
array”, X. Chen on behalf of the Tibet ASγ Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
46. ”Gamma-ray Observation of SNR G106.3+2.7 with the Tibet Air Shower Array”, M. Ohnishi on behalf of the Tibet ASγ
Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
47. ”Modeling of the TeV cosmic-ray anisotropy based on intensity mapping in an MHD-simulated heliosphere”, T. K. Sakoon behalf of the Tibet ASγ Collaboration, and N. V. Pogorelov, 37th Intrenational Cosmic Ray Conference, JUL 12-23
2021
48. ”Current status of ALPACA for exploring sub-PeV gamma-ray sky in Bolivia”, T. Sako on behalf of the ALPACA
Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
49. ”Half ALPACA and its sensitivity to sub-PeV gamma rays from the Galactic Center”, Y. Yokoe on behalf of the ALPACA
Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
50. ”A simulation study on the performance of the ALPAQUITA experiment”, S. Kato on behalf of the ALPACA Collaboration, 37th Intrenational Cosmic Ray Conference, JUL 12-23 2021
51. ”Ultralight dark matter searches with KAGRA gravitational wave telescope”, Tomohiro Fujita, Jun’ya Kume, Yuta
Michimura, Soichiro Morisaki, Koji Nagano, Hiromasa Nakatsuka, Atsushi Nishizawa, Ippei Obata,, TAUP 2021, AUG
26-SEP 23, 2021
52. ”A nonparametric method to assess the significance of events in the search for gravitational waves with false discoveryrate”, Yuzurihara, H; Mano, S; Tagoshi, H, Conference on Space-Time Topology Behind Formation of Micro-Macro
Magneto-Vortical Structure Manifested by Nambu Mechanics, SEP 28-OCT 01, 2020

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D. Doctoral Theses
1. Search for Neutrinos associated with Solar Flares in Super-Kamiokande,
OKAMOTO, Kouhei,
Ph.D Thesis, Mar. 2022
2. Novel technique to purify liquid xenon for the XENONnT dark matter search experiment,
KATO, Nobuyuki,
Ph.D Thesis, Mar. 2022
3. Exploring the Frontiers of Low-mass Galaxy Formation over the Cosmic History,
KIKUCHIHARA, Shotaro,
Ph.D Thesis, Mar. 2022
4. Search for UHECR sources considering the deflection by the galactic magnetic field,
HIGUCHI, Ryo,Ph.D Thesis, Mar. 2022
5. Imprints on Cosmic Microwave Background by WIMP and Axion,
NAKATSUKA, Hiromasa,
Ph.D Thesis, Mar. 2022
6. Concrete Models and Phenomenologies of Composite Asymmetric Dark Matter,
KOBAYASHI, Shin,Ph.D Thesis, Mar. 2022

E. Public Relations(a) ICRR NewsICRR News is a quarterly publication written in Japanese about scientific and educational activities at ICRR.
Below lists the main topics in the issues published in FY 2021:
No.111 (2021 Summer)• Features : Professor Masato Takita Wins 67th Nishina Memorial Prize for ”Establishment of the sub-PeV gamma ray
astronomy and elucidation of the origin of galactic cosmic rays”• Features : Special Contribution by Jiro Aarafune, Former ICRR Director, ”From Kamiokande To Super-Kamiokande ∼Remembering Prof. Masatoshi Koshiba ∼”• Features : Gathering to remember Prof. Masatoshi Koshiba Held at Koshiba Hall in Hongo Campus• Reports : About 2,300 People Enjoy Online Kashiwa Campus Open Days
• Reports : Super-Kamiokande and KAGRA Open Days Held Online• Reports : ICRR’s Alumnae, Ms. Momoko Kashino Talks at Event for Female Students, ” Yappari Buturiga Suki”• Topics : Dr. Oishi’s Recommended Book and Her Comment Displayed at Tamarokuto Scicence Center• Topics : ICRR Calls for Participation in Spring School 2022
• Topics : Dr. Fedynitch (Telescope Array Group) Wins Best Poster Award at ICRC2021• Topics : ICRR Calendar 2021 On Sale
• Topics : Annual Report 2020 Published• Information : Staff Reassignment• Information : ICRR Seminar

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No.112 (2021-22 Winter)• Features : ”Elucidating Origin of Matter and Evolution of Universe” Hyper-Kamiokande Co-Spokesperson, Prof. Masato
Shiozawa• Features : Associate Professor, Koji Noda Wins JSPS Prize for ”Study of Very High Energy Gamma Ray Emission from
Gamma Ray Bursts”• Reports : ICRR Holds UTokyoNY Event ”Exploring the Universe with Multi-Messengers”• Reports : Project Assistant Prof. Takashi Sako Talks at Science Café held with Tamarokuto Science Center• Reports : Assistance Professor Kyohei Kawaguchi Talks at 25th ICRR x IPMU Public Lecture• Topics : FY2021 ICRR Master and Doctor Thesis Workshop Held Online• Topics : ICRR and KEK Sign MOU with NCBJ in Poland to Promote Hyper-Kamiokande Project• Topics : Professor Masami Ouchi Selected as a Highly Cited Researcher 2021
• Topics : ICRR Holds Online Lecture Meeting Commemorating Prof. Takita’s winning of Nishina Memorial Prize• Topics : Dr. Seiji Fujimoto (formerly affiliated Observational Cosmology Group) Wins Inoue Research Award for Young
Scientists• Topics : CTA LST-1 Awarded European Technology Awards 2021
• Information : Staff Reassignment• Information : ICRR Seminar(b) Public Lectures• ”Central Asia Nobel Fest,” Apr. 14, 2021, Online, Takaaki Kajita (ICRR, The University of Tokyo).
• ”5th Workshop on Korean Neutrino Observatory,” May. 10, 2021, Online, Takaaki Kajita (ICRR, The University ofTokyo).
• ”Public Lecture commemorating the 20th Anniversary of Museum Opening,” Jul. 3, 2021, Children’s Science Musium,
Itano-Gun, Tokushima, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Summer Festival for the 30th Anniversary of JSPS Washington Office,” Sep. 3, 2021, Online, Takaaki Kajita (ICRR,The University of Tokyo).
• ”Public Lecture for Junior High School Students in Higashimatsuyama City,” Sep. 28, 2021, Higashimatsuyama Citizen’s Culture Center, Higashimatsuyama-City, Saitama, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Tsukuba Conference 2021,” Sep. 29, 2021, Online, Takaaki Kajita (ICRR, The University of Tokyo).
• ”All Russican Science Festival,” Oct. 9, 2021, Online, Takaaki Kajita (ICRR, The University of Tokyo).
• ”IEEE Nuclear Science Symposium plenary talk,” Oct. 18, 2021, Online, Takaaki Kajita (ICRR, The University ofTokyo).
• ”World Alliance Forum in San Francisco,” Nov. 16, 2021, Online, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Public Lecture commemorating the 25th Anniversary of SK’s Observation,” Nov. 28, 2021, Spirit Garden Hall, HidaCity, Gifu, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Nishina Memorial Public Lecture,” Dec. 5, 2021, Online, Takaaki Kajita (ICRR, The University of Tokyo).
• ”Forum for Graduate School Educational Reform 2021,” Jan. 8, 2022, Osaka University, Suita-City, Osaka, Takaaki
Kajita (ICRR, The University of Tokyo).
• ”ISS/KIBO Utilization Symposium 2022,” Feb. 9, 2022, Online, Takaki Kajita (ICRR, The University of Tokyo).
• ”100th Anniversary of the municipal system of Kawagoe City,” Feb. 19, 2022, Westa Kawagoe, Kawagoe-City, Saitama,
Takaaki Kajita (ICRR, The University of Tokyo).

121• ”Summer Science Workshop for Tokatsu Techno Plaza,” Aug. 6, 2021, Online, Makio Nakamura (ICRR, The Universityof Tokyo).
• ””Let’s Talk with Researchers” for Kashiwa Campus Open Day,” Oct. 22, 2022, Online, ICRR, The University of Tokyo.
• ”Kashiwa Campus Open Day,” Oct. 23, 2021, Online, Takaaki Kajita, Yuhang ZHAO, Marcel STRZYS (ICRR, The
University of Tokyo).
• ”Kashiwa Campus Open Day,” Oct. 24, 2022, Online, Kazumasa Kawata, Moritz Huetten, Masataka Miwa, Takuya
Tashiro, Teruyoshi Kawashima (ICRR, The University of Tokyo).
• ”Tokyo Municipal Musashi High School,” Nov. 17, 2021, Musashi High School, Musashino-City, Tokyo, Yoshiaki Ono(ICRR, The University of Tokyo).
• ”The 25th ICRR X IPMU Public Lecture,” Nov. 28, 2021, Online, Kyohei Kawaguchi (ICRR, The University of Tokyo).
• ”Science Café for Tamarokuto Science Center,” Jan. 30, 2022, Online, Takashi Sako (ICRR, The University of Tokyo).
• ”UTokyoNY Event,” Feb. 12, 2022, Online, Hiroyuki Sagawa, Masato Takita, Masahiro Teshima, Hideyuki Tagoshi(ICRR, The University of Tokyo).
• ”The 24th ICRR X IPMU Public Lecture,” Apr. 10, 2021, Online, Yoichi Asaoka (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Kyoto Prefectural Rakuhoku High School,” Jun. 3, 2021, Online, Yosuke Kataoka (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Toyama High School,” Jul. 5, 2021, Toyama High School, Toyama-City, Toyama, Jun Kameda (Kamioka Observatory,
ICRR, The University of Tokyo).
• ”Gifu Prefectural Yoshiki High School,” Jul. 9, 2021, Skydome Kamioka, Hida-City, Gifu, Ko Abe (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Nanao High School & NUS High School,” Jul. 14, 2021, Online, Christophe Bronner (Kamioka Observatory, ICRR,The University of Tokyo).
• ”Fuzoku Shimada Junior High School,” Jul. 17, 2021, Online, Masato Shiozawa (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Osaka Seiko Gakuin,” Jul. 19, 2021, Online, Hidekazu Tanaka (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Kofu Minami High Shool / Senior High School Attached to Kyoto University of Education,” Jul. 27, 2021, Online,
Makoto Miura (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Yamanashi Prefectural Nirasaki High School,” Sep. 22, 2021, Online, Yosuke Kataoka (Kamioka Observatory, ICRR,The University of Tokyo).
• ””Let’s Talk with Researchers” for Kashiwa Campus Open Day,” Oct. 22, 2021, Online, Kamioka Observatory, ICRR,The University of Tokyo.
• ”Gifu Prefectural Museum’s Public Lecture,” Oct. 24, 2021, Gifu Prefectural Museum, Seki-City, Gifu, Masayuki
Nakahata (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Gathering in Memory of Professor Koshiba,” Nov. 7, 2021, Online, Masayuki Nakahata (Kamioka Observatory, ICRR,The University of Tokyo).
• ”Kyoto Prefectural Rakuhoku High School,” Nov. 11, 2021, Online, Motoyasu Ikeda (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Public Lecture,” Nov. 13, 2021, Fuchu Fureai Kan, Toyama-City, Toyama, Masayuki Nakahata (Kamioka Observatory,
ICRR, The University of Tokyo).
• ”Super-Kamiokande Open House,” Nov. 20, 2021, Online, Motoyasu Ikeda (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Super-Kamiokande Open House,” Nov. 20, 2021, Online, Yuki Nakano, Shintaro Miki, Yuri Kashiwagi (Kamioka
Observatory, ICRR, The University of Tokyo).

122• ”Super-Kamiokande Open House,” Nov. 20, 2021, Online, Masayuki Nakahata (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Super-Kamiokande Open House,” Nov. 20, 2021, Online, Kei Ieki (Kamioka Observatory, ICRR, The University ofTokyo).
• ”Super-Kamiokande Open House,” Nov. 20, 2021, Online, Hidekazu Tanaka (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Super-Kamiokande Open House,” Nov. 20, 2021, Online, Masayuki Nakahata (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Public Lecture commemorating the 25th Anniversary of SK’s Observation,” Nov. 28, 2021, Spirit Garden Hall, HidaCity, Gifu, Masayuki Nakahata (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Gunma Prefectural Ota High School & Ota Girl’s High School,” Dec. 9, 2021, Online, Kei Ieki (Kamioka Observatory,
ICRR, The University of Tokyo).
• ”Aichi Prefectural Okazakikita High School & Kariya Senior High School,” Dec. 28, 2021, Online, Hiroyuki Sekiya(Kamioka Observatory, ICRR, The University of Tokyo).
• ”Hyper-Kamiokande Online Lecture,” Jan. 15, 2022, Online, Masato Shiozawa (Kamioka Observatory, ICRR, The
University of Tokyo).
• ”Public Lecture,” Jul. 21, 2021, Online, Yoshinari Hayato (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Gifu Prefectural Association of Chairpersons of City Council,” Jul. 9, 2021, Furukawa Community Center, Hida-City,
Gifu, Masayuki Nakahata (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Public Lecture,” Feb. 25, 2022, Online, Masayuki Nakahata (Kamioka Observatory, ICRR, The University of Tokyo).
• ” 51st Astrophysics / Astrophysics Young Summer School,” Aug. 25, 2021, Online, Hiroyuki Sekiya (Kamioka Observatory, ICRR, The University of Tokyo).
• ”Public Lecture,” Oct. 8, 2021, Online, Hiroyuki Sekiya (Kamioka Observatory, ICRR, The University of Tokyo).
• ”UTokyo’s Lab Tour for High School Students,” Mar. 23, 2022, Online, Yoshinari Hayato (Kamioka Observatory, ICRR,The University of Tokyo).
• ”Open College in Hida,” Oct. 24, 2021, Hida Earth Wisdom Center, Masatake Ohashi (KAGRA Observatory, ICRR,The University of Tokyo).
• ”Open College in Hida,” Oct. 24, 2021, Hida Earth Wisdom Center, Shoichi Oshino (KAGRA Observatory, ICRR, The
University of Tokyo).
• ”KAGRA Open House,” Nov. 20, 2021, Online, Masatake Ohashi (KAGRA Observatory, ICRR, The University ofTokyo).
• ”KAGRA Open House,” Nov. 20, 2021, Online, Hirotaka Yuzurihara (KAGRA Observatory, ICRR, The University ofTokyo).
• ”KAGRA Open House,” Nov. 20, 2021, Online, Kenta Tanaka and Chiaki Hirose (KAGRA Observatory, ICRR, The
University of Tokyo)(KAGRA Collaborator)(Niigata University).
• ”Osaka Seiko Gakuin Junior/Senior Higsh School,” Jul. 20, 2021, Online, Takafumi Ushiba (KAGRA Observatory,
ICRR, The University of Tokyo).
• ”Yamanashi Prefectural Nirasaki High School,” Sep. 29, 2021, Online, Takashi Uchiyama (KAGRA Observatory, ICRR,The University of Tokyo).
• ”Hida Kamioka High School,” Oct. 12, 2021, Online, Tatsuki Washimi (KAGRA Collaborator)(National Astronomical
Observatory of Japan).
• ”Fukuoka Prefectural Tochiku Senior High School,” Mar. 28, 2022, Online, Masatake Ohashi (KAGRA Observatory,
ICRR, The University of Tokyo).

123• ”Toyama High School,” Jul. 5, 2021, Toyama High School, Shoichi Oshino (KAGRA Observatory, ICRR, The Universityof Tokyo).
• ”Gifu Prefecutral Yoshiki Senior High School,” Jul. 9, 2021, Skydome Kamioka, Hida-City, Gifu, Takaaki Yokozawa(KAGRA Observatory, ICRR, The University of Tokyo).
• ”Hida Kamioka High School,” Jun. 29, 2021, Hida Kamioka High School, Osamu Miyakawa (KAGRA Observatory,
ICRR, The University of Tokyo).

(c) Visitors
Kashiwa Campus (Total: 4 groups, 43 people)• Junior High and High schools: 0 group• Universities and Graduate schools: 0 group• Researchers: 2 group• Inspections: 1 groups• Press: 1 groups• Others: 0 group
KAMIOKA Observatory (Total: 32 groups, 155 people)• Junior High and High schools: 0 group• Universities and Graduate schools: 3 groups• Researchers: 8 groups• Inspections: 14 groups• Press: 3 groups• Others: 4 groups
KAGRA Observatory (Total: 31 groups, 143 people)• Junior High and High schools: 0 group• Universities and Graduate schools: 7 groups• Researchers: 2 groups• Inspections: 13 groups• Press: 3 group• Others: 6 groups

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F.

Inter-University Research Activities

Numbers of Researchers
Number ofApplications
Facility Usage
Kamioka Observatory
Akeno Observatory
Norikura Observatory
Primary Cosmic rays, Kashiwa underground facility
KAGRA Observatory
High Energy Astrophysics Facility in Canarias
Laboratorial Facilities in Kashiwa
Large-Scale Comping System in Kashiwa
Conference Facilities in Kashiwa
Overseas Facilities (Utah, Tibet and Bolivia)
Annual Sums

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

Domestic
International
Domestic
International

Domestic
International
Domestic
International
Domestic
International
Domestic
International
Domestic
International
Domestic
International

Domestic
International
Domestic
International
Domestic
International
Domestic
International

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

Number of
Adoptions

Number of
Researchers

45
9
6
2
11
3
22
6
11
1
1
14
1
6
1
17
5
136
25

45
9
6
2
11
3
22
6
11
1
1
14
1
6
1
17
5
136
25

1,755
48
181
27
82
14
612
27
416
2
7
269
10
267
5
207
40
3,810
159

45
10
60
9
25
6
6
136
25

45
10
60
9
25
6
6
136
25

1,755
58
1,306
74
687
27
62
3,810
159

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10. Search for proton decay via e+ π 0 mode
11. Study of proton decay p → νK +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. Neutrino search associated with astronominal transient events
15. Measurement of charge ratio and polarization of cosmic ray muon using undergraund particle physics experiments
16. Tokai to Kamioka Long Baseline Experiment T2K
17. Neutrino interaction study using accelerator data
18. Study for improvement of neutrino oscillation measurement in T2K expriment
19. Joint Oscillation Analysis With the T2K and Super-Kamiokande Experiments
20. Energy calibration for Super-Kamiokande
21. Development of a new LINAC for Super-Kamiokande energy calibration
22. Research and development of computer simulation of Super-Kamiokande detector
23. Development of low concentration radon detection system
24. Hyper-Kamiokande project
25. Development of the Large Aperture Photodetector for a next-generation neutrino detector
26. Development of software for the next generation neutrino detector
27. Measurement of radon in underground laboratories and evaluate of effects from radon to experiments
28. Study for lowering backgrounds of radioisotopes in large volume detectors
29. Development of a radioactivity assay system for underground experiments
30. Measurement of neutron flux at the Kamioka underground laboratory
31. Trace level radio-activity measurements for SK and underground experiments with an ICP-mass spectrometer
32. RI measurement for SK-Gd project with HPGe detector
33. Generation three direct dark matter search experiment
34. A Search for Dark Matter using Liquid Xenon Detector
35. Detector structure study for future direct dark matter search experiment
36. Development of low radioactivity molecular sieves for dark matter search experiment
37. Research and Development for XENONnT and future Dark Matter searches
38. Direction-sensitive dark matter search
39. A study on the near-infrared emission of liquid xenon
40. Study of double beta decay of 48Ca
41. Studies on the background evaluation using laser spectroscopy analysis
42. Integration of crustal activity observation around the Atotsugawa fault
43. Strain, tilt, seismic measurement in Kamioka-mine
44. Searches for neutrinoless double beta decay with high-pressure Xenon gas detector
45. Soft-error-rate estimation for semiconductor device at underground laboratory

126

46. R&D for a Small Atmospheric Cherenkov Telescope in Akeno Observatory
47. Development of new surface detector for observation
48. Multi-Color Imager for Transients, Survey and Monstrous Explosions
49. Observation of Galactic Cosmic Ray Intensities using Large Area Muon Telescopes
50. Observation of solar neutrons in solar cycle 25
51. Space weather observation using muon hodoscope at Mt. Norikura
52. Observation of cosmogenic nuclides concentrations at Mt. Norikura
53. Study of secondary cosmic rays from Thundercloud at Mt. Norikura
54. Development of high energy proton irradiation technique for devices used in spaceship
55. Investigation of alpine plants on Mt. Norikura
56. Symbiosis betweem Pinus pumila and Nucifraga caryocatactes on Mt.Norikura
57. Atmospheric mercury speciation dynamics and mercury wet deopsition monitoring at the high-altitude
58. Patch dynamics of Pinus pumila and Vaccinium vitis-idaea at the ridge site of Mt. Norikura
59. Evaluation of atmospheric SO2 to cloud acidification at free troposhere through atmospheric observation at Mt. Norikura
60. Test exepriment of the Fuel cell in Norikura
61. CTA Project
62. Development of Focal Plane Instruments for the CTA Large Sized
63. Development of the readout system for the CTA large sized telescopes
64. Engineering runs of the first Large Size Telescope of CTA and construction of LST2-4 in La Palma Canary Islands, Spain
65. Set-up and Commissioning of the onsite data center for CTA North in La Palma, Spain
66. Localization of very high energy gamma-ray emission region in an active galactic nuclei
67. Development of SiPM modules for CTA-LST
68. Development of the CTA/LST telescope control system
69. Integration and operation of the optical and power systems in CTA LST
70. CTA-Japan Physics Research
71. Development of analysis method and initial observation with CTA Large-Sized Telescope
72. The extreme Universe viewed in very-high-energy gamma rays 2021
73. Development of camera for CTA small-sized telescopes
74. CTA Monte Carlo Simulation
75. Study of High Energy Gamma-ray Objects with the MAGIC telescope
76. Study of Extremely-high Energy Cosmic Rays by Telescope Array
77. R&D of the measurement of atmospheric transparency at the Telescope Array site
78. Research and development of surface detectors for the stable run of the TAx4 experiment
79. Study of very high energy cosmic rays around 1017 eV with the TALE hybrid experiment
80. Evaluation of optical system calibration for fluorescence detector optical system with standard light source mounted on
UAV

127

81. Development and analysis of night cloud observation by CCD camera for automatic observation of air fluorescence
detector
82. Development of the local center for the fully remote operation of TA-FD
83. Observation of airshower fluorescence light at the TA FD site by using an Imaging UV telescope
84. Observing ultrahigh-energy cosmic rays with new fluorescence detectors at Telescope Array site
85. Research and development of a Fresnel lens air fluorescence telescope for the next generation UHECR observation
86. The position measurement of radiation by one-side readout scintillation detector
87. Experimental Study of High-energy Cosmic Rays in the Tibet ASγ experiment
88. Sidereal daily variation of ∼10TeV galactic cosmic ray intensity observed by the Tibet air shower array
89. Study of the composition of cosmic-rays at the Knee
90. A study on variation of interplanetary magnetic field with the cosmic-ray shadow by the sun
91. Air shower observation for high-energy gamma ray and cosmic ray detections at the Chacaltaya Cosmic Ray Observatory
92. Study of High Energetic Radiation from Thundercloud in the Altiplano93. Study of solar magnetic fields using the cosmic-ray Sun’s shadow observed at the southern hemishpere
94. Study of high-energy cosmic-ray anisotropy at the Chacaltaya Cosmic Ray Observatory
95. Development of an advanced Compton camera using SOI pixel semiconductor
96. Design study of a Compton camera for study of cosmic rays
97. Study on High Energy Cosmic-ray Sources by Observations in Space with CALET
98. comprehensive study of high-energy astrophysical phenomena
99. Development of a new code for cosmic-ray air shower simulation
100. Cosmic ray interactions in the knee and the highest energy regions
101. Study of Fast Moving Dark Matters and Meteoroids using High Sensitivity CMOS Camera System
102. Preparatory Activities by the Local Organizing Committee for the 38th International Cosmic Ray Conference
103. YMAP symposium 2021 (Basic Part)
104. Observation with Ahsra
105. Integration of the optical fiber trigger system for Ashra
106. Research of Large-scale Gravitational wave Telescope (XI)
107. Study of Gravitational-wave by cryogenic laser interferometer CLIO in KAMIOKA Mine(V)108. Development of High Performance Cryogenic Mirror Control System
109. Research on cryogenic payload for KAGRA
110. Deveropment of conduction cooling schemes for KAGRA
111. R&D for the intensity stabilization of the laser system in KAGRA
112. Development of scattered light measurement technique and noise reduction for improving sensitivity in KAGRA II
113. Development of optical cavity for ultranarrow stable lasers
114. Study of Environmental Noise Reduction in KAGRA
115. Study of the investigation methed for the envieronmental noises by injection in KAGRA detector

128116. Noise characteristics analysis using machine learning and deep learning and its application to interferometer diagnosis
117. Improving the reliability of KAGRA gravitational wave data by using machine learning technique for glitch detection
118. Calorimetric measurement of absorption coefficient of sapphire at cryogenic temperatures
119. Numerical Simulation of Electro-Magnetic Wave Propagation in Gravitational wave Detector IX
120. Development of a high performance sapphire mirror suspension
121. Control and automatic operation for KAGRA
122. Construction of KAGRA data transfer and storage system (7)123. Promotion of collaborative research to incorporate KAGRA data into the low latency international gravitational wave
search network
124. Construction of the interferometer diagnostic system for gravitational waves searches
125. Promotion of KAGRA scientific collaboration, and development of new methods in gravitational data analysis
126. Search for poorly modeled gravitational wave signal using KAGRA II
127. Environmental investigation for underground gravity measurement at micro-meter scale
128. Precise geophysical observation at the Kamioka underground site and modeling of crustal activities
129. Cosmic Reionization and Galaxy Formation Probed with Large Optical Near-Infrared Telescopes
130. Evolution of the universe and particle physics
131. Detection of time variations for cosmogenic nucleid Be-7
132. Time profile of radioactive Cs concentration and its aerosol size distribution in local area
133. Development of the observation templates of supernova neutrinos considering nuclear equations of state
134. Behavior of radionuclides in the marine environment
135. Frontier of the planetary material science
136. Precise calculation of the atmospheric neutrino flux
137. Light scattering measurement in the water using the Super-Kamiokande detector
138. Search for the nucleon decay including multi pions in Super-Kamiokande
139. New Photogrammetry Calibration and Machine Learning Event Reconstruction for Super-Kamiokande and HyperKamiokande
140. Study of supernova neutrinos in Super-Kamiokande
141. Data Taking, Calibrations, Measurements and Analysis with Super-Kamiokande I - VI
142. Constraining systematics at T2K and SuperKamiokande oscillation analyses using neutrino-nucleus interaction models
143. Development and testing of cost-effective, high-performance PhotoDetector anti-implosion covers for HyperKamiokande
144. Outer Detector for the Hyper-Kamiokande experiment145. Seal, mechanical and functional tests on the mPMT prototype for external vessel optimizzations
146. Towards a multiphysics liquid Xe TPC with scintillation and charge multiplication readouts
147. A joint observation of near earth space through ∼100 GeV cosmic rays using the Akeno and the GRAPES-3 muon
telescope148. Neutrino Telescope Array Light Collector Prototype Test
149. Ultra-high-energy cosmic-ray origin studies with the Telescope Array and TAx4 surface detector

129150. Study of the Origin and Nature of Ultra High Energy Cosmic Rays with the Telescope Array
151. Study of Anisotropy of Ultra High Energy Cosmic Rays with Telescope Array and TAx4
152. Understanding the signatures of cosmic ray feedback in the diffuse extra-galactic gamma-ray background with CTA
153. Kashiwa Dark Matter symposium 2021
154. Study of high-energy cosmic rays at a high altitude in Tibet, China
155. Study for Galactic CR origin using the ALPACA air shower array in Bolivia
156. Noise Evaluation and Reduction of Cryogenic Mirror Suspension System in KAGRA
157. Intermediate stage for future cryogenic payload development
158. Filter cavity experiments for the frequency dependent squeezed light source for KAGRA
159. Implementing Sophisticated Data Analysis Methods on KAGRA Data
160. Collaboration for gravitation wave observatory in study dark matter with compact binary coalescence161. Experimental research on gravitational-wave astrophysics with TOBA

130

G. List of Committee Members(a) Board of Councillors
KAJITA, Takaaki
SHIOZAWA, Masato
TAKITA, Masato
KAWASAKI, Masahiro
HOSHINO, Masahiro
SAITO, Nobuhito
TOKUSHUKU, Katsuo
AOKI, Shinya
TSUNETA, Saku
TORII, Shoji
MORI, Masaki
YOKOYAMA, Junichi
YAMADA, Toru
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
Ritsumeikan University
Research Center for the Early Universe, The University of Tokyo
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
IOKA, Kunihito
ISHITSUKA, Masaki
OZAWA, Kyoichiro
YOKOYAMA, Masashi
TASHIRO, Makoto
OGIO, Shoichi
NISHIJIMA, Kyoshi
ITOW, Yoshitaka
KANDA, Nobuyuki
NAKAHATA, Masayuki
SAGAWA, Hiroyuki
KAWASAKI, Masahiro
SHIOZAWA, Masato
TAKITA, Masato
OHASHI, Masatake
TESHIMA, Masahiro

ICRR, The University of Tokyo
KEK Theory Center
Yukawa Institute for Theoretical Physics, Kyoto University
Tokyo University of Science
Institute of Particle and Nuclear Studies, KEK
Graduate School of Science, The University of Tokyo
Graduate School of Science and Engineering, Saitama University
Graduate School of Science, Osaka City University
Tokai University
Institute for Space-Earth Environmental Research, Nagoya 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
ITOW, Yoshitaka
ASO, Yoichi
MORI, Masaki
TAJIMA, Hiroyasu
KUBO, Hidetoshi
HIBINO, Kinya
MIUCHI, Kentaro
TSUNESADA, Yoshiki
NAKAHATA, Masayuki
TAKITA, Masato
TAGOSHI, Hideyuki
NODA, Koji
UCHIYAMA, Takashi (Organizer)
KAJITA, Takaaki (Observer)

Institute for Space-Earth Environmental Research, Nagoya University
National Astronomical Observatory of Japan
Ritsumeikan University
Institute for Space-Earth Environmental Research, Nagoya University
Kyoto University
Kanagawa University
Graduate School of Science, Kobe 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

131

H. List of Personnel

Director

KAJITA, Takaaki,

Vice-Director

NAKAHATA, Masayuki,

SAGAWA, Hiroyuki,

ICRR HQ in Kashiwa Campus- High Energy Cosmic Ray Division
Scientific Staff
ASANO, Katsuaki,
FIASSON, Armand,
KINUGAWA, Tomoya,
NODA, Koji,
OHISHI, Michiko,
SAKO, Takashi,
SITAREK, Julian,
TESHIMA, Masahiro,
YOSHIDA, Tatsuo,
Technical Staff
AOKI, Toshifumi,
SEKINO, Koichi,
Research Fellow
ANZORENA, Marcos,
COLOMBO, Eduardo,
HUETTEN, Moritz,
MARQUEZ, Paniagua Patricia,
STRZYS, Marcel,
ZHEZHER, Yana Valeryevna,
Secretary
IDOMURA, Takako,- Astrophysics and Gravity Division
Scientific Staff
HARIKANE, Yuichi,
ONO, Yoshiaki,
Research Fellow
AOYAMA, Shohei,

DE LA FUENTE, Eduardo,
HADASCH, Daniela,
KUBO, Hidetoshi,
NONAKA, Toshiyuki,
OHNISHI, Munehiro,
SAKO, Takashi,
TAKEDA, Masahiro,
VOVK, Ievgen,
YOSHIKOSHI, Takanori,
MARUYAMA, Tatsuya,

ENOMOTO, Ryoji,
KAWATA, Kazumasa,
MAZIN, Daniel,
OGIO, Shoichi,
SAITO, Takayuki,
SASAKI, Makoto,
TAKITA, Masato,
YAMAMOTO, Tokonatsu,

CERIBELLA, Giovanni,
FEDYNITCH, Anatoli,
INADA, Tomohiro,
NAKAMURA, Yoshiaki,
TAKAHASHI, Mitsunari,

CHIKAWA, Michiyuki,
FUKAMI, Satoshi,
KAWASHIMA, Tomohisa,
OHMURA, Takumi,
TAKEISHI, Ryuji,

SHIRAGA, Ryoko,

SUGAHARA, Midori,

IBE, Masahiro,
OUCHI, Masami,
EIJIMA, Shintaro,

KAWASAKI, Masahiro,

OHOKA, Hideyuki,

UMEHATA, Hideki,

Kamioka Observatory
Director
Scientific Staff

Administrative Staff
Public Relations Staff
Technical Staff
Research Administrator
Research Fellow
Secretary

NAKAHATA, Masayuki,
ABE, Ko,
GONIN, Michel,
HOSOKAWA, Keishi,
ITOW, Yoshitaka,
MIURA, Makoto,
NAKAYA, Tsuyoshi,
PRONOST, Guillaume,
SHIOZAWA, Masato,
TANAKA, Hidekazu,
YOSHIDA, Takuto,
TAKENAGA, Yumiko,
HAMA, Satoshi,
NOZAWA, Noriyuki,
KURACHI, Masafumi,
TAKENAKA, Akira,
DOI, Kyoko,
NISHIDA, Hiromi,

ASAOKA, Yoichi,
HAYATO, Yoshinari,
IEKI, Kei,
KAMEDA, Jun,
MORIYAMA, Shigetaka,
NAKAYAMA, Shoei,
SATO, Kazufumi,
TAKEDA, Atsushi,
YANO, Takatomi,

BRONNER, Christophe,
HIRAIDE, Katsuki,
IKEDA, Motoyasu,
KATAOKA, Yosuke,
NAKANO, Yuki,
NISHIMURA, Yasuhiro,
SEKIYA, Hiroyuki,
TAKEMOTO, Yasuhiro,

HIGASHI, Tetsuji,
ONOUE, Tatsuya,

ISHITA, Katsumi,

KAMIKAWATO, Rie,

MAEDA, Yukari,

132

KAGRA Observatory
Director
Scientific Staff

Administrative Staff
Technical Staff

Research Fellow

Secretary

OHASHI, Masatake,
KAWAGUCHI, Kyohei,
MIYAKAWA, Osamu,
TAGOSHI, Hideyuki,
UCHIYAMA, Takashi,
YAMAMOTO, Takahiro,
FUNADA, Shinya,
AOUMI, Masakazu,
NODE, Ayako,
SAKAI, Akiko,
TAKAHASHI, Masahiro,
YOSHIMURA, Mitsuharu,
MIYO, Koseki,
SAITO, Yoshio,
TANIOKA,Satoshi
YUZURIHARA, Hirotaka,
KIKUCHI, Rie,

KIMURA, Nobuhiro,
MIYOKI, Shinji,
TAKAHASHI, Hirotaka,
USHIBA, Takafumi,
YOKOZAWA, Takaaki,
OKINAKA, Mihoko,
KAMIIZUMI, Masahiro,
OMAE, Akio,
SAKAMOTO, Eri,
TAKASE, Takashi,

MIO, Norikatsu,
OSHINO, Shoichi,
TOMARU, Takayuki,
YAMAMOTO, Kazuhiro,
YOSHII, Yuzuru,

NARIKAWA, Tatsuya,
SURESH, Jishnu,
TUYENBAHEV, Darkhan,
ZHAO, Yuhang,
KUDO, Naomi,

OHHARA, Kenichi,
SUZUKI, Toshikazu,
UCHIKATA, Nami,

TASHIRO, Takuya,

TOKANAI, Fuyuki,

NAKAGAKI, Koji,
PENA ARELLANO, Fabian,
SHIMODE, Katsuhiko,
YASUI, Hiromi,

NOJIRI, Midori,High Energy Astrophysics Facility in Canarias
Director

TESHIMA, Masahiro,

Research Center for Cosmic Neutrinos
Director
Scientific Staff
Technical Staff
Secretary

OKUMURA, Kimihiro,
KAJITA, Takaaki,
SHINOHARA, Masanobu,
KITSUGI, Atsuko,

MASHIMA, Chieko,

TAKITA, Masato,
AWAI, Kyosuke,
INOME, Yusuke,
TOMURA, Tomonobu,

HAYAKAWA, Hideaki,
OKAZAKI, Nao,
USHIMARU, Tsukasa,

IMANISHI, Hidenori,
SHIMODAIRA, Hideaki,

FUJISUE, Kozo,
HSIEH, Bin-Hua,
KATO, Sei,
KOBAYASHI, Shin,
MURAI, Kai,NISHIWAKI, Kosuke,
OTANI, Francis,
SONODA, Yutaro,
XIA, Junjie,

HAN, Seungho,
ISOBE, Yuki,
KATO, Takashi,
KOBAYASHI, Yukiho,
NAKATSUKA, Hiromasa,
OHTANI, Yoshiki,
SAKURAI, Shunsuke,
TANAKA, Kenta,
YOKOE, Yoshichika,

Norikura Observatory
Director
Technical Staff

Akeno Observatory
Director
Technical Staff

SAGAWA, Hiroyuki,
KOBAYASHI, Ryoichi,

Graduate Students
Doctor

ABE, Hyuga,
HIGUCHI, Ryo,
KATO, Nobuyuki,
KIKUCHIHARA, Shotaro,
LEE, Eunsub,
NAKAYAMA, Yuhei,
OKAMOTO, Kohei,
SHIN, Heungsu,
TANG, Shenli,
ZHANG, Yechi,

133

Master

ABE, Shotaro,
CHIBA, Renichi,
HASHIYAMA, Kazuaki,
KANAMORI, Shotaro,
KASAI, Kentaro,
KIBUKAWA, Hirofumi,
MIKI, Shintaro,
MIYAZAKI, Kazuyoshi
SHIBA, Hayato,
SUGIYAMA, Takumu,
TAKAHASHI, Kaoru,
UMEDA, Hiroya,
WATANABE, Keiichi,
YOSHIDA, Shunsuke,

ABOUNNASR-MARTINS,Théo,GARG, Suyog,
IMAGAWA, Kaname,
KANEMURA, Yuki,
KASHIWAGI, Yuri,
KUSAFUKA, Yo,
MISHIMA, Masataka,
OKIMOTO, Naoya,
SHIMIZU, Kotaro,
SUN, Dongsheng,
TAMAKI, Masahide,
WANG, Xubin,
XU, Yi,BAXTER, Joshua Ryo,

AKASHI, Yoko,
HIRAGA, Takuya,
NAKAYA, Yukiko,
SAEKI, Yuya,
TAKAMICHI, Ryo,
YAMAGISHI, Yukinao,

AKIYAMA, Makiko,
ISHIZUKA,Yasushi,
OKANO, Yuka,
SAITO, Akiko,
WATANABE, Shinji,
YAMAGUCHI, Akiko,

SATO, Ritsuko,
NAKAMURA, Makio,

SUGIMOTO, Kumiko,

GOTO, Ryota,
IWAYA, Masaki,
KANESHIMA, Ryota,
KAWASHIMA, Teruyoshi,
MATSUMOTO, Akinori,
MIWA, Masataka,
SATO, Yuta,
SHIMODATE, Karin,
SUZUKI, Ryota,
TOMIYA, Takuya,
WATANABE, Shuhei,
YAN, Haochen,

Administrative Division
Administrative Staff

Research Administrator
Public Relations Staff
Archive RoomAKAIDA, Yohei,
FUKUHARA, Nana,
MARUMORI, Yasuko,
ONO, Kazumi,
SATO, Hiroki,
YAMADA, Takaharu,
YAMASUE, Akiko,
ITO, Michiru,
ITO, Yoriko,
NAKAMURA, Kenzo,