Monte Carlo generation of two body resonant states

For Monte Carlo calculations of elementary particle processes, the transition rate integral can be written in terms of intermediate mass variables. Events generated in this way bear a weight factor consisting of the product of the squared transition amplitude and several two-body phase space factors. Importance sampling, using a Breit--Wigner density, can improve efficiency in the case that one or more mass variables of the transition density are well approximated by a Breit--Wigner resonance. With the further restriction that the resonance occurs close to threshold and decays to two final-state particles, the efficiency was improved by sampling a probability density which is the product of a Breit--Wigner density times the associated two-body phase space factor