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Spin correlations in $τ$-lepton pair production due to anomalous magnetic and electric dipole moments

We present a simple algorithm for the calculation of event weights embedding the effects of anomalous electric and magnetic dipole moments in simulation of $e^-e^+\to τ^-τ^+ (nγ)$ events, and the subsequent decay of the $τ$ leptons produced. The impact of these weights on the spin-correlation matrix and the total cross-section is taken into account. The algorithm is prepared to work in-situ the {\tt KKMC} Monte Carlo, without the need for introducing any external change to the generator libraries. As an example, $e^-e^+ \to τ^-τ^+ (nγ), \; τ^- \to ρ^- ν_τ\to π^- π^0 ν_τ, \ τ^+ \to ρ^+ \barν_τ\to π^+ π^0 \barν_τ$ events were simulated at a center-of-mass energy of 10.58 GeV. The distributions of the acoplanarity angle between the planes spanned by the $π^- π^0$ and the $π^+ π^0$ momenta of respectively $ρ^-$ and $ρ^+$ decays and in the rest-frame of the entirely visible $ρ^-ρ^+$ system, are presented for different values of the coupling constants incorporating anomalous electric and magnetic dipole moments in the $τ^- τ^+ γ$ vertex.