  The Fermi constant, G_F, describes the strength of the weak force and is
determined most precisely from the mean life of the positive muon, tau_mu.
Advances in theory have reduced the theoretical uncertainty on G_F as
calculated from tau_mu to a few tenths of a part per million (ppm). The
remaining uncertainty on G_F is entirely experimental, and is dominated by the
uncertainty on tau_mu. The MuLan experiment is designed to measure the muon
lifetime to part-per-million precision, a better-than twenty-fold improvement
over the previous generation of experiments. In 2007, we reported an
intermediate result, tau_mu=2.197013(24) microseconds (11 ppm), which is in
excellent agreement with the previous world average. This mean life was
measured using a pulsed surface muon beam stopped in a ferromagnetic target,
surrounded by a symmetric scintillator detector array. Since this intermediate
measurement, the detector was instrumented with waveform digitizers, the muon
beam rate and beam extinction were increased, and two data sets were acquired
on different targets, each containing over 10^12 muon decays. These data will
lead to a new determination of G_F to better than a part per million.
