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

Abstract

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 ∼4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). 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 ≫ 5σlevel for 100 keV < 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.

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