Neutrino oscillations: ILL experiment revisited

TL;DR

This paper re-analyzes the ILL reactor neutrino experiment data using standard chi-square methods, strengthening evidence for a sterile neutrino with a specific mass squared difference around 0.9 eV$^2$ and significance up to 3.3 sigma.

Contribution

It introduces a conventional spectral chi-square analysis for the ILL experiment, providing more robust evidence for sterile neutrinos compared to previous methods.

Findings

Evidence for sterile neutrinos is significantly enhanced.

Best-fit mass squared difference around 0.9 eV$^2$.

Statistical significance up to 3.3 sigma.

Abstract

The ILL experiment, one of the "reactor anomaly" experiments, is re-examined. ILL's baseline of 8.78 m is the shortest of the short baseline experiments, and it is the experiment that finds the largest fraction of the electron anti-neutrinos disappearing -- over 20%. Previous analyses, if they do not ignore the ILL experiment, use functional forms for chisquare which are either totally new and unjustified, are the magnitude chi-square (also termed a "rate analysis"), or utilize a spectral form for chi-square which double counts the systematic error. We do an analysis which utilizes the standard, conventional form for chi-square as well as a derived form for a spectral chi-square. Results for the ILL, Huber, and Daya Bay fluxes are given. We find that the implications of the ILL experiment in providing evidence for a sterile, fourth neutrino are significantly enhanced. Moreover, we find that the ILL experiment provides a set of choices for specific values of the mass squared difference. The value of the mass square difference for the deepest minimum and its statistical significance, using the flux independent spectral chi-square and the Huber or Daya Bay flux are 0.92 eV$^2$ and 2.9 $\sigma$; for the conventional chi-square and the Daya Bay flux are 0.95 eV$^2$ and 3.0 $\sigma$; and for the conventional chi-square and Huber flux are 0.90 eV$^2$ and 3.3 $\sigma$. These probabilities are significantly larger than the 1.8 $\sigma$ found for the ILL experiment using a rate analysis.