Speaker
Description
The $(e, e'K^+)$ reaction experiments with several nuclear targets performed recently at the Jefferson Lab have provided high-resolution spectroscopic data. They are very fruitful in disclosing hypernuclear structure details and also in understanding hyperon-nucleon fundamental interaction properties.
Among others, the $^{10}$B $(e, e'K^+)$ $_{\,\Lambda}^{10}$Be reaction data are quite interesting, because they have shown extra subpeaks which seem difficult to be explained within the conventional model with the $p$-shell nuclear natural-parity configurations. In order to describe this novel hypernuclear structure, we have extended the model space by introducing the new configurations which include unnatural-parity nuclear core-excited states. In the extended model for each hypernuclear state of $J^{\pm}$, we take four types of configurations, $[J_{\mathrm{c}(i)}^+ \otimes s^{\Lambda}]_{J^+}$, $[J_{\mathrm{c}(i)}^+ \otimes p^{\Lambda}]_{J^-}$, $[J_{\mathrm{c}(i)}^- \otimes s^{\Lambda}]_{J^-}$, and $[J_{\mathrm{c}(i)}^- \otimes p^{\Lambda}]_{J^+}$, where $J_{\mathrm{c}(i)}^{\pm}$ denotes all the possible spin-parity states of core-nucleus, and $s^{\Lambda}$ and $p^{\Lambda}$ denote the single-particle states of $\Lambda$ hyperon. We take into account all the states of core-nuclei in the $0\hbar\omega$ and $1\hbar\omega$ space, which are labeled with $i$. It is interesting to point out that the nuclear core eigenstates with different parities are mixed by the $\Lambda$ participation. For the $_{\,\Lambda}^{10}$Be hypernucleus, natural- and unnatural-parity nuclear-core configurations, $[ J_{\mathrm{c}(i)}^+ \otimes p^{\Lambda} ]_{J^-}$ and $[ J_{\mathrm{c}(i)}^- \otimes s^{\Lambda} ]_{J^-}$, can be mixed easily by the $\Lambda N$ interaction at appropriate excitation energy. We emphasize that, for the first time, this configuration mixing successfully explains the extra subpeaks in the $^{10}$B $(e, e'K^+)$ reaction experiments.
For adjacent hypernuclei with $A=9$-$12$, we will show the energy levels and the DWIA cross-sections of $(K^-, \pi^-)$, $(\pi^+, K^+)$, and $(\gamma, K^+)$ reactions that are calculated within the extended model space. We focus on the interesting behavior of $\Lambda$ $p$-state. In $_{\Lambda}^{9}$Be, it is well known that the $\Lambda$ $p$-state splits into two orbital states, $p$-perpendicular and $p$-parallel states, which is due to the strong coupling with nuclear core deformation having the $\alpha$-$\alpha$ structure. We will discuss the theoretical spectrum of $^{10}$B $(K^-, \pi^-)$ $_{\,\Lambda}^{10}$B and $^{11}$B $(K^-, \pi^-)$ $_{\,\Lambda}^{11}$B reactions, and will show the two peaks corresponding to the $p$-perpendicular and $p$-parallel states. The extended model study will be useful for new projects of $(K^-, \pi^-\gamma)$ and $(\pi^+, K^+\gamma)$ reaction experiments with high-intensity and high-resolusion that are being planned at J-PARC.
