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

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arxiv 1803.10890 v1 pith:EB3XAPAV submitted 2018-03-29 nucl-ex physics.ins-det

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

X. Sun , E. Adamek , B. Allgeier , M. Blatnik , T. J. Bowles , L. J. Broussard , M. A.-P. Brown , R. Carr
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S. Clayton C. Cude-Woods S. Currie E. B. Dees X. Ding B. W. Filippone A. Garc\'ia P. Geltenbort S. Hasan K. P. Hickerson J. Hoagland R. Hong G. E. Hogan A. T. Holley T. M. Ito A. Knecht C.-Y. Liu J. Liu M. Makela R. Mammei J. W. Martin D. Melconian M. P. Mendenhall S. D. Moore C. L. Morris S. Nepal N. Nouri R. W. Pattie Jr. A. P\'erez Galv\'an D. G. Phillips II R. Picker M. L. Pitt B. Plaster J. C. Ramsey R. Rios D. J. Salvat A. Saunders W. Sondheim S. Sjue S. Slutsky C. Swank G. Swift E. Tatar R. B. Vogelaar B. VornDick Z. Wang W. Wei J. Wexler T. Womack C. Wrede A. R. Young B. A. Zeck
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classification nucl-ex physics.ins-det
keywords decayneutronchanneldarklimitsmatterbranchingdiscrepancy
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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.

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