Japan’s Vision for the Future of Particle Physics

Japan’s high-energy physics community is entering a crucial phase as it maps out its long-term vision.  Researchers from across the country, working together through the Japan Association of High Energy Physicists (JAHEP), are discussing the next big steps for the field. JAHEP has set up a group to prepare a full report on future projects in high-energy physics, aiming to complete it by the end of 2026. Recently, JAHEP also submitted the input to the European Strategy for Particle Physics, based on these discussions.

“The last report on the Japanese future project was published in 2017, and since then, the landscape of particle physics has changed dramatically,” said Tsuyoshi Nakaya, Chair of JAHEP. That earlier report urged the community to pursue all possible avenues to realize two major projects: the International Linear Collider (ILC) and Hyper-Kamiokande. It also emphasized the start of SuperKEKB and the launch of physics operations of Belle II as planned, along with continuing the search for CP violation via the T2K experiment and upgrades to the J-PARC neutrino beam.

Today, SuperKEKB is operational and holds the world record for luminosity in electron–positron collisions. J-PARC has achieved key performance goals. Hyper-Kamiokande, officially approved in 2020, is under construction. “Among the major projects highlighted in the 2017 report, the one that has yet to advance to construction is the ILC,” Nakaya noted.

The discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 prompted global recognition of the need for a Higgs factory, which produces large numbers of Higgs bosons, enabling precise studies of their properties. The 2012 and 2017 reports reflected this, stating that “should a new particle such as a Higgs boson with a mass below approximately 1 TeV be confirmed at LHC, Japan should aim for an early realization of an e⁺ e⁻ linear collider.” At the time, the ILC was the only candidate being considered in Japan.

While international agreement around the importance of Higgs research remains unchanged, the past decade has seen the emergence of significant alternatives to the ILC, including China’s CEPC and Europe’s FCC-ee. “None of these projects have yet been approved, but they have become serious options on the table,” Nakaya said. In preparing Japan’s input to the latest European Strategy Update, JAHEP took these developments into account.

The input paper was compiled by the Committee on Future Projects in High Energy Physics under JAHEP. This committee is composed of relatively early-career researchers under the age of 50 from a broad range of experimental backgrounds. “Although the full report is not expected until 2026, our input for the European Strategy Update was due this March. So we asked the committee to prioritize discussions around how Japan should engage with a future Higgs factory,” Nakaya explained.

Historically, Japan has seen the Higgs factory as essentially synonymous with the ILC, and thus alternative projects were not seriously considered. “This time, however, the committee was explicitly tasked with exploring all options,” Nakaya said. Until recently, Japan’s research community was not deeply involved with the CEPC or FCC-ee, partly because these projects are relatively new and Japanese participation has been limited. The committee began its work by studying these initiatives in depth.

Following these discussions, the committee concluded that the early realization of a Higgs factory remains the highest priority for Japan’s high-energy physics community. While it remains unclear which project will move forward globally, Japan should support the timely construction of a Higgs factory, wherever it may be.

The report outlines two key recommendations:

• We prioritize efforts to realize the ILC as Global Project, taking a leading role in advancing ongoing initiatives. We will engage with international partners to discuss governance, responsibilities, and site selection. We intend to develop and expand our scientific and promotional activities to host the ILC as Global Project in Japan .

• We also extend our activities in other Higgs factory proposals as a collective approach to maximize the chances of timely realizing a Higgs factory.

The community concluded that, particularly considering technical readiness and cost, the ILC remains the most feasible option for near-term realization. Furthermore, it should not be pursued as a purely national endeavour. “From the beginning, the vision has been for the ILC to be a global project,” Nakaya said. “There are people in the international science community who seem to think that the ILC is a Japanese initiative, and while Japan does wish to host it, this has always been on the basis of global consensus and collaboration.”

The second recommendation marks a significant shift. A decade ago, there was no discussion of Japan contributing to other Higgs factory initiatives. Now, the community is expressing a strong desire to engage. Though no Japanese groups are currently participating in overseas projects for Higgs Factories other than the ILC, the report encourages starting with collaborative technical efforts—particularly in areas where Japan has unique expertise.

One such contribution could be SuperKEKB’s nanobeam technology, which is expected to play a key role in future circular Higgs factories. “Rather than immediately committing to a specific project, we hope to begin with technical collaborations,” Nakaya said.

He also emphasized that building multiple Higgs factories has been unrealistic, especially given the current global situation and rising costs. Whichever project advances, it will require robust international dialogue on issues like funding and location. In this context, international cooperation becomes more crucial than ever.

“If any project becomes isolated or competitive in a negative sense, the entire field will suffer,” he said. “We must avoid exclusivity and pursue a more inclusive, cooperative approach.”

He pointed out that unproductive rivalries—for instance, assuming support for the ILC means one must oppose other ongoing projects such as B Factory or J-PARC , or vice versa—can be damaging. “If we start off by isolating ourselves, the project is unlikely to succeed,” he warned.

Positive signs are emerging. The recent Higgs Factory Study under ECFA signals growing interest in cross-border collaboration.  “When things become difficult in your own country, it’s essential to have allies and supporters abroad,” Nakaya said. “We hope this mindset spreads across the entire community.”

The discussion on Japan’s long-term strategy is ongoing. As the 2026 report takes shape, broader conversations will include other major initiatives. Japan must consider how best to align the ILC with future plans for the B Factory, J-PARC, and more—and how to prioritize among them.

“Priorities change with time,” Nakaya concluded. “That’s why we need inclusive, forward-looking discussions that reflect both evolving circumstances and the needs of the community.”

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