Precision Measurements and CKM Unitarity
Abstract
Determinations of $\vud$ and $\vus$ along with their implications for the unitarity test $\vud^2+\vus^2+\vub^2=1$ are discussed. The leading two loop radiative corrections to neutron
β
-decay are evaluated and used to derive a refined relationship $\vud^2 \tau_n (1+3g_A^2)=4908(4)$sec. Employing $\vud =0.9740(5)$ from superallowed nuclear decays and the measured neutron lifetime
τ
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 $\vus$ are described and updated. The long accepted Particle Data Group value of $\vus$ 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.