Making dark matter out of light: Freeze-in from plasma effects

Dark matter (DM) could couple to Standard Model (SM) particles through a light mediator and this could be responsible for the observed DM abundances through $f\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}z\phantom{\rule{0}{0ex}}e\ensuremath{-}i\phantom{\rule{0}{0ex}}n$. Here, the authors identify an additional production channel through the decay of photons that have acquired an in-medium plasma mass. They contribute to the relic abundance, lowering by an order of magnitude the target cross-section for the lowest dark matter masses.