Sterile neutrinos or flux uncertainties? — Status of the reactor anti-neutrino anomaly

Abstract

The ∼ 3σ discrepancy between the predicted and observed reactor antineutrino 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.

Reevaluating reactor antineutrino anomalies with updated flux predictions

“…We cross-check our results with the HM flux model against the rate results in Refs. [1,6,[55][56][57][58]. We find general good agreement; the resulting 95% C.L.…”

“…The 1σ, 2σ and 3σ preferred regions are shown in dark, medium and light green, respectively, and are consistent with similar results in Refs. [6,29,57]. The sensitivity is primarily driven by DANSS; the total evidence for a sterile neutrino is 3.1σ.…”

Thermalisation of sterile neutrinos in the early universe in the 3+1 scheme with full mixing matrix

“…Thus, when the active neutrinos decouple from the rest of the plasma the sterile state is not fully populated. For instance, we get N eff 3.5 for |U e4 | 2 = 0.001 (cyan line), which is just outside the 3σ allowed region by DANSS and NEOS [12,44,45].…”

A testable hidden-sector model for Dark Matter and neutrino masses

“…1 The considered model is an ESS neutrino mass model [57][58][59], an extension of the type I seesaw. It was originally introduced to gives rise to a eV sterile neutrino to explain the observed anomalous disappearance of electron neutrinos at reactors [60,61], and in experiments using intense radioactive sources [62,63] and appearance of electron neutrinos from a muon neutrino beam at short baseline experiments like LSND [64] and MiniBooNE [65-67] 2 . Its usual particle content consists of three additional right-handed neutrinos and one gauge fermion singlet.…”