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 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.

Late-time interacting cosmologies and the Hubble constant tension

“…The presence of new physics related to the three standard neutrinos, such as nonstandard interactions (NSI, see e.g. [13]) between neutrinos and electrons [14,15], or a non-unitary neutrino mixing matrix [16] can lead to deviations from N std eff . These effects are rather small and do not significantly impact the H 0 value, although they may be tested by next-generation CMB measurements [17,18].…”

Standard and Non-Standard Aspects of Neutrino Physics

“…Essentially, the full N ν × N ν matrix that regulates the mixing between all the neutrino species is unitary, but when restricted to the 3 × 3 block corresponding to the PMNS matrix, unitarity is lost. A non-unitary PMNS matrix can affect CC and NC interactions, neutrino oscillations, electroweak precision data, and flavour observables as well as the history of the Universe, resulting in bounds on such deviations [98][99][100].…”

Neutrino decoupling in standard and non-standard scenarios

“…In the years to come, Simons Observatory [1] and CMB-S4 [2], could significantly improve this measurement to 𝜎(𝑁 eff ) = 0.02 − 0.03. With such unprecedented precision, it will be possible to test some non-standard scenarios of neutrino decoupling [3,4].…”