# Weak Magnetism Correction to Allowed Beta-decay for Reactor Antineutrino Spectra

## TL;DR

This paper evaluates the common approximation for weak magnetism correction in reactor antineutrino spectra, finding it valid for relevant beta-decays and that the main uncertainty stems from two-body effects.

## Contribution

It clarifies the validity of the one-body weak magnetism approximation for fission fragment beta-decays and quantifies the associated uncertainty in reactor antineutrino spectra.

## Key findings

- The approximation is valid for relevant beta-decays.
- Uncertainty from one-body weak magnetism is less than 1%.
- Main uncertainty arises from two-body meson-exchange currents.

## Abstract

The weak magnetism correction and its uncertainty to nuclear beta-decay play a major role in determining the significance of the reactor neutrino anomaly. Here we examine the common approximation used for one-body weak magnetism in the calculation of fission antineutrino spectra, wherein matrix elements of the orbital angular momentum operator contribution to the magnetic dipole current are assumed to be proportional to those of the spin operator. Although we find this approximation invalid for a large set of nuclear structure situations, we conclude that it is valid for the relevant allowed beta-decays between fission fragments. In particular, the uncertainty in the fission antineutrino due to the uncertainty in the one-body weak magnetism correction is found to be less than 1%. Thus, the dominant uncertainty from weak magnetism for reactor neutrino fluxes lies in the uncertainty in the two-body meson-exchange magnetic dipole current.