Abstract
We analyse how dark matter (DM) can be produced in the early universe, working in the framework of a hidden sector charged under a U(1)' gauge symmetry and interacting with the Standard Model through kinetic mixing. Depending on the masses of the dark matter particle and of the dark photon, as well as on the hidden U(1)' gauge coupling and the kinetic mixing parameter, we classify all the distinct regimes along which the observed dark matter relic density can be accounted for. We find that 9 regimes are potentially operative to produce the DM particles and this along 5 distinct dynamical mechanisms. Among these, 4 regimes are new and correspond to regimes in which the DM particles are produced by on-shell dark photons. One of them proceeds along a new dynamical mechanism, which we dub sequential freeze-in. We argue that such regimes and the associated dynamical mechanisms are characteristic of DM models for which, on top of the Standard Model and the dark sector, there are other massive, but relatively light particles -- akin to the dark photon -- that interact both with the SM and the DM sectors.
Highlights
As the nature of dark matter (DM) remains a mystery, it is possible that DM is a particle that belongs to a whole new hidden sector
We find that nine regimes are potentially operative to produce the DM particles, and these operate along five distinct dynamical mechanisms
We argue that such regimes and the associated dynamical mechanisms are characteristic of DM models for which, on top of the Standard Model and the dark sector, there are other massive, but relatively light particles—akin to the dark photon—that interact with both the SM and the DM sectors
Summary
As the nature of dark matter (DM) remains a mystery, it is possible that DM is a particle that belongs to a whole new hidden sector. It has been shown in that work that the observed relic density could be reached along four distinct dynamical mechanisms depending on the values of the parameters of the model These dynamical mechanisms are freeze-in (regime Ia in the sequel) [2,15,16,17], reannihilation (IIIa) [2,16], secluded freeze-out (IVa), and the standard textbook thermal freeze-out mechanism (Va and Vb). The three sectors (depicted as blobs) consist of the SM, the dark matter particle χ, and the massive dark photon γ0, while the connections between the sectors (depicted as lines) are parametrized by the mixing ε (or more precisely εeff, which will be defined below), the hidden fine structure constant α0, and the “millicharge” κ Considering such a structure, we will point out the existence of four new regimes. The case in which the mass of the dark photon arises through the Brout-EnglertHiggs mechanism is considered in Appendix B
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