Is time reversal invariance violated in muon decay? A measurement of the transverse positron polarization

In the standard model (SM) of electroweak interactions the positron from the decay of polarized positive muons is mainly longitudinally polarized. However the model also predicts a small transverse polarization component PT1, which lies in the plane spanned by muon-spin and positron momentum. Interference with additional, scalar couplings would result in substantial values for PT1 as well as in a non-zero value of the transverse component PT2 which is perpendicular to the above mentioned plane. A nonzero component PT2, proportional to the imaginary part of a possible scalar coupling, would be the first observation of time reversal violation in a purely leptonic decay. Measuring PT1, which is proportional to the real part, amounts to a model independent determination of the Fermi-coupling constant. The μPT experiment [1] at the Paul Scherrer Institute will improve the current experimental limits PT2=(16±22)×10−3,PT2=(7±23)×10−3 by almost one order of magnitude. First preliminary results of the experiment a...