Thermalisation of light sterile neutrinos in the early universe

TL;DR

This paper investigates the thermalisation process of light sterile neutrinos in the early universe by solving quantum kinetic equations, revealing that assumptions of full thermalisation are only valid for small initial lepton asymmetries.

Contribution

It provides a detailed analysis of sterile neutrino thermalisation using full quantum kinetic equations, challenging simplified assumptions in previous cosmological constraints.

Findings

Full thermalisation occurs only with small initial lepton asymmetry.

Partial or null thermalisation happens with large initial lepton asymmetry.

The study refines constraints on sterile neutrinos based on thermalisation dynamics.

Abstract

Recent cosmological data favour additional relativistic degrees of freedom beyond the three active neutrinos and photons, often referred to as 'dark' radiation. Light sterile neutrinos is one of the prime candidates for such additional radiation. However, constraints on sterile neutrinos based on the current cosmological data have been derived using simplified assumptions about thermalisation of the sterile neutrino at the Big Bang Nucleosynthesis (BBN) epoch. These assumptions are not necessarily justified and here we solve the full quantum kinetic equations in the (1 active + 1 sterile) scenario and derive the number of thermalised species just before BBN begins (T~1MeV) for null (L=0) and large (L=0.01) initial lepton asymmetry and for a range of possible mass-mixing parameters. We find that the full thermalisation assumption during the BBN epoch is justified for initial small lepton asymmetry only. Partial or null thermalisation occurs when the initial lepton asymmetry is large.