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

We report on a search for a bound di-neutron by comparing electron-induced proton-knockout (e,e^′p) measurements from Helium-3 (³He) and Tritium (³H). The measurements were performed at Jefferson Lab Hall A with a 4.326 GeV electron beam, and kinematics of large momentum transfer (〈Q²〉≈1.9 (GeV/c)²) and x_B>1, to minimize contributions from non quasi-elastic (QE) reaction mechanisms. Analyzing the measured ³He 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 ³H 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 ³H is less than 0.9% of that on ³He in the measured kinematics at the 95% confidence level. This limit implies that the di-neutron content of the tritium spectral function is less than 1.5%. With a dedicated measurement using similar high resolution spectrometers, but lower beam energy and vacuum coupling, significantly better energy missing energy resolution could be achieved, extending the sensitivity of the method to search for a di-neutron with far smaller binding energy.