Dineutron structure in ${}^8$He

The ground and excited states of ${}^8$He were investigated with a method of antisymmetrized molecular dynamics(AMD). We adopted effective nuclear interactions which systematically reproduce the binding energies of ${}^4$He, ${}^6$He and ${}^8$He. The ground state of ${}^8$He has both the $j$-$j$ coupling feature($p_{3\mathrm{/}2}$ closure) and the $L$-$S$ coupling feature(${}^4$He$+2n+2n$) with a slight tail of dineutron at the long distance region. The theoretical results give an indication of the $0_2^{+}$ state with dineutron gas-like structure. The dineutron structure, ${}^4$He+$2n$+$2n$, of this state is similar to the $3\alpha$-cluster structure of the ${}^{12}$C($0_2^{+}$) state which has been interpreted as an $\alpha$ condensate state. Since the ${}^8$He($0_2^{+}$) state has a significant overlap with the dineutron condensate wave function where two dineutrons are moving in $S$ wave around the $\alpha$ core with a dilute density, we suggest that this theoretically predicted $0_2^{+}$ state is a candidate of the dineutron condensate state.