4.7 CP violation in the open $t \overline {t}$ region

Linear colliders can provide a clean environment for the study of CP violation in top-quark couplings in the process $e^+e^- \rightarrow$ $t \overline {t}$ and also in $\gamma\gamma\rightarrow$ $t \overline {t}$. CP violation in the production process can lead to a definite pattern of deviation of t and $\overline{t}$ polarizations from the predictions of SM. A specific example is the asymmetry between the rate of production of $t_L\overline{t}_L$ and $t_R\overline{t}_R$, where L,R denote helicities. Since top polarization is measured only by studying distributions of the decay products, it is advantageous to make predictions for these distributions, as far as possible, without reference to details of top reconstruction.

The usual procedure is to study either CP-violating asymmetries or correlations of CP-violating observables to get a handle on the CP-violating parameters of the underlying theory. For a given situation, correlations of optimal CP-violating variables correspond to the maximum statistical sensitivity. It is however convenient sometimes to consider variable which are simpler and can be handled more easily theoretically and experimentally.

CP-violation can be studied in e⁺e^- collisions as well in the $\gamma \gamma$collider option. Both these approaches are outlined below. In either case, it is seen that polarized beams help to increase the sensitivity.