Thermal Production of Massless Dark Photons

TL;DR

This paper calculates the thermal production rate and cosmological impact of massless dark photons within a general effective field theory framework, providing predictions consistent with current bounds and testable by future experiments.

Contribution

It introduces the most general dimension-six effective field theory for massless dark photons interacting with Standard Model particles and computes their thermal production and cosmological effects.

Findings

Predicted dark photon yield consistent with current cosmological bounds.

Calculated contribution to effective number of neutrinos.

Framework applicable to generic models with massless dark photons.

Abstract

A dark photon is predicted by several well-motivated Standard Model extensions and UV completions. Here the most general effective field theory up to dimension-six operators describing the interactions of a massless dark photon with all Standard Model particles is considered. This captures the predictions of a generic model featuring this type of vector boson at sufficiently low energies. In such framework the thermal production rate of dark photons is computed at leading order, including the contributions of all SM particles. The corresponding cosmological yield of the dark photon and its contribution to the effective number of neutrinos are also calculated. These predictions satisfy the current observational bounds and will be tested by future measurements.