Abstract
We consider, in general terms, the possible parameter space of thermal dark matter candidates. We assume that the dark matter particle is fundamental and was in thermal equilibrium in a hidden sector with a temperature $T'$, which may differ from that of the Standard Model temperature, $T$. The candidates lie in a region in the $T'/T$ vs. $m_{\rm dm}$ plane, which is bounded by both model-independent theoretical considerations and observational constraints. The former consists of limits from dark matter candidates that decoupled when relativistic (the relativistic floor) and from those that decoupled when non-relativistic with the largest annihilation cross section allowed by unitarity (the unitarity wall), while the latter concerns big bang nucleosynthesis ($N_{\rm eff}$ ceiling) and free streaming. We present three simplified dark matter scenarios, demonstrating concretely how each fits into the domain.
Highlights
Of all the scenarios for dark matter (DM), one of the most appealing possibilities is that it is made of elementary particles that were in thermal equilibrium in the early universe
Thermal relic dark matter candidates may come in many forms, and there is a vast literature concerning this class of models
For scenarios in which the candidate was in equilibrium with the SM bath, the allowed range of dark matter masses is well known, as reviewed in Sec
Summary
Of all the scenarios for dark matter (DM), one of the most appealing possibilities is that it is made of elementary particles that were in thermal equilibrium in the early universe. In the absence of an asymmetry, the thermal abundance would be far below observations Translating this to the maximum possible cross section set by unitarity considerations, the mass of the asymmetric DM must be below the standard GK bound (4), mdm;asym < mdm;GK. Composite DM candidates [4] with radius much larger than their Compton wavelength, rdm ≫ 1=mdm, can have a geometric cross section, σ ∼ πr2dm (see, for instance, [20] for a recent concrete example) With this possible exception kept in mind, we assume that generic thermal particle DM candidates all fall in the domain (5). We use ξ ≡ T0=T for the ratio of temperatures of the hidden and visible sectors
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