Weak magnetism, predicted by Murray Gell-Mann nearly seven decades ago as a direct consequence of the conserved vector current (CVC) hypothesis, has been observed for the first time in the decay of the free neutron. This is the simplest β-decay system that exists, yet until now the key observable, the β asymmetry, could not disentangle the weak-magnetism form factor g_M from the axial-to-vector ratio λ. By combining β-asymmetry measurements with the neutron lifetime and the Ft value of superallowed 0⁺→0⁺ transitions, the authors resolve that degeneracy.

Current data yield g_M = 4.47(87), in good agreement with the CVC prediction of 4.71. The hypothesis of no weak magnetism at all is excluded at the 5.1σ level. The robustness of the result is striking: the significance remains between 5.0σ and 6.1σ across all considered variations of the neutron dataset, despite the well-known internal tensions that these measurements carry.

The work also assesses how far a detailed analysis of the energy dependence of the β asymmetry could refine the extraction of g_M. The conclusion is that, under present experimental conditions, that route would improve the determination by only a small margin, just a few percent. The more promising path forward, therefore, lies not in dissecting the shape of the spectrum but in improving the precision of the quantities that feed the global combination.

The result matters on two counts. On the one hand, it confirms a pillar of the Standard Model —the structure of the hadronic weak current and its conservation— in the cleanest possible nuclear setting. On the other, it closes a chapter open since 1958, when Gell-Mann formulated the prediction. The measurement of g_M also becomes a useful tool to cross-check future determinations of the ratio g_A/g_V and of the neutron lifetime, two quantities that continue to fuel debate in nuclear and particle physics.