First observation of weak magnetism in neutron decay
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 $\mathcal{F}t$ value of superallowed $0^+\to0^+$ 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.