Next-to-leading-logarithmic QCD corrections to the cross section σ(e+e-→t<over bar>t<over bar>H) at 500 GeV

We determine the next-to-leading logarithmic (NLL) QCD corrections to the cross section sigma(e(+)e(-)-> t (t) over barH) for center-of-mass (c.m.) energies up to 500 GeV. The dynamics is dominated by nonrelativistic effects, and the summation of terms singular in the relative t (t) over bar velocity is mandatory to all orders in the strong coupling constant alpha(s) using an effective theory. The summations lead to an enhancement of the tree-level predictions by about a factor of 2 and are important for the determination of the top Yukawa coupling. We also study the impact of polarization of the electron-positron beams and provide a fast approximation formula for the known O(alpha(s)) QCD fixed-order prediction.