CP property of the Higgs at the gamma gamma colliders using $t \bar t$ production

Abstract

We present results of an investigation to study CP violation in the Higgs sector in $t\bar t$ production at a $\gamma\gamma$-collider, via the process $ \gamgam \ra \phi \ra t \bar t$ where the $\phi$ is a scalar with indeterminate CP parity. The study is performed in a model independent way parametrising the CP violating couplings in terms of six form factors $\{\Re(S_{\gamma}), \Im(S_{\gamma}), \Re(P_{\gamma}), \Im(P_{\gamma}), S_t, P_t\}$. The CP violation is reflected in the polarisation asymmetry of the produced top quark. We use the angular distribution of the decay lepton from $t/\bar t$ as a diagnostic of this polarisation asymmetry and hence of the CP mixing, after showing that the asymmetries in the angular distribution are indpendent of any CP violation in the $tbW$ vertex. We construct combined asymmetries in the initial state lepton (photon) polarization and the final state lepton charge and study how well different combinations of these form factors can be probed by measurements of these asymmetries, using circularly polarized photons. We demonstrate the feasibility of the method to probe CP violation in the Higgs sector at the level induced by loop effects in supersymmetric theories, using realistic photon spectra expected for a TESLA like $e^+ e^-$ collider. We investigate the sensitivity of our method for for different widths of the scalar as well as for the more realistic backscattered laser photon spectrum resulting from the inlcusion of the nonlinear effects.