Valence QCD: Connecting QCD to the quark model - art. no. 112001

A valence QCD theory is developed to study the valence quark properties of hadrons. To keep only the valence degrees of freedom, the pair creation thr ough the Z graphs is deleted in the connected insertions, whereas the sea q uarks are eliminated in the disconnected insertions. This is achieved with a new "valence QCD" Lagrangian where the action in the time direction is mo dified so that the particle and antiparticle decouple. It is shown in this valence version of QCD that the ratios of isovector to isoscalar matrix ele ments (e.g., F-A/D-A and F-S/D-S ratios) in the nucleon reproduce the SU(6) quark model predictions in a lattice QCD calculation. We also consider how the hadron masses are affected on the lattice and discover new insights in to the origin of dynamical mass generation. It is found that, within statis tical errors, the nucleon and the Delta become degenerate for the quark mas ses we have studied (ranging from 1 to 4 times the strange mass). The pi an d rho become nearly degenerate in this range. It is shown that valence QCD has the C, P, T symmetries. The lattice version is reflection positive. It also has the vector and axial symmetries. The latter leads to a modified pa rtially conserved axial Ward identity. As a result, the theory has a U(2N(F )) symmetry in the particle-antiparticle space. Through lattice simulation, it appears that this is dynamically broken down to U-q(N-F)xU(q)(-)(N-F). Furthermore, the lattice simulation reveals spin degeneracy in the hadron m asses and various matrix elements. This leads to an approximate U-q(2N(F))x U(q)(-)(2N(F)) symmetry which is the basis for the valence quark model. In addition, we find that the masses of N, Delta, rho, pi, a(1), and a(0) all drop precipitously compared to their counterparts in the quenched QCD calcu lation. This is interpreted as due to the disappearance of the ''constituen t'' quark mass which is dynamically generated through tadpole diagrams. The origin of the hyperfine splitting in the baryon is largely attributed to t he Goldstone boson exchanges between the quarks. Both of these are the cons equences of the lack of chiral symmetry in valence QCD. We discuss its impl ications concerning the models of hadrons. [S0556-2821(99)01009-7].