Incoming director-general of KEK, Shoji Asai, describes the latest activities in Japan’s diverse high-energy physics programme and how to build on the field’s successes in international collaboration.
What has been your career trajectory so far?
I completed my PhD at the University of Tokyo (U-Tokyo) in 1995 on precise measurements of the orthopositronium decay rate, in which I solved the problem of the orthopositronium lifetime puzzle. The measurement ultimately confirmed second-order QED predictions with an accuracy of 100 ppm, and positronium’s hyperfine structure and Bose–Einstein condensation are ongoing projects in the Tokyo group. I remained at U-Tokyo as an assistant, associate and then full professor, and in 1995 I joined the OPAL experiment at LEP and then ATLAS at the LHC. At OPAL I took an initiative in electroweak gaugino searches and performed a new search for scalar top quarks. I continued to work on supersymmetry searches at ATLAS, and also made a contribution to the discovery of the Higgs boson. From 2017, I became director of the International Center for Elementary Particle Physics at U-Tokyo, and on 31 March 2024 I will leave the university after close to 40 years to take up my new role at KEK.
How does it feel to be taking over as KEK director general, and what will be your priorities in the coming years?
I am honoured and feel a sense of humility at the same time. We are at a critical time to determine future project(s), and strong international collaborations are crucial. I want to have fun and do my best! The successful accomplishment of ongoing programmes (SuperKEKB, J-PARC upgrade and Hyper-Kamiokande) is the top priority in the coming years. KEK also has photon factories, and upgrades to these are urgent. The International Linear Collider (ILC) is the top priority after SuperKEKB and the construction of Hyper-Kamiokande (Hyper-K).
Will you still play a role in ATLAS?
Personally, I will leave the ATLAS experiment. I thank all ATLAS collaborators with whom I have had a wonderful and exciting time for more than 20 years. Japan has contributed to the HL-LHC projects and the associated ATLAS upgrades, as it did for the first phase of LHC/ATLAS. Now we begin an additional contribution to HL-LHC concerning the power supply for the quench heater and radio-frequency generators for the crab cavities. The Japanese high-energy physics community and MEXT (Ministry of Education, Culture, Sports, Science and Technology) would like to continue their large contributions to CERN, the LHC and ATLAS.
How is data collection progressing at SuperKEKB, and what are the current luminosity targets?
SuperKEKB represents a new generation of electron–positron colliders based on nanobeam technology. The highest instantaneous luminosity achieved so far (5 × 1034 cm–2 s–1, a record for an e+e– machine) was obtained with only half the beam current of its predecessor KEKB. Now, SuperKEKB is emerging from a long shutdown and will restart in December. The first target is to reach higher than 1035 cm–2 s–1 with the nominal beam current, after which the beam will be squeezed further to reach a final target that is a factor of 10 higher. SuperKEKB opens up opportunities for the discovery of a new CP phase and phenomena beyond the Standard Model. Many new baryon and meson states will be discovered, and a deep understanding of QCD at low energy will be obtained.
What is the current situation with the ILC, and do you expect any advances in the near future regarding Japan’s hosting of the facility?
The Japanese community considers the ILC as the top-priority project after SuperKEKB and the neutrino CP-violation programme at Hyper-K. We would like to realise the ILC as a “global project” built up through a worldwide collaborative effort in which all decisions (such as the construction decision itself, cost sharing, the construction location, risk management and organisation scheme) are taken collectively by all partners from the beginning. This is a new approach in particle physics. We are setting up the ILC technology network and a global discussion framework in collaboration with the IDT (the International Development Team established by the International Committee for Future Accelerators).
Moving to neutrinos, how are things going with the T2K upgrade and Hyper-K projects, and how strongly do these relate to LBNF/DUNE in the US?
A megawatt power-upgrade of the drive accelerator at J-PARC and the construction of the Hyper-K detector are ongoing without any serious problems. We expect to start the neutrino programme with Hyper-K in 2027, with the main goal of establishing the CP phase in the neutrino sector. We have much experience with T2K and water Cherenkov detectors, which are an advantage for this programme. We can also share our experience of the target of the high-power proton beam with LBNF. DUNE and Hyper-K are quite different detectors, so we can cover each other.
What are KEK’s major collaborations in the broader region, for example JUNO and the Super Charm-Tau factory?
These are very interesting programmes. The Japanese high-energy physics community has contributed to many ongoing programmes overseas, and we also have many important projects in Japan. Human resources are limited and focussing on these ongoing programmes is the priority.
The SuperKEKB and neutrino programmes, in addition to the muon programmes in Japan, always open a window for the world. New collaborators are always welcome to these programmes. As for involvement in other proposed future-collider projects, for example FCC and CEPC, it depends on the realisation of the ILC and the collaboration frameworks that will be proposed.
How do you view the current global picture of high-energy physics?
My happy time as a scientist in OPAL and with ATLAS, and the enormous success of LEP and the LHC prove that international collaboration is very successful in our field. I am afraid that our next major project has become too large and will cost more than one country can afford, which is why we need the ILC to be a global project. I understand that this approach will not be easy, but we have fantastic experience to build on. Now we face problems in international relations generally, such as war, pandemics and budget tensions in many counties. We can overcome them, I hope.