Non-unitary three-neutrino mixing in the early Universe

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 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 eff fromforthcoming cosmological observations can provide independent and complementary limits on thedepartures from unitarity. For completeness, we relate the scenario of small deviations fromunitarity to non-standard neutrino interactions and compare the forecasted constraints to otherexisting limits in the literature.