Search for dark matter decay of the free neutron from the UCNA experiment: n $\to \chi +e^{+}{e}^{-}$

Abstract

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($\chi$) 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 $\chi$ 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 $\sim 4\pi$ 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 $\chi$ mass, on the branching fraction for this decay channel. For $\chi$ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at $\gg 5\sigma$ level for $100\text{keV}<E_{e^{+}{e}^{-}}<644\text{keV}$. If the $\chi +e^{+}{e}^{-}$ 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.

arXiv comment5 pages, 5 figures

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