Michel parameters

The Michel parameters, usually denoted by ${\displaystyle \rho ,\eta ,\xi }$ and ${\displaystyle \delta }$, are four parameters used in describing the phase space distribution of leptonic decays of charged leptons, ${\displaystyle l_{i}^{-}\rightarrow l_{j}^{-}\nu _{i}{\bar {\nu _{j}}}}$. They are named after the physicist Louis Michel. Sometimes instead of ${\displaystyle \delta }$, the product ${\displaystyle \xi \delta }$ is quoted. Within the Standard Model of electroweak interactions, these parameters are expected to be

${\displaystyle \rho ={3 \over 4},\quad \eta =0,\quad \xi =1,\quad \xi \delta ={3 \over 4}.}$

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.