Sterile Neutrino Oscillations: The Global Picture

TL;DR

This work performs a comprehensive global analysis of oscillations with one or two eV-scale sterile neutrinos (3+1, 3+2, 1+3+1) using short- and long-baseline accelerator, reactor, radioactive source, atmospheric, and solar data. It derives oscillation probabilities in both SBL and LBL regimes, articulates a CP-violating phase structure, and integrates diverse experimental constraints including reactor and Gallium anomalies and LSND/MiniBooNE appearance signals. The results reveal persistent tension between appearance and disappearance data in all schemes, with the 1+3+1 scenario offering the best (but still limited) global compatibility (PG ~ 0.2%), while 3+1 and 3+2 remain strongly disfavored by combined datasets. The analysis highlights the need for new experiments and possibly new cosmological considerations to resolve whether eV-scale sterile neutrinos exist and how they fit into the broader neutrino sector.

Abstract

Neutrino oscillations involving eV-scale neutrino mass states are investigated in the context of global neutrino oscillation data including short and long-baseline accelerator, reactor, and radioactive source experiments, as well as atmospheric and solar neutrinos. We consider sterile neutrino mass schemes involving one or two mass-squared differences at the eV^2 scale denoted by 3+1, 3+2, and 1+3+1. We discuss the hints for eV-scale neutrinos from nu_e disappearance (reactor and Gallium anomalies) and nu_mu->nu_e appearance (LSND and MiniBooNE) searches, and we present constraints on sterile neutrino mixing from nu_mu and neutral-current disappearance data. An explanation of all hints in terms of oscillations suffers from severe tension between appearance and disappearance data. The best compatibility is obtained in the 1+3+1 scheme with a p-value of 0.2% and exceedingly worse compatibilities in the 3+1 and 3+2 schemes.

Figures (11)

• Figure 1: Left: Allowed regions of oscillation parameters from SBL reactor data in the 3+1 scheme for a rates only analysis (contours) as well as a fit including Bugey3 spectral data (colored regions). Right: Event rates in SBL reactor experiments compared to the predictions for three representative sets of oscillation parameters. The thick (thin) error bars correspond to uncorrelated (total) experimental errors. The neutrino flux uncertainty is not included in the error bars. The Rovno and SRP data points at 18 m have been shifted for better visibility.
• Figure 2: Allowed regions at 95% CL (2 dof) for 3+1 oscillations. We show SBL reactor data (blue shaded), Gallium radioactive source data (orange shaded), $\nu_e$ disappearance constraints from $\nu_e$--$^{12}$$\text{C}$ scattering data from LSND and KARMEN (dark red dotted), long-baseline reactor data from CHOOZ, Palo Verde, DoubleChooz, Daya Bay and RENO (blue short-dashed) and solar+KamLAND data (black long-dashed). The red shaded region is the combined region from all these $\nu_e$ and $\bar{\nu}_e$ disappearance data sets.
• Figure 3: Constraints on $\nu_e$ and $\bar{\nu}_e$ disappearance in a $3+1$ model at two different fixed values of $\Delta m^2_{41}$. Regions are shown at 95% CL (2 dof) with respect to the minimum $\chi^2$ at the fixed $\Delta m^2_{41}$. We show constraints from the radio chemical Gallium experiments using radioactive sources (orange band), from short-baseline reactor experiments (blue band), from the KARMEN and LSND measurements of the $\nu_e$--${^{12}\text{C}}$ cross section (dark red dotted line), from long-baseline reactor experiments (blue dashed line), from the combined solar+KamLAND data (black dashed line), and from the the combination of all aforementioned experiments (red shaded region).
• Figure 4: Left: Constraints in the plane of $|U_{\mu 4}|^2$ and $\Delta m^2_{41}$ at 99% CL (2 dof) from CDHS, atmospheric neutrinos, MiniBooNE disappearance, MINOS CC and NC data, and the combination of them. We minimize with respect to $|U_{\tau 4}|$ and the complex phase $\varphi_{24}$. In red we show the region preferred by LSND and MiniBooNE appearance data combined with reactor and Gallium data on ${\overset{ \raisebox{-0.15em}{(} \raisebox{-0.3em}{--} \raisebox{-0.15em}{)}} {\nu}}_e$ disappearance, where for fixed $|U_{\mu 4}|^2$ we minimize with respect to $|U_{e4}|^2$. Right: Constraints in the plane of $|U_{\tau 4}|^2$ and $\Delta m^2_{41}$ at 99% CL (2 dof) from MINOS CC + NC data (green) and the combined global ${\overset{ \raisebox{-0.15em}{(} \raisebox{-0.3em}{--} \raisebox{-0.15em}{)}} {\nu}}_\mu$ and NC disappearance data (blue region, black curves). We minimize with respect to $|U_{\mu 4}|$ and we show the weakest ("best phase") and strongest ("worst phase") limits, depending on the choice of the complex phase $\varphi_{24}$. In both panels we minimize with respect to $\Delta m^2_{31}$, $\theta_{23}$, and we fix $\sin^22\theta_{13} = 0.092$ and $\theta_{14} = 0$ (except for the evidence regions in the left panel).
• Figure 5: Constraints in the plane of $|U_{\mu 4}|^2$ and $|U_{\tau 4}|^2$ for three fixed values of $\Delta m^2_{41}$ from MINOS CC + NC data (green), atmospheric neutrinos (orange), CDHS + MiniBooNE ${\overset{ \raisebox{-0.15em}{(} \raisebox{-0.3em}{--} \raisebox{-0.15em}{)}} {\nu}}_\mu$ disappearance + LBL reactors (red), and the combination of those data (blue). The constraint from solar neutrinos is shown in magenta. Regions are shown at 90% and 99% CL (2 dof) with respect to the $\chi^2$ minimum at the fixed $\Delta m^2_{41}$. We minimize with respect to complex phases and include effects of $\theta_{13}$ and $\theta_{14}$ where relevant. The gray region is excluded by the unitarity requirement $|U_{\mu 4}|^2 + |U_{\tau 4}|^2 \le 1$. Note the different scale on the axes.