# Self-consistent calculation of the reactor antineutrino spectra including forbidden transitions

## TL;DR

This paper presents a self-consistent calculation of reactor antineutrino spectra that includes forbidden transitions, addressing the reactor neutrino anomaly by refining theoretical models of beta decay contributions.

## Contribution

It introduces a comprehensive method to incorporate forbidden beta decay transitions into reactor antineutrino spectrum calculations, improving upon previous allowed-only approximations.

## Key findings

- Forbidden transitions significantly affect the predicted spectra.
- The new calculations help explain the reactor neutrino anomaly.
- Results suggest a need to revise theoretical models for better accuracy.

## Abstract

With the goal of determining the $\theta_{13}$ neutrino oscillation mixing angle, the measurements of reactor antineutrino fluxes at the Double Chooz, RENO and Daya Bay experimental facilities have uncovered a systematic discrepancy between the number of observed events and theoretical expectations. In the \emph{ab initio} approach, the total reactor antineutrino spectrum is a weighted sum of spectra resulting from all $\beta$ branches of all fission products in the reactor core. At all three facilities a systematic deviation of the number of observed events from the number of predicted events was noticed, i.e., approximately 6\% of the predicted neutrinos were not observed. This discrepancy was named the reactor neutrino anomaly. In theoretical studies it is assumed that all the decays are allowed in shape, but a quarter of all transitions are actually forbidden and may have a complex energy dependence that will affect the total reactor antineutrino spectrum. In order to estimate the effect of forbidden transitions, we perform a fully self-consistent calculation of spectra from all contributing transitions and compare the results with a purely allowed approximation.