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

Abstract

Using the complex scaling method, the low-lying three-body resonances of ${}^6$He, ${}^6$Li, and ${}^6$Be are investigated in a parameter-free microscopic three-cluster model. In ${}^6$He a 2${}^{+}$, in ${}^6$Li a 2${}^{+}$ and a 1${}^{+}$, and in ${}^6$Be the 0${}^{+}$ ground state and a 2${}^{+}$ excited state is found. The other experimentally known 2${}^{+}$ state of ${}^6$Li cannot be localized by our present method. We have found no indication for the existence of the predicted 1${}^{-}$ soft dipole state in ${}^6$He. We argue that the sequential decay mode of ${}^6$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 ${}^{11}$Li, too. We propose an experimental analysis, which can decide between the soft dipole mode and the sequential decay mode.

Comments: REVTEX, Submitted to Phys. Rev. C, 12 pages, 2 postscript figures are available upon request. CALTECH, MAP-166