标题： 寻找来自UCNA实验的自由中子暗物质衰变：n $\rightarrow χ+ e^+e^-$ 显示英文标题

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

摘要： 最近有提议认为，一种之前未被观察到的中子衰变分支到暗物质粒子（$\chi$）可能解释了使用两种不同测量技术的实验中观测到的中子寿命差异。 讨论的可能末态之一包括一个$\chi$以及一个$e^{+}e^{-}$对。 我们使用UCNA（超冷中子不对称性）实验的数据来设定此衰变通道的限制。 利用一对探测器检测到具有$\sim 4\pi$接受度的同步电子样事件，这些探测器观察存储的超冷中子（UCN）体积。 此类事件的总动能（$E_{e^{+}e^{-}}$）被用来根据$\chi$质量设定此衰变通道的分支比限制。 对于与解决中子寿命差异一致的$\chi$质量，我们排除了这是主导的暗物质衰变通道，在$\gg~5\sigma$水平上对于$100~\text{keV} < E_{e^{+}e^{-}} < 644~\text{keV}$。 如果$\chi+e^{+}e^{-}$最终态不是唯一的，我们设定了上述$E_{e^{+}e^{-}}$范围内其分支比为$< 10^{-4}$在$> 90\%$置信水平上的限制。 显示英文摘要

摘要： 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.