Another Experiment Disfavors Sterile Neutrinos

As attractive as sterile neutrinos are as an easy explanation for any anomaly that crops up in a neutrino oscillation measurement, it doesn't hold water on closer examination.

Most of the LSDN anomaly parameter space for a sterile neutrino is ruled out (except from about 0.2 to 2 eV sterile neutrinos with sine squared two theta parameters of 0.01 to 0.001 in scenarios that are not "appearance only"), some of gallium anomaly sterile neutrino parameter space (almost all of it in a "disappearance only" scenario) and some of the already tiny remaining Neutrino-4 parameter space (which has to be an almost exactly 2.8 eV sterile neutrino that can't have a sine squared two theta mixing parameter more than about 0.3), is also excluded.

The sterile neutrino parameter space suggested by the reactor anomaly does not overlap with the sterile neutrino parameter space suggested by the Neutrino-4 anomaly, disfavoring both a hints of new physics.

The conclusion of the paper notes that: "the MicroBooNE BNB Run 1–3 data show no evidence of sterile neutrino oscillations and are found to be consistent with the 3ν hypothesis within 1σ significance."

The introduction to the paper explains the state of the research to date related to the anomalies at other experiments that have given rise to sterile neutrino explanations:

The discoveries of solar and atmospheric neutrino oscillations have motivated a broad experimental program dedicated to studying neutrino mixing. While most measurements are consistent with three-flavor (3ν) neutrino oscillations as described by the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) formalism, several experimental anomalies hint at the existence of a sterile neutrino with a mass at the eV scale. 

The SAGE and GALLEX experiments, and more recently, the BEST experiment, have observed lower than expected ν(e) rates from radioactive sources, which is known as the gallium anomaly. 

Reactor neutrino experiments have measured lower anti-ν(e) rates than the expectation based on reactor anti-neutrino flux calculations. This observation is referred to as the reactor anomaly. 

An oscillation signal in the reactor anti-ν(e) energy spectrum over distances of a few meters was reported by the Neutrino-4 collaboration. 

In addition to these observed anti-ν(e) deficits, excesses of anti-ν(e)-like events were also observed in some anti-ν(µ) dominated accelerator neutrino experiments. The LSND collaboration observed an anomalous excess of anti-ν(e)-like events, and the MiniBooNE collaboration observed an excess of low-energy electron-like events. 

These anomalies are in strong tension with other experimental results within the 3(active) + 1(sterile) oscillation framework as seen in a global fit of the data. 

In addition, recent experimental measurements and improvements of the reactor anti-neutrino flux calculation lead to a plausible resolution of the reactor anti-neutrino anomaly. 

The Neutrino-4 anomaly is largely excluded by the results from other very short baseline reactor neutrino experiments, for example, PROSPECT, STEREO, DANSS, NEOS, although it is consistent with the gallium anomaly.

The parameter space to the right of the red lines are excluded by the latest MicroBooNE data reported in the paper below at a 95% confidence level. The shaded area are parameters for a sterile neutrino in a 3-1 model that are not ruled out by the referenced prior experiments due to anomalies in their data.

We present a search for eV-scale sterile neutrino oscillations in the MicroBooNE liquid argon detector, simultaneously considering all possible appearance and disappearance effects within the 3+1 active-to-sterile neutrino oscillation framework. We analyze the neutrino candidate events for the recent measurements of charged-current νe and νμ interactions in the MicroBooNE detector, using data corresponding to an exposure of 6.37×10^20 protons on target from the Fermilab booster neutrino beam. We observe no evidence of light sterile neutrino oscillations and derive exclusion contours at the 95% confidence level in the plane of the mass-squared splitting Δm241 and the sterile neutrino mixing angles θμe and θee, excluding part of the parameter space allowed by experimental anomalies. Cancellation of νe appearance and νe disappearance effects due to the full 3+1 treatment of the analysis leads to a degeneracy when determining the oscillation parameters, which is discussed in this paper and will be addressed by future analyses.

MicroBooNE collaboration, "First constraints on light sterile neutrino oscillations from combined appearance and disappearance searches with the MicroBooNE detector" arXiv:2210.10216 (October 18, 2022).