New determination of the Michel parameter in tau decay

We have used the leptonic decays of the tau to determine the Lorentz structure of its decay vertex. We find the Michel parameter {ρ} to be 0.71±{}0.09±{}0.03, which is consistent with V-A ({ρ}=0.75) and inconsistent with V+A. This measurement represents the first such analysis using muons in the final state.