The MuLan Experiment: A New Measurement of the Fermi Constant at the Level of 1 ppm

The Fermi constant, GF is best determined through measurements of the positive muon lifetime, τμ. Until about 10 years ago, incomplete theoretical calculations limited the precision with which GF could be extracted from experiment - the publication of those results paved the way for a series of new measurements of τμ. The MuLan collaboration published its first result in 2007 (11 ppm on τμ, 5.5 ppm on GF) and will soon publish the final results from large production runs in 2006 and 2007. In my talk, I presented a preliminary result for τμ on behalf of the collaboration. We expect that the overall error on τμ will be about 1 ppm. I have also tried to explain how this very simple measurement is made and describe our strategies for controlling the systematic errors.