Analytic results for e(+)e(-) -> t(t)over-bar and gamma gamma -> t(t)over-bar observables near the threshold up to the next-to-next-to-leading order of NRQCD

We present an analytic description of top-antitop pair production near the threshold in e(+)e(-) annihilation and gamma gamma collisions. A set of bas ic observables being considered includes total cross sections, forward-back ward asymmetry, and top-quark polarization. Threshold effects relevant to t he basic observables are described by three universal functions related to S-wave production, P-wave production, and S-P interference. These functions are computed analytically up to the next-to-next-to-leading order of NRQCD . The total e(+)e(-) --> t (t) over bar cross section near the threshold is obtained in the next-to-next-to-leading order in a closed form including t he contribution originating from the axial coupling of top quarks that is m ediated by the Z boson. The effects of the running of the strong coupling c onstant and the finite top-quark width are taken into account analytically for the P-wave production and S-P wave interference. (C) 2001 MAIK "Nauka/I nterperiodica".