Direct observation of three-neutron emission from $^7$He$^*$ and the search for the trineutron

TL;DR

This study reports the first direct measurement of three-neutron emission from $^7$He, identifying a resonance structure and analyzing decay mechanisms, with no evidence of a trineutron resonance.

Contribution

First direct observation of three-neutron emission from $^7$He and detailed analysis of decay pathways and correlations.

Findings

Resonance at 2.08 MeV above threshold with a width of 3.9 MeV.

Decay predominantly via sequential decay through $^6$He $2_1^+$ state.

No evidence of a trineutron resonance beyond known two-body interactions.

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 $J^{\pi}=3/2^{-}_2$ 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$^*$ 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.