2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45197Multiparticle production in (2+1) dimensions is investigated. We show that in a small region around the threshold the perturbation theory becomes unapplicable due to infrared divergencies in a class of Feynman graphs with rescattering in final state. We develop a technique based on renormalization group for summing up leading logarithms and apply it to the $ϕ^4$ models with and without symmetry breaking and to the $O(N)$ $ϕ^4$ theory.based on talk at International Seminar Quarks-94, 11-18 May 1994, Vladimir, Russia, REVTEX 3.0, 13pp, 3 figures included (use bezier.sty)High Energy Physics - Phenomenologytext