The EDGES Collaboration has recently reported the detection of a stronger-than-expected absorption feature in the global 21-cm spectrum, centered at a frequency corresponding to a redshift of z ~ 17. This observation has been interpreted as evidence that the gas was cooled during this era as a result of scattering with dark matter. In this study, we explore this possibility, applying constraints from the cosmic microwave background, light element abundances, Supernova 1987A, and a variety of laboratory experiments. After taking these constraints into account, we find that the vast majority of the parameter space capable of generating the observed 21-cm signal is ruled out. The only range of models that remains viable is that in which a small fraction, ~ 0.3-2%, of the dark matter consists of particles with a mass of ~ 10-80 MeV and which couple to the photon through a small electric charge, epsilon ~ 10^{-6}-10^{-4}. Furthermore, in order to avoid being overproduced in the early universe, such models must be supplemented with an additional depletion mechanism, such as annihilations through a L_μ-L_τ gauge boson or annihilations to a pair of rapidly decaying hidden sector scalars.
Constraining minimal anomaly free $\mathrm{U}(1)$ extensions of the Standard Model
本文考虑一类具有三代右手中微子和一个复标量的标准模型的最小反常自由$\mathrm{U}(1)$扩展。使用电弱精度约束，新的13 TeV LHC数据，并考虑到理论上的限制，如微扰，我们表明，它是可能的，以约束广泛的一类模型。通过用一个参数$κ$对这些模型进行分类，我们可以在新的$\mathrm{U}(1)$规范耦合$g_z$上设置一个与模型无关的上界。我们发现，新的双轻子数据提出了强大的边界上的参数，特别是在质量区域$M_{Z'}\lesssim 3~ \mathrm{TeV}$。
我们构造了一个无异常扩展的左右对称模型，其中的最大味群是规范的，并通过添加新的类矢量费米子态来保证异常消除。我们解决的问题，最低允许的味道对称尺度与数据一致。由于Grinstein等人最近指出的机制，树水平的味道改变中性电流原来起着非常弱的约束作用。在我们的模型中，对于电弱精密观测量也是如此。主要的约束原来来自WR介导的风味改变中性电流盒图，主要是K-Kbar混合。在离散宇称对称的情况下，目前在TeV尺度，这一约束意味着较低的边界上的矢量费米子和味玻色子的质量分别为5和10 TeV。然而，当在TeV尺度上只恢复SU（2）_R x U（1）_{B-L}而不恢复宇称时，这些限制被削弱了。例如，假设SU（2）规范耦合的gR/gL比约为0.7，则上述限制对于矢量费米子和味玻色子都可以降低一半。我们的模型提供了一个框架，容纳中微子的质量，在宇称对称的情况下，提供了一个解决方案的强CP问题。由于大爆炸核合成的限制，轻子味标度上的束缚更强一些。然而，我们认为，这些限制的适用性取决于在工作中产生中微子质量的机制。
