Perturbative formulae for relativistic interactions of effective   particles
  The concept of effective particles as degrees of freedom in a relativistic quantum field theory is defined using a non-perturbative renormalization group procedure for Hamiltonians. However, every candidate for a basic physical theory appears to require an initial perturbative search for the set of interaction terms that may provide a basis with which the full effective theory Hamiltonian could be constructed in a series of successive approximations. This article describes the required perturbative expansion and illustrates it with a set of general 4th-order formulae. 
Stanislaw D. Glazek
hep-th