A Constituent Picture of Hadrons from Light-Front QCD
  It may be possible to derive a constituent approximation for bound states in QCD using hamiltonian light-front field theory. Cutoffs that violate explicit gauge invariance and Lorentz covariance must be employed. A similarity renormalization group and coupling coherence are used to compute the effective hamiltonian as an expansion in powers of the canonical QCD running coupling constant. At second order the QCD hamiltonian contains a confining interaction, which is being studied using bound state perturbation theory. Explicit constituent masses appear because of symmetry violations, and confinement also produces mass gaps, leading to the possibility of an accurate non-perturbative constituent approximation emerging in light-front QCD. 
Robert J. Perry