Computing singlet scalar freeze-out with plasmon and plasmino states

The final-state particles from cosmological dark matter co-annihilation are expected to equilibrate. As dictated by Hard Thermal Loop resummation, the spectrum of equilibrated quasiparticles is richer than in vacuum, with a massless gauge field possessing three independent polarization states (“plasmons”), and a massless fermion developing a novel branch (“plasmino”). Furthermore, once the Higgs phenomenon sets in, vacuum and thermal mass corrections interfere. We collect together the corresponding poles and residues for the Standard Model around its crossover temperature. Choosing its singlet scalar extension for illustration, we subsequently demonstrate, both numerically and via power counting, and in accordance with general theoretical expectations, how in the freeze-out of TeV-scale dark matter, these effects remain well hidden in the inclusive annihilation cross section. In particular, the dominant (longitudinal) gauge channel is shown to be practically temperature-independent. Cosmological constraints on TeV-scale singlet scalars are reconfirmed.