Tension between neutron-decay determinations of g_A/g_V · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
Tension between neutron-decay determinations of $g_A/g_V$
Plain
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 · changed · core
Revision 1: 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.
Revision 2: 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 $\mid \lambda \mid = 1.2677(28)$ (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 $3 \times 10^{-4}$, or a nonzero exotic coupling.
Settled by
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.
Kind
well-posed
Answer type
identification
References
(empty)
Literature status
open
Status note
No measurement of a more precise than aSPECT had been published, to our knowledge, as of mid-2026.
Posed since
2020

Decisions