Wednesday, May 10, 2017

Reactor Anomaly Ascribed To Sterile Neutrinos Isn't Real

The apparent reactor anomaly signal of a sterile neutrino is actually due to failure to properly model fission reactions from Uranium-235 and not from a sterile neutrino after all. This is in accord with other recent evidence pointing to the same conclusion. Furthermore, by not just ruling out this hypothesis, but providing an alternative explanation, this paper really puts the nail in the coffin of this idea. And, the lack of a sterile neutrino singlet not only impacts neutrino physics but also removes one of the more plausible dark matter candidates.
The Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 GWth reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective 239Pu fission fractions, F239, from 0.25 to 0.35, Daya Bay measures an average IBD yield, σ¯f, of (5.90±0.13)×1043 cm2/fission and a fuel-dependent variation in the IBD yield, dσf/dF239, of (1.86±0.18)×1043 cm2/fission. 
This observation rejects the hypothesis of a constant antineutrino flux as a function of the 239Pu fission fraction at 10 standard deviations. The variation in IBD yield was found to be energy-dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at 3.1σ. 
This discrepancy indicates that an overall deficit in measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes 235U, 239Pu, 238U, and 241Pu. Based on measured IBD yield variations, yields of (6.17±0.17) and (4.27±0.26)×1043 cm2/fission have been determined for the two dominant fission parent isotopes 235U and 239Pu. A 7.8% discrepancy between the observed and predicted 235U yield suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.
F.P. An, et al., "Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay" (April 4, 2017).

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