Decay-lepton angular asymmetries in polarized e+e- -> t tbar as a measure of CP-violating dipole couplings of the top quark

Abstract

In the presence of an electric dipole coupling of the top quark to a photon, and an analogous "weak" dipole coupling to the Z, CP violation in the process e+e- -> t tbar leads to the polarization of the top and anti-top. This polarization can be analyzed by studying the angular distributions of decay charged leptons when the top or anti-top decays leptonically. We have obtained analytic expressions for these distributions when either t or tbar decays leptonically, including O(alpha_s) QCD corrections in the soft-gluon approximation. The angular distributions are insensitive to anomalous interactions in top decay. We study two types of simple CP-violating polar-angle asymmetries and two azimuthal asymmetries which do not need the full reconstruction of the t or tbar. We have evaluated independent 90% CL limits that may be obtained on the real and imaginary parts of the electric and weak dipole couplings at a linear collider operating at centre-of-mass energy 500 GeV with integrated luminosity 200 inverse fb and also at 1000 GeV with integrated luminosity 1000 inverse fb. The effect of longitudinal beam polarization has been included.