Non-unitary three-neutrino mixing in the early Universe

TL;DR

This paper investigates how non-unitary neutrino mixing, due to potential new physics beyond the Standard Model, affects early Universe neutrino decoupling and the effective number of neutrinos, with implications for cosmological observations.

Contribution

It provides a detailed calculation of the cosmological energy density considering non-unitary neutrino mixing and explores how future measurements of $N_{\rm eff}$ can constrain such deviations.

Findings

Non-unitarity modifies neutrino decoupling and $N_{\rm eff}$.

Future cosmological data can set independent limits on neutrino mixing deviations.

Relates non-unitarity to non-standard neutrino interactions.

Abstract

Deviations from unitarity in the three-neutrino mixing canonical picture are expected in many physics scenarios beyond the Standard Model. The mixing of new heavy neutral leptons with the three light neutrinos would in principle modify the strength and flavour structure of charged-current and neutral-current interactions with matter. Non-unitarity effects would therefore have an impact on the neutrino decoupling processes in the early Universe and on the value of the effective number of neutrinos, $N_{\rm eff}$. We calculate the cosmological energy density in the form of radiation with a non-unitary neutrino mixing matrix, addressing the possible interplay between parameters. Highly accurate measurements of $N_{\rm eff}$ from forthcoming cosmological observations can provide independent and complementary limits on the departures from unitarity. For completeness, we relate the scenario of small deviations from unitarity to non-standard neutrino interactions and compare the forecasted constraints to other existing limits in the literature.