The tau weak-magnetic dipole moment

We calculate the prediction for the anomalous weak-magnetic form factor of the tau lepton at q² = M² within the Standard Model. With all particles on-shell, this is an electroweak gauge-invariant quantity. Its value is a_τ^w(M_Z²)= − (2.10+0.61 i) × 10⁻⁶. We show that the transverse and normal components of the single-tau polarization of tau pairs produced at e⁺e⁻ unpolarized collisions are sensitive to the real and absorptive parts of the anomalous weak-magnetic dipole moment of the tau. The sensitivity one can achieve at LEP in the measurement of this dipole moment is discussed.