Limits on tensor coupling from neutronβdecay

Abstract

Limits on the tensor couplings generating a Fierz interference term $b$ in mixed Gamow-Teller Fermi decays can be derived by combining data from measurements of angular correlation parameters in neutron decay, the neutron lifetime, and ${G}_{\mathrm{V}}={G}_{\mathrm{F}}{V}_{ud}$ as extracted from measurements of the $\mathcal{F}t$ values from the ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ superallowed decay data set. These limits are derived by comparing the neutron $\ensuremath{\beta}$-decay rate as predicted in the standard model with the measured decay rate while allowing for the existence of beyond the standard model (BSM) couplings. We analyze limits derived from the electron-neutrino asymmetry $a$, or the beta asymmetry $A$, finding that the most stringent limits for ${C}_{\mathrm{T}}/{C}_{\mathrm{A}}$ under the assumption of no right-handed neutrinos is $\ensuremath{-}0.0026<{C}_{\mathrm{T}}/{C}_{\mathrm{A}}<0.0024$ (95% C.L.) for the two most recent values of $A$. The derived limits on scalar and tensor couplings have the useful property that they are independent of BSM extensions with vector or axial-vector symmetry to first order.