Decay of Scalar Condensation in Quantum Field Theory

We consider decay processes of scalar-field condensation in the framework of well-established quantum field theory. We postulate that the quantum state corresponding to the scalar-field condensation is so-called coherent state with discussing the validity of such a treatment. We show that, by using the unitarity relation of the scattering matrix, decay rate of the coherent state is systematically calculated. We apply our procedure to derive explicit formulae of decay rates for two cases: (i) we study the case where the scalar condensation decays into a pair of scalar particles and show that our formalism reproduces the results obtained from the parametric-resonance analysis, and (ii) we also calculate the decay rate when the coherent state decays via anomaly.