The magnetic spectrometer "S-2S" system for the missing-mass spectroscopy of Ξ hypernuclei.

The experimental data for the strangeness S=−2 system are limited by compared to the S=−1 system such as ΛN and ΣN interactions. Recently, infomations for ΛΛ and ΞN interactions were observed by emulsion experiments and femtoscopy in heavy ion collision. However, to understand the ΞN interaction in detail, the spectroscopic data of Ξ hypernuclei which have higher statistics and higher precision are awaited. This will play impotant role to understand the extension of the baryon-baryon interaction including hyperon.

At the K1.8 beamline in Hadron experimental facility of J-PARC, a missing-mass spectroscopic experiment for Ξ hypernuclei (J-PARC E70) started. In this experiment, high-purity and high intensity K⁻ beams with 1.8 GeV/c are irradiated into a ¹²C target and produces Ξ hypernuclei, ¹²_ΞBe via the (K^–, K⁺) reaction. The missing-mass for ¹²_ΞBe are given by momenta of scattered K⁺ which measured by the new magnetic spectrometer "S-2S" which expected to have great momentum resolution Δp/p = 6.0×10^–4. The resolution of missing-mass is expected to be less than 2 MeV which is higher than past spectroscopy and can be evaluated for the peak structure of hypernuclei. We had been preparing for the experiment for many years, and the first commissioning run with hadron beams was carried out in June, 2023.

In this talk, we will report performance evaluation for the S-2S system on the first commissioning run. In addition, we will introduce momentum analysis for the S-2S by using machine learning.