Muon lifetime measurements at PSI

Abstract

The Fermi constant, GF, is most precisely determined by the muon lifetime, τμ. Calculations of the two‐loop terms in the extraction of GF from τμ in the late 1990’s and early 2000’s reduced the theoretical uncertainty on the extraction by two orders of magnitude and motivated a new generation of muon lifetime experiments. The FAST and MuLan experiments have the most ambitious precision goals, and take place at Paul Scherrer Institute (PSI). Both have released intermediate results, and have compatible final precision goals of 2 ppm and 1 ppm respectively. Their intermediate measurements have improved the world average muon lifetime to τμ = 2.197035 μs is (8 ppm), and new results at the precision goals are expected in 2010. Although the goals are similar, the experiments have different systematic uncertainties and provide an excellent cross‐check on each other.