HEP Theory Journal Club

Photon self-energy at all temperatures and densities in all of phase space

Hugo Schérer

McGill University

Dark matter could reside in a dark sector that contains a dark photon, a spin-1 gauge boson that would couple weakly to the Standard Model photon. This coupling results in interesting phenomenology in a variety of physical contexts. In many environments, such as the early universe or stars, finite temperature effects modify the properties of particles, including their dispersion relations. In particular, the Standard Model photon acquires an effective mass, or self-energy. This impacts its coupling to the dark photon in qualitatively new ways. In an isotropic background comprised of free charges, the transverse and longitudinal modes of the photon acquire large corrections to their dispersion relations, described by the in-medium photon self-energy. Previous work has developed simple approximations that describe the propagation of on-shell photons in plasmas of varying temperatures and densities. However, off-shell excitations can also receive large medium-induced corrections, and the on-shell approximations have often been used in an effort to capture these effects. In this work we show that the off-shell self-energy can be qualitatively very different than the on-shell case. We develop analytic approximations that are accurate everywhere in phase space, especially in classical and degenerate plasmas.