Big Bang Nucleosynthesis with Independent Neutrino Distribution Functions

TL;DR

This paper introduces a flexible Big Bang Nucleosynthesis model that incorporates arbitrary, time-dependent neutrino distribution functions, enabling analysis of various scenarios involving neutrino spectral distortions and their impact on primordial element abundances.

Contribution

It develops a self-consistent BBN calculation framework that allows for arbitrary neutrino spectra, including scenarios with sterile neutrinos and lepton asymmetries, extending previous models.

Findings

Constraints on lepton numbers from primordial abundances

Ability to model neutrino spectral distortions in BBN

Open-source code for community use

Abstract

We have performed new Big Bang Nucleosynthesis calculations which employ arbitrarily-specified, time-dependent neutrino and antineutrino distribution functions for each of up to four neutrino flavors. We self-consistently couple these distributions to the thermodynamics, the expansion rate and scale factor-time/temperature relationship, as well as to all relevant weak, electromagnetic, and strong nuclear reaction processes in the early universe. With this approach, we can treat any scenario in which neutrino or antineutrino spectral distortion might arise. These scenarios might include, for example, decaying particles, active-sterile neutrino oscillations, and active-active neutrino oscillations in the presence of significant lepton numbers. Our calculations allow lepton numbers and sterile neutrinos to be constrained with observationally-determined primordial helium and deuterium abundances. We have modified a standard BBN code to perform these calculations and have made it available to the community.