Probing oscillations into sterile neutrinos with cosmology, astrophysics and experiments

TL;DR

This study provides a comprehensive, multi-disciplinary test of one extra sterile neutrino within a general 4-neutrino framework. It combines cosmological (BBN, CMB, LSS), solar, supernova, atmospheric, reactor, and beam neutrino data to map the sterile parameter space defined by a single mixing angle s and mass splittings with active states; it employs a density-matrix formalism to handle oscillations in diverse environments. Across all probes, the authors find no compelling evidence for sterile mixing and delineate still-allowed regions that future sub-MeV solar experiments, improved BBN/CMB measurements, and future SN observations could probe. They show that the LSND hint is strongly disfavoured by standard cosmology, and they provide quantitative bounds on sterile admixtures in each sector (e.g., solar bound _s  0 ± 0.2 and BBN/LSS constraints on _ u h^2). Overall, the work emphasizes how complementary observables tightly constrain sterile neutrinos while outlining concrete experimental paths to explore remaining parameter space.

Abstract

We perform a thorough analysis of oscillation signals generated by one extra sterile neutrino, extending previous analyses done in simple limiting cases and including the effects of established oscillations among active neutrinos. We consider the following probes: solar, atmospheric, reactor and beam neutrinos, Big-Bang Nucleosynthesis (helium-4, deuterium), Cosmic Microwave Background, Large Scale Structure, supernovae, neutrinos from other astrophysical sources. We find no evidence for a sterile neutrino in present data, identify the still allowed regions, and study which future experiments can best probe them: sub-MeV solar experiments, more precise studies of CMB or BBN, future supernova explosions, etc. We discuss how the LSND hint is strongly disfavoured by the constraints of (standard) cosmology.

Figures (14)

• Figure 1: Basic kinds of four neutrino mass spectra. Left: sterile mixing with a flavour eigenstate ($\nu_\mu$ in the picture). Right: sterile mixing with a mass eigenstate ($\nu_2$ in the picture).
• Figure 2: Isocurves to the effective number of neutrinos produced by 2 neutrino oscillations in the cases $\nu_e/\nu_{\rm s}$ (left plot) and $\nu_{\mu,\tau}/\nu_{\rm s}$. Solar and atmospheric oscillations are included in the 3 neutrino plots of fig. \ref{['fig:BBN']}, where the meaning of the various isolines is precisely explained.
• Figure 3: Cosmological effects of sterile neutrino oscillations. We compare four different signals. The continuous red line refers to the $^4\hbox{\rm He}$ abundancy (we shaded as 'disfavoured' regions where its value corresponds to $N_\nu > 3.8$), the violet dotted line to the deuterium abundancy, and the dashed blue line to the effective number of neutrinos at recombination. We plotted isolines of these three signals corresponding to an effective number of neutrinos $N_\nu=3.2$ and $3.8$. The precise meaning of the parameter $N_\nu$ in the three cases is explained in the text. The upper (lower) dot-dashed orange lines corresponds to $\Omega_\nu h^2= 10^{-2}$$(10^{-3})$, where $\Omega_\nu$ is the present energy density in neutrinos.
• Figure 4: We compare the usual 'active only' global fit of solar (shaded regions in the left plot) and of solar plus KamLAND data (shaded regions in the right plot), with the Gaussian approximation employed in this paper (dashed lines)
• Figure 5: Level crossing schemes. The right plot shows qualitatively the effective anti-neutrino masses in a supernova. The left plot shows the effective neutrino masses in the sun. We assumed hierarchical active neutrinos (i.e. $m_1 = 0$ and $m_2 = (\Delta m^2_{\rm sun})^{1/2}$) and plotted the two matter eigenstates that give rise to LMA oscillations for two different values of $E_\nu$: $10\,\hbox{\rm MeV}$ and $0.86\,\hbox{\rm MeV}$, the energy of the main Beryllium line. Colors indicate the flavour composition. An extra sterile neutrino with small mixing is represented by an horizontal line with height equal to its mass.