Search for anomalous weak dipole moments of the τ lepton

The anomalous weak dipole moments of the τ lepton are measured in a data sample collected by ALEPH from 1990 to 1995 corresponding to an integrated luminosity of 155 pb^-1. Tau leptons produced in the reaction e ⁺e^- → τ⁺τ^- at energies close to the Z mass are studied using their semileptonic decays to π, ρ, a₁ → π2π⁰ or a₁ → 3π. The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment, Re μ _τ, Im μ_τ, Re d_τ and Im d _τ, are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained (95% CL): |Re μ_τ| < 1.14 × 10^-3, |Im μ_τ| < 2.65 × 10^-3, |Re d_τ| < 0.91 × 10^-3, and |Im d_τ| < 2.01 × 10^-3.