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The Fermi Constant, G_F, describes the strength of the weak force and isdetermined most precisely from the mean life of the positive muon, tau_mu.Advances in theory have reduced the theoretical uncertainty on G_F ascalculated from tau_mu to a few tenths of a part per million (ppm). Untilrecently, the remaining uncertainty on G_F was entirely experimental anddominated by the uncertainty on tau_mu. We report the MuLan collaboration'srecent 1.0 ppm measurement of the positive muon lifetime. This measurement isover a factor of 15 more precise than any previous measurement, and is the mostprecise particle lifetime ever measured. The experiment used a time-structuredlow-energy muon beam and an array of plastic scintillators read-out by waveformdigitizers and a fast data acquisition system to record over 2 times 10^{12}muon decays. Two different in-vacuum muon-stopping targets were used inseparate data-taking periods. The results from these two data-taking periodsare in excellent agreement. The combined results givetau_{mu^+}({MuLan})=2196980.3(2.2) ps. This measurement of the muon lifetimegives the most precise value for the Fermi Constant: G_F({MuLan}) = 1.1663788(7) \times 10^{-5} {GeV}^{-2} (0.6 ppm). The lifetime is also used to extractthe mu^-p singlet capture rate, which determines the proton's weak inducedpseudoscalar coupling g_P.

A new part-per-million measurement of the positive muon lifetime and determination of the Fermi Constant