Complete 2-loop quantum electrodynamic contributions to the muon   lifetime in the Fermi model
  The complete 2-loop QED contributions to the muon lifetime have been calculated analytically in the Fermi theory. The exact result for the effects of virtual and real photons, virtual electrons, muons and hadrons as well as e+e- pair creation is   Delta Gamma^(2)=Gamma_0(alpha/pi)^2[(156815/5184)-(1036/27)zeta(2)   -(895/36)zeta(3)+(67/8)zeta(4)   +53zeta(2)ln(2)-(0.042+/-0.002)]   where Gamma_0 is the tree-level width. This eliminates the theoretical error in the extracted value of the Fermi coupling constant, G_F, which was previously the source of the dominant uncertainty. The new value is   G_F=(1.16637 +/- 0.00001) x 10^-5 GeV^-2   with the error being entirely experimental. Several experiments are planned for the next generation of muon lifetime measurements and these can proceed unhindered by theoretical uncertainties. 
Robin G. Stuart