Recent progress at J-PARC: &[Xi] hypernuclei*

We have measured an excitation energy spectrum of the ¹²C(K^-,K⁺)X reaction at 1.8 GeV/c with an energy resolution of 5.4 MeV (FWHM), which is the best energy resolution ever achieved in studying this reaction. The measurement was performed at the K1.8 beam line of the J-PARC hadron experimental hall by using the SKS spectrometer, as a pilot run of J-PARC E05 experiment. The K^-beam intensity at the primary proton beam power of 39 kW was typically 6x10⁵/spill with 5.5-sec. beam cycle. The energy resolution was estimated from the peak observed  in the p(K^-,K⁺)Ξ^-reaction from a 9.54-g/cm²CH₂target. We took the data on the ¹²C(K^-,K⁺)X reaction with a 9.4-g/cm²C target for about 10 days. We have observed about 60k events of quasi-free Ξ^-production, and several tens of events in the bound region. While the analysis is near the final stage, we can see clear enhancements in the bound region above a flat background. Fitting to the enhancements with a few models suggest that the binding energy of  ¹²_ΞBe would be larger than the previously estimated value of about 4.5 MeV.