Gravitational Production of Completely Dark Photons with Nonminimal Couplings to Gravity

TL;DR

This paper investigates how nonminimal gravitational couplings affect the production of dark photons in the early universe, revealing potential enhancements in relic density and discussing stability constraints.

Contribution

It extends previous gravitational particle production models for dark photons to include nonminimal couplings, analyzing their impact on particle generation and stability.

Findings

Nonminimal couplings can enhance dark photon production.

Instability constraints limit the parameter space.

Production can account for dark matter relic density.

Abstract

Dark photons are a theorized massive spin-1 particle which can be produced via various mechanisms, including cosmological gravitational particle production (GPP) in the early universe. In this work, we extend previous results for GPP of dark photons to include nonminimal couplings to gravity. We find that nonminimal couplings can induce a ghost instability or lead to runaway particle production at high momentum and discuss the constraints on the parameter space such that the theory is free of instabilities. Within the instability-free regime we numerically calculate the particle production and find that the inclusion of nonminimal couplings can lead to an enhancement of the particle number. As a result, GPP of nonminimally coupled dark photons can open the parameter space for production of a cosmological relevant relic density (constituting all or part of the dark matter) as compared to the minimally-coupled theory. These results are independent of the choice of inflation model, which we demonstrate by repeating the analysis for a class of rapid-turn multi-field inflation models.