Low-mass dark sector searches with deuteron photodisintegration

Abstract: Recent years have seen much activity in searches for dark-sector messenger particles in the 10-100 MeV mass range, especially in view of a potential new light boson conjectured by the ATOMKI collaboration, X17. Under the assumption that the messenger particle has definite parity and either zero or unit spin, quite stringent bounds already exist on its coupling to electrons and protons. Equally stringent bounds on the neutron coupling do not exist yet, but are nonetheless desirable. We explore how measurements of deuteron photodisintegration with a quasi-free neutron can yield bounds on the neutron coupling, and compute projections for a potential measurement at the lowenergy high-intensity electron scattering experiment MAGIX@MESA. The projected bounds are found to be competitive for an axial-vector or pseudoscalar scenario, but not for a vector or scalar scenario.