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The two most precise such measurements differ by about 3 standard deviations.","posed_since":"2020","precise":"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.","problem_ref":"5c69388e4c24c3c9742b11210c9fe730ac253e4cf1e3782ed7c8419bc9265c33","references":"","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.","status_note":"No measurement of a more precise than aSPECT had been published, to our knowledge, as of mid-2026.","title":"Tension between neutron-decay determinations of 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