Probing the electromagnetic dipole moment of the τ lepton in the e^+e^- γ^*/Z τ^+ τ^- reaction

High-precision measurements of the lepton's electromagnetic dipole moments provide a powerful probe for testing the Standard Model. A non-zero value of the τ lepton's electric (weak) dipole moment (d_τ^γ, d_τ^Z) would serve as a smoking gun of the new physics. On the one hand, current and future high energy colliders offer an ideal environment for such measurements. On the other hand, it is essential to investigate the optimal measurement method for extracting not only d_τ^γ and d_τ^Z, but also the anomalous magnetic dipole moment (a_τ). In this work, we analyze the precision of observables, particularly optimal observables, for determining these physics quantities through the process e^+e^- γ^*/Z τ^+ τ^- with τ leptons undergoing (semi-) leptonic and hadronic decays at the centre-of-mass energies at the Z pole mass and significantly below mZ. By considering the full kinematic information of the decay products, we find that the sensitivities to Imd_τ and Red_τ can reach 10^-21 ecm at CEPC s = mZ, compared to 10^-20 ecm at Belle-II Υ (4S) resonance and 10^-17 ecm at BEPCII ψ (2S) resonance. For a_τ, we find that Ima_τ and Rea_τ with a precision of 10^-5 at Belle-II s=10. 58 GeV can be attained.