Light dark matter (DM) with mass around the GeV scale faces weaker bounds from direct detection experiments. If DM couples strongly to a light mediator, it is possible to have observable direct detection rate. However, this also leads to a thermally under-abundant DM relic due to efficient annihilation into light mediators. We propose a novel scenario where a first-order phase transition (FOPT) occurring at MeV scale can restore GeV scale DM relic by changing the mediator mass sharply at the nucleation temperature. The MeV scale FOPT predicts stochastic gravitational waves with nano-Hz frequencies within reach of pulsar timing array (PTA) based experiments like NANOGrav. In addition to enhancing direct detection rate, the light mediator can also give rise to the required DM self-interactions necessary to solve the small scale structure issues of cold dark matter. The existence of light scalar mediator and its mixing with the Higgs keep the scenario verifiable at different particle physics experiments.

中文翻译：

---

轻热暗物质的新认识 增强探测前景


质量在 GeV 尺度附近的轻暗物质 （DM） 在直接探测实验中面临较弱的界限。如果 DM 与光介质强烈耦合，则有可能具有可观察到的直接检出率。然而，由于有效地湮灭到光介质中，这也导致了热丰度不足的 DM 遗迹。我们提出了一种新的情景，即在 MeV 尺度上发生的一阶相变 （FOPT） 可以通过在成核温度下急剧改变介质质量来恢复 GeV 尺度的 DM 遗迹。MeV 标度 FOPT 预测了基于脉冲星计时阵列 （PTA） 的实验（如 NANOGrav）所能达到的具有纳米 Hz 频率的随机引力波。除了提高直接探测率外，光介质还可以产生解决冷暗物质的小尺度结构问题所需的 DM 自相互作用。光标量介质的存在及其与 Higgs 的混合使该场景在不同的粒子物理实验中是可验证的。