Study of CP Property of the Higgs at a Photon Collider using   $\gamma\gamma\to t\bar t\to l X $
  We study possible effects of CP violation in the Higgs sector on $t\bar t$ production at a $\gamma\gamma$-collider. These studies are performed in a model-independent way in terms of six form-factors $\{\Re(S_{\gamma}), \Im(S_{\gamma}), \Re(P_{\gamma}), \Im(P_{\gamma}), S_t, P_t\}$ which parametrize the CP mixing in the Higgs sector, and a strategy for their determination is developed. We observe that the angular distribution of the decay lepton from $t/\bar t$ produced in this process is independent of any CP violation in the $tbW$ vertex and hence best suited for studying CP mixing in the Higgs sector. Analytical expressions are obtained for the angular distribution of leptons in the c.m. frame of the two colliding photons for a general polarization state of the incoming photons. We construct combined asymmetries in the initial state lepton (photon) polarization and the final state lepton charge. They involve CP even ($x$'s) and odd ($y$'s) combinations of the mixing parameters. We study limits up to which the values of $x$ and $y$, with only two of them allowed to vary at a time, can be probed by measurements of these asymmetries, using circularly polarized photons. We use the numerical values of the asymmetries predicted by various models to discriminate among them. We show that this method can be sensitive to the loop-induced CP violation in the Higgs sector in the MSSM. 
Rohini M. Godbole, Saurabh D. Rindani and Ritesh K. Singh