Are there sterile neutrinos at the eV scale?

TL;DR

A reanalysis of global short-baseline neutrino oscillation data in a framework with one or two sterile neutrinos finds that, with the new reactor flux prediction, the global fit improves considerably when two sterile Neutrino states are introduced.

Abstract

New predictions for the anti-neutrino flux emitted by nuclear reactors suggest that reactor experiments may have measured a deficit in the anti-neutrino flux, which can be interpreted in terms of oscillations between the known active neutrinos and new sterile states. Motivated by this observation, we perform a re-analysis of global short-baseline neutrino oscillation data in a framework with one or two sterile neutrinos. While one sterile neutrino is still not sufficient to reconcile the signals suggested by reactor experiments and by the LSND and MiniBooNE experiments with null results from other searches, we find that, with the new reactor flux prediction, the global fit improves considerably when the existence of two sterile neutrinos is assumed.

Figures (5)

• Figure 1: Comparison of sterile neutrino models to reactor data: energy spectra from Bugey3 and the rate measurement of Bugey4 (inset). The data points correspond to the ratio of the observed event numbers to the predicted event number for no oscillations using the new reactor anti-neutrino fluxes Mueller:2011nm. For the Bugey3 spectra we show statistical errors only, whereas the error on the Bugey4 rate is dominated by systematics. The green solid curve shows the prediction for the no oscillation hypothesis, the blue solid and red solid curves correspond to the 3+1 and 3+2 best fit points for SBL reactor data (Tab. \ref{['tab:react-bfp']}), and the dashed curve corresponds to the 3+2 best fit point of global SBL data (Tab. \ref{['tab:global-bfp']}).
• Figure 2: $\chi^2$ from global SBL data (upper panel) and from SBL reactor data alone (lower panel) for the 3+1 (blue) and 3+2 (red) scenarios. Dashed curves were computed using the old reactor anti-neutrino flux prediction Schreckenbach:1985ep, solid curves are for the new one Mueller:2011nm. All undisplayed parameters are minimized over. The total number of data points is 137 (84) for the global (reactor) analysis.
• Figure 3: Global constraints on sterile neutrinos in the 3+1 model. We show the allowed regions at 90% and 99% CL from a combined analysis of the LSND Aguilar:2001ty and MiniBooNE anti-neutrino AguilarArevalo:2010wv signals (filled regions), as well as the constraints from the null results of KARMEN Armbruster:2002mp, NOMAD Astier:2001yj and MiniBooNE neutrino AguilarArevalo:2007it appearance searches (blue contour). The limit from disappearance experiments (green contours) includes data from CDHS Dydak:1983zq, atmospheric neutrinos, and from the SBL reactor experiments. For the latter we compare the results for the new anti-neutrino flux prediction from Mueller:2011nm (solid) and the previous ones Schreckenbach:1985ep (dashed). The region to the right of the curves is excluded at 99% CL.
• Figure 4: Predicted spectra for MiniBooNE data and the transition probability for LSND (inset). Solid histograms refer to the 3+2 global best fit point (Tab. \ref{['tab:global-bfp']}), dashed histograms correspond to the best fit of appearance data only (LSND, MiniBooNE $\nu/\bar{\nu}$, KARMEN, NOMAD). For MiniBooNE we fit only data above 475 MeV.
• Figure 5: The globally preferred regions for the neutrino mass squared differences $\Delta m_{41}^2$ and $\Delta m_{51}^2$ in the 3+2 (upper left) and 1+3+1 (lower right) scenarios.