Muon lifetime and Fermi constant: an update

We present an updated prediction for the lifetime of the muon including a detailed analysis of all relevant uncertainties. Our prediction includes QED corrections up to order α³ and state-of-the-art hadronic contributions based on dispersive methods. Radiative corrections and finite-electron-mass effects are parametrized by the correction factor $Δq$ in τ_μ⁻¹=GF²m_μ⁵(1+Δq)/(192π³), for which we obtain Δq=(-4\, 384\, 678 ± 34)× 10⁻⁹. This reduces the uncertainty associated with $Δq$ by an order of magnitude compared to the previous prediction at order α². We use our results to provide an updated value of the Fermi coupling constant, GF=1.166\,378\, 59 \, (59) × 10⁻⁵ \, GeV⁻². Further improvements will require better measurements of the muon lifetime and mass.