Measurement of the transverse polarization of positrons from the decay of polarized muons: A status report
A new experiment  to measure both transverse components of the positron polarization from the decay of polarized muons starts data taking at PSI in Switzerland. The experiment aims at an accuracy of ΔPT1 = ΔPT2 = 3 × 10-3 and by that at an improvement of the existing limit by nearly one order of magnitude. These results will be sensitive tests for the contribution of exotic couplings to muon decay. In particular, the measurement of PT1 allows the extraction of the so-called low energy Michel parameter η, the accuracy of which presently limits a more precise model independent determination of the Fermi coupling constant GF. The measurement of PT2 is at present the only possibility to test time reversal invariance in a purely leptonic reaction. The physics motivation is discussed together with a description of the experimental setup and results from test measurements.