Modified perturbation theory approach for tt-bar production and decay

Abstract

The modified perturbation theory (MPT), based on direct expansion of probabilities instead of amplitudes, allows one to avoid divergences in the phase-space integrals resulting from production and decay of unstable particles. In the present paper the range of applicability of MPT is determined numerically in the case of the process $e^{+}{e}^{-}\to (\gamma ,Z)\to t\bar{t}\to W^{+}b:W^{-}\bar{b}$. It is shown that with the complete expansion in powers of the coupling constant (without Dyson resummation) MPT operates best at the energies located near the maximum of the cross-section and slightly above the maximum. In this region the MPT expansion within the next-to-leading order considerably exceeds in accuracy well-known DPA approach.

Comments: LaTeX, 11 pages, 3 eps figures