First-forbidden transitions in the reactor anomaly

TL;DR

This study calculates the impact of forbidden nuclear transitions on reactor antineutrino spectra, revealing they significantly influence spectral features and could explain anomalies observed in experiments.

Contribution

It provides the first detailed shell model calculations of forbidden transitions and proposes a spectral correction that impacts reactor neutrino spectrum interpretations.

Findings

Forbidden transitions dominate the spectral flux in most accessible ranges.

Spectral corrections reduce the significance of the reactor antineutrino anomaly.

Results align with the observed spectral shoulder in modern experiments.

Abstract

We study the dominant forbidden transitions in the antineutrino spectra of the fission actinides from 4 MeV onward using the nuclear shell model. Through explicit calculation of the shape factor, taking into account Coulomb corrections, we show the expected changes on cumulative electron and antineutrino spectra. Compared to the usual allowed approximation this results in a minor decrease of electron spectra from 4 MeV and onward, whereas an increase of several percent is observed in antineutrino spectra. We show that, despite their limited number, forbidden transitions dominate the spectral flux for most of the experimentally accessible range. Based on the shell model calculations we attempt a parametrization of forbidden transitions and propose a spectral correction for all forbidden transitions. We enforce correspondence with the ILL dataset using a summation+conversion approach. When compared against modern reactor neutrino experiments, the resultant spectral change is observed to be of comparable magnitude and shape as the reported spectral shoulder, drastically decreasing the statistical significance of the latter.