Search for neutron dark decay: n → χ + e⁺e⁻

Abstract

In January, 2018, Fornal and Grinstein proposed that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in two different types of experiments. 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). We use the timing information of coincidence events to select candidate dark sector particle decays by applying a timing calibration and selecting events within a physically-forbidden timing region for conventional n → p + e- + ν̅e decays. The summed kinetic energy (Ee+e−) from such events is reconstructed and used to set limits, as a function of the χ mass, on the branching fraction for this decay channel.

New Opportunities for the Study of Baryon Number Violation at Low-Energy Accelerators

“…Precision tests of the SM charged-weak current [22,23,24], as shown in Fig. 2, direct searches [25,26,27], and the existence of neutron stars of 2M ⊙ in mass [28,29,30] all constrain the dark decay scenario, though modes such as n → 3χ ′ [31] evade the last two constraints. Generally, the neutron dark decay rate can grossly exceed that of visible |∆B| = 1 decays by many orders of magnitude [24].…”