Spectroscopy of $\Lambda$ Hypernuclei: Recent Progress and Future Prospects

Recently, great progress has been made in spectroscopic studies of $\Lambda$ hypernuclei. In this talk, the present status and future plans of $\Lambda$ hypernuclear spectroscopy are presented, particularly on precision $\gamma$-ray spectroscopy. We have been investigating detailed structure of $\Lambda$ hypernuclei by means of the precision $\gamma$-ray spectroscopy technique with a large germanium detector array, Hyperball. We have studied $\gamma$ transitions in various p-shell $\Lambda$ hyerpnuclei ($^7_\Lambda$Li, $^9_\Lambda$Be, $^{10}_\Lambda$B, $^{11}_\Lambda$B, $^{15}_\Lambda$N, and $^{16}_\Lambda$O) via the $(\pi^+,K^+\gamma)$ reaction at KEK and the $(K^-,\pi^-\gamma)$ reaction at BNL. The precise level structure data of these hypernuclei allowed us to determine all the spin-dependent (spin-spin, spin-orbit, and tensor) $\Lambda$$N$ interaction strengths, which give stringent constraints to baryon-baryon interaction models. The upgraded apparatus (Hyperball2) is now ready for further studies, where one of the most important subjects is the study of magnetic moment of a $\Lambda$ in a nucleus through measurement of $B(M1)$ values for $\Lambda$-spin-flip $M1$ transitions in hypernuclei. In near future, hypernuclear $\gamma$ spectroscopy will be greatly developed by using strong beams from the 50 GeV proton synchrotron at J-PARC. In addition, recent progress and future plans are summarized for high-resolution $\Lambda$ hypernuclear spectroscopy via the $(e,e'K^+)$ reaction at Jlab and spectroscopy of neutron-rich $\Lambda$ hypernuclei via the $(\pi^-,K^+)$ reaction at KEK.