Three-body resonances in 6He, 6Li, and 6Be, and the soft dipole mode problem of neutron halo nuclei.

Abstract

Using the complex scaling method, the low-lying three-body resonances of $^{6}\mathrm{He}$, $^{6}\mathrm{Li}$, and $^{6}\mathrm{Be}$ are investigated in a parameter-free microscopic three-cluster model. In $^{6}\mathrm{He}$ a ${2}^{+}$, in $^{6}\mathrm{Li}$ a ${2}^{+}$ and a ${1}^{+}$, and in $^{6}\mathrm{Be}$ the ${0}^{+}$ ground state and a ${2}^{+}$ excited state are found. The other experimentally known ${2}^{+}$ state of $^{6}\mathrm{Li}$ cannot be localized by our present method. We have found no indication for the existence of the predicted ${1}^{\mathrm{\ensuremath{-}}}$ soft dipole state in $^{6}\mathrm{He}$. We argue that the sequential decay mode of $^{6}\mathrm{He}$ through the resonant states of its two-body subsystem can lead to peaks in the excitation function. This process can explain the experimental results in the case of $^{11}\mathrm{Li}$, too. We propose an experimental analysis, which can decide between the soft dipole mode and the sequential decay mode.