Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant to Part-per-Million Precision

We report a measurement of the positive muon lifetime to a precision of 1. 0 ppm; it is the most precise particle lifetime ever measured. The experiment used a time-structured, low-energy muon beam and a segmented plastic scintillator array to record more than 210^12 decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give ^{+} (MuLan) =2 196 980. 3 (2. 2) ps, more than 15 times as precise as any previous experiment. The muon lifetime gives the most precise value for the Fermi constant: G₅ (MuLan) =1. 166 378 8 (7) 10^-5 GeV^-2 (0. 6 ppm). It is also used to extract the ^-p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling g₏.