异常EDGES 21 cm信号暗物质散射的更严格限制
Tighter Limits on Dark Matter Explanations of the Anomalous EDGES 21cm Signal
摘要：
我们研究了暗物质和重子之间的库仑型相互作用解释了最近由EDGES实验测量到的在红移z ~ 17处的21 cm亮温极小值的假说。特别是，我们重新评估的情况下，总DM的一小部分是millicharged的有效性，侧重于普朗克2015年宇宙微波背景（CMB）数据的新衍生的约束。重要的是，如果相互作用DM的分数大于（或相当于）重子能量密度的分数不确定性，CMB功率谱对DM-重子散射敏感。同时，有一个依赖于质量的下限的分数，所需的相互作用，以充分冷却重子是如此之强，它驱动的相互作用DM温度的重子温度之前，他们从CMB去耦。如果这种情况早在再结合时就发生，则冷却饱和。我们精确地确定了毫荷电DM的可行参数空间，并发现只有一小部分（m_chi/MeV）整个DM含量的0.0115% <~ f <~ 0.4%，并且仅对于0.5 MeV - 35 MeV之间的DM粒子质量，可以在边缘解释异常所需的水平充电，而不违反SLAC，CMB，大爆炸核合成（BBN），或恒星和SN 1987 A冷却。然而，在现实中，我们证明，至少需要适度的微调，以符合测量的吸收曲线，并克服各种天体物理热源。最后，我们指出，在重子复合过程中与重子紧密耦合的~0.4%的毫荷电DM分量可以解决目前BBN和CMB测定的重子能量密度之间的2 σ张力.未来的CMB-S4测量将能够直接探测这种情况。
英文摘要：
We investigate the hypothesis that Coulomb-type interactions between dark matter (DM) and baryons explain the anomalously low 21cm brightness-temperature minimum at redshift z ~ 17 that was recently measured by the EDGES experiment. In particular, we reassess the validity of the scenario where a small fraction of the total DM is millicharged, focusing on newly derived constraints from Planck 2015 cosmic microwave background (CMB) data. Crucially, the CMB power spectrum is sensitive to DM-baryon scattering if the fraction of interacting DM is larger than (or comparable to) the fractional uncertainty in the baryon energy density. Meanwhile, there is a mass-dependent lower limit on the fraction for which the required interaction to cool the baryons sufficiently is so strong that it drives the interacting-DM temperature to the baryon temperature prior to their decoupling from the CMB. If this occurs as early as recombination, the cooling saturates. We precisely determine the viable parameter space for millicharged DM, and find that only a fraction (m_chi/MeV) 0.0115% <~ f <~ 0.4% of the entire DM content, and only for DM-particle masses between 0.5 MeV - 35 MeV, can be charged at the level needed to marginally explain the anomaly, without violating limits from SLAC, CMB, Big-Bang nucleosynthesis (BBN), or stellar and SN1987A cooling. In reality, though, we demonstrate that at least moderate fine tuning is required to both agree with the measured absorption profile and overcome various astrophysical sources of heating. Finally, we point out that a ~0.4% millicharged DM component which is tightly coupled to the baryons at recombination may resolve the current 2-sigma tension between the BBN and CMB determinations of the baryon energy density. Future CMB-S4 measurements will be able to probe this scenario directly.
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