Precision Measurements and CKM Unitarity

Abstract

Determinations of $\text{vud}$ and $\text{vus}$ along with their implications for the unitarity test ${\text{vud}}^2+{\text{vus}}^2+{\text{vub}}^2=1$ are discussed. The leading two loop radiative corrections to neutron $\beta$-decay are evaluated and used to derive a refined relationship ${\text{vud}}^2{\tau }_n(1+3g_A^2)=4908(4)$sec. Employing $\text{vud}=0.9740(5)$ from superallowed nuclear decays and the measured neutron lifetime ${\tau }_n=885.7(7)$sec, leads to the precise prediction $g_A=1.2703(8)$ which is compared with current direct experimental values. Various extractions of $\text{vus}$ are described and updated. The long accepted Particle Data Group value of $\text{vus}$ from fitted $K_{e3}$ decay rates suggests a deviation from CKM unitarity but it is contradicted by more recent experimental results which confirm unitarity with good precision. An outlook for possible future advances is given.

Comments: 15 pages, version accepted for publication in the Physical Review D