Measurement of the transverse polarization of positrons from the decay of polarized muons: A status report

A new experiment [1] 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 Delta P-T1 = Delta P-T2 = 3 x 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 cont ribution of exotic couplings to muon decay. In particular, the measurement of P-T1 allows the extraction of the so-called low energy Michel parameter eta, the accuracy of which presently limits a more precise model independen t determination of the Fermi coupling constant Gr. The measurement of P-T1 is at present the only possibility to best time reversal invariance in a pu rely leptonic reaction. The physics motivation is discussed together with a description of the experimental setup and results from test measurements.