Abstract
Dark matter (DM) could couple to particles in the Standard Model (SM) through a light vector mediator. In the limit of small coupling, this portal could be responsible for producing the observed DM abundance through a mechanism known as freeze-in. Furthermore, the requisite DM-SM couplings provide a concrete benchmark for direct and indirect searches for DM. In this paper, we present updated calculations of the relic abundance for DM produced by freeze-in through a light vector mediator. We identify an additional production channel: the decay of photons that acquire an in-medium plasma mass. These plasmon decays are a dominant channel for DM production for sub-MeV DM masses, and including this channel leads to a significant reduction in the predicted signal strength for DM searches. Accounting for production from both plasmon decays and annihilations of SM fermions, the DM acquires a highly non-thermal phase space distribution which impacts the cosmology at later times; these cosmological effects will be explored in a companion paper.
Highlights
One of the most well-studied mechanisms for setting the observed dark matter (DM) abundance is thermal freezeout, where DM is in equilibrium with the Standard Model (SM) thermal bath at very early times
While freeze-in from electron-positron annihilations via a light vector mediator has been studied in the past [24,45], in this work we thoroughly explore a previously overlooked production mechanism: freeze-in through plasma effects
II by reviewing the arguments for the simplest viable freeze-in models in the keV–MeV mass range: either pure millicharged DM arising from a DM hypercharge or effectively millicharged DM that is coupled to an ultralight dark photon mediator
Summary
One of the most well-studied mechanisms for setting the observed dark matter (DM) abundance is thermal freezeout, where DM is in equilibrium with the Standard Model (SM) thermal bath at very early times. Producing the observed relic abundance requires a particular thermally averaged annihilation cross section in most thermal freeze-out scenarios, hσvi ∼ 10−26 cm3=s This weak-scale cross section provides a target that can be probed by direct and indirect detection experiments. References [43,44] studied the possible direct detection cross sections in models of sub-MeV DM, finding that it would be difficult to observe thermal freeze-out scenarios (even purely within a dark sector) due to a combination of BBN, cosmic microwave background (CMB), fifth force, and stellar emission constraints. II by reviewing the arguments for the simplest viable freeze-in models in the keV–MeV mass range: either pure millicharged DM arising from a DM hypercharge or effectively millicharged DM that is coupled to an ultralight dark photon mediator These two scenarios are almost phenomenologically identical, with the key difference being that DM-DM scattering can be parametrically larger when dark photon interactions are present. We find that existing cosmological constraints restrict mχ ≳ tens of keV for freezein via a light mediator, and it will be possible to probe even higher masses with planned experiments
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