CP-violating Zγγ and top-quark electric dipole couplings in

An effective anomalous CP-violating Z coupling can give rise to observable CPodd effects in ! tt. We study certain asymmetries in the decay lepton distributions in ! tt arising from top decay in the presence of a CP-violating Z coupling as well as a top-quark electric dipole coupling. We find that a photon linear collider with geometric luminosity of 20 fb −1 can put limits of the order of 0.1 on the imaginary part of the CP-violating anomalous Z coupling using these asymmetries. While gauge-boson couplings to fermions have been measured with great accuracy and agreement of these measurements with predictions from the Standard Model (SM) is overwhelmingly precise, the area of pure gauge bososn couplings is not explored with that precision. Deviation of the gauge boson couplings from the SM values could be used to infer the presence of new physics. Such couplings arising from new physics could even be CP-violating. There have been detailed discussions on the anomalous triple gauge boson couplings WWV and ZV , where V = ,Z, in the literature [1, 2, 3]. Experimental bounds on these couplings obtained at LEP [4, 5, 6, 7] and at the Tevatron [8, 9] are fairly weak, and are found to be consistent with SM. While future experiments would improve these limits, effects of these couplings are expected to be more visible at higher energies, for example, at the proposed linear e + e − colliders. CP-violating triple gauge boson couplings get contributions only beyond one loop in SM. This makes them good candidates to study new physics effects. Theoretical studies have largely concentrated on W + W − and ZW + W − couplings, and less attention has been paid to neutral gauge-boson self-couplings. In particular, CP-violating Z and ZZ couplings have been the subject of few discussions. These couplings are absent at the tree level in SM, and any observation of these at a substantial level would signal new physics beyond SM. Our work concerns the measurement of the CP-violating Z coupling.