Abstract: BS>A Hamiltonian model of Lorentz invariant particle interactions is used as a framework in an attempt at describing massless particle decay. By restricting to the two particle sector an exact solution is found for the time development of the survival probability. Several modes are obtained and discussed. In particular it is shown that the presence of a persistent mode, i.e., the case in which the survival probability has a nonzero limiting value, has to be interpreted as indicating that the massless particle is a stable bound state. (auth)