Neutrino-Dark Sector Equilibration and Primordial Element Abundances

TL;DR

This paper investigates how neutrino interactions with a dark sector after decoupling affect primordial element abundances, providing new limits on dark sector properties using Big Bang Nucleosynthesis calculations.

Contribution

It introduces a detailed calculation of light element abundances considering neutrino-dark sector equilibration, incorporating these effects into the PRIMAT code, and deriving new constraints on dark sector parameters.

Findings

Limits on neutrino-dark sector interactions down to 100 keV.

Dark sector $N_{\rm{eff}}$ step can impose $ ext{omega}_b$-dependent constraints.

Future CMB data can further tighten these limits.

Abstract

After neutrinos decouple from the photon bath, they can populate a thermal dark sector. If this occurs at a temperature above ~100 keV, this can have measurable impacts on light element abundances. We calculate light element abundances in this scenario, studying the impact from rapid cooling of the Standard Model neutrinos, and from an increase in the number of relativistic degrees of freedom $N_{\rm{eff}}$, which can occur in the presence of a mass threshold. We incorporate these changes in the publicly available BBN code PRIMAT, using the reaction networks from PRIMAT and from the BBN code PArthENoPE, to calculate Y$_{\rm{P}}$ and D/H. We provide limits from the two different reaction networks as well as with expanded errors to include both results. If electron neutrinos significantly participate in the cooling, we find limits down to temperatures as low as 100 keV. If electron neutrinos are weakly participating (for instance if only the mass eigenstate $\nu_3$ equilibrates), cooling places no limits. However, if the dark sector undergoes a "step" in $N_{\rm{eff}}$, there can be additional, $\omega_b$-dependent constraints. These limits can vary from strong (for low values of $\omega_b$) to a mild preference for new physics (for high values of $\omega_b$). Future analyses including upcoming CMB data should improve these limits.