arXiv:2203.13979v1 [hep-ex] 26 Mar 2022

Japan’s Strategy for Future Projects
in High Energy Physics
Japan Association of High Energy Physicists (JAHEP)
March 20th, 2022

Editors:
M. Endo, K. Hamaguchi, M. Ibe, T. Ishibashi, A. Ishikawa, M. Ishino, M. Ishitsuka,
S. Kanemura, M. Kuriki, T. Mori1 , S. Moriyama, H. Nanjo, W. Ootani, and Y. Sato

Submitted to the Proceedings of the US Community Studyon the Future of Particle Physics (Snowmass 2021)

1

Contact: mori@icepp.s.u-tokyo.ac.jp

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Executive Summary
The current strategy for future projects of the Japanese high energy physics community remains as described in the Final Report of the Committee on Future Projects
in High Energy Physics, published in 2017 [1]. The Recommendation part of the
Final Report is excerpted in the next page.
This document updates the Final Report by adding developments and advancesthat have occurred since 2017. It is summarized as follows:• An e+ e− Higgs Factory is the most important next large-scale particle physics
facility. The International Linear Collider (ILC), being the most advanced
candidate, should be realized in Japan as early as possible. Carry out the timecritical development work and support international discussions to start the
preparatory phase in a timely manner. In parallel, continuing studies of new
physics should be pursued using the LHC and its upgrades.
• Hyper-Kamiokande, currently under construction, should be completed as scheduled to study CP symmetry in the lepton sector as well as proton decays. In
the meantime search for CP violation should be continued with the T2K experiment and upgrades of the J-PARC neutrino beam.
The High Energy Physics Committee should pursue all available options to achieve
the early realisation of these key, large-scale projects.
It is important to accumulate an increasing amount of data as planned for physics
studies at SuperKEKB that started full operation in 2019. Some of the medium- andsmall-scale projects currently under consideration have implicit potential to develop
into important research fields in the future, as happened with neutrino physics. They
should be promoted in parallel in order to pursue new physics from various directions. Flavor physics experiments, such as muon experiments at J-PARC, searches
for dark matter and neutrinoless double beta decay, observations of CMB B-mode
polarization and dark energy, are considered to be projects that have such potential.
Furthermore, accelerator R&D should be continued to dramatically increase particle collision energies in preparation for future experimental efforts that may indicate
the existence of new particles and new phenomena at higher energy scale.

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Recommendation (September 2017) [1]
In 2012, not only was a Higgs boson with a mass of 125 GeV discovered at
the LHC, but three-generation neutrino mixing was also established. Taking
full advantage of the opportunities provided by these discoveries, the committee makes the following recommendations concerning large-scale projects,
which comprise the core of future high energy physics research in Japan.
• With the discovery of the 125 GeV Higgs boson at the LHC, constructionof the International Linear Collider (ILC) with a collision energy of 250
GeV should start in Japan immediately without delay so as to guide thepursuit of particle physics beyond the Standard Model through detailed
research of the Higgs particle. In parallel, continuing studies of new
physics should be pursued using the LHC and its upgrades.
• Three-generation neutrino mixing has been established and has provided a path to study CP symmetry in the lepton sector. Therefore,
Japan should promote the early realization of Hyper-Kamiokande as aninternational project due to its superior proton decay sensitivity, andshould continue to search for CP violation with the T2K experimentand upgrades of the J-PARC neutrino beam.
The High Energy Committee should pursue all available options to achieve
the early realization of these key, large-scale projects.
It is important to complete the construction of SuperKEKB and startphysics studies as scheduled. Some of the medium- and small-scale projects
currently under consideration have implicit potential to develop into important research fields in the future, as happened with neutrino physics. They
should be promoted in parallel in order to pursue new physics from various directions. Flavor physics experiments, such as muon experiments at J-PARC,
searches for dark matter and neutrinoless double beta decay, observationsof CMB B-mode polarization and dark energy, are considered to be projects
that have such potential.
Furthermore, accelerator R&D should be continued to dramatically increase particle collision energies in preparation for future experimental efforts
that may indicate the existence of new particles and new phenomena at higher
energy scale.

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Contents
1 Status and Prospects for Particle Physics

5

2 Energy Frontier Physics
2.1 International Linear Collider . . . . . . . . . . . . . . . . . . . . . . .
2.2 LHC Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Future Accelerators and Novel Acceleration Techniques . . . . . . . .

9
10
13
17

3 Neutrino Oscillation and Proton Decay
19
3.1 Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Future Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4 Flavor Physics
23
4.1 SuperKEKB / Belle II Experiment . . . . . . . . . . . . . . . . . . . 23
4.2 Kaon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3 Muon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.4 Neutron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5 Non-accelerator Particle Physics
29
5.1 Particle Physics in Underground Laboratories . . . . . . . . . . . . . 29
5.2 Cosmological Observations . . . . . . . . . . . . . . . . . . . . . . . . 31

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1

Status and Prospects for Particle Physics

The standard model of particle physics is the current best theory to describe themost basic building blocks of the universe. It explains how all known matters are
made up and how the electromagnetic, strong and weak forces act on them. The
theory has succeeded in explaining almost all experimental results and predicting awide variety of phenomena very precisely.
In spite of these great successes, the standard model is known to describe physics
processes inadequately up to the Planck scale. With the discovery of the Higgs bosonin 2012, experimental investigations of the Higgs sector at high energy accelerators
have become the most solid path to approach more fundamental theories. To verify
the mass generation mechanism of the standard model, it is really essential to makethe best possible measurements of Higgs boson couplings to gauge bosons and quarks
and leptons. Deviations from the standard model predictions, if experimentallyconfirmed, would determine the direction of new physics beyond the standard modelthrough their patterns and magnitudes. To this end, precise measurements at thelevel of a percent or better are required, which can be realized only by lepton colliders
such as the International Linear Collider (ILC).
The Higgs potential, the essential ingredient for the electroweak symmetry breaking, remains largely unknown and must be experimentally scrutinized. The Higgs
potential does not only connect to the dynamics of the spontaneous symmetry breaking, but also determines aspects of the electroweak phase transition in the early
Universe, which is critically important to understand the scenario of electroweak
baryogenesis. The triple Higgs boson coupling, the most important parameter of the
Higgs potential, can be measured at the high luminosity LHC (HL-LHC), but forsufficient precision the energy-upgraded ILC or next-generation high-energy hadroncolliders would be absolutely necessary. The electroweak phase transition can be further explored through the synergy with future gravitational wave observations suchas the LISA.
Direct searches for new particles predicted by a variety of new physics models,such as supersymmetry or extra dimensions, should be performed at future energy
frontier experiments. New particles, if discovered at LHC or HL-LHC, should bethoroughly studied at the ILC or the upgraded ILC to further deepen our understanding of the Universe. It will open a new era of particle physics.
Another superior approach to probe higher energy scales, even beyond the kinematic reach of experiments, is to examine huge numbers of particles with sensitive de5

2012

2015

2018

2021

2024

2027

2030

2033

2036

2039

2042

2045

ghZZ ~0.4%,
Yb,Yc,Yτ~1-2%, ghgg ~2%

preparatory
phase

e+e- (ILC)

Br (H → inv.) < 0.3%,∆ΦCP∼ 4°,Γh~3%

construction

Λ(Higgs composite)< 3TeVmA(SUSY) < 1TeV

energyfrontier*ILC future ugrade

SUSY < 2.5TeV

SUSY < 3 TeV

p-p (HL-)LHC
Yt , Yb , Yτ , Yμ ~ 10%

Yt , Yb , Yτ , Yμ ~ 20%
Proton Decay
2.4x1034 yr

Super K
neutrino ・
proton decay

SN relic
neutrino

T2K

CPV 3σHyper K

5 ab-1SuperKEKB

Proton Decay ~1035 yr

Mass Ordering

SN relic neutrino 4-5σ

construction

50 ab-1

Φ2 : ± 4°± 2°± 0.6°

R(D*) : ± 9%

± 3.2%± 2.1%

Br(τ→μγ) : 45x10-9< 14.7x10-9< 4.7x10-9< 8x10-5< 4x10-5

Br(B→K(*)νν)± 10%invisible dark photon 1GeV : e < 10-3

KOTO

10-9

10-8

flavorμ → eγMuon rare decayO(10-11)

O(40 events)

MEG II
10-14

MEG
10-13

μ→e
conv.

DeeMe
10-13

COMET
10-17

COMET
10-15

Muon g-2/EDM

μ g-2 : 0.4ppm, μ EDM : 10-21 e・cm
n : 10-26 e・cm

Neutron EDM0ν ββ

CPV 5σconstructionn : 10-27 e・cm

n : 10-28 e・cm

inverted hierarchy

degenerated~ 40meVundergroundexperiments
10-45cm2

10-46cm2~ 20meV

10-47cm2

10-49cm2

10-48cm2

Dark Matter
POLARBEAR-2

Simons Observatory

GroundBIRD Simons ArrayCMB

LiteBIRDCMB Stage-4

σ(r) = O ( 0.01 ) ℓ > 20σ ( Σ mν) = 40 meV

Cosmology

Roman

EuclidDark Energy
DESI

Subaru HSC
2012

2015

VRO LSST

Subaru PFS
2018

2021

2024

σ(r) = O ( 0.001 ) ℓ ≥ 2σ ( Σ mν) = 15 meVσ(Nνν)~)~ O
O (( 0.01
0.01 ))σ(N
σ(w) = 0.01

PBH
σ ( Σ mν) = 20 meV
2027

2030

2033

2036

2039

2042

2045

*ILC future upgrade
Luminosity upgrade (2040-)
Energy upgrade (2045-)

running / approved experiments∆mt ∼50MeV, Yt ∼3−6%, λhhh∼15−30%,m(WIMP, stop, Higgsino) < 250GeV, gttZ(gttγ) < 1%

proposing / planning experiments

Figure 1: Timeline of ongoing and future projects described in the report. Sensitivities are overlaid at the time when they will be achieved (2012 – 2040).

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tectors. It encompasses searches for extremely rare processes and for tiny deviations
from standard model expectations induced by quantum effects. Recently, the Fermilab Muon g − 2 experiment announced a new measurement of the magnetic momentof the muon and confirmed the discrepancy found two decades earlier at Brookhaven
National Laboratory, supporting possible existence of physics beyond the standard
model. A striking pattern of anomalies, such as lepton-flavor universality violations,have also been emerging in rare B meson decays, hinting at characteristic aspects of
new physics. Experimental explorations of these anomalies is therefore essential. It
is also important to search for other physical processes, such as flavor violations andelectric dipole moments, as they give us clues to physics beyond the standard model.
Japan’s two world-class accelerators, SuperKEKB and J-PARC, are among the bestexperimental facilities to study these key processes of flavor physics.
The discovery of neutrino oscillations established tiny masses of the neutrinos,
which imply the existence of physics beyond the standard model. The pattern offlavor mixing in the neutrino sector, being totally different from that in the quark
sector, may suggest that the origins of the neutrino and quark masses may be totallydifferent. Essential questions in neutrino physics include whether or not oscillations
violate CP, whether neutrinos are Dirac or Majorana fermions, what are the absolutevalues of the neutrino masses, and whether the neutrino mass hierarchy is normal
or inverted. Precise measurements of CP violation and the determination of theneutrino mass hierarchy will be carried out by future experiments, namely HyperKamiokande in Japan and the DUNE experiment in the U.S. Searches for neutrinolessdouble β (0νββ) decays are important to settle the issue of whether the neutrinomass is Majorana- or Dirac-type. If neutrinos are Majorana fermions, their masses
could be related to physics at extremely high energy scales, as predicted by theseesaw mechanism. The Majorana nature and the CP violation in the neutrino
sector would strongly imply that the leptogenesis scenario might have given birth tothe matter-antimatter asymmetry.
The search for proton decay, which is considered a signature of Grand Unification,
has been explored by Super-Kamiokande with the world’s highest sensitivity. So far
a proton decay signal has not been observed and the lower limit on the proton
lifetime has been established as 1034 years for the decay mode p → e+ + π 0 and p →ν̄ + K + . Hyper-Kaiokande, DUNE and JUNO experiments will reach unprecedented
sensitivities to the proton lifetime.
Connections between cosmology and particle physics have never been so strong.
Cosmological puzzles, such as the origin of the matter-antimatter asymmetry in theuniverse, the nature of the dark matter and dark energy and the cosmological inflation, seem to require some new ingredients in particle physics. Naturally, any
7

new ingredients beyond the standard model must have played important roles in theevolution of the universe. Running and future experiments will continue to provide
indispensable knowledge and insights to solve these cosmological problems. For example, various experimental programs are being carried out or prepared to explore
a broader range of dark matter candidates, including weakly interacting massive
particles (WIMPs), coherent oscillation of scalar condensation such as axions, andprimordial black holes. Weakly interacting dark matter particles can be detectedby ongoing or future direct dark matter detection experiments, and their properties
may be later investigated at high energy colliders such as LHC, HL-LHC and theILC. Observations of the cosmic microwave background (CMB), in particular, theB-mode polarization, will help deepen our understanding of the cosmological inflation and its energy scale. Progress in the study of dark energy and the origin of
the accelerating expansion of the universe is also expected by upcoming programs toprecisely measure the Hubble expansion rate and the angular distance over a wide
redshift range.

82Energy Frontier Physics

Overview
Research at energy frontier aims to discover new particles and phenomena as wellas to determine their properties using state-of-the-art high-energy accelerators, andthereby to explore the laws of the nature governing them. Theories beyond thestandard model are clearly necessary to explain dark matter, baryon asymmetry of
the Universe and neutrino oscillation as well as the hierarchy problem. The proposed
hypothetical ideas must be thoroughly tested by energy frontier experiments. A solid
target is to precisely measure the Higgs boson couplings to other particles. The
Higgs self-coupling must also be measured to explore the nature of the electroweak
symmetry breaking and the phase transition at the early Universe. Direct searches
for new particles and phenomena must be continuously pursued.
In order to study higher energy phenomena at hadron colliders such as LHC,
either the beam energy must be increased with stronger bending magnets to keep
particles on their circular trajectory, or the luminosity must be increased to obtaina higher collision rate of high-energy partons. For lepton colliders, collision energy
can be greatly extended at an electron-positron linear collider without acceleration
losses due to synchrotron radiation. Alternatively a circular muon collider may be
developed.
At hadron colliders, the beams consist of protons or antiprotons, which are composite particles, so that a wide variety of phenomena can be studied from a broad
perspective depending on the collision energies, including production and decays ofthe Higgs boson and searches for new particles. There are, however, issues frommultiple interactions in the same beam bunch (pile-up) and large backgrounds. At
lepton colliders, signals are clean because all the beam energy is converted into thereaction, so that the Higgs boson can be precisely studied. Once new particles are
discovered, their properties can also be thoroughly measured. In particular, at alinear collider like the ILC, the beam energy and polarization can be adjusted todetermine detailed properties of the new particles.
There is an international consensus for an electron-positron Higgs factory as thenext highest-priority collider to be built. Several electron-positron collider projects
are in progress including linear colliders such as the ILC and the CLIC with futureenergy extendability beyond the Higgs factory, and circular colliders such as the
FCC-ee/CEPC as Higgs Factories in the first step, followed by 100 TeV-scale hadroncolliders, FCC-hh/SppC in the second step.

9

2.1

International Linear Collider

While the importance of precision Higgs measurements is unquestionable, the physics
case for higher collision energies has become increasingly strong. Among all the Higgs
factory candidates, only linear colliders can extend the collision energy to far beyond
the Higgs factory. Their capabilities to provide longitudinally polarized electron
and positron beams are also great advantages in probing new physics. The ILC iscurrently the most technically mature and ready-to-be-built Higgs factory, which
can be extended to much higher energies at later stages. The 20 km machine with acollision energy of 250 GeV can be extended to 30 km (50 km) with 500 GeV (1 TeV)
with the currently available superconducting accelerator technologies. Recent rapid
advances in accelerator technologies mean that even higher energy up to multi-TeV
can be envisaged in the future.
The properties of the Higgs boson will be studied in great detail at the initialstage of the ILC with 250 GeV. Many of the Higgs couplings to gauge bosons andfermions will be measured with a precision of 1% or below, which allows to pin
down underlining new physics beyond the standard model. If the Higgs boson is acomposite particle, its energy scale can be probed up to 3 TeV. If the Higgs boson isan elementary particle as in SUSY models, the mass of the heavy Higgs boson canbe probed up to 1 TeV. The measurement of Higgs CP properties will shed light onbaryogenesis models that can account for the asymmetry between matter and antimatter in the universe. At the upgraded stages of the ILC with 500 GeV or above,the measurement of triple Higgs self-coupling will provide a direct probe of the Higgs
potential which will help reveal the deepest mystery in the electroweak symmetry
breaking.
The ILC will also dramatically improve the precision of our understanding ofelectroweak reactions. Already at the initial stage of the ILC the radiative returnprocess e+ e− → γZ will provide 90 million Z-pole events. Taking advantage of thebeam polarizations, this process will improve the measurements of weak mixing angle(sin2 θw ) as well as Left-Right Asymmetry (ALR ) by a factor of 10 over LEP and SLC.
The measurements of anomalous triple gauge couplings and top-quark electroweak
couplings can also be improved at the ILC by one order of magnitude over what wasmeasured at LEP2 and the projections at the HL-LHC.
There is ample room for the ILC to directly discover SUSY particles with massesof up to half of the center-of-mass energies, being able to close up the loop-holes
in the models with nearly degenerate mass spectra of the Lightest SupersymmetricParticles (LSP) and the next to LSP. The latest experimental result on (g − 2)µindicates larger deviation with the SM prediction. Light supersymmetric particles
that can explain such deviations will be within the reach of the ILC. Many models
10beyond the standard model require new gauge symmetries for which the clean difermion processes at the ILC will offer a great sensitivity, being able to directly0
produce new gauge bosons such as Z with a mass up to the collision energy or toprovide indirect sensitivity up to around 10 TeV.
The ILC can also make available the most intense and highest-energy electron
and positron beams for beam dump and dedicated fixed-target experiments that will,
for example, study light weakly-interacting particles. This potential of the ILC iscurrently under active and intensive discussion among a broader scientific community.
A more detailed introduction of the physics potential at the ILC can be found in
the ILC White Paper for Snowmass 2021 [2].
Recent Progress and Overall Plan
In October 2012, immediately after the Higgs discovery, JAHEP proposed a staging approach to start the ILC with 250 GeV [3]. Later in July 2017, after observing
that no new particle had been discovered at the early LHC, JAHEP made anotherproposal [4], calling for the early realization of the ILC as a Higgs factory.
In June 2020, the European Strategy for Particle Physics was updated [5]. It
identifies an electron-positron Higgs factory as the highest-priority next collider. It
is stated that the timely realisation of the ILC in Japan would be compatible withthe strategy and the European particle physics community would wish to collaborate. The international consensus for an electron-positron Higgs factory as the next
collider has strengthened the efforts towards the realization of the ILC in Japan withan increasing number of people from various sectors such as scientists, politicians,
industry members, business circles, and local communities.
To advance the ILC project forward to the next step, the ILC International Development Team (IDT) was established by the International Committee for Future
Accelerators (ICFA) in August 2020 [6]. The IDT, hosted by the KEK, is mandated
to prepare the ILC Preparatory Laboratory (Pre-lab), based on MOUs among participating laboratories, as the first step. It is envisaged that, if the ILC is approved
based on an inter-governmental agreement during the Pre-lab phase of about four
years, the construction phase of about ten years will start by establishing the ILC
Laboratory among the partner countries/regions.
To facilitate the transition to the preparatory phase, two new community organizations were established by JAHEP: the ILC Steering Panel in October 2020
and the ILC Japan in May 2021. The ILC Japan, in close cooperation with KEK,
communicates and discusses matters related to the ILC with the Japanese government and other stakeholders to advance the ILC project on behalf of JAHEP, while11the ILC Steering Panel leads bottom-up activities to promote the project, cooperating with other scientific communities, government authorities, industry, and regional
partners [7].
The IDT completed the ILC Pre-lab proposal in June 2021, which describes
the organizational framework and the work plan of the Pre-lab [8]. At the sametime JAHEP and KEK summarized the key issues of the project in a report, andsubmitted it together with the ILC Pre-lab proposal to the Ministry of Education,
Culture, Sports, Science and Technology (MEXT). In response MEXT started asecond round of the MEXT Expert Panel in July 2021. The Panel’s recommendations
were published in February 2022 [9].
ILC Detector Technology R&D
ILC detector technology R&D has long been carried out internationally for two
detector concepts; ILD and SiD. Both are designed to achieve unprecedented performance of the hadron jet reconstruction with the particle flow approach, which
requires high-precision pixel vertex detector, large-solid-angle tracking detector, andhigh-granularity calorimeter. Japanese physicists play an important role in the global
detector R&D effort including the international detector collaborations such as the
LCTPC and the CALICE, with particular emphasis on the vertex detector, the TPCbased tracking detector, and the calorimeters for the ILD. In close collaboration withtheory groups, sensitivity studies and optimization of the detector concept are being
advanced. The Japanese detector operation experience with the Belle II, the ATLAS
and the T2K near detector has provided useful information to develop important ILC
detector components.
ILC Accelerator R&D
Important technologies for ILC construction such as polarized electron sources, positron
beams, flat ultra-low emittance beams, high gradient superconducting accelerating
cavities, and nano-beam focusing are well established after more than 10 years of
R&D effort. The current aim of ILC accelerator R&D is further improvement of
performance and fabrication technology, detailed engineering design, and reliability
studies by prototyping.
In recent years, various large scale superconducting accelerators have been constructed and operated. The successful operations of these machines have demonstrated the high performance and reliability of the ILC super-conducting technology.
At KEK-STF (Superconducting RF Test Facility), beam acceleration with an average
acceleration gradient of 33.0 MV/m, exceeding the ILC specification, was achieved.
Hi-Q cavity with the nitrogen treatment and the flux expulsion had been established. New treatment methods using oxygen diffusion which is simpler than that12

with nitrogen demonstrated high-Q and high accelerating gradient.
R&D work for cost reduction has proceeded under the collaboration between
Japan and the United States. An example is cavity fabrication with direct sliced Nb
sheet leading to a cost reduction by omitting several processes. Prototype cavities
showed more than 35 MV/m.
The feasibility of the beam focusing at the interaction point of the ILC has been
demonstrated, by achieving a vertical beam size of 41 nm at ATF2. Investigation
for the emittance growth for a large bunch charge at ATF2 showed that the effect issufficiently suppressed at the ILC with much higher beam energy and shorter bunch
length. A study to improve the luminosity is in progress by applying corrections tohigher-order components of magnet at ATF2.
Polarized electron source is technically established as the strain compensated
GaAs/GaAsP super-lattice (SCSL) cathode. The laser system for spin polarized
electron generation with a high stability and reliability has to be designed.
Positron source development shows significant progress. For E-driven positronsource, a full-scale target prototype has been fabricated and a long-term stable operation by keeping 7 × 10−7 Pa is confirmed. The cooling design of the target, the flux
concentrator, and the first accelerator tube has been progressed, and the prototype
will be fabricated in 2022. The heavy beam loading at the capture linac is compensated by a phase modulation on the input RF. In the Undulator Positron source, thevitality of the target material (Ti alloy) was examined with an electron beam.
Green ILC
Recently carbon neutral and sustainability are becoming an increasingly important issue to consider. “Green ILC” is a series of initiatives to address these issues andstarted in 2013. The expected CO2 emission from the ILC is 320 kton/year, which
would increase emission from the surrounding area (Ichinoseki city) by 24%. Many
studies on urban design, such as energy management with waste heat recycling andreuse and CO2 absorption by regional forests, and developments on new technologies,
such as high efficiency klystron and energy effective accelerator operation, have been
conducted by the Green ILC Working Group (WG) of the Advanced Accelerator
Association (AAA), which is a tight collaboration among industry, academia, andlocal governments. The WG contributes to the international panel on sustainable
colliders and accelerators that was established by ICFA in 2015.

2.2

LHC Upgrade

Introduction
Large-Hadron-Collider (LHC) has been running and producing physics results since13

2010, and the center-of-mass energy reached 13 TeV at the beginning of Run-2 in
2015. The discovery of the Higgs boson with a mass of 125 GeV in 2012 is oneof the outstanding results, and the properties, especially the coupling to the third
generation fermions, has been confirmed and measured. The reach of new physics
searches is extended by LHC, and the energy frontier as well as the luminosity frontierof the particle physics are being continuously developed. The final focusing magnetof the LHC was designed and constructed by the collaboration of KEK (Japan) andFermilab (USA) and it played a key role in achieving the instantaneous luminosityof 2.1×1034 cm−2 s−1 in Run-2, which is twice as high as the design value. The
performance and stability of the final focusing magnet is one of the key factors for
success.
The ATLAS Japan collaboration consists of 12 universities and KEK and has been
making significant contributions to the design, construction, commissioning, andoperation of various systems including the solenoid magnet, the silicon-strip detector,
and the muon-trigger system. The grid computing system has become a vital tool
for processing large-scale data and obtaining physics results. Japan has been making
significant contributions as a Tier-2 center. On top of these contributions, Japanese
ATLAS collaborators are achieving outstanding physics results, examples of which
are the discovery of the Higgs boson and continuous studies on the production anddecaying processes. Further, important physics topics have been explored by young
Japanese researchers and students, including contributions to searches for SUSY and
new particles as well as precision measurements of the standard model particles.
Physics program in the LHC upgrade project
After the discovery of the Higgs boson, a new era in the exploration of physics
beyond the standard model has begun. Direct observations of new particles as wellas precise measurements of the Higgs boson are effective approaches. In terms ofprecision measurements of the Higgs boson, its couplings to the W/Z-boson, top
quark, bottom quark, τ and µ will be measured with a precision of 1 to 10% with an
integrated luminosity of 3000 fb−1 . It is crucial to test whether the couplings of the
Higgs boson with other elementary particles are consistent with the predictions of
the standard model or a different rule is found. The differential cross-section of Higgs
boson is a probe to explore new physics; especially the high-pT tails are sensitive to
new physics. The measurement of the self-coupling of Higgs is also coming into viewin the HL-LHC era.
A direct search for new particles is another main subject of the LHC upgrade
program, and the strategy is to extend the reach by increasing the integrated luminosity significantly. The leading target of the new physics search in the HL-LHC era
14

is the SUSY particle observation, which is motivated by the presence of dark matter in the universe. In particular, mass degenerated Gauginos or Higgsinos are theprimary focus, and the extension of the reaches within the parameter space whichis consistent with the relic density of the dark matter is one of the critical goals.
In addition, the naturalness still well motivates the third generation SUSY particle
searches, although the LHC search results have excluded a part of the bulk parameter spaces. The high statistics HL-LHC data will allow us to explore the remaining
phase spaces in the third generation SUSY searches. Searches for other types of newparticles will be continued and the reaches will be further extended by introducing
innovative analysis techniques, trigger-level analysis, etc. To understand the vacuum
stability, the precise measurement of the top-quark mass is an important subject.
In case no new signal will be observed, a cross-section anomaly of the vector-boson
scattering may tell us the energy scale to be explored with future accelerators.
Development of the High-Luminosity LHC
For the High-Luminosity LHC (HL-LHC) physics program, the accelerator complex
will be upgraded and operated. The major upgrades on the injectors have been
performed during the Long-Shutdown 2 (2019 – 2021), and the brightness of the
proton bunch is increased by a factor of two. During the Long-Shutdown 3 (2026– 2028), various major upgrades of the accelerator components are scheduled tokick off the High-Luminosity LHC project (HL-LHC) starting from the year 2029,
including the replacement of the final focusing magnets, the beam-separation dipole
magnets, and the upgrade of the beam collimators. In terms of machine operation,
the instantaneous luminosity is controlled to 7.5×1034 cm−2 s−1 by introducing theluminosity leveling technique (β ∗ -leveling) to maximize the integrated luminositywith longer beam lifetimes. The plan is to accumulate 3000 - 4000 fb−1 in about ten
years. The HL-LHC project has been officially approved by the CERN council in
June 2016.
The key components for the success of the HL-LHC project are the performanceof the magnets placed around the beam collision points, notably the final focusing
quadrupole magnets and the beam separation dipole magnets. To increase the luminosity significantly, the beam will be defocused before the collision point and thenstrongly squeezed (β ∗ = 0.25 m in 2018 will be changed to 0.15 m). To cope with thestrong beam focusing, the bore size of the final focusing magnet needs to be enlarged
and at the same time the coil peak field needs to be increased from 8.6 Tesla to11.4 Tesla. This will be realized with a new superconducting material, Nb3 Sn, which
will be applied as an accelerator component for the first time. The final focusing
quadrupoles magnets are produced by the collaboration of CERN and the U.S. DOE
laboratories, while the beam separation dipole magnets, whose coil is made of NbTi,
15

will be constructed by KEK. After producing several 2-m long model magnets, afull-scale prototype of the beam separation dipole was produced in 2021 and theperformance was tested. The nominal operational field of 5.6 Tesla was confirmedwith a reasonable margin and is ready for series production.
Detector developments for HL-LHC
There are two major upgrade tasks for the ATLAS spectrometer, one is to replace
all the inner trackers and the other is to revise all the trigger and DAQ systems.
Aiming to keep the high tracking and vertexing performance in the high radiation
background of HL-LHC, five layers of pixel detectors and four layers of strip detectors
with highly radiation tolerant silicon sensors will be installed. The Japanese grouphas experience in assembling the ATLAS silicon detector and there is a major siliconsensor vendor, Hamamatsu (HPK), in Japan. Taking these advantages, Japaneseinstitutes have made strong contributions to the R&D program of the pixel and strip
sensors. The production of strip sensor is progressing in HPK, and is about to start
for the pixel sensor. Further contributions to the pixel module assembly, installation,commissioning, operation are foreseen.
Concerning the triggering and DAQ, in order to record the target physics objects
efficiently under the high luminosity condition, the first stage hardware trigger isdesigned with a longer latency of 10 µs as well as the increased acceptance rate of
1 MHz. The numbers of the current system are 2.5 µs and 100 kHz, respectively.
Thanks to the longer latency, sophisticated trigger algorithms, that are usually used
in offline analyses, can be introduced to the first stage trigger. The Japanese grouphas been working on the muon hardware trigger electronics. The system design
is completed, and the production of several types of electronics modules and theirintegration tests are progressing. Developments of firmware and software are donein parallel and the team is in full swing to be ready for the system installation at
the beginning of LS3 (2026).
Towards future energy frontier of hadron collider
Technology developments for the HL-LHC project may open the door to higher beamenergy for future hadron colliders. There is a plan to newly excavate a large-scaletunnel and increase the center-of-mass energy to 100 TeV, Future Circular Collider,
FCC-hh. A few Japanese institutes join the FCC collaboration and are working on
the physics performance as well as the spectrometer design. The key technology forrealizing the project is the high field accelerator magnets generating 16 Tesla, whichis twice as the current LHC magnet, or even higher field. A consensus can be found in
the report of European Strategy 2020 that the highest priority is to identify the next
technology including the aspects of performance, stability, cost for mass-production.
16

The R&D program of the superconducting material of Nb3 Sn is progressing at
the cryogenic science center in KEK collaborating with Japanese universities andcompanies with supports of CERN.

2.3

Future Accelerators and Novel Acceleration Techniques

X-band High-Gradient Accelerating-Structure
The Compact Linear Collider (CLIC) is a potential future linear collider which aims
to collide electrons and positrons at 3 TeV at the final stage of the project. It was
proposed and developed as an international collaboration between CERN and otherlaboratories including KEK. The total length of CLIC is 11.4 km at the first stage
with 380 GeV and 50 km at the final state with 3 TeV. The acceleration frequency
and gradient of the main linac are 12 GHz (X-band) and 72 MV/m (380 GeV) /100 MV/m (3 TeV), respectively, in order to realize the compact size. It should be
noted that the first 380 GeV stage can be realized with X-band klystrons feeding
the high-power microwave to the main linac instead of the two-beam acceleration.
The key technologies were demonstrated at the CLIC Test Facility and collaborating
laboratories.
The high-energy community in the US has proposed a new concept of a normalconducting electron-positron linear collider, named Cool Copper Collider (C3 ), witha compact footprint of 8 km based on the following two innovative technologies:(1) a distributed coupling accelerating structure, in which RF power is distributed
to individual cells through a waveguide manifold, allowing a high degree of freedom in accelerating-structure design; (2) cryogenic-temperature operation of copper
structures, which enables higher acceleration efficiency and higher-field operation,
simultaneously. The basic design is completed for a center-of-mass energy of 250
GeV (first stage, 70 MV/m) and 550 GeV (second stage, 120 MV/m) with a total
length of 8 km. The key technologies need to be demonstrated at a demonstration
facility in the near future.
For future normal-conducting linear colliders, X-band (11.4 GHz) acceleratingstructure development has been conducted by KEK in collaboration with SLAC and
CERN for many years. X-band high-gradient accelerating tubes can be fabricatedby KEK and high-power tested at Nextef (New X-band Test Facility) in KEK. The
recent focus is to develop this mature technology for various applications, e.g. formedical/industrial use.
Muon Collider
A muon collider is another candidate for a future energy-frontier lepton collider. The
muon projects at J-PARC developed essential technologies that would be useful for17

a muon collider in the MAP scheme [10]. The technology of high-intensity positron
beam developed for the ILC was investigated as a way to realize a muon collider inthe LEMMA scheme [11]. Also, a new scheme of muon collider that would use thelow-emittance positive muon beam [12] discussed in Section 4.3 and negative muon
cooling utilizing the reaction of muon-catalyzed fusion is under discussion.
Laser-plasma accelerator, novel accelerator technologies
A laser-plasma accelerator has a potential to realize much higher gradients than conventional technology. Recently high-quality beams with percent-level energy spreadand micrometer-level emittance have been obtained. It has been pointed out that theincrease in emittance in plasma acceleration is due to a mismatch of the betatronwith the strong focusing field in the plasma at the injection and extraction. The
mismatch can be eliminated and the emittance growth can be suppressed by applying a gradient to the plasma density. Thus design studies of laser-acceleration-based
practical machines have been started.
In recent years, research on thin-film superconducting cavities using high Tc superconductor such as Nb3 Sn and MgB2 have been actively conducted all over theworld to realize performances that exceed Nb cavities. The Nb3 Sn cavity, in particular, was demonstrated to achieve accelerating gradient of larger than 20 MV/mwith Q value of higher than 1 × 1010 at 4.2 K. It is expected that a future Nb3 Sn accelerator could be operated by cryo-coolers, without liquid helium. Also recently an
increasing number of investigations on superconducting cavities with multi-layered
thin film structure have been carried out. It is a very promising technology that canexceed Nb cavities, as high accelerating gradient of around 100 MV/m is theoretically
predicted.

18

3

Neutrino Oscillation and Proton Decay

Overview
The Super-Kamiokande and T2K experiments have been producing world-leading
results in neutrino physics. As these projects continue the research, the successor
experiment, Hyper-Kamiokande [13], is currently being constructed by the international collaboration to start operation in 2027. The Hyper-Kamiokande project ishosted by the University of Tokyo and KEK in Japan.

3.1

StatusNeutrino Oscillation
Three flavor mixing is now established while the existence of CP violation in the neutrino sector has been indicated by T2K. Measurements of mixing and CP violationin the lepton sector will provide insight into the underlying symmetry mechanisms
in comparison with that of quark sector. In addition, if the unitarity is violated inneutrino mixing, it will be taken as a hint of new physics at an ultra-high energy
scale. In addition, the sign of the mass squared difference ∆m232 = m23 − m22 is still
unknown. The ordering of the neutrino masses is an important ingredient to understand the origin of extremely small neutrino masses in comparison with the otherfermions.

Proton Decay
Searches for proton decay are currently dominated by Super-Kamiokande (SK), whichhas placed 90% C.L. limits on the proton lifetime as 2.4 × 1034 years for p → e+ + π 0
and 8.2 × 1033 years for p → ν̄ + K + . Some GUT models predict p → e+ + π 0 as thedominant decay mode, while p → ν̄ + K + is considered as a major decay mode insupersymmetric GUT models. These models predict proton lifetimes at around 1034
to 1035 years, which is a target of the future projects to search for.

3.2

Future Plans

The T2K experiment has reported a hint of CP violation from the measurementof neutrino and antineutrino oscillation probabilities using the J-PARC neutrino
beam [14]. Therefore, it is necessary to continue the search for CP violation by
T2K to confirm the existence of CP violation with higher statistics, and in parallel,

19

complete the upgrade of the J-PARC neutrino beam and construction of the HK
detector to achieve the precision measurement.
Hyper-Kamiokande (HK) is currently under construction to study neutrino CP
violation over a wide range of δCP with the combination of an upgraded J-PARC
neutrino beam. The HK long-baseline project can accumulate more than 20 times
larger statistics than the T2K experiment with a larger fiducial volume of the HK
detector than that of the SK detector, and upgrade of J-PARC accelerator beam
power. This is a tremendous improvement as the current sensitivity of T2K is limitedby the statistical uncertainties.
Sensitivity to CP violation is enhanced once the mass ordering is determined. The
strategy of HK is to determine the mass ordering by the observation of atmospheric
neutrinos. Future projects, such as JUNO and DUNE, have sensitivity to determine
the mass ordering with different approaches from HK using reactor neutrinos andneutrino beam, respectively. Therefore, these future projects are complementary to
HK. DUNE has the sensitivity to CP violation as well.
Construction of the HK detector has started in 2020 and the operation will begin
in 2027. HK can extend the search for proton decay by an order of magnitude andthereby has a good opportunity to provide direct evidence of the GUT models.
As for the sterile neutrino search, JSNS2 [15] has started data taking in 2020 at
MLF in J-PARC, aiming to test the oscillation parameter regions suggested by LSND
and MiniBooNE. Investigation of Majorana properties is another important subjectof neutrino physics. Future projects for neutrinoless double beta decay (0νββ) search
will be explained in Section 5.1.
J-PARC Accelerator and Neutrino Beam
The neutrino beam at J-PARC is produced from 30-GeV protons accelerated byits Main Ring (MR). By 2016 the beam power reached ∼ 500 kW. The J-PARC
accelerator upgrade was budgeted and is progressing toward a beam power of 1.3 MW
for the MR, which exceeds its original design value of 750 kW. In 2022 the repetition
rate will be increased by almost a factor of two from the current value of 2.5 sec
with an upgrade of the power supplies for the main magnets and the introduction ofhigher gradient RF cavities. These upgrades will result in a beam power greater than
750 kW by 2023. Further upgrades of the RF power supplies and the beam monitors
are scheduled by 2025. The proton beam will be further optimized and stabilizedwith these upgrades, and the number of protons per spill will be increased. The
beam power is expected to reach 1.3 MW by around 2028. The J-PARC neutrino
beam facility upgrade is also progressing to operate with the 1.3-MW beam powerin the high repetition rate.
20

For the long-term future, further upgrades towards the realization beyond 1.3MW accelerator are being considered. One possibility is to construct a new 8-GeV
booster ring between the current Rapid Cycling Synchrotron (RCS) and MR to allow more protons to be injected with smaller emittance. The design of the booster
ring is under development in view of location and cost. As a possible operational
improvement, the parallel operation of the fast extraction beam for neutrino experiments and slow extraction beam for hadron experiments can be realized if a stretcher
ring for the slow extraction is newly constructed as an addition to the current MR
accelerator. The stretcher ring can be constructed inside the same tunnel of the MR.
There is also an idea to arrange linac accelerators in the KEKB tunnel to make a
10-MW-scale proton driver after the completion of the SuperKEKB project. The
driver front-end to 1.2 GeV was designed in detail, including the electromagnetic
design of half wave resonator at the front-end. The challenges for high duty target
horn to receive the 10-MW beam are under discussion.
Hyper-Kamiokande
Hyper-Kamiokande (HK) is a next-generation large-scale water Cherenkov detector
based on the technology established in SK with the upgrades of the detector scale andperformance. The detector is designed as a cylindrical structure with 68 m diameterand 71 m height, with a fiducial volume of about 188 kiloton, which is approximately
8.4 times larger than SK. The physics capabilities of HK cover a broad range of
topics as summarized in this section.
HK will be able to precisely measure neutrino oscillation parameters using the
J-PARC neutrino beam with extremely high sensitivity to CP violation. Statisticaluncertainty of νµ → νe and ν µ → ν e appearance signals will be 5% (3%) in 5 (10)years operation. In order to suppress the systematic uncertainties in the measurements and further enhance the sensitivity to CP violation, upgrades of the T2K neardetectors and a construction of new 1 kton scale water Cherenkov detector (IWCD)
are planned in HK. As a supplementary project, test beam experiment (WCTE) is
proposed at CERN to be conducted in 2023 with a 50 ton scale water Cherenkov detector to validate the performance of small water Cherenkov detector implemented
with multi-PMT modules which consist of multiple small PMTs encapsulated in a
vessel. Accumulating data for 10 years with the 1.3MW J-PARC neutrino beam, theδCP measurement precision is estimated to be better than 23◦ (7◦ for δCP = 0◦ ).
Determination of neutrino mass ordering is one of the major physics targets ofHK, not only because it may give hints toward the origin of the extremely lightneutrino mass, but also it is also related to studies of Majorana neutrinos being
explored by the neutrinoless double beta decay searches. Although the sensitivity
depends on the value of mixing angles, HK will be able to determine the mass21

hierarchy at a significance of 4–6σ with observations of atmospheric neutrinos and
J-PARC beam neutrinos in 10 years of operations. It should be noted that thesensitivity to CP violation is enhanced once the mass ordering is determined by HK
or the other experiments due to the degeneracy of the parameters.
HK also has an unprecedented sensitivity to proton decays. In addition to theincreased fiducial volume, atmospheric neutrino backgrounds will be largely suppressed by the improved performance of the detector with new photosensor. The 3σ
discovery potential reaches to 1 × 1035 years for the p → e+ + π 0 and 3 × 1034 for
p → ν̄ + K + decay modes, approximately one order better than the current limits,and can therefore test various GUT models.
Because of its huge fiducial volume, HK’s observations of astrophysical neutrinos,such as solar neutrinos, supernova neutrinos, as well as searches for dark matter canlead to new discoveries. In terms of the supernova neutrino observations, HK has
an outstanding capability to observe neutrinos from supernova bursts within thegalaxy or its near surroundings. SK has started operation with Gd loaded waterin 2020 aiming to discover as-yet unmeasured supernova relic neutrinos, and HK is
expected to observe them with even higher statistics. As the flux of the supernova
relic neutrinos depends on the frequency of supernova bursts in the early universe,
its observation may advance the study of the evolution of the universe.
These physics capabilities can be further enhanced by the international contribution such as additional PMTs.

22

4

Flavor Physics

Overview
Flavor-physics experiments can probe physics at high mass scales, even beyond thosekinematic reaches. Currently, several experiments have reported hints of phyiscs
beyond the standard model as in measurements of the muon anomalous magnetic
moment (g − 2) and a variety of measurements at the B-factories and the LHCb experiments. It is highly important to further test these signals, and a wide program offlavor measurements are also required to shed light on physics beyond the standard
model because they are complementary to other searches. Japan is playing an essential role in the field of flavor experiments, as represented by SuperKEKB/Belle II,
KOTO, MEG II, COMET, J-PARC muon g-2/EDM, TUCAN, neutron lifetime at
J-PARC, and NOPTREX.

4.1

SuperKEKB / Belle II ExperimentAmong the results by KEKB/Belle, several anomalies are seen and could be interpreted as new physics in high energy scale, for example, lepton flavor universality (LFU) in B → D(∗) τ ν [16, 17, 18] and isospin sum rule in B → Kπ [19].
LHCb experiment at CERN also found an anomaly of the LFU in B + → K + `+ `−in 2021 [20]. To exploit the anomalies and new physics in flavor physics, the SuperKEKB/Belle II project had been started.
SuperKEKB/Belle II is the only operational e+ e− B-factory. It has a rich, uniqueand diverse physics program [21, 22].
Flavor Physics
The primary aim of the Belle II experiment is to make new physics discoveries at ahigh-energy scale through flavor physics with B, D mesons and τ leptons with highprecision. A test of unitarity triangle can probe new physics in B 0 − B̄ 0 mixing. Only
Belle II can provide the precise measurements of all six observables for the triangle(e.g., three angles and three sides) to test the Kobayashi-Maskawa theory with anorder of 1 % precision which probe new physics scale up to 2000 TeV. New sources ofCP violation in quark and lepton sectors might explain the baryon asymmetry of theuniverse. Belle II exploited various types of CP violation, such as time dependent andintegrated CP asymmetries, triple product correlation, and electric dipole momentusing B, D and τ decays. Extensions of Higgs sector, which are required in many
new physics models, predict the existence of charged Higgs bosons. The decays23

of heavy flavor quarks and leptons, such as inclusive B → Xs γ, B + → τ + ν andτ − → KS0 π − ντ decays, are sensitive to the contributions of charged Higgs bosons.
The tests of LFU in b → cτ ν and b → s`+ `− are one of the central issues in B physics.
If the current central values of R(D(∗) ) and RK (∗) are unchanged, 5σ observations
are possible around middle of 2020s. Further, angular analyses can constrain theLorentz structure of the new physics. The yet unobserved decays b → sν ν̄ can give
another inputs to the new physics which explains anomalies in b → s`+ `− . Belle II
can observe the B → K (∗) ν ν̄ with 10% precision. Studies of charged lepton flavor
violation and electric dipole moment of τ lepton are only possible at Belle II among
the running experiments. If these signatures are seen, it is a clear new physics signal.
Dark Sector and Hadron Physics
Measurements of radiative return processes especially in e+ e− → π + π − γ can improve
the theoretical uncertainty of hadronic vacuum polarization contributions to muon
g−2 in the SM. Belle II will perform the measurement of e+ e− → π + π − γ with similaror better sensitivity as that for BaBar. The dark matter and its portal particle, which
is light around MeV∼GeV mass scale and has smaller coupling to the SM particles,
can be searched for at clean and high luminosity e+ e− collider environment. Oneof the flagship search signatures at Belle II is the e+ e− → γ + nothing which ispredicted in many new physics models. The mechanism of hadron formation will be
studied which will lead to advances in elementary particle physics, hadron physics,
astrophysics and their associated interdisciplinary fields.
Apparatus
SuperKEKB is an asymmetric-energy electron-positron double-ring collider with a
3-km circumference that aims to achieve a peak luminosity of 6 × 1035 cm−2 s−1 .
The SuperKEKB design is based on the “nanobeam” scheme, wherein the beam
size is reduced to 50-60 nm in the vertical direction and 10 µm in the horizontal
direction at the interaction point (IP). Belle II is a multi-purpose 4π detector aiming
to collect 50 ab−1 of data to provide the world’s highest sensitivities for searches for
new phenomena by around 2031. The Belle II consists of several sub-detectors tomeasure the decay vertices of B mesons, and energy of charged and neutral particles
precisely, and to identify charged particle flavor efficiently. The clean environment
at e+ e− collider allows reconstruction of neutral particles (e.g., photon, π 0 , KS0 , andKL0 ) efficiently, which is crucial for inclusive measurements and tagging of the otherB mesons.
Operation
The operation of SuperKEKB Phase 3 began in March 2019 with the Belle II de24

tector. The crab waist scheme was implemented to improve the beam-beam performance in April 2019. In 2020, SuperKEKB surpassed the luminosity record of
2.11×1034 cm−2 s−1 achieved by its predecessor KEKB with the vertical and horizontal
beta functions at the IP set at 1 mm and 80(LER)/60(HER) mm, respectively. Owing
to the innovative nanobeam scheme, the peak luminosity record was surpassed withsmaller beam currents than were used at KEKB. In December 2021, SuperKEKB set
a new world record for a peak luminosity of 3.81 × 1034 cm−2 s−1 . Since 2019, Belle II
has collected 268 f b−1 of integrated luminosity and published world-leading physics
results such as dark sector, B → Kν ν̄ and charm lifetimes [23, 24, 25, 26].
UpgradesWhile achieving steady progress, SuperKEKB encounters some challenges as a luminosity frontier machine, such as the efficiencies and stabilities of the beam injection
to the main ring, collimation and machine protection strategies, safe increase of beam
currents, lowering the beta functions at the IP, and overcoming various beam instabilities. An International Task Force has been organized and an active exchange of ideas
and opinions has been conducted in order to solve problems that hinder luminosity
improvement. SuperKEKB continues to improve its performances in the frameworkof the SuperKEKB/Belle-II collaboration and with the accelerator community in theworld.
The Belle II detector upgrades are planned to improve its performance significantly even under higher beam-induced background during long shutdown 1 (LS1)from 2022 summer and LS2 around 2026. The upgrade in LS1 is being prepared to
install the second layer of DEPFET pixel detector and to replace the conventional
MCP-PMTs in TOP to life-extended ones if performance requires. The major detector upgrade is proposed in LS2 to replace almost all sub-detectors and add newparticle identification detector [27]. The worldwide R&D effort for the major upgrade is ongoing and EOI’s had been submitted to the upgrade advisory committee
in 2021.
Consideration of further luminosity upgrade and electron polarization capabilityof SuperKEKB are started for ultimate new physics searches with heavy flavor quarks
and leptons including τ lepton g − 2 in the light of muon g − 2 anomaly [28].

4.2

Kaon

Measurements of the branching ratios of the decays KL → π 0 νν and K + → π + ννare among the most important in the search for new physics with rare kaon decays.
The branching ratios are sensitive to new physics, since those are extremely small
and precisely predicted in the standard model.
25

The NA62 experiment at CERN SPS measured the branching ratio of K + →−11
π + νν with the data collected in 2016-18 to be (10.6+4.0, which is−3.4 ± 0.9) × 10
−11
consistent with the standard model branching ratio 8 × 10 . NA62 will measure itwith a precision of ∼ 10% in the 2020s. After that, the NA62 group is considering
comprehensive programs to study rare decay modes for both K + and KL , including
measurements of the branching ratios of K + → π + νν at 5% level, and KL → π 0 ννat 20% level (based on the design in the KLEVER project) [29].
In Japan, the KOTO experiment is searching for the decay KL → π 0 νν withthe high-intensity proton beam from J-PARC. In the standard model, the branchingratio of KL → π 0 νν is predicted to be 3 × 10−11 . The KOTO experiment set thecurrent upper limit 3.0 × 10−9 at 90% C.L. using the data collected in 2015, andreached a single event sensitivity of 7.2 × 10−10 with the data collected in 2016-18.
KOTO will reach a sensitivity equivalent to the branching ratio of O(10−11 ) in the
2020s.
To go beyond the goals of the current KOTO experiment and explore the effects
of new physics at the level of 10% deviations from the standard model prediction,a new experiment, called KOTO step-2, is proposed with a new detector and a newbeam line [30]. For KL → π 0 νν in the standard model, 35 events are expected so
far with a signal-to-background ratio of 0.6, which will give a discovery of the decay.
The new beam line requires an extension of the Hadron Experimental Facility
at J-PARC with a second production target (T2) in addition to the current one(T1) [30]. In order to use the T1 and T2 simultaneously for also other experiments,
the beam power of 150 kW is desirable in the slow extraction at J-PARC Main Ring(MR).
J-PARC MR has achieved 64 kW beam power in its slow extraction mode withan extraction efficiency of 99.5%. To increase the beam power, the MR is to be
operated in faster cycle and circulating protons are increased by suppressing thebeam instability. To aim over 100 kW, further suppression of the beam instability andhigher extraction efficiency are necessary. A new scheme, setting a diffuser in frontof the extraction system, has successfully demonstrated increasing the extraction
efficiency.

4.3

Muon

Charged Lepton Flavor Violation
Discovery of charged lepton flavor violation would be a clear signature of new physics
and such experiments should be advanced intensively. The MEG experiment set an
upper limit on the branching ratio of the µ → eγ process at 4.2 × 10−13 , which is 30
26

times more stringent than the previous limit from MEGA experiment. An upgradedexperiment, MEG II aims for ten-times better sensitivity with improved detectors
and the increased beam intensity. The upgraded detectors have been successfullycommissioned during the engineering run in 2021. The physics data taking will startfrom summer 2022. The projected sensitivity for three DAQ years is 6 × 10−14 .
Preparations for experiments searching for µ − e conversion at a sensitivity of−1410∼ 10−17 are underway both in Japan and in the U.S. The DeeMe experiment at
the MLF facility in J-PARC is aiming at the early realization of O(10−14 ) sensitivities.
The construction of new beamline (H-line) was completed in January 2022. The
COMET experiment at J-PARC and the Mu2e experiment at Fermilab are now in theconstruction phase and they are aiming for sensitivities of O(10−17 ). The COMET
is one of the flagship experiments of J-PARC employing a staging approach. The
first phase of the experiment (Phase-I), which is aiming for a sensitivity of O(10−15 ),is under construction with a priority defined by the KEK-PIP 2016. The COMET
will realize a high-intensity pulsed proton beam with an extremely small extinction
factor in addition to large-acceptance pion capture and transport solenoids for the
first time. Prior to Phase-I, the commissioning of the proton beam and the muontransport with a low-intensity beam (Phase-α) is under preparation.
muon g − 2 and EDM
The Fermilab g − 2 experiment released the first results in April 2021 and confirmed
the BNL-E821 result. The combined results give 4.2σ deviation from the standard
model prediction. The improvements in the precision of the standard model prediction are foreseen by community efforts under the muon g − 2 theory initiative.
The J-PARC muon g − 2/EDM experiment [31] will measure g − 2 and the electric
dipole moment simultaneously. It employs a completely new method by using a lowemittance positive muon beam from a thermal-energy muon source accelerated bylinear accelerators. The experiment is possible to achieve a precision of 0.4 ppm withthe current technical achievements, and the project is aiming at a final precision of
0.1 ppm. Construction of the frontend muon beamline (H-line) has been completed.
The first beam of surface muon was delivered to H-line in January 2022. Further
extension of the beamline and construction of muon source and LINAC are currently
in progress.

4.4

Neutron

Neutron EDM
The neutron electric dipole moment (EDM) is sensitive to new physics, since the SM
prediction is highly suppressed to be 10−32 e·cm (without strong CP-violation). An
27

upper limit on the neutron EDM of |dn | < 1.8 × 10−26 e · cm (90% C.L.) was obtainedat PSI using ultra-cold neutrons (UCNs) produced with solid deuterium converter.
The intensity of the UCNs was less than the design value due to technical problems.
They are now working on the upgrade of the UCN source and the measurement
apparatus. Developments of high-density UCN sources to study the neutron EDM of
10−27 to 10−28 e·cm is becoming a ground for international competition. At TRIUMF,
TUCAN collaboration successfully produced the UCNs by using a superfluid helium
converter in 2017 [32]. The UCN source has been upgraded to have higher UCN
density by Japanese institutes KEK and RCNP in charge of the development, and is
now under commissioning. The EDM measurement apparatus is also being developedin Japan.
Neutron lifetime
The neutron lifetime is an important parameter in big bang nucleosynthesis and canbe an independent systematic test for Vud determination. In 2021, the US groupreported a new value of neutron lifetime in a precision of 0.34 s by using UCNs,
however, this did not solve the “neutron lifetime puzzle”, 8.5-s deviation in the
lifetime between two methods: “bottle” method counting stored UCNs and “beam”
method counting protons from the neutron decay. At J-PARC, the lifetime has beenmeasured using a pulsed neutron beam with a third method counting electrons fromthe neutron decay. The method was demonstrated in the first result published in
2021 [33], although the accuracy was not enough. Upgrade and commissioning ofboth of beamline and apparatus will improve the accuracy better than 1 s to solve
the puzzle in the near future.
T-violation in neutron reaction with compound nuclei
A large enhancement of T-violation in compound nuclei has been theoretically suggested. An international collaboration NOPTREX plans to search for such Tviolation in resonant neutron-nuclei capture reactions. One candidate of the nuclei is139
La, because detailed studies with pulsed neutrons in J-PARC have shown that theviolation could be enhanced by a factor of 106 [34, 35]. Polarizations for both nuclear
target and neutron beam are necessary, and the techniques of the polarization arebeing developed.
Exotic force in neutron interaction
Neutron interaction is sensitive to exotic interactions mediated by a massive boson
which couples to mass, because deviation from the gravity low is possible in the shortrange and the neutron is massive neutral particle. New limits of the Yukawa-type
interaction in the range of nanometer were obtained by using neutron scattering [36]and interference [37].
285Non-accelerator Particle Physics

5.1

Particle Physics in Underground Laboratories

Dark Matter Searches
XMASS-I, a single phase liquid xenon detector, had been taking data over five years
looking for WIMPs and other new physics and completed its data taking in 2019.
XMASS-I provided results on WIMPs, axions, dark photons, exotic properties ofneutrinos, a rare decay of a xenon nucleus and possible coincidence events withgravitational wave events. Some of the non-WIMP searches in liquid xenon detectors
were pioneered and a broader range of dark matter candidates started to be sought
by utilizing the low background of electronic recoils in XMASS-I. At present final
results on WIMPs are being prepared.
In this community, experimental searches for a wide range of WIMP masses havebeen conducted with various approaches. For the heavy WIMPs, the final result
from the XENON1T experiment excluded WIMP models with a nucleon interaction
cross section greater than 4.1 × 10−47 cm2 at 30 GeV/c2 and more recently PandaX4T improved it (3.8 × 10−47 cm2 at 40 GeV/c2 ). XENON1T is also providing thebest constraints at lighter WIMPs down to about 85 MeV by looking for electronic
recoils. No significant excess has been observed in the WIMP searches.
Based on the future plan described in the document in 2017 [1], members of the
XMASS group started to contribute to the worldwide efforts on future WIMP search
programs by joining the XENON experiment. In 2021, XENONnT started to takedata. It is expected to provide a sensitivity down to 1.4 × 10−48 cm2 at 50 GeV/c2
with an exposure of 20 t yr [38].
An excess observed at low energies of XENON1T is consistent with a solar axion
signal, a solar neutrino signal from an enhanced neutrino magnetic moment, anda bosonic dark matter signal with a particle mass of 2.3 keV/c2 . However, thesolar axion and neutrino hypotheses are in strong tension with stellar constraints.
A tritium background in the detector is also possible and XENON1T is unable toconfirm or exclude the presence of tritium at this time. XENONnT is expected toprovide new information on this topic soon.
A part of the XENON members in Japan joined DARWIN [39] collaboration in
2020. The DARWIN collaboration aims at building the ultimate, liquid xenon-based
direct detection dark matter detector, with a dark matter sensitivity of 10−49 cm2 forthe spin-independent case, limited by irreducible neutrino backgrounds. The detector will have a 40 ton liquid xenon target operated as a dual-phase time projection
chamber. R&D of the detector and the experimental site study are underway, and29

its conceptual design report will be available within a few years. The members inJapan have been developing a hermetic TPC design [40] to reduce background originating from radon, and photosensors based on a legacy of XMASS low-backgroundtechnology [41, 42]. In 2021, groups in Japan have joined in signing a Memorandumof Understanding between members of the XENON/DARWIN and LUX-ZEPLIN
Collaborations towards building this Next-Generation Liquid Xenon Experiment together.
R&D has been done for production of radio-pure sodium iodide crystals and theconstruction of a detector with 150 kg of NaI(Tl), PICOLON, has been approved.
This is expected to check the result of the DAMA/LIBRA experiment.
As for directional dark matter measurements, NEWAGE started to join a worldwide collaborative TPC development framework, CYGNUS, and is one of its coregroups now. Further development of this international cooperation is expected.
An experiment based on cutting-edge technology developed for interferometersin gravitational wave detection was proposed to search for light dark matter axions.
This is expected to explore a new area for parameter space for axions and new gaugebosons.
0νββ decay
One of the most important advances in 0νββ decay experiments since June 2017 is
the recent result from KamLAND-Zen 800. The experiment started in January 2019
after the installation of a larger inner balloon, which houses 136 Xe dissolved in liquid
scintillator to increase the amount of this isotope to 745 kg. In March 2022, thecollaboration reported that no signal was observed and imposed a lower limit for the0ν0νββ half life of 136 Xe, T1/2
> 2.0 × 1026 yr (90% C.L.) [43]. The limit combined0ν> 1.07 × 1026 yr)
with the previous experimental limit by KamLAND-Zen 400 (T1/20ν> 2.3 × 1026 yr (90% C.L.), which is the most stringent limit in the world.
is T1/2
This result corresponds to hmββ i < 36–156 meV and this means that we entered theinverse hierarchy region. The KamLAND-Zen 800 continues physics run, and further
improvement is expected.
To probe down into the inverse hierarchy region, the demand for detectors withhigh energy resolution is increasing because the 2νββ events are an unavoidable
background. The KamLAND group is proceeding with a project targeting mββ =20
meV, KamLAND2-Zen, where the energy resolution is improved to about 2%. In
the CANDLES experiment, a 0νββ decay experiment using 48 Ca, collaborative work
started with a Korean group for intensive R&D on a scintillating bolometer, which
will provide higher energy resolution and good particle discrimination.
Experiments using different nuclei and different methods are important in future30

0νββ experiments, andnew experimental ideas are being developed in Japan such as
AXEL and ZICOS.

5.2

Cosmological Observations

Inflation Physics
CMB polarization experiments provide information about physics beyond the Standard Model, such as the inflation physics. The current best upper limit on thetensor-to-scalar ratio comes from the combined analysis using the BICEP/Keck andPlanck data, as r < 0.032 (95% C.L.).
The successful POLARBEAR experiment leads to an upgrade project called as
Simons Array (SA), which contains three telescopes and corresponding receivers. The
development of the first SA receiver, PB-2A, has been led by Japanese institutions
including KEK and Kavli IPMU, and deployed in 2018. The expected sensitivity ofSA after the three years of observations is σ(r) = 0.006 (68% C.L.).
A novel observational concept is proposed as GroundBIRD that plans to observea large angular scale by a scan strategy with continuous rotation in azimuth from theground. This is the first Japanese-born CMB polarization experiment that reachedto the first light in 2019 and currently observing at Tenerife, Spain.
Two US-led and yet Japanese participating projects are called Simons Observatory (SO) and CMB Stage-IV (CMB-S4). SO is the upcoming ground telescopesin Atacama, Chile. The telescopes are scheduled to start science observations in
2023. Japanese team, utilizing its unique expertise, has been providing key telescope
components such as the 1-K optics tube, cryogenic half-wave plate, and calibrators.
The expected baseline sensitivity is σ(r) = 0.003 [44]. CMB-S4 is a proposed groundtelescope array by unifying efforts in the ground-based CMB experiment community
with a prospective deployment in late 2020s. One of the major science goals is to set
an upper limit of r < 0.001.
Space-borne CMB missions, WMAP and Planck, have played a key role to probe
the inflationary Universe. In May 2019, LiteBIRD was selected as the second L-classmission of the Institute of Space and Astronautical Science (ISAS) Japan Aerospace
Exploration Agency (JAXA). The prospective launch is in the late 2020s, and currently it is under the conceptual design phase with international partners for thedesign sensitivity of σ(r) = 0.001 [45]. LiteBIRD has been adopted as a priority
project of the Science Council Master Plan 2014, 2017, and 2020. It has also been
selected as one of the plans on the MEXT Road Map 2014, 2017, and 2020. As
seen by the joint constraint on r by the ground telescopes, BICEP/Keck, and Planck
today, we anticipate the joint analysis using the data from LiteBIRD and CMB-S4
31

in the future. The combined data should cover large angular ranges and frequency
bands, and thus it opens up even deeper sensitivity to the inflationary signal in the2030s.
Dark Energy, Dark Matter and Neutrino Mass
The nature of dark energy is one of the most tantalizing problems in cosmology.
The upcoming Subaru Prime Focus Spectrograph (PFS) cosmology program [46],which has been led by Kavli IPMU and will start in 2024, allows us to measure
the Hubble expansion rate H(z) and the angular diameter distance DA (z) to 3%
fractional accuracies in each of 7 redshift bins over 0.6 < z < 2.4, which can be used
to explore the nature of dark energy over the wide redshift range. The “early” darkenergy model can be a solution to the H0 tension, and the PFS survey can address
whether this is the case. The same PFS dataset allows us to robustly measureredshift-space distortion (RSD) effects in the distribution of galaxies. The RSD
measurements can be used to not only constrain the strength of gravitational field(matter clustering) in large-scale structure, but also make a stringent test of gravity
theory on cosmological scales, which can be used to address whether modification ofgravity is an explanation of the cosmic acceleration without the need of introducing
mysterious dark energy.
The nature of dark matter is another unresolved mystery. Primordial black holes(PBH), which could form in the early universe, are a viable candidate of dark matter. Recent observational results have triggered various studies including theoreticalstudies that build a model for PBH formation in the early universe such as inflationary scenario. Any observational confirmation of PBH promises transformative
progress of our understanding of the early universe physics. The ultimate microlensing surveys by the Subaru Hyper Suprime-Cam and other telescopes in the coming
decade will provide us with a conclusive answer to the PBH existence.
Some dwarf galaxies in the Milky Way exhibit a “core” in the central regionof their mass profile. The cored profile can be a hint of ultralight dark matter of
10−21 eV mass scales, such as axion-like particles inspired by string theory. The
unique wide-field and massively-multiplexed spectroscopic capability of PFS enables
high-precision reconstruction of dark matter distribution for each of the representative dwarf galaxies. The PFS measurements of dwarf galaxies can give a robust testof “wave” or “fuzzy” dark matter scenario.
Neutrino mass takes part in the role of dark matter in structure formation due
to its weak interaction
Pnature. The expected sensitivity of SA after the three yearsof observations is σ( mν )= 40 meV (68% C.L.) for the sum of neutrino masses
by combining the DESI results of baryon acoustic oscillations. The Subaru PFS
and US-led DESI surveys are both designed to independently achieve a precision of32

Pσ( mν ) ' 20 meV
P until around 2025. Hence these cosmology programs will enable a
5σ detection of mν , rather than the upper limit,
P if the neutrinos follow the inverted
mass hierarchy that predicts the lower limit ofmν ' 100 meV. Any claim of theneutrino mass detection will need independent confirmation, which
is what PFS
P
and DESI are expected to attain. The expected precision of σ( mν ) ' 20 meV iscurrently limited by the uncertainty in the optical depth τ from the Planck data. The
high-precision τ measurements by upcoming CMB polarization experiments probing
large angular scales, including GroundBIRD and eventually cosmic variance limited
measurements byPLiteBIRD, will further improve the determination of the neutrino
mass, such as σ( mν ) ' 15 meV after 2030.
The upcoming ultimate panoramic surveys such as the Vera C. Rubin Observatory’s LSST (2024), the ESA Euclid (2023) and the NASA Roman Space Telescope(2026) promise to advance our understanding of the Universe, including explorationof the nature of dark matter and dark energy with unprecedented precision. Even in
the coming decade, only the PFS has the wide-field and massively-multiplex spectroscopic capabilities among other 8m-class telescopes, and is uniquely positioned toplay synergetic and complementary roles with these cosmology surveys.

33

Acknowledgements
The editors thank J. Tian, T. Nakamoto, K. Terashi, M. Togawa, Y. Okumura,
K. Sakashita, M. Yokoyama, M. Masuzawa, Y. Suetsugu, M. Tobiyama, Y. Ushiroda,
Michael Roney, Soeren Prell, T. Mibe, M. Kitaguchi, M. Yamashita, K. Martens,
Y. Kishimoto, M. Takada, T. Matsumura, K. Nakayama, T. Abe, T. Higo, andM. Otani for helpful inputs.

34

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