Precisely measured electron spectra from fission products, the missing piece in the reactor antineutrino puzzle

The Daya Bay collaboration has recently published a precise measurement of the inverse beta decay cross section folded antineutrino spectrum, revealing a 5.4% antineutrino deficit beyond the three-flavor neutrino oscillation as well as a spectral distortion. This deficit is compatible with earlier nuclear reactor antineutrino experiments as analyzed by Mention et al, who showed a systematic deficit of antineutrinos at short distances, an effect referred to as the 'reactor neutrino anomaly'. The obvious question is whether this deficit is due to the presence of one (or more) sterile neutrinos, or simply an unknown component in the underlying nuclear physics used in the predictions. In this work, we explore three possibilities in the latter group. First, we study the feasibility of the ²³⁸U contribution and the effective Z used in the conversion method as possible sources of the disagreement by adjusting them to match the measured Daya Bay spectrum. Both scenarios are considered unlikely: the adjusted ²³⁸U antineutrino spectrum has an unphysical shape and smaller integral than expected from systematic trends, while the fitted Z effective would correspond to fission products with negligible fission yield. Motivated by the fact that 75% of a reactor antineutrino spectrum in the region relevant to the anomaly is accounted for by the decay of fewer than 50 nuclides, it is then possible that individual shape corrections arising from first forbidden transitions or second order terms such as weak magnetism may not cancel out, which led us to explore the sensitivity of the Daya Bay spectrum to experimental shape factor corrections. We find that using a linear term equal to +6%/MeV in the conversion method results in a much closer agreement with the Daya Bay spectrum. Unlike the two previous cases, this scenario cannot be ruled out as there is currently an absence of precisely measured electron spectra from the relevant fission products. Previously, it was noted how important beta intensities and fission yields are for the summation method. This sensitivity analysis highlights the importance of experimental shape factors for both conversion and summation calculations. One possible way forward to confirm the existence of the anomaly, or even quantify it, would be to precisely measure the electron spectra for the relevant nuclides, incorporate these data in summation calculations to understand their effect, and finally, through an average procedure, include them in a conversion calculation. This document is made up of an abstract and the slides of the presentation. (authors)