Structure of few-body hypernuclei

Recently, in hypernuclear physics, we had three neutron-rich $\Lambda$ hypernuclei, $nn \Lambda$, $^6_{\Lambda}$H and $^7_{\Lambda}$He. These observation are very important by following reason: One of the research goal in hypernuclear physics is to study new dynamical features by injecting a $\Lambda$ particle into a nucleus. Since there is no Pauli principle between nucleons and a $\Lambda$ particle, the $\Lambda$ participation gives rise to more bound states and significant contraction of nuclear cores, especially in light systems. If a $\Lambda$ particle is added to neutron-rich nuclei to have a weakly bound state or resonant one, a resultant hypernucleus will become more stable against neutron decay. Three observed hypernuclei are such systems. Currently, it is important to investigate structure of these $\Lambda$ hypernuclei theoretically. For this purpose, I will report these hypernuclei within the framework of $nn\Lambda$, $tnn\Lambda$ and $\alpha \Lambda NN$ three- and four-body model. The following will be reported: (1) To study $nn\Lambda$ system, the coupled channel calculation of $NN\Lambda$ and $NN\Sigma$ is performed. We do not find any $nn\Lambda$ bound state, which is inconsistent with the interpretation of the data. (2) Interactions among the constituent subunits in $^6_{\Lambda}$H are determined so as to reproduce reasonably well the observed low energy properties of the $tn$, $t\Lambda$ and $tnn$. As long as we reproduce the energy and width of $^5$H within the error bar, the ground state of $^6_{\Lambda}$H is obtained as a resonant state. (3) In our previous work, we predicted the ground state, $1/2^+$ and the excited states, $3/2^+_1$ and $5/2^+$. And the recent observed data at Jlab are in good agreement with our prediction. Here, I will report another new states, $3/2_2$ and $5/2^+_2$ in $^7_{\Lambda}$He which is second $2^+$ state of $^6$He coupled to $0s$-orbit of $\Lambda$ particle. And I will report the structure of these spectra.