Top quark polarization as a probe of $t\bar{t}$ threshold dynamics
  We study the spin properties of top quarks produced in collisions of polarized photons in the threshold region. For a relatively heavy top quark the influence of non-perturbative effects is small and its polarization parameters can be predicted in perturbative QCD. The measurements of the top polarization may allow a novel test of QCD in the $t\bar{t}$ system. In particular, they may provide a new way to determine the precise value of $\alpha_s$ and to study the properties of the top quark. 
V.S. Fadin, V.A. Khoze and M.I. Kotsky