A precision measurement of the Michel parameter ξ" in polarised muon decay

The present form of the Standard Model (SM) describes weak process in terms of vector and axial-vector type interaction, with fully left-handed massless neutrons. Any experimental deviation from the SM predictions will in special cases reveal the need for extensions to the SM. The longitudinal polarization of the positron emitted from polarized muon present a simple and sensitive case. We present hereafter the status of a longitudinal polarization measurement of these positrons.