Precision Measurements and CKM Unitarity

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  • 29 June 2004
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 beta-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, 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 |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.

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