Abstract
We develop a systematic approach to describe the finite lifetime effects in the threshold production of top quark-antiquark pairs. It is based on the nonrelativis-tic effective field theory with an additional scale ρ1/2m t characterizing the dynamics of the top-quark decay, which involves a new expansion parameter ρ = 1 − mW /mt. Our method naturally resolves the problem of spurious divergences in the analysis of the unsta-ble top production. Within this framework we compute the next-to-leading nonresonant contribution to the total cross section of the top quark-antiquark threshold production in electron-positron annihilation through high-order expansion in ρ and confirm the recently obtained result. We extend the analysis to the next-to-next-to-leading O(αs) nonreso- nant contribution which is derived in the leading order in ρ. The dominant nonresonant contribution to the top-antitop threshold production in hadronic collisions is also obtained.
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