Threshold t(t)over-bar cross section at next-to-next-to-leading logarithmic order

The total cross section for top quark pair production close to threshold in e(+)e(-) annihilation is investigated. Details are given about the calculation at next-to-next-to-leading logarithmic order. The summation of logarithms leads to a convergent expansion for the normalization of the cross section, and a small residual dependence on the subtraction parameter nu. A detailed analysis of the residual v dependence is carried out. A conservative estimate for the remaining uncertainty in the normalization of the total cross section from QCD effects is deltasigma(t (t) over bar)/sigma(t (t) over bar)=+/-3%. This makes precise extractions of the strong coupling and top width feasible, and further studies of electroweak effects mandatory.