γ transitions inA=7hypernuclei and a possible derivation of hypernuclear size

Abstract

On the basis of the ${}_{\ensuremath{\Lambda}}^{5}\mathrm{He}+N+N$ three-body model which has successfully been applied to a systematic study of the energy and nucleon halo structure of the $A=7$ isotriplet hypernuclei, strengths of $\ensuremath{\gamma}$ transitions in ${}_{\ensuremath{\Lambda}}^{7}\mathrm{Li}$ and ${}_{\ensuremath{\Lambda}}^{7}\mathrm{He}$ are calculated. The new model confirms that the ${B(E2;5/2}^{+}\ensuremath{\rightarrow}{1/2}^{+})$ value in ${}_{\ensuremath{\Lambda}}^{7}\mathrm{Li}$ is reduced remarkably in comparison with the corresponding ${B(E2;3}^{+}\ensuremath{\rightarrow}{1}^{+})$ in the core nucleus ${}^{6}\mathrm{Li}.$This is due to the gluelike role of the $\ensuremath{\Lambda}$ particle which induces a contraction of the core nuclear size. It is suggested that a measurement of the ${5/2}^{+}\ensuremath{\rightarrow}{1/2}^{+}E2$ transition rate in ${}_{\ensuremath{\Lambda}}^{7}\mathrm{Li}$ (ongoing at KEK as E419) provides a unique opportunity to derive the hypernuclear size and hence to confirm the size contraction experimentally. The $E2$ and $M1$ transition strengths are also predicted for low-lying states in the hypernucleus ${}_{\ensuremath{\Lambda}}^{7}\mathrm{He}$ whose core nucleus ${}^{6}\mathrm{He}$ is known to have a neutron halo. Another prediction is made of much enhanced $E2$ transitions in ${}_{\ensuremath{\Lambda}}^{7}\mathrm{Li}$ from the ${5/2}^{+}$ and ${3/2}^{+}(T=1)$ states which are expected to have a proton halo structure.