Unstable-particles pair production in modified perturbation theory in NNLO
We consider pair production and decay of fundamental unstable particles in the framework of a modified perturbation theory (MPT), which treats resonant contributions of unstable particles in the sense of distributions. The cross-section of the process is calculated within the NNLO of the MPT in a model that admits exact solution. Universal massless-particles contributions are taken into consideration. The calculations are carried out by means of FORTRAN code with double precision, which ensures per mille accuracy of computations. A comparison of the outcomes with the exact solution reveals an excellent convergence of the MPT series at the energies close to and above the maximum of the cross-section. Near the maximum of the cross-section the discrepancy of the NNLO approximation makes up a few per mille.