Hypernuclear physics via $\gamma$-ray spectroscopy

A series of hypernuclear $\gamma$-ray spectroscopy experiments performed at KEK-PS and BNL-AGS using a germanium detector array, Hyperball, have accumulated precise data on various p- shell $\Lambda$ hypernuclei. We observed ``hypernuclear fine structure'' in various hypernuclei and extracted the strengths of all the spin-dependent parts (spin-spin, spin-orbit, and tensor terms) of the $\Lambda$-$N$ interaction. The obtained strengths reproduced structure of most of the p-shell $\Lambda$ hypernuclei quite well, and also played important roles in testing and improving baryon-baryon interaction models. We also measured a B(E2) value of $^7_\Lambda$Li and confirmed ``shrinking effect'' of a $\Lambda$ hyperon. In the J-PARC facility, further experiments of hypernuclear $\gamma$-ray spectroscopy are planned to be performed. In the first experiment (E13), we will extend our study to s-shell and sd-shell hypernuclei and investigate the $\Lambda$-$N$ interaction more in detail. We also plan to study the g factor of a $\Lambda$ hyperon in a nucleus by measuring a B(M1) value of $\Lambda$-spin-flip transitions in hypernuclei in order to study possible modification of baryon properties in nuclear matter. In future, we will also investigate ``impurity effect'' of nuclear structure induced by a $\Lambda$, such as the shrinking effect and a possible change of nuclear deformation.