Assuming that Dark Matter (DM) is made of fermions in the sub-GeV mass range with interactions dominated by electromagnetic moments of higher order, such as the electric and magnetic dipoles or the anapole moment, we show that direct detection experiments searching for atomic ionisation events in xenon targets can shed light on whether DM is a Dirac or Majorana particle. Specifically, we find that between about 45 (120) and 610 (1700) signal events are required to reject Majorana DM in favour of Dirac DM with a statistical significance corresponding to 3 (5) standard deviations. The exact number of DM signal events corresponding to a given significance depends on the relative size of the anapole, magnetic dipole and electric dipole contributions to the expected rate of DM-induced atomic ionisations under the Dirac hypothesis. Our conclusions are based on Monte Carlo simulations and the likelihood ratio test. While the use of asymptotic formulae for the latter is standard in many applications, here it requires a non-trivial extension to the case where one of the hypotheses lies on the boundary of the parameter space. Our results constitute a solid proof of concept about the possibility of using direct detection experiments to reject the Majorana DM hypothesis when the DM interactions are dominated by higher-order electromagnetic moments.

中文翻译：

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用电子散射实验拒绝暗物质的马约拉纳性质

假设暗物质 (DM) 由亚 GeV 质量范围内的费米子组成，相互作用受高阶电磁矩支配，例如电和磁偶极子或阿纳极矩，我们表明寻找原子电离的直接检测实验氙目标中的事件可以阐明 DM 是狄拉克粒子还是马约拉纳粒子。具体来说，我们发现需要大约 45 (120) 到 610 (1700) 个信号事件来拒绝 Majorana DM 以支持 Dirac DM，其统计显着性对应于 3 (5) 个标准偏差。对应于给定显着性的 DM 信号事件的确切数量取决于在狄拉克假设下对 DM 诱导的原子电离预期速率的贡献的 anapole、磁偶极子和电偶极子的相对大小。我们的结论基于蒙特卡罗模拟和似然比检验。虽然后者的渐近公式的使用在许多应用中是标准的，但在这里它需要对假设之一位于参数空间边界的情况进行非平凡的扩展。当 DM 相互作用由高阶电磁矩主导时，我们的结果构成了关于使用直接检测实验来拒绝 Majorana DM 假设的可能性的可靠概念证明。