Abstract
Freeze-in dark matter (DM) mediated by a light (≪ keV) weakly-coupled dark-photon is an important benchmark for the emerging low-mass direct detection program. Since this is one of the only predictive, detectable freeze-in models, we investigate how robustly such testability extends to other scenarios. For concreteness, we perform a detailed study of models in which DM couples to a light scalar mediator and acquires a freeze-in abundance through Higgs-mediator mixing. Unlike dark-photons, whose thermal properties weaken stellar cooling bounds, the scalar coupling to Standard Model (SM) particles is subject to strong astrophysical constraints, which severely limit the fraction of DM that can be produced via freeze-in. While it seems naively possible to compensate for this reduction by increasing the mediator-DM coupling, sufficiently large values eventually thermalize the dark sector with itself and yield efficient DM annihilation to mediators, which depletes the freeze-in population; only a small window of DM candidate masses near the ∼ GeV scale can accommodate the total observed abundance. Since many qualitatively similar issues arise for other light mediators, we find it generically difficult to realize a viable freeze-in scenario in which production arises only from renormalizable interactions with SM particles. We also comment on several model variations that may evade these conclusions.
Highlights
Traditional direct detection experiments, designed for WIMP scattering off heavy nuclei with ∼ keV recoil energy thresholds, many of these techniques exploit observable transitions between small internal energy levels to probe much lighter dark matter (DM), which typically deposits ∼ meV − few eV per scatter
Unlike dark-photons, whose thermal properties weaken stellar cooling bounds, the scalar coupling to Standard Model (SM) particles is subject to strong astrophysical constraints, which severely limit the fraction of DM that can be produced via freeze-in
The only predictive, UV complete example in which this has been demonstrated (DM coupled to an ultra-light dark-photon) has subtly special properties that may not generalize to other mediators
Summary
Where A and κ are the renormalizable portal couplings. After electroweak symmetry breaking (EWSB) the neutral component of the doublet h mixes with φ and, in the small mixing limit, the system is diagonalized with the shift h → h + sin θ φ, so φ acquires massproportional couplings to SM fermions. Where f is a SM fermion of mass mf and v 246 GeV is the SM Higgs vacuum expectation value This is only one of several possible scalar mediated scenarios, as we will see, it captures much of the essential physics and many of the issues encountered here apply to a much broader class of variations on this simple setup (see section 4 for a discussion and [29, 30] for a complementary studies of thermal freeze-out with the same field content). It may be interesting to explore whether DM with a scalar mediator in the mass range considered here mφ g αme could preserve sombe of these features (as in [29] for heavier scalars), but this question is beyond the scope of the present work
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