Abstract
Two anomalies at nuclear reactors, one related to the absolute antineutrino flux, one related to the antineutrino spectral shape, have drawn special attention to the field of reactor neutrino physics during the past decade. Numerous experimental efforts have been launched to investigate the reliability of flux models and to explore whether sterile neutrino oscillations are at the base of the experimental findings. This review aims to provide an overview on the status of experimental searches at reactors for sterile neutrino oscillations and measurements of the antineutrino spectral shape in mid-2021. The individual experimental approaches and results are reviewed. Moreover, global and joint oscillation and spectral shape analyses are discussed. Many experiments allow setting of constraints on sterile oscillation parameters, but cannot yet cover the entire relevant parameter space. Others find evidence in favour of certain parameter space regions. In contrast, findings on the spectral shape appear to give an overall consistent picture across experiments and allow narrowing down of contributions of certain isotopes.
Highlights
During the last 20 years, measurements of neutrino oscillations in the three-flavour framework have determined all mixing angles and the magnitudes of the neutrino mass splittings [1,2,3]
The Reactor Antineutrino Anomaly (RAA) was discovered in a re-evaluation of the prediction of antineutrino spectra emitted by nuclear reactor cores in 2011 [5,6,7]
One can look at the common deficit of the 235 U component of the lowly enriched U cores (LEU) experiments Daya Bay and RENO by conducting a fit, where the isotopic inverse beta decay reaction (IBD) yields of 235 U and 239 Pu are free, while those of 238 U and 241 Pu are constrained to the prediction [49]
Summary
During the last 20 years, measurements of neutrino oscillations in the three-flavour framework have determined all mixing angles and the magnitudes of the neutrino mass splittings [1,2,3]. A deficit in the detected absolute antineutrino flux is observable at those baselines Such a sterile neutrino could explain the lower than predicted electron neutrino rates measured in the calibration runs of the gallium solar neutrino experiments [12,13]. With a significance of 2.7 standard deviations, the flux deficit triggered a new set of reactor antineutrino experiments at very short baselines of about 10 m They are searching for characteristic oscillation patterns in connection to a sterile neutrino state. An updated calculation, which includes forbidden decays via nuclear shell model calculations leads to an increase in the flux at energies above 4 MeV [20] While this reduces the observed shape anomaly, it yields an increase in the overall flux prediction and thereby an increased rate deficit. Aside from nuclear physics, it was argued that common uncertainties in detector calibrations could contribute to the observed spectral shape anomaly [22]
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