Search for a bound di-neutron by comparing $^3$He(e,e'p)d and $^3$H(e,e'p)X measurements

TL;DR

This study searches for a bound di-neutron by comparing electron-induced proton knockout reactions from Helium-3 and Tritium, finding no evidence for a bound di-neutron with similar binding energy to the deuteron.

Contribution

First experimental comparison of $^3$He and $^3$H reactions at high momentum transfer to search for bound di-neutrons, setting new exclusion limits.

Findings

No signature of a bound di-neutron detected in $^3$H under the studied conditions.

The two-body break-up cross section on $^3$H is less than 0.9% of that on $^3$He at 95% confidence.

Bound di-neutrons with binding energy similar to the deuteron are excluded at the measured kinematics.

Abstract

We report on a search for a bound di-neutron by comparing electron-induced proton-knockout $(e,e'p)$ measurements from Helium-3 ($^3$He) and Tritium ($^3$H). The measurements were performed at Jefferson Lab Hall A with a 4.326 GeV electron beam, and kinematics of large momentum transfer $Q^2 \approx 1.9$ (GeV/$c$)$^2$ and $x_B>1$, to minimize contributions from non quasi-elastic (QE) reaction mechanisms. Analyzing the measured $^3$He missing mass ($M_{miss}$) and missing energy ($E_{miss}$) distributions, we can distinguish the two-body break-up reaction, in which the residual proton-neutron system remains bound as a deuteron. In the $^3$H mirror case, under the exact same kinematic conditions, we do not identify a signature for a bound di-neutron with similar binding energy to that of the deuteron. We calculate exclusion limits as a function of the di-neutron binding energy and find that, for binding equivalent to the deuteron, the two-body break-up cross section on $^3$H is less than 0.9% of that on $^3$He in the measured kinematics at the 95% confidence level.