Kaon identification in Λ hypernuclear spectroscopy with S-2S at J-PARC (E94 Experiment)

We are preparing for a high-precision spectroscopy experiment of Λ hypernuclei by (π⁺,K⁺) reaction using S-2S (Strangeness -2 Spectrometer) at J-PARC (J-PARC E94 experiment). The aim of this experiment is to measure Λ binding energies (B_Λ) of ⁷_ΛLi, ¹⁰_ΛB, and ¹²_ΛC with a resolution of 1 MeV (FWHM). With this high resolution, ⁷_ΛLi can be used as a reference for B_Λ, and the accuracy of B_Λ including systematic errors is expected to be 100 keV. The measurement of the ¹⁰_ΛB with such a high accuracy would allow us to compare with their mirror nuclei ¹⁰_ΛBe measured in the (e,e'K⁺) reaction spectroscopy to study charge symmetry breaking in the Lambda-N interaction.

Two types of Cherenkov detector, aerogel and water Cherenkov counters (AC, WC) are equipped in S-2S for an identification of K⁺ (KID). Replacing WC by another Cherenkov detector is one of options to improve the KID performance in the E94 experiment. We obtained data of 1.4-GeV/c protons on WC in a commissioning run of the J-PARC E70 experiment. The commissioning data are useful for the K-proton separation study by WC in E94 because the number of photoelectrons is comparable to that of K⁺ at 0.72 GeV/c.

In the talk, kaon identification by Cherenkov counters in E94 is going to be introduced along with a performance result of the counters in the commissioning run in June 2023.