Unraveling the Dirac neutrino with cosmological and terrestrial detectors

Peter Adshead,Yanou Cui,Andrew J Long,Michael Shamma

doi:10.1016/j.physletb.2021.136736

Abstract

We point out a correlation between the effective number of relativistic degrees of species Neff, the cosmologically measured mν,sterileeff, and the terrestrially measured neutrino mass sum and effective electron neutrino mass, Σmν and mνe, which arises in the Dirac neutrino hypothesis. If the neutrinos are Dirac particles, and if the active neutrinos' sterile partners were once thermalized in the early universe, then this new cosmological relic would simultaneously contribute to the effective number of relativistic species, Neff, and also lead to a correlation between the cosmologically-measured effective sterile neutrino mass mν,sterileeff and the terrestrially-measured active neutrino mass sum Σmν. We emphasize that specifically correlated deviations in Neff≳3, mν,sterileeff and Σmν above their standard predictions could be the harbinger revealing the Dirac nature of neutrinos. We provide several benchmark examples, including Dirac leptogenesis, that predict a thermal relic population of the sterile partners, and we discuss the relevant observational prospects with current and near-future experiments. If the correlation highlighted in this work is observed in future surveys, it could be interpreted as supporting evidence of Dirac neutrino masses.

Highlights

The phenomenon of neutrino flavor oscillations [1,2,3,4,5] requires that at least two neutrinos are massive
We present the derivation here, both for readers who are unfamiliar with cosmological relic calculations and to emphasize the special role played by the degeneracy of active and sterile neutrinos in the Dirac Neutrino Hypothesis (DNH)
Recall that g∗S(ta,dec) ≈ 10.75 is the effective number of relativistic species when the active neutrinos decouple near Ta,dec 1 MeV, and it is g∗S(ts,dec) > 10.75 when the sterile neutrinos decouple

Summary

Introduction

The phenomenon of neutrino flavor oscillations [1,2,3,4,5] requires that at least two neutrinos are massive. Confirmation of the DNH is far harder to achieve This is because the DNH’s new degrees of freedom are light gauge-singlet sterile neutrinos, which do not interact with the W ± and Z bosons in the same way as the active neutrinos. There are many compelling beyond the SM scenarios in which the sterile states acquire a thermal distribution in the early universe and survive today as cosmological relics We demonstrate how this leads to deviations in the cosmological neutrino observables (namely, Neff and meνf,fsterile) that are correlated with one another, as expected in general for eV-scale relics [17], and correlated with terrestrial neutrino observables (namely, mνe and Σmν), as pointed out recently in Ref. We demonstrate how this leads to deviations in the cosmological neutrino observables (namely, Neff and meνf,fsterile) that are correlated with one another, as expected in general for eV-scale relics [17], and correlated with terrestrial neutrino observables (namely, mνe and Σmν), as pointed out recently in Ref. [18]

Models with thermal sterile neutrinos

Terrestrial and cosmological probes of neutrinos

Correlated terrestrial and cosmological observables

Results and discussion