Anomalous population of ${}^{10}$He states in reactions with ${}^{11}$Li

Abstract

Structure with the lowest energy observed in the ${}^{10}$He spectrum populated in the proton knockout reaction with ${}^{11}$Li beam has a peak at $1.2-1.5$ MeV. This peak is usually interpreted as a resonant $0^{+}$ ground state of ${}^{10}$He. Our theoretical calculations indicate that this peak is likely to be a pileup of $1^{-}$, $0^{+}$, and $2^{+}$ excitations with very similar shapes. %We predict a very specific nature of the $1^{-}$ excitation in ${}^{10}$He. Moreover, the ``soft'' $1^{-}$ excitation appears to be the lowest one in energy. Such an anomalous continuum response is traced to the halo structure of ${}^{11}$Li providing extreme low energy shift to all the expected continuum excitations. Competitions of the initial state structure (ISS) and the final state interaction (FSI) effects on the spectrum and three-body correlations in ${}^{10}$He are discussed. Analogous effect of the extreme low-energy shift could also be expected in other cases of $2n$ emitters populated in reactions with halo nuclei. Simplified example of the ${}^{10}$He spectrum in $\alpha$ knockout from ${}^{14}$Be, is given. We also discuss limits on the properties of ${}^9$He stemming from the observed ${}^{10}$He spectrum.

Comments: 10 pages, 13 figures