Tension between neutron-decay determinations of $g_A/g_V$
In plain words
The ratio $\lambda$ of the two weak couplings in neutron decay can be read from the electron's preferred direction relative to the neutron spin, or from the angle between the electron and the antineutrino. The two most precise such measurements differ by about 3 standard deviations.
Precise statement
PERKEO III, from the $\beta$ asymmetry $A$, gives $\lambda = -1.27641(45)(33)$ (Markisch et al., PRL 122, 242501, 2019); aSPECT, from the electron-antineutrino correlation $a = -0.10430(84)$, gives $|\lambda| = 1.2677(28)$ (Beck et al., PRC 101, 055506, 2020). Determine whether the difference is an experimental systematic or a sign of scalar or tensor couplings that shift A, a and the Fierz term $b$ differently. Answer: a consistent $\lambda$ from $A, a \text{ and } \tau_{n}$ with uncertainty below $3e-4$, or a nonzero exotic coupling.
What would settle it
The Nab measurement of a and b at the Spallation Neutron Source, combined with $A, a, b\ \text{and}\ \tau_n$ in a global fit.
Status in the literature
Unverified note
No measurement of a more precise than aSPECT had been published, to our knowledge, as of mid-2026.