Neutrino oscillations: The ILL experiment revisited

The ILL experiment, one of the “reactor anomaly” experiments, is reexamined. The ILL’s baseline of 8.78 m is the shortest of the reactor anomaly short baseline experiments, and it is the experiment that finds the largest fraction of the electron antineutrinos disappearing-about 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 chisquare (also termed a “rate analysis”), or utilize a spectral form for chisquare which double counts the systematic error. We do an analysis which utilizes the standard, conventional form for chisquare as well as a derived functional form for a spectral chisquare. We find that when analyzed with a conventional chisquare that includes spectral information or with a spectral chisquare that is independent of the magnitude of the flux, the ILL experiment finds a substantial distortion in the neutrino spectra as compared to conventional no-oscillation spectra. Interpreting this in terms of a fourth neutrino, rather than an error in some aspect of the analysis such as the energy calibration, the results are a set of specific values for possible mass-squared differences of the fourth neutrino, and where the minimum chisquare difference values are significantly enhanced over previous analyses. For the Huber flux and the conventional chisquare, the two most preferred values are mass-squared differences of 0.90 and 2.36 eV2 preferred at Δχmin2 values of −12.1 and −13.0 (3.5 and 3.6σ), respectively. For the Daya Bay flux and conventional chisquare we find 0.95 and 2.36 eV2 preferred at Δχmin2 of −10.5 and −11.7 (3.2 and 3.4σ), respectively. For the spectral chisquare and either flux these values are 0.95 and 2.36 eV2 preferred at Δχmin2 of −8.22 and −9.45 (2.9 and 3.1σ), respectively. These are to be compared to −4.4 (2.1σ) found in the original reactor anomaly analysis for all of the experiments except the ILL experiment.