\textit{Ab initio} calculations of first-forbidden $\beta$ transitions in the reactor antineutrino anomaly

TL;DR

This paper presents the first ab initio calculations of first-forbidden beta transitions crucial for understanding the reactor antineutrino anomaly, revealing significant deviations from approximate models and contributing to explaining the 5 MeV spectral bump.

Contribution

It introduces a self-consistent ab initio approach to compute forbidden beta transition rates and shape factors, improving the accuracy of reactor antineutrino spectrum predictions.

Findings

Reasonable agreement of calculated log ft values with experimental data

Significant deviations in shape factors from allowed transition approximations

Enhanced reactor antineutrino spectrum around 5 MeV potentially explaining the bump

Abstract

We present the first \textit{ab initio} calculations of first-forbidden $\beta$ transitions which are important for understanding the reactor antineutrino anomaly. Starting from chiral two- plus three-nucleon forces, we derive both valence-space effective Hamiltonians and forbidden transition operators self-consistently. Focusing on the dominant first-forbidden transitions relevant to reactor antineutrino spectra, we compute 20 key transitions and obtain $\log ft$ values that show reasonable agreement with experimental data. Obtained shape factors exhibit significant deviations from the values approximated with forbidden transitions treated as allowed transitions, indicating the importance of explicit treatments of forbidden transitions in reactor antineutrino studies. By incorporating these microscopic shape factors into the calculation of the ${}^{235}$U antineutrino spectrum, we observe a pronounced spectrum enhancement around 5 MeV of the antineutrion energy, which may partially explain the well-known ``5 MeV bump'' observed in reactor experiments.