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 2×10¹² decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give τ_μ⁺(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: GF(MuLan)=1.166 378 8(7)×10^-5 GeV^-2 (0.6 ppm). It is also used to extract the μ^-p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling gP.