Precision measurement of the positive muon lifetime by the MuLan collaboration

We report the result of a measurement of the positive muon lifetime τμ to one part-per-million (ppm) by the MuLan collaboration using a low-energy pulsed muon beam and a segmented array of plastic scintillators to record over 2×1012 decay positrons. Two different in-vacuum muon-stopping targets were used in separate data-taking periods. The combined result gives . This measurement of the muon lifetime provides the most precise determination of the Fermi constant, , and will be used to extract the capture rates of the negative muon on the proton and the deuteron in the ongoing MuCap and MuSun experiments.