The energy spacing between the ground-state spin doublet of 4 Λ He(1 + ,0 + ) was determined to be 1406 ± 2 ± 2 keV, by measuring γ rays for the 1 + → 0 + transition with a high efficiency germanium detector array in coincidence with the 4 He(K − , π − ) 4Λ He reaction at J-PARC. In comparison to the corresponding energy spacing in the mirror hypernucleus 4 Λ H, the present result clearly indicates the existence of charge symmetry breaking (CSB) in ΛN interaction. It is also found that the CSB effect is large in the 0 + ground state but is by one order of magnitude smaller in the 1 + excited state, demonstrating that the ΛN CSB interaction has spin dependence.
The missing mass spectroscopy of Ξ − hypernuclei with the (K − ,K + ) reaction is planned to be performed at the J-PARC K1.8 beam line by using a new magnetic spectrometer, Strangeness −2 Spectrometer (S-2S). AČerenkov detector with a radiation medium of pure water (refractive index of 1.33) is designed to be used for on-line proton rejection for a momentum range of 1.2 to 1.6 GeV/c in S-2S. Prototype waterČerenkov detectors were developed and tested with positron beams and cosmic rays to estimate their proton-rejection capability. We achieved an average number of photoelectrons of greater than 200 with the latest prototype for cosmic rays, which was stable during an expected beam time of one month. The performance of the prototype in the cosmic-ray test was well reproduced with a Monte Carlo simulation in which some input parameters were adjusted. Based on the Monte Carlo simulation, we expect to achieve > 90% proton-rejection efficiency while maintaining > 95% K + survival ratio in the whole S-2S acceptance. The performance satisfies the requirements to conduct the spectroscopic study of Ξ − hypernuclei at J-PARC.
Spectroscopy of strangeness −2 systems has not been greatly in progress, because of the limited K − beam intensity to produce such systems and the limited energy resolution to identify them. Increasing K − beam intensity at J-PARC will enable us to carry out the spectroscopy by improving the energy resolution of the spectrometers in one order of magnitude from 14 MeV to 2 MeV. The status of the new spectrometer S − 2S is reported.
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