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

TL;DR

This study uses UCNA experiment data to search for a hypothesized dark matter decay mode of the neutron involving an electron-positron pair, setting stringent limits on its occurrence and contribution to neutron lifetime discrepancies.

Contribution

It provides the first experimental limits on the neutron decay channel to dark matter involving an electron-positron pair, constraining its role in neutron lifetime anomalies.

Findings

Excluded this decay channel as the dominant dark matter decay at >5σ for specific energy range.

Set upper limits on the branching fraction of this decay mode at <10^{-4} for relevant energies.

Provided constraints on dark matter decay hypotheses related to neutron lifetime discrepancy.

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.