Measuring the lifetime of the muon to 1 ppm with MuLan at PSI

The MuLan collaboration$^{1}$ will measure the positive muon lifetime to 1~ppm. This precision will determine the Fermi coupling constant $G_F$, which sets the strength of the weak interaction, to 0.5~ppm. In addition to its fundamental connection to the structure of the standard model, the $\mu^+$ lifetime is needed as a normalization for muon capture experiments. A $\pm$ 12.5~kV electric kicker is used to impose a pulsed structure on the continuous muon beam at the Paul Scherrer Institute, with a bunch of $\sim 50$ muons arriving within a 5 $\mu$s beam-on time followed by a 22 $\mu$s beam-off measuring period. Muons are stopped in a depolarizing target, either amorphous sulfur or Arnokrone-3, to reduce the effects of spin rotation; an external magnetic field is applied to dephase the muons over the accumulation time. A scintillator hodoscope with 170 tile pairs arranged in a truncated icosohedral (``soccer ball'') geometry provides a high level of segmentation to minimize pulse pileup. Similarly, new waveform digitizers will resolve pulses at separations of 4~ns. A dataset is in hand that should provide an initial result with 7 to 10~ppm precision, a factor of $\sim$ 2 better than the current world average, and major production running is scheduled in 2005 and 2006. MuLan is supported by the National Science Foundation and the Department of Energy $^{1}$ U.C. Berkeley - Boston U. - U. Illinois - Istanbul Tech. U. - James Madison U. - U. Kentucky - KVI - PSI