Michel parameters

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The Michel parameters, usually denoted by $ρ,η,ξ$ and $δ$ , are four parameters used in describing the leptonic decays of charged leptons, $l_{i}^-\rightarrow l_{j}^{-}\nu_{i}\bar{\nu_{j}}$ . Sometimes instead of $δ$ , the product $ξδ$ is quoted. They are named after the physicist Louis Michel.

Within the Standard Model of electroweak interactions, these parameters are expected to be:

$\rho={3\over4} \quad \eta=0 \quad \xi=1 \quad \xi\delta={3\over4}$

Precise measurements of the energy and angular distributions of the product lepton in decays of polarized muons and taus are so far in good agreement with these predictions of the Standard Model.