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Isolating CP-violating γZZ coupling in e+e- \to γZ with transverse beam polarizations

We revisit the process $e^+e^- \to γZ$ at the ILC with transverse beam polarization in the presence of anomalous CP-violating $γZ Z$ coupling $λ_1$ and $γγZ$ coupling $λ_2$. We point out that if the final-state spins are resolved, then it becomes possible to fingerprint the anomalous coupling {\rm Re}$λ_1$.90% confidence level limit on {\rm Re}$λ_1$ achievable at ILC with center-of-mass energy of 500 GeV or 800 GeV with realistic initial beam polarization and integrated luminosity is of the order of few times of $10^{-2}$ when the helicity of $Z$ is used and $10^{-3}$ when the helicity of $γ$ is used. The resulting corrections at quadratic order to the cross section and its influence on these limits are also evaluated and are shown to be small. The benefits of such polarization programmes at the ILC are compared and contrasted for the process at hand. We also discuss possible methods by which one can isolate events with a definite helicity for one of the final-state particles.