New precision determination of gP and GF : the MuXperiments at PSI

We discuss two precision experiments which will measure fundamental weak interaction parameters: MuLAN’s goal is the measurement of the positive muon lifetime to 1 ppm, which will in turn determine the Fermi coupling constant GF to 0.5 ppm precision. MuCAP is the first experiment which will unambiguously determine the induced pseudoscalar form factor of the proton, gP . While contradictory experimental results for gP are under discussion, firm theoretical calculations on the percent level within the framework of Chiral Perturbation Theory are now challenging the measurements. We will describe our experimental efforts and latest achievements.