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

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. ; Mendenhall, M. P. ; Moore, S. D. ; Morris, C. L. ; Nepal, S. ; Nouri, N. ; Pattie, R. W. ; Galván, A. Pérez ; Phillips, D. G. ; Picker, R. ; Pitt, M. L. ; Plaster, B. ; Ramsey, J. C. ; Rios, R. ; Salvat, D. J. ; Saunders, A. ; Sondheim, W. ; Sjue, S. ; Slutsky, S. ; Swank, C. ; Swift, G. ; Tatar, E. ; Vogelaar, R. B. ; VornDick, B. ; Wang, Z. ; Wei, W. ; Wexler, J. ; Womack, T. ; Wrede, C. ; Young, A. R. ; Zeck, B. A.

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.

Publication Date:
May 21 2018
Date Submitted:
Jun 28 2019
Physical Review C
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 Record created 2019-06-28, last modified 2019-08-06

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