Accepted Paper

First ab initio calculations of first-forbidden $β$ transitions relevant for the reactor antineutrino anomaly

X. Y. Xu, Z. Y. Meng, Z. C. Xu, and F. R. Xu

Phys. Rev. C - Accepted 29 September, 2026

DOI: https://doi.org/10.1103/3mp8-3dj4

Abstract

Forbidden $β$ transitions are important for understanding the reactor antineutrino anomaly. Using many-body perturbation theory with two- plus three-nucleon interactions, we have derived the valence-space effective Hamiltonian and effective operators for first-forbidden transitions. 20 dominant first-forbidden transitions have been investigated, which provide important contributions to the reactor antineutrino spectrum anomaly. Calculated $log f t$ values are in reasonable agreement with experimental data. Obtained shape factors exhibit significant deviations from the values approximated with forbidden transitions treated as allowed transitions. The "5 MeV bump" observed in the experimental $^{235}$ U-fission antineutrino spectrum was discussed with self-consistent shape factors obtained in the present work. No empirical quenching factors are required in the present calculations of the first-forbidden transitions.

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Physical Review C

First ab initio calculations of first-forbidden $β$ transitions relevant for the reactor antineutrino anomaly

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First ab initio calculations of first-forbidden β transitions relevant for the reactor antineutrino anomaly

Abstract

First ab initio calculations of first-forbidden $β$ transitions relevant for the reactor antineutrino anomaly