Abstract
We consider a dark sector model containing stable fermions charged under an unbroken $U(1)$ gauge interaction, with a massless dark photon as force carrier, and interacting with ordinary matter via scalar messengers. We study its early Universe evolution by solving a set of coupled Boltzmann equations that track the number density of the different species, as well as entropy and energy exchanges between the dark and visible sectors. Phenomenologically viable realizations include: i) a heavy (order 1 TeV or more) lepton-like dark fermion playing the role of the dark matter candidate, with various production mechanisms active depending on the strength of the dark-visible sector portal; ii) light (few GeV to few tens of GeV) quark-like dark fermions, stable but with suppressed relic densities; iii) an extra radiation component in Universe due to dark photons, with temperature constrained by cosmic microwave background data, and in turn preventing dark fermions to be lighter than about 1 GeV. Extra constraints on our scenario stem from dark matter direct detection searches: the elastic scattering on nuclei is driven by dipole or charge radius interactions mediated by either Standard Model or dark photons, providing long-range effects which, however, are not always dominant, as usually assumed in this context. Projected sensitivities for next-generation detectors cover a significant portion of the viable parameter space and are competitive with respect to the model-dependent constraints derived from the magnetic dipole moments of leptons and cooling of stellar systems.
Highlights
The existence of a multicomponent dark sector has been extensively discussed in the literature
Viable realizations include: (i) a heavy leptonlike dark fermion playing the role of the dark matter candidate, with various production mechanisms active depending on the strength of the dark-visible sector portal; (ii) light quarklike dark fermions, stable but with suppressed relic densities; (iii) an extra radiation component in Universe due to dark photons, with temperature constrained by cosmic microwave background data, and in turn preventing dark fermions to be lighter than about 1 GeV
Taking into account the emerging picture, we will consider a scenario with: (i) scalar messengers as the heaviest states in the dark-sector, (ii) a leptonlike dark fermion χ playing the role of dark matter, lighter than scalar messengers but at a comparable mass scale, and (iii) two dark fermions QU and QD coupled to the quarks, which are much lighter than χ and representative of the light dark sector
Summary
The existence of a multicomponent dark sector has been extensively discussed in the literature (see [1,2] for two recent reviews). The others are much lighter, have suppressed relic abundances, but concur in determining the ratio between dark and visible photon temperatures at late times; such ratio is constrained by cosmic microwave background (CMB) data, given that dark photons contribute as an extra radiation component to the Universe’s dynamics In this respect, the role of portal interactions between dark and visible sectors is important: we consider scalar messengers mediating Yukawa-like interactions. While long-range interactions are present and boost the recoil spectrum at low recoil energies, the correlated contact terms are contributing to the cross section and may be dominant (contrary to standard lore that contact interactions can be neglected in the presence of long-range effects)
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