TIME-DEPENDENT VARIATIONAL PRINCIPLE FOR PHI(4) FIELD-THEORY .1. RPA APPROXIMATION AND RENORMALIZATION

We investigate the time-dependent variational equations in phi(4) field theory. We show that the standard method for renormalization applies to these time-dependent equations. The crucial point is to use the hamiltonian nature of the variational principle. In particular we have considered small oscillations about equilibrium and shown that these give the two meson modes of the theory. The two meson equation has a closed solution leading to a single bound state for attractive renormalized coupling and a complete form for the scattering amplitude in the continuum. This form is easily adapted to the usual running coupling constant in the two meson energy. We also find that the massless solution is the lowest minimum for a range of renormalized coupling constant and that this minimum is not stable, implying: that the actual lowest solution is not homogeneous. We have examined our equations for so called runaway solution where one of the physical parameters goes to infinity. Using the ''potential'' part of our variational hamiltonian we are able to show that conservation of energy prohibits any unphysical runaway. (C) 1995 Academic Press, Inc.