Precision measurements and CKM unitarity

TLDR

CKM unitarity tests rely on precise determinations of |V.

Abstract

Determinations of $|{V}_{ud}|$ and $|{V}_{us}|$ along with their implications for the unitarity test $|{V}_{ud}{|}^{2}+|{V}_{us}{|}^{2}+|{V}_{ub}{|}^{2}=1$ are discussed. The leading two-loop radiative corrections to neutron $\ensuremath{\beta}$ decay are evaluated and used to derive a refined relationship $|{V}_{ud}{|}^{2}{\ensuremath{\tau}}_{n}(1+3{g}_{A}^{2})=4908(4)\text{ }\mathrm{s}$. Employing $|{V}_{ud}|=0.9740(5)$ from superallowed nuclear decays and the measured neutron lifetime ${\ensuremath{\tau}}_{n}=885.7(7)\text{ }\mathrm{s}$ leads to the precise prediction ${g}_{A}=1.2703(8)$ which is compared with current direct experimental values. Various extractions of $|{V}_{us}|$ are described and updated. The long accepted Particle Data Group value of $|{V}_{us}|$ 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.