Relic neutrino decoupling with flavour oscillations revisited

TL;DR

This paper revisits the decoupling process of neutrinos in the early universe, incorporating three-flavour oscillations and detailed collision integrals, confirming the standard N_eff value and exploring effects of non-standard interactions.

Contribution

It introduces a comprehensive calculation of neutrino decoupling with proper collision integrals for all density matrix elements, refining previous models and including non-standard interactions.

Findings

N_eff = 3.045, consistent with previous results

Non-standard interactions can slightly alter N_eff between 3.040 and 3.059

Decoupling process remains robust against neutrino mass ordering

Abstract

We study the decoupling process of neutrinos in the early universe in the presence of three-flavour oscillations. The evolution of the neutrino spectra is found by solving the corresponding momentum-dependent kinetic equations for the neutrino density matrix, including for the first time the proper collision integrals for both diagonal and off-diagonal elements. This improved calculation modifies the evolution of the off-diagonal elements of the neutrino density matrix and changes the deviation from equilibrium of the frozen neutrino spectra. However, it does not vary the contribution of neutrinos to the cosmological energy density in the form of radiation, usually expressed in terms of the effective number of neutrinos, N_eff. We find a value of N_eff=3.045, in agreement with previous theoretical calculations and consistent with the latest analysis of Planck data. This result does not depend on the ordering of neutrino masses. We also consider the effect of non-standard neutrino-electron interactions (NSI), predicted in many theoretical models where neutrinos acquire mass. For two sets of NSI parameters allowed by present data, we find that N_eff can be reduced down to 3.040 or enhanced up to 3.059.