Effective theory approach to unstable particle production

A theory that allows calculations for resonant processes to be systematically expanded in powers of the coupling α and γ÷M was described. A toy model that involved a massive scalar field φ and two fermion fields was used in the study. The hierarchy of scales γ<<M was used in resonant particle production, where M is the pole mass. The method was illustrated by computing the next-to-leading order line shape of a scalar resonance in a Abelian gauge-Yukawa model.