MuLan Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant

We report results from the MuLan measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured surface muon beam and a segmented plastic scintillator array. Two different in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field and a quartz crystal in a moderate external magnetic field. From a total of 1.6 x 10^{12} decays, we obtained the muon lifetime tau_mu = 2196980.3(2.2) ps (1.0 ppm) and Fermi constant G_F = 1.1663787(6) x 10^{-5} GeV^{-2} (0.5 ppm).