Probing dark photons in the early universe with big bang nucleosynthesis

TL;DR

This paper models dark photon production and decay during the early universe's weak decoupling and BBN, revealing their impact on light element abundances and radiation content, and suggesting new constraints and future experimental prospects.

Contribution

It provides a comprehensive, self-consistent calculation of dark photon effects on early universe physics, including entropy transfer and element synthesis, extending previous constraints.

Findings

Dark photon decay affects light element abundances.

Entropy injection alters the time-temperature relation during BBN.

Certain dark photon parameters remain unconstrained but detectable in future experiments.

Abstract

We perform calculations of dark photon production and decay in the early universe for ranges of dark photon masses and vacuum coupling with standard model photons. Simultaneously and self-consistently with dark photon production and decay, our calculations include a complete treatment of weak decoupling and big bang nucleosynthesis (BBN) physics. These calculations incorporate all relevant weak, electromagnetic, and strong nuclear reactions, including charge-changing (isospin-changing) lepton capture and decay processes. They reveal a rich interplay of dark photon production, decay, and associated out-of-equilibrium transport of entropy into the decoupling neutrino seas. Most importantly, the self-consistent nature of our simulations allows us to capture the magnitude and phasing of entropy injection and dilution. Entropy injection-induced alteration of the time-temperature-scale factor relation during weak decoupling and BBN leads to changes in the light element abundance yields and the total radiation content (as parametrized by $N_{\rm eff}$). These changes suggest ways to extend previous dark photon BBN constraints. However, our calculations also identify ranges of dark photon mass and couplings not yet constrained, but perhaps accessible and probable, in future Stage-4 cosmic microwave background experiments and future high precision primordial deuterium abundance measurements.