2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/189058The notion that elementary systems correspond to irreducible representations of the Poincare group is the starting point for this paper, which then goes on to discuss how a semigroup for the time evolution of unstable states and resonances could emerge from the underlying Poincare symmetry. Important tools in this analysis are the Clebsch-Gordan coefficients for the Poincare group.17 pages, 1 figureHigh Energy Physics - Phenomenologytext