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The Michel parameters, usually denoted by
Precise measurements of energy and angular distributions of the daughter leptons in decays of polarized muons and tau leptons are so far in good agreement with these predictions of the Standard Model.
Muon decay
Let us consider the decay of the positive muon:
In the muon rest frame, energy and angular distributions of the positrons emitted in the decay of a polarised muon expressed in terms of Michel parameters are the following, neglecting electron and neutrino masses and the radiative corrections:
where
For the decay of the positive muon, the expected decay distribution for the Standard Model values of Michel parameters is
Integration of this expression over electron energy gives the angular distribution of the daughter positrons:
The positron energy distribution integrated over the polar angle is