Measurement of the transverse polarization of the e+ from the decay of polarized mu

The transverse polarization components of the positrons from the decay of polarized muons have been measured at the Paul Scherrer Institute by annihilating in flight the decay positrons with polarized electrons. The use of a stroboscopic method greatly reduces systematic errors. The energy dependence of the transverse polarization component PT1, which lies in the plane spanned by muon spin and positron momentum, yields the low energy parameter and thus an improved model-independent value of the Fermi coupling constant. A non-zero value of the transverse component PT2, which is perpendicular to the plane mentioned above, would be the first observation of time reversal violation in a purely leptonic decay. Part of the data (40 * 108 annihilation events) have been evaluated. Both PT1 and PT2, averaged over the positrons' energy, are consistent with zero with a precision improved by a factor of three as compared with the previous measurement. The preliminary results are PT1 > align=middle border=0 > = (7 +/- 8) * 10-3, PT2 > align=middle border=0 > = (-1 +/- 8) * 10-3 in full agreement with the standard model.