Generalized Unitarity Method for Unstable Particles

In theories with unstable particles, unitarity is satisfied by the inclusion of only stable states in unitarity sums. Therefore, unitarity cuts are not to be taken through unstable particles. This presents a challenge to the generalized unitarity method, whose aim is to reconstruct amplitudes by analyzing sets of unitarity cuts. Nevertheless, under some general physical conditions, and perhaps some methodological modifications, we prove that the method is still reliable for one-loop amplitudes containing resonances. We discuss some simple examples which illustrate these features.