Snowmass2021 - Letter of Interest

Update of the Japanese Strategy for Particle Physics
Japan High Energy Physics Committee1 :Shoji Asai, Kazunori Hanagaki, Toru Iijima, Kiyotomo Kawagoe, Tadashi Koseki, Shigeki Matsumoto,Toshinori Mori (chair), Mikihiko Nakao, Tsuyoshi Nakaya, Masashi Otani, and Makoto Tomoto
Contact: Toshinori Mori [mori@icepp.s.u-tokyo.ac.jp]
31 August 2020

Preface:
In September 2017, the Japan Association of High Energy Physicists (JAHEP) updated the Japanese strategyfor future particle physics by endorsing the recommendations of the Committee on Future Projects in HighEnergy Physics. Since then there have been important developments in the field that include the formationof the ILC International Development Team to prepare the ILC Pre-Lab in Japan, gradual implementationof the HL-LHC project in MEXT’s Large Scientific Research Projects in Japan, the official approval andstart of construction of the Hyper-Kamiokande, the beginning of the Belle II physics program at the newly
operating SuperKEKB, and the recent update of the European Strategy for Particle Physics. Here we briefly
summarize all these recent developments. More detailed accounts will be reported in our submissions to theSnowmass 2021 Proceedings.

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Representing the Japan Association of High Energy Physicists (JAHEP)

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The Japanese Strategy for Particle Physics 2017

In September 2017, the Committe on Future Projects made the following recommendations to update theJapanese Strategy, 1 following the JAHEP’s proposal of early realization of the ILC as a Higgs factory 2,3 :In 2012, not only was a Higgs boson with a mass of 125 GeV discovered at theLHC, but three-generation neutrino mixing was also established. Taking full advantageof the opportunities provided by these discoveries the committee makes the following
recommendations concerning large-scale projects, which comprise the core of futurehigh energy physics research in Japan.
• With the discovery of the 125 GeV Higgs boson at the LHC, construction of theInternational Linear Collider (ILC) with a collision energy of 250 GeV should startin Japan immediately without delay so as to guide the pursuit of particle physics
beyond the Standard Model through detailed research of the Higgs particle. Inparallel, continuing studies of new physics should be pursued using the LHC andits upgrades.
• Three-generation neutrino mixing has been established and has provided a pathto study CP symmetry in the lepton sector. Therefore, Japan should promote theearly realization of Hyper-Kamiokande as an international project due to its superior proton decay sensitivity, and should continue to search for CP violation with
the T2K experiment and 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 construction of SuperKEKB and start physics studies asscheduled. Some of the medium- and small-scale projects currently under consideration have implicit potential to develop into important research fields in the future, ashappened 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 beprojects 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 theexistence of new particles and new phenomena at higher energy scale.

2Recent Developments in Implementing the Strategy

The international consensus for an electron-positron Higgs factory as the highest-priority next collider has
helped further strengthen efforts to realize the ILC in Japan during the past year. An increasing number ofpeople from various sectors, including scientists, politicians, industry members, business circles, and local
communities, are proactively working together towards the realization of the ILC. The recent progress anddevelopments in various sectors of Japan were summarized in a document 4 that was sent to the European
Strategy Group in December, 2019.
2This August ICFA announced the formation of the ILC International Development Team 5 as the firststep towards the preparatory phase of the ILC project, with a mandate to prepare the ILC Pre-Lab in Japanby the end of 2021. The Team has replaced the LCB/LCC organization that had ended its mandate at theend of June, 2020. KEK now hosts the Team and provides necessary support.
A large part of the Japanese community are working at the energy frontier and are strongly committed
to the LHC and its upgrade. Japan’s participation in the HL-LHC has been officially listed in the MEXT
roadmap under the title of “Particle Physics with the HL-LHC” since 2017 and the budget for the HL-LHC
has been allocated since 2019. Taking advantage of Japan’s expertise in the current LHC program, Japan’scontributions to the HL-LHC consist of the R&D and construction of beam separation dipole magnets, andthe inner tracking system and the muon trigger system of the ATLAS experiment.
This year the Hyper-Kamiokande (Hyper-K) project 6 was officially approved by the Japanese government. The construction has already started and the physics program is expected to start in 2027. The
Hyper-K is a world-leading international project hosted by Japan aiming to elucidate the origin of matterand the Grand Unified Theories. It consists of the Hyper-K detector, which has 8.4 times larger fiducial
mass than the Super-Kamiokande, and a high-intensity neutrino beam produced by the J-PARC accelerator
facility that is being upgraded in stages towards 1.3 MW beam power along with the T2K physics program.
The Hyper-K collaboration is being formed by interested international groups, currently from 19 countries
including the United States that has historically contributed significantly to neutrino programs in Japan.
It is anticipated that international contributions will cover additional inner-detector photosensors, sensor
covers and light collectors, outer-detector photosensors, readout electronics and data acquisition system,
water system upgrade, detector calibration systems, downstream offline computing systems, and the nearand intermediate detector complex.
After the completion of construction and commissioning with collidng beams in 2018, the physics program of the SuperKEKB/Belle II started in the spring of 2019. The SuperKEKB has already achieved theworld’s highest instantaneous luminosity of 2.40 × 1034 cm−2 s−1 by using the “nano-beam” technology,
and the Belle II continues to take data with tolerable background level. Major milestones of reaching peak
and integrated luminosities of Lpeak ∼ 1 × 1035 cm−2 s−1 and Lint ∼ 5 ab−1 are expected within the next
few years, and that will bring significant physics results with a large sample of heavy falvor particle decays.
It is the aim of the project to achieve Lpeak ∼ 6 × 1035 cm−2 s−1 and accumulate Lint ∼ 50 ab−1 by as earlyas 2030, by upgrading machine and detector components, such as the interaction region, superconducting
final focus, as well as detector susbsystems. Feasibility studies are in progress for much higher luminosity
beyond 50 ab−1 and for electron beam polarization as possible future upgrades.
Kaon and muon experiments at the J-PARC provide opportunities to search for new physics through rare
processes and precision measurements. The KOTO is a unique ongoing experiment that aims to measure
the KL → π 0 ν ν̄ process, and the COMET Phase-I experiment is under construction to search for muonelectron conversion with sensitivity better than 10−14 . Outside Japan the MEG II experiment starts looking
for flavor-violating muon decays at the PSI.
Accelerator R&D aiming for higher energy collisions is crucial for future high energy physics. There
have been continued efforts on developments of superconducting magnets, higher gradient accelerating
structure, muon collider, and novel accelerator technologies such as laser-plasma accelerator. Among others,
the Japanese community intends to contribute to R&D for superconducting magnets as preparations for anext-generation hadron collider beyond the LHC and superconducting RF technologies which could be usedfor future upgrades of the ILC.
The Japanese community hopes to continue to work together with international colleagues to achieve
global scientific goals of particle physics.

3References[1] Masaki Ishitsuka, Hirokazu Ishino, Masaya Ishino (chair), Wataru Ootani, Takashi Obina,
Masaaki Kitaguchi, Ryuichiro Kitano, Hiroshi Sakai, Tomoyuki Sanuki, Nanae Taniguchi, Makoto
Tomoto, Takeshi Nakadaira, Hajime Nanjo, Takeo Higuchi, Shigeki Matsumoto, Shigetaka Moriyama,and Tamaki Yoshioka, “Final Report of the Committee on Future Projects in High Energy Physics,”
September 6, 2017.
http://www.jahep.org/files/20170906-en.pdf[2] Shoji Asai (chair), Junichi Tanaka, Yutaka Ushiroda, Mikihiko Nakao, Junping Tian, Shinya Kanemura, Shigeki Matsumoto, Satoshi Shirai, Motoi Endo, and Mitsuru Kakizaki, “Report by the Committee on the Scientific Case of the ILC Operating at 250 GeV as a Higgs Factory,” arXiv:1710.08639,
July 22, 2017.
http://www.jahep.org/files/ILC250GeVReport-EN-FINAL.pdf[3] Japan Association of High Energy Physicists,“Scientific Significance of ILC and Proposal of its Early
Realization in light of the Outcomes of LHC Run2,” August 16, 2017.
http://www.jahep.org/files/JAHEP-ILCstatement-170816-EN.pdf[4] Toshinori Mori, “Recent Progress Towards the Realization of the ILC in Japan: Cooperative Efforts
by Academia, Industry, and Local Region,” December 13, 2019.
http://www.jahep.org/files/input_JapanHEPC_20191213.pdf[5] International Committee for Future Accelerators (ICFA), “ICFA announces a new phase towardspreparation for the International Linear Collider,” August 2, 2020.
https://icfa.fnal.gov/wp-content/uploads/ICFA_Statement_August_2020.pdf[6] K. Abe et al., “Hyper-Kamiokande Design Report,” May, 2018, arXiv:1805.04163.

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