Top quark longitudinal polarization near the threshold in l+ l-   annihilation
  We show that the longitudinal polarization of the top quarks produced in the annihilation of e+ e- or mu+ mu- into tbar t at energies near the threshold is not affected by the large Coulomb-type corrections, which greatly modify the total cross section. Thus the longitudinal polarization, although small, may provide an independent information on the mass and the width of the top quark, largely independent of the uncertainty in alpha_s. 
B.M. Chibisov, M.B. Voloshin