Antonio Pagnamenta

The N-quantum approximation in static models

Abstract The N -quantum approximation (NQA) is a method designed to construct approximate operator solutions to the field equations. We show in detail how this method works and can be made compatible with the renormalization program in the Lee model and for the scalar field interacting with a fixed fermion source. The terminated in-field expansions can still be applied successfully in the presence of bound states, for which one will introduce new infields, or if one deals with resonances. This is discussed on the examples of the V -θ-bound state and the unstable V -particle.