First-forbidden transitions in the reactor anomaly

TL;DR

This paper presents detailed nuclear shell model calculations of forbidden transitions in reactor antineutrino spectra, revealing significant deviations from previous models and emphasizing their importance in reactor anomaly analysis.

Contribution

It introduces a high-fidelity shape factor calculation for forbidden transitions, proposes a new parametrization, and assesses their impact on reactor antineutrino spectra and anomalies.

Findings

Forbidden transitions cause several percent differences in antineutrino spectra.

Proper treatment of forbidden transitions is crucial for reactor anomaly analysis.

New parametrization improves modeling of forbidden transition spectra.

Abstract

We describe here microscopic calculations performed on the dominant forbidden transitions in reactor antineutrino spectra above 4 MeV using the nuclear shell model. By taking into account Coulomb corrections in the most complete way, we calculate the shape factor with the highest fidelity and show strong deviations from allowed approximations and previously published results. Despite small differences in the ab initio electron cumulative spectra, large differences on the order of several percents are found in the antineutrino spectra. Based on the behaviour of the numerically calculated shape factors we propose a parametrization of forbidden spectra. Using Monte Carlo techniques we derive an estimated spectral correction and uncertainty due to forbidden transitions. We establish the dominance and importance of forbidden transitions in both the reactor anomaly and spectral shoulder analysis. Based on these results, we conclude that a correct treatment of forbidden transitions is indispensable in both the normalization anomaly and spectral shoulder.