Abstract
The ∼ 3σ discrepancy between the predicted and observed reactor anti-neutrino flux, known as the reactor anti-neutrino anomaly, continues to intrigue. The recent discovery of an unexpected bump in the reactor anti-neutrino spectrum, as well as indications that the flux deficit is different for different fission isotopes seems to disfavour the explanation of the anomaly in terms of sterile neutrino oscillations. We critically review this conclusion in view of all available data on electron (anti)neutrino disappearance. We find that the sterile neutrino hypothesis cannot be rejected based on global data and is only mildly disfavored compared to an individual rescaling of neutrino fluxes from different fission isotopes. The main reason for this is the presence of spectral features in recent data from the NEOS and DANSS experiments. If state-of-the-art predictions for reactor fluxes are taken at face value, sterile neutrino oscillations allow a consistent description of global data with a significance close to 3σ relative to the no-oscillation case. Even if reactor fluxes and spectra are left free in the fit, a 2σ hint in favour of sterile neutrinos remains, with allowed parameter regions consistent with an explanation of the anomaly in terms of oscillations.
Highlights
We find that the sterile neutrino hypothesis cannot be rejected based on global data and is only mildly disfavored compared to an individual rescaling of neutrino fluxes from different fission isotopes
The spectral distortions observed in NEOS and DANSS prefer sterile neutrino oscillations over flux rescaling and the preference for the flux-free fit obtained by Daya Bay flux data decreases
The bound on θ14 is mainly driven by the good agreement between the theoretical expectation of the 8B neutrino flux, which is predicted by the Standard Solar Model, and its precise determination in high-energy solar experiments
Summary
In ref. [29], the Daya Bay collaboration has for the first time presented independent measurements of the νe fluxes from 235U and 239Pu fission. We will perform a “free fluxes” analysis, where ξ235 and ξ239 are allowed to vary freely In this analysis, χ2flux(ξi) still imposes a weak 1σ constraint of 10% relative to the Huber & Mueller predictions on the subleading isotopes (ξ238 and ξ241) to avoid unphysical results. The reason for the slightly lower value for Tobs in eq (2.4) compared to the value of 7.9 obtained in [29] is that our “fixed fluxes” analysis includes the uncertainties in the Huber & Mueller flux prediction for the four isotopes (encoded in ξi factors in eq (2.1)), whereas ref. While the Daya Bay flux measurements favour the “free fluxes” hypothesis over the sterile neutrino hypothesis, the latter still provides a good fit to the data (gof of 18%). We proceed with the sterile neutrino analysis and combine the Daya Bay flux data with all other reactor data
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