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

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. 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 $100<E_{e^{+}{e}^{-}}<644\text{keV}$. 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.

Figure 1

Schematic diagram of the UCNA spectrometer.

Figure 2

Background-subtracted and calibrated timing distributions of events that trigger both East and West detectors, separated according to which detector triggered first. Overlaid is a geant4 simulation of the time-of-flight for all backscattering $\beta$-decay events that trigger both East and West detectors. The simulation includes a timing resolution of 3 ns which best matches the data. The time cutoff near 120 ns for the data is an instrumental effect. Dashed lines indicate the chosen analysis time-window (see text).

Figure 3

Energy spectrum of (a) background and (b) foreground runs, for three separate time-windows. Clear structure of a neutron $\beta$-decay backscattering peak at 300 keV is visible for time-windows $>12$ ns in the foreground runs. Dashed lines at 0 and 800 keV indicate the energy region of interest used for the present analysis.

Figure 4

(a) Background-subtracted $e^{+}{e}^{-}$ pair kinetic energy spectra for events in the analysis time-window. For comparison, simulated positive dark matter decay signals at 322 and 644 keV are overlaid. (b) Total $e^{+}{e}^{-}$ pair acceptance (see text) as a function of summed kinetic energy.

Figure 5

Confidence limits on the branching ratio of the neutron dark decay channel, as a function of the kinetic energy of the produced $e^{+}{e}^{-}$ pair. This is directly related to the proposed $\chi$ mass by $m_{\chi }=m_n-2m_e-E_{e^{+}{e}^{-}}$, which has a range of $937.900<m_{\chi }<938.543\text{MeV}$. A branching ratio of ${10}^{-2}$, which would be required to explain the neutron lifetime anomaly if $n\to \chi +e^{+}{e}^{-}$ were the only allowed final state, is shown by the dashed line.