The strongest bounds on active-sterile neutrino mixing after Planck data

Abstract

Light sterile neutrinos can be excited by oscillations with active neutrinos in the early universe. Their properties can be constrained by their contribution as extra-radiation, parameterized in terms of the effective number of neutrino species N eff , and to the universe energy density today Ων h 2 . Both these parameters have been measured to quite a good precision by the Planck satellite experiment. We use this result to update the bounds on the parameter space of (3+1) sterile neutrino scenarios, with an active-sterile neutrino mass squared splitting in the range (10 −5 − 10 2 ) eV 2 . We consider both normal and inverted mass orderings for the active and sterile states. For the first time we take into account the possibility of two non-vanishing active-sterile mixing angles. We find that the bounds are more stringent than those obtained in laboratory experiments. This leads to a strong tension with the short-baseline hints of light sterile neutrinos. In order to relieve this disagreement, modifications of the standard cosmological scenario, e.g. large primordial neutrino asymmetries, are required.

Light sterile neutrinos

“…Therefore, in spite of the stringent upper bound (69) on m eff s , it is still possible to have a neutrino with m s ≈ 1 eV if ∆N eff is small. However, if the sterile neutrinos are generated by active-sterile oscillations in the early Universe [256][257][258][259][260][261], they are fully thermalized well before CMB decoupling, resulting in ∆N eff 1, which is disfavored by the bound (69) 19 . This problem led several authors to propose new mechanisms that can relieve the tension: a large lepton asymmetry [259,[300][301][302][303][304][305][306][307][308][309][310], new neutrino interactions [311][312][313][314][315][316][317][318][319][320][321], entropy production after neutrino decoupling [322], neutrino decay [323], very low reheating temperature [324,325], time varying dark energy components [298], a larger cosmic expansion rate at the time of sterile neutrino production [326], inflationary freedom [327].…”

Status of Neutrino Properties and Future Prospects—Cosmological and Astrophysical Constraints

“…A light sterile neutrino has been proposed as an explanation of the anomalies observed in short-baseline (SBL) experiments (see e.g., [199] and references therein). However, a sterile neutrino with the mass (m s ≃ 1 eV) and coupling required to explain reactor anomalies would rapidly thermalize in the early Universe (see e.g., [200,201]) and lead to N eff = 1, strongly at variance with cosmological constraints (excluded at more than 99% confidence considering the above combination of Planck and BAO data). We conclude this section by quoting the forecasts for future cosmological probes.…”

Neffin low-scale seesaw models versus the lightest neutrino mass

“…For recent analyzes of eV scale neutrinos, with and without lepton asymmetries, see [35]- [41]. In [42] we explored systematically the contribution to N eff of the minimal Type I seesaw models with just two extra singlets, N = 2.…”