On the Evaluation of Threshold Effects in Processes Involving Heavy Quarks

A detailed evaluation is presented of production of heavy fermions (particularly ttbar quarks) by a vector current, in the region around threshold. This includes bound states as well as the region above threshold, to the same degree of accuracy; in both cases radiative and nonperturbative corrections are included. The contribution of this region to the vacuum polarization (``threshold effects’') is calculated and compared with a perturbative calculation; the importance of the threshold effects is found substantially smaller than claimed by some authors. Open fermion-antifermion production is also discussed.