The tau weak-magnetic dipole moment

Abstract We calculate the prediction for the anomalous weak-magnetic form factor of the tau lepton at q 2 = M 2 within the Standard Model. With all particles on-shell, this is an electroweak gauge-invariant quantity. Its value is a τ w ( M Z 2 )= − (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.