The Michel parameters, usually denoted by ρ ρ , η η , ξ ξ and δ δ , are four parameters used in describing the phase space distribution of leptonic decays of charged leptons, l i − − → → l j − − ν ν i ν ν j ¯ ¯ . They are named after the physicist Louis Michel. Sometimes instead of δ δ , the product ξ ξ δ δ is quoted. Within the Standard Model of electroweak interactions, these parameters are expected to be