First observation of weak magnetism in neutron decay
Abstract
Weak magnetism, introduced by Gell-Mann as a hallmark consequence of the conserved vector current (CVC) hypothesis, has remained unobserved in the decay of the free neutron, the simplest β-decay system. The reason is that the relevant observable, the β asymmetry, cannot separate the weak-magnetism form factor gM from the axial-to-vector ratio λ. We show that this degeneracy is lifted by combining the β asymmetry measurements with the neutron lifetime and the ℱt value of superallowed 0+→0+ decays. Current data yield gM = 4.47(87), in agreement with the CVC prediction of $4.71$ and excluding the absence of weak magnetism at the 5.1σ level. Importantly, the significance remains between 5.0σ and 6.1σ for all considered variations of the neutron dataset, despite its well-known internal tensions. We further show that, under present experimental conditions, a full analysis of the energy dependence of the asymmetry would improve the gM extraction by only a few percent. These results constitute the first observation of weak magnetism in the decay of the free neutron nearly seven decades after Gell-Mann's prediction.
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BibTeX
@misc{camarasadomene2026first,
title = {{First observation of weak magnetism in neutron decay}},
author = {Miriam Camarasa-Domene and Martín González-Alonso and Martín Novoa-Brunet},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.26792},
eprint = {2610.26792},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.26792},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - First observation of weak magnetism in neutron decay
AU - Miriam Camarasa-Domene
AU - Martín González-Alonso
AU - Martín Novoa-Brunet
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2610.26792
UR - https://arxiv.org/abs/2610.26792
AB - Weak magnetism, introduced by Gell-Mann as a hallmark consequence of the conserved vector current (CVC) hypothesis, has remained unobserved in the decay of the free neutron, the simplest β-decay system. The reason is that the relevant observable, the β asymmetry, cannot separate the weak-magnetism form factor g_M from the axial-to-vector ratio λ. We show that this degeneracy is lifted by combining the β asymmetry measurements with the neutron lifetime and the ℱt value of superallowed 0^+→0^+ decays. Current data yield g_M = 4.47(87), in agreement with the CVC prediction of $4.71$ and excluding the absence of weak magnetism at the 5.1σ level. Importantly, the significance remains between 5.0σ and 6.1σ for all considered variations of the neutron dataset, despite its well-known internal tensions. We further show that, under present experimental conditions, a full analysis of the energy dependence of the asymmetry would improve the g_M extraction by only a few percent. These results constitute the first observation of weak magnetism in the decay of the free neutron nearly seven decades after Gell-Mann's prediction.
ER -