The degenerate spin-flip doublet (3+/2,5+/2) ofΛ9Be

Abstract

The energy of the degenerate doublet $({3}^{+}/2,{5}^{+}/2)$ of ${}_{\ensuremath{\Lambda}}^{9}\mathrm{Be}$, treating it as a partially nine-body system in the $\ensuremath{\Lambda}\ensuremath{\alpha}\ensuremath{\alpha}$ cluster model, has been calculated in the variational Monte Carlo framework. A simplified treatment, with the central two-body Urbana type $\ensuremath{\Lambda}N$ and the three-body dispersive and two-pion exchange $\ensuremath{\Lambda}\mathit{NN}$ forces along with the central two- and three-body correlations, is found to be adequate in explaining the energy of observed $\ensuremath{\gamma}$-ray transition from the excited degenerate doublet to the ground state. The hypernucleus ${}_{\ensuremath{\Lambda}}^{9}\mathrm{Be}$ is highly deformed and has an oblate shape in the excited state. The results of the present work are consistent with the earlier three-body cluster model analyzes of ${}_{\ensuremath{\Lambda}}^{9}\mathrm{Be}$.