First <i>ab initio</i> calculations of first-forbidden <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>β</mi></math> transitions relevant for the reactor antineutrino anomaly

X. Y. Xu; Z. Y. Meng; Z. C. Xu; F. R. Xu

doi:10.1103/3mp8-3dj4

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

X. Y. Xu ¹, Z. Y. Meng ¹, Z. C. Xu ^2,3, and F. R. Xu ^1,4,*

¹School of Physics, and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
²Key Laboratory of Nuclear Physics and Ion-beam Application (MOE), Institute of Modern Physics, Fudan University, Shanghai 200433, China
³Shanghai Research Center for Theoretical Nuclear Physics, NSFC and Fudan University, Shanghai 200438, China
⁴Southern Center for Nuclear-Science Theory, Institute of Modern Physics, Chinese Academy of Sciences, Huizhou 516000, China

^*Contact author: frxu@pku.edu.cn

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Phys. Rev. C 114, 044317 – Published 14 October, 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.

Physical Review C

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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

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