Systematic Uncertainties in the Analysis of the Reactor Neutrino Anomaly

Abstract

We examine uncertainties in the analysis of the reactor neutrino anomaly, wherein it is suggested that only about 94% of the emitted antineutrino flux was detected in short baseline experiments. We find that the form of the corrections that lead to the anomaly are very uncertain for the 30% of the flux that arises from forbidden decays. This uncertainty was estimated in four ways, is larger than the size of the anomaly, and is unlikely to be reduced without accurate direct measurements of the antineutrino flux. Given the present lack of detailed knowledge of the structure of the forbidden transitions, it is not possible to convert the measured aggregate fission beta spectra to antineutrino spectra to the accuracy needed to infer an anomaly. Neutrino physics conclusions based on the original anomaly need to be revisited, as do oscillation analyses that assumed that the antineutrino flux is known to better than approximately 4%.The term "reactor neutrino anomaly" first appeared in a publication by G. Mention et al. [1], where it generally referred to the 3σ deficit of neutrinos detected in short-baseline reactor neutrino experiments relative to the number predicted. The predicted number of detected neutrinos has evolved upward over time, largely as a consequence of a predicted increase in the energy of the neutrino flux and an increasedν e + p → n + e + cross section associated with smaller values for the neutron lifetime. This cross section is used to infer the neutrino flux in a presumably well-characterized detector. The changes in the predicted neutrino flux are mostly associated with improved knowledge of the beta decays of the isotopes created in fission reactors. Such an anomaly would potentially be extremely significant, if a shortfall in the detected neutrino flux could be ascribed toν e oscillation into a light sterile neutrino with a mass of about 1 eV.There is an extensive recent literature dealing with the reactor anomaly, starting with a seminal paper by Mueller et al. [2] that reexamined the reactor antineutrino flux. The latter publication sought to improve the earlier flux estimates based on the ILL on-line measurements [3][4][5] of the integral beta spectrum of the fission products. An antineutrino spectrum can be inferred from a beta spectrum provided one knows the linear combination of operators involved in the decay, the end-point energy, and the nuclear charge. The fission beta spectra involve about 6000 beta transitions, of which about 1500 are forbidden [6]. Clearly some assumptions are required in order to infer the fission antineutrino flux. The improvements [1, 2] on the earlier analyses of ILL integral measurements led to an increased energy of the antineutrino flux, which was subsequently verified in an independent analysis [7].The present contribution examines the consequences of the forbidden transitions known to be present (at the 30% level) in the beta decay of fission products. We analyze the antineutrino flux, using a first-principles derivation of the finite size (FS) and weak ...

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