The part-per-million measurement of the positive muon lifetime and
determination of the Fermi constant by the MuLan experiment at the Paul
Scherrer Institute is reviewed. The experiment used an innovative,
time-structured, surface muon beam and a
near-4\piπ,
finely-segmented, plastic scintillator positron detector. Two 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. The experiment acquired a dataset of
1.6 \times 10^{12}1.6×1012
positive muon decays and obtained a muon lifetime
\tau_{\mu} = 2\, 196\, 980.3(2.2)τμ=2196980.3(2.2)~ps
(1.0~ppm) and Fermi constant G_F = 1.166\, 378\, 7(6) \times 10^{-5}F=1.1663787(6)×10−5
GeV^{-2}−2
(0.5~ppm). The thirty-fold improvement in \tau_{\mu}τμ
has proven valuable for precision measurements in nuclear muon capture
and the commensurate improvement in G_FF
has proven valuable for precision tests of the standard model.
Detailed report of the MuLan measurement of the positive muon lifetime and determination of the Fermi constant
