New Directions in Quantum Chromodynamics∗

I review the light-cone Fock state represention and its associated light-cone factorization scheme as a method for encoding the flavor, momentum, and helicity properties of hadrons in the form of universal process-independent and frame-independent amplitudes. Discrete light-cone quantization (DLCQ) provides a matrix representation of the QCD Hamiltonian and a nonperturbative method for computing the quark and gluon bound state wavefunctions. A number of applications of the light-cone formalism are discussed, including an exact light-cone Fock state representation of semileptonic B decay amplitudes. Hard exclusive and diffractive reactions are shown to be sensitive to hadron distribution amplitudes, the valence Fock state hadronic wavefuctions at small impact separation. Semi-exclusive reactions are shown to provide new flavor-dependent probes of distribution amplituds and new types of deep inelastic currents. “Self-resolving” diffractive processes and Coulomb dissociation are discussed as a direct measure of the light-cone wavefunctions of hadrons. Alternatively, one can use Coulomb dissociation to resolve nuclei in terms of their nucleonic and mesonic degrees of freedom. I also discuss several theoretical tools which eliminate theoretical ambiguities in perturbative QCD predictions. For example, commensurate scale relations are perturbative QCD predictions based on conformal symmetry which relate observable to observable at fixed relative scale; such relations have no renormalization scale or scheme ambiguity. I also discuss the utility of the αV coupling, defined from the QCD heavy quark potential, as a useful physical expansion parameter for perturbative QCD and grand unification. New results on the analytic fermion masses dependence of the αV coupling at two-loop order are presented. Invited talk presented at International Summer School on Particle Production Spanning MeV and TeV Energies (Nijmegen 99) Nijmegen, The Netherlands 8–20 August 1999 ∗Work supported by Department of Energy contract DE–AC03–76SF00515.