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. The summed kinetic energy (Ee+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 ≫ 5σ level for 100 < Ee+e− < 644keV. 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 Ee+e− range at > 90% confidence level.
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This work is supported in part by the US Department of Energy, Office of Nuclear Physics (DE-FG02-08ER41557, DE-SC0014622, DE-FG02-97ER41042) and the National Science Foundation (NSF-1002814, NSF-1005233, NSF-1102511, NSF-1205977, NSF-1306997, NSF-1307426, NSF-1506459, and NSF-1615153). We gratefully acknowledge the support of the LDRD program (20110043DR) and the AOT division of the Los Alamos National Laboratory.
Sun, X.; Adamek, E.; Allgeier, B.; Blatnik, M.; Bowles, T. J.; Broussard, L. J.; Brown, M. A. -P.; Carr, R.; Clayton, S.; Cude-Woods, C.; Currie, S.; Dees, E. B.; Ding, X.; Filippone, B. W.; García, A.; Geltenbort, P.; Hasan, S.; Hickerson, K. P.; Hoagland, J.; Hong, R.; Hogan, G. E.; Holley, A. T.; Ito, T. M.; Knecht, A.; Liu, C. -Y.; Liu, J.; Makela, M.; Mammei, R.; Martin, J. W.; Melconian, D.; Nepal, Subash C.; and Plaster, Bradley R., "Search for Dark Matter Decay of the Free Neutron from the UCNA Experiment: n → χ + e+e−" (2018). Physics and Astronomy Faculty Publications. 569.