BBN Photodisintegration Constraints on Gravitationally Produced Vector Bosons

TL;DR

This paper investigates how gravitationally produced light vector bosons, especially dark photons, can be constrained by Big Bang Nucleosynthesis observations, revealing stronger bounds than thermal production scenarios.

Contribution

It introduces the impact of gravitational production of feebly interacting vector bosons on BBN constraints, extending previous thermal-only analyses.

Findings

Gravitational production dominates over thermal production for Hubble scales above 10^8 GeV.

Stronger BBN constraints are derived on the mass and lifetime of vector bosons due to photodisintegration effects.

Constraints on dark photon parameters are established from light element abundance observations.

Abstract

Gravitational production of massive particles due to cosmic expansion can be significant during the inflationary and reheating period of the Universe. If the particle also has non-gravitational interactions that do not significantly affect its production, numerous observational probes open up, including cosmological probes. In this work, we focus on the gravitational production of light vector bosons that couple feebly to the Standard Model (SM) particles. Due to the very feeble coupling, the light vector bosons never reach thermal equilibrium, and if the Hubble scale at the end of inflation is above $10^8$ GeV, the gravitational production can overwhelm the thermal production via the freeze-in mechanism by many orders of magnitude. As a result, much stronger constraints from the Big Bang Nucleosynthesis (BBN) can be placed on the lifetime and mass of the vector bosons compared to the scenario where only thermal production is considered. As an example, we study the sub-GeV scale dark photons, which couple to the SM only through kinetic mixing, and derive constraints on the mass and kinetic mixing parameter of the dark photon from the photodisintegration effects on the light element abundances relevant at the end of the BBN when the cosmic age was around $10^4$ s.