Lattice QCD had allowed to compute spectrum of bounded states of hadrons. Accurate measures of the mass of a hadron can be obtained by Monte-Carlo methods and Lattice QCD (see here). On the other hand, classical perturbative methods in QED and Weinberg–Salam theory, are usually applied to non-bounded (collision states).
- What alternative is there to find bound states of a quantum field theory, other than lattice theories?
- Is there a case where we can use perturbative methods or Feynman diagrams to find bound states of an interacting field theory?