Search for dark matter decay of the free neutron from the UCNA experiment: n →χ +e+e-

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle (χ ) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single χ along with an e⁺e^- pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼4 π acceptance using a pair of detectors that observe a volume of stored ultracold neutrons. The summed kinetic energy (E_e⁺e^-) from such events is used to set limits, as a function of the χ mass, on the branching fraction for this decay channel. For χ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at ≫5 σ level for 100 <E_e⁺e^-<644 keV . If the χ +e⁺e^- final state is not the only one, we set limits on its branching fraction of <10^-4 for the above E_e⁺e^- range at >90 % confidence level.