The thermal freeze-out mechanism in its classical form is tightly connected to physics beyond the Standard Model around the electroweak scale, which has been the target of enormous experimental efforts. In this work we study a dark matter model in which freeze-out is triggered by a strong first-order phase transition in a dark sector, and show that this phase transition must also happen close to the electroweak scale, i.e. in the temperature range relevant for gravitational wave searches with the LISA mission. Specifically, we consider the spontaneous breaking of a U(1)′ gauge symmetry through the vacuum expectation value of a scalar field, which generates the mass of a fermionic dark matter candidate that subsequently annihilates into dark Higgs and gauge bosons. In this set-up the peak frequency of the gravitational wave background is tightly correlated with the dark matter relic abundance, and imposing the observed value for the latter implies that the former must lie in the milli-Hertz range. A peculiar feature of our set-up is that the dark sector is not necessarily in thermal equilibrium with the Standard Model during the phase transition, and hence the temperatures of the two sectors evolve independently. Nevertheless, the requirement that the universe does not enter an extended period of matter domination after the phase transition, which would strongly dilute any gravitational wave signal, places a lower bound on the portal coupling that governs the entropy transfer between the two sectors. As a result, the predictions for the peak frequency of gravitational waves in the LISA band are robust, while the amplitude can change depending on the initial dark sector temperature.

中文翻译：

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用 LISA 寻找 WIMP：关联暗物质和引力波信号


经典形式的热冻结机制与弱电尺度标准模型之外的物理学紧密相关，这一直是大量实验工作的目标。在这项工作中，我们研究了一个暗物质模型，其中冻结是由暗区中强烈的一级相变触发的，并表明这种相变也必须发生在接近电弱尺度，即在相关的温度范围内。与 LISA 任务一起进行引力波搜索。具体来说，我们考虑通过标量场的真空期望值自发破缺 U(1)' 规范对称性，这会产生费米子暗物质候选者的质量，随后湮灭成暗希格斯粒子和规范玻色子。在这种设置中，引力波背景的峰值频率与暗物质遗迹丰度紧密相关，并且将后者的观测值强加意味着前者必须位于毫赫兹范围内。我们的设置的一个独特之处是，暗区在相变期间不一定与标准模型处于热平衡，因此两个区的温度独立变化。然而，宇宙在相变后不会进入长时间的物质统治的要求（这会强烈稀释任何引力波信号）为控制两个扇区之间熵传递的门户耦合设置了下限。因此，对 LISA 波段引力波峰值频率的预测是稳健的，而幅度可能会根据初始暗区温度而变化。