New benchmarks for direct detection of freeze-in dark matter in vector portal models

We investigate the freeze-in of MeV-scale fermionic dark matter (DM) that couples to the Standard Model via a new vector mediator to assess the potential that future direct detection experiments have to observe new physics in either the DM or neutrino sectors. We study the minimal kinetic mixing dark photon of a secluded U(1) D as well as gauge bosons of the anomaly-free U(1) L i-L j, with i,j=e,μ,τ, and U(1) B-L gauge extensions, exploring the impact of low reheating temperatures on the DM production rates. For the ultralight dark photon scenario, we show that current experimental constraints from electron recoil data in DAMIC-M and PandaX-4T can be avoided if the DM fermion is only a subcomponent (smaller than 40%) of the total cold DM and that future detectors can be sensitive to a DM fraction below 1% for masses above 1 MeV. For a massive dark photon, there are allowed regions of the parameter space with masses in the range 50 MeV m DM 500 MeV that can be within the reach of direct detection experiments through nuclear recoils if freeze-in occurred at a low reheating temperature. Finally, the case of U(1) L i-L j and U(1) B-L is particularly interesting since the discovery of new physics can come from either the DM or the neutrino sector, which features new interactions. We find that freeze-in at low reheating temperatures can reproduce the observed abundance in large parts of the parameter space up to gauge couplings of g X about10-2 for MeV DM. Most notably, direct detection experiments will be sensitive to considerable parts of this parameter space in nuclear recoils for 50 MeV m DM 500 MeV. Additionally, the enhanced signal from solar neutrino coherent scattering is observable in these scenarios, which can serve as a further handle to identify the underlying particle physics model.