Direct observation of three-neutron emission from ${}^7$He${}^{\ast }$ and the search for the trineutron

Abstract

Three-neutron emission from ${}^7$He has been directly measured for the first time, following neutron knockout from a ${}^8$He beam at 156 MeV/nucleon. A resonance-like structure at $2.08(4)$ MeV above the ${}^4$He+$3n$ threshold [$E_x=2.68(4)$ MeV] with a width of $3.9(2)$ MeV was observed and deduced to arise predominately from the predicted $J^{\ddot{I}}€=3/2_2^{-}$ level. The three-neutron invariant-mass spectrum was reconstructed and found to peak at around 1 MeV and could, through complete simulations incorporating neutron-neutron correlations, be very well described by the sequential decay of ${}^7$He${}^{\ast }$ via the $2_1^{+}$ excited state of ${}^6$He. No evidence was found for any significant three-neutron correlations beyond those expected from well-established two-body interactions, including a trineutron resonance.

Accepted for publication in Physical Review Research