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

TL;DR

The paper investigates whether light sterile neutrinos that mix with active flavors can coexist with Planck constraints on the relativistic radiation content and late-time neutrino energy density. It extends the cosmological bounds to a (3+1) framework with two active-sterile mixing angles by solving averaged-momentum density-matrix equations that describe early-universe neutrino oscillations, collisions, and matter effects. The authors derive 95% CL exclusions in the plane spanned by $\Delta m^2_{41}$ and $\sin^2\theta_{i4}$ for NH/SNH and IH/SIH, finding that Planck data strongly constrain sterile production especially at larger mixings, and that regions favored by short-baseline hints are largely excluded, implying a tension that would demand non-standard cosmology such as a sizable lepton asymmetry $L_\nu$. Consequently, sterile neutrinos with $m_s \sim \mathcal{O}(1)$ eV are disfavored at more than $4\sigma$, highlighting the need for new physics to reconcile laboratory hints with cosmological observations.

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 Ω_νh^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.

Figures (4)

• Figure 1: Active and sterile neutrinos mass orderings with the scheme of possible resonances.
• Figure 2: Evolution of the sterile neutrino density matrix $\rho_{ss}$ in the case of $\sin^2\theta_{14}= 10^{-2}$, for $\Delta m^2_{41}= 10^{-5}$ eV$^{2}$ (continuous curve), $\Delta m^2_{41}= -10^{-5}$ eV$^{2}$ (dashed curve) and $\Delta m^2_{41}= 5\times 10^{-2}$ eV$^{2}$ (dotted curve).
• Figure 3: Active normal mass hierarchy NH. Exclusion plots for the active-sterile neutrino mixing parameter space for SNH (upper panels) and SIH (lower panels) cases from $N_{\rm eff}$ (black curves) and $\Omega_\nu h^2$ (red curves) at 95 % C.L. The contours refer to different values of $\sin^2 \theta_{i4}$: $\sin^2 \theta_{i4} =0$ (continuous curves), $\sin^2 \theta_{i4} =10^{-3}$ (dashed curves), $\sin^2 \theta_{i4} =10^{-2}$ (dotted curves), $\sin^2 \theta_{i4} =10^{-1.5}$ (dot-dashed curves). (see the text for details).
• Figure 4: Active inverted mass hierarchy IH. Exclusion plots for the active-sterile neutrino mixing parameter space for SNH (upper panels) and SIH (lower panels) cases from $N_{\rm eff}$ (black curves) and $\Omega_\nu h^2$ (red curves) at 95 % C.L. The contours refer to different values of $\sin^2 \theta_{i4}$: $\sin^2 \theta_{i4} =0$ (continuous curves), $\sin^2 \theta_{i4} =10^{-2}$ (dotted curves), $\sin^2 \theta_{i4} =10^{-1.5}$ (dot-dashed curves). (see the text for details).