Constraining Electroweak Physics

The most fundamental observable related to the weak interaction is the muon lifetime, τμ. With the electromagnetic two-loop contribution known1, τμ can be used unambiguously to extract the Fermi constant, GF = 1.16637(1)× 10 −5 GeV, where the uncertainty is completely dominated by experiment. Adding the fine structure constant, α, one can obtain two relations between the intermediate gauge boson masses, MW,Z , and the weak mixing angle2, sin θ̂W ≡ ŝ 2 = A M2 W (1−∆r̂W ) , ŝ(1− ŝ) = A M2 Z(1−∆r̂Z) . (1)