Thresholdtt¯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 ν. A detailed analysis of the residual ν dependence is carried out. A conservative estimate for the remaining uncertainty in the normalization of the total cross section from QCD effects is $\delta {\sigma }_{t\overline{t}}/{\sigma }_{t\overline{t}}=\pm 3\%.$ This makes precise extractions of the strong coupling and top width feasible, and further studies of electroweak effects mandatory.