First observation of weak magnetism in neutron decay
Miriam Camarasa-Domene, Martín González-Alonso, Martín Novoa-Brunet · arXiv: 2610.26792v1 · Summary page
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A missing signature in neutron decay
An effect predicted by the theory of the weak interaction has finally emerged from measurements of free-neutron decay, according to a new preprint. Known as weak magnetism, it connects the weak force responsible for beta decay with the electromagnetic properties of nucleons. The obstacle was not simply measurement precision: the beta asymmetry, which describes how electron emission depends on neutron spin, cannot by itself distinguish weak magnetism from another parameter of the decay. The researchers break that ambiguity by combining asymmetry measurements with the neutron lifetime and information from superallowed nuclear decays.
They obtain a weak-magnetism form factor of 4.47(87), consistent with the predicted 4.71. The significance remains strong across the alternative neutron datasets they consider, despite known disagreements among neutron measurements. The result turns a familiar theoretical ingredient into an experimentally isolated feature of the simplest nuclear beta-decay system—and illustrates how combining different observables can reveal something none can establish alone.
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