The Michel parameter for the decay τ→eνν

Abstract The Crystal Ball detector at the Doris II storage ring at DESY has been used to study the electron energy spectrum of the decay τ→evv. In the standard model the Lorentz structure of the decay matrix element is of the well known V-A type, resulting in a Michel parameter of p =0.75. Our spectrum is found to be consistent with this value. A fit yields the value of 0.64±0.07.