TOP PAIR PRODUCTION IN e + e − AND γγ PROCESSES

In 1994, the top quark was discovered by the CDF and D0 collaborations . The measured top quark mass is approximately 175 GeV, which is nearly twice the mass scale of electro-weak symmetry breaking. So the top quark with large mass brings a good opportunity to understand electro-weak symmetry breaking and to search hints of any new physics. The top quark decays electroweakly before hadronizing because its width is much greater than the hadronization timescale set by ΛQCD . Therefore, there are significant angular correlations between the decay products of the top quark and the spin of the top quark. These angular correlations depend sensitively on the top quark couplings to the Z boson and photon, and to the W boson and b quark. This aspect is unique for the top quark sector. Actually, there are many works 6 on the angular correlations for the top quark events produced at ee colliders. In most of these works, the top quark spin is decomposed in the helicity basis. However, G. Mahlon and S. Parke 4 suggested a more optimal decomposition of the top quark spin to find spin correlations at hadron colliders. S. Parke and Y. Shadmi 5 extended their work to the ee annihilation process at the leading order in perturbation theory and concluded that the “Off-Diagonal” basis is the best decomposition of top pair spins.