Spectroscopy of $S=-2$ hypernuclei at J-PARC with a new spectrometer S-2S

The study of $S=-2$ hypernuclei is important for understanding baryon-baryon interaction and strange nuclear matter. However, experimental data of $S=-2$ systems are very limited. We will obtain the spectroscopic information of $S=-2$ hypernuclei using the $(K^-,K^+)$ reaction. As a first step, We plan to perform the J-PARC E05 experiment by using the $^{12}$C$(K^-,K^+)$ reaction. Following this experiment, we plan to carry out further studies on $\Xi$- and double $\Lambda$-hypernuclei with various targets. We will utilize the high intensity $K^-$ beam and high resolution beam spectrometer at J-PARC K1.8 beam line. To achieve both enough statistics and better resolution, a new spectrometer S-2S for scattered $K^+$ is under construction. It consists of a QQD-type configuration. It is designed to have a momentum resolution of better than $5\times10^{-4}$ (FWHM), which corresponds to energy resolution of 1.5 MeV. The construction of Q1 and Q2 has already been finished. We measured magnetic field of Q1, and obtained a field gradient of 8.7 T/m enough to achieve an acceptance of 60 msr at 1.3 GeV/$c$. We are also developing detectors, especially a water Cherenkov counter for on-line $K/p$ separation. The magnets and the detectors will be ready for installation in the next year.