English Summary

Our group is experimentally investigating baryon–baryon interactions beyond the ordinary nucleon–nucleon sector, extending them to systems that include strange quarks, such as hyperon–nucleon and hyperon–hyperon interactions. The J-PARC E94 experiment aims to perform high-precision spectroscopy of A = 7, 10, and 12 Lambda hypernuclei in order to address one of the long-standing questions in Lambda–nucleon interaction studies: charge symmetry breaking, namely the possible difference between Lambda–proton and Lambda–neutron interactions.

At the J-PARC K1.8 beamline, the new high-resolution magnetic spectrometer S-2S, constructed under the leadership of the Kyoto University group, started operation in 2025 and has begun taking physics data. The E94 experiment will use this state-of-the-art spectrometer to approach the long-standing CSB problem with a level of precision that was previously difficult to achieve.

In this experiment, Lambda hypernuclei are produced via the (π⁺, K⁺) reaction and measured by missing-mass spectroscopy. Since Lambda-hypernuclear production events are extremely rare, efficient and clean identification of K mesons in S-2S is essential. Background particles such as protons, pions, and muons from beam-particle decays must be rejected with high efficiency.

To improve K-meson purification, we are introducing new acrylic Cherenkov detectors using ultraviolet-transmitting acrylic radiators into the existing S-2S detector system. On May 13, 2026, Tomohiro Taniguchi, a D1 student at Kyoto University, together with Daigo Watanabe, a D3 student at Tohoku University, started mass production of the acrylic Cherenkov detectors at Kyoto University.

The detector design is based on prototype studies and performance tests carried out by Taniguchi as part of his master’s thesis research in the previous academic year. We plan to complete the full detector production by this summer and prepare for installation at the K1.8 beamline in the J-PARC Hadron Experimental Facility this autumn.