First-forbidden transitions in the reactor anomaly

Understand

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.