  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.
