Hadron and nuclear physics on the light front

The QCD light-front (LF) Hamiltonian equation H_LF|Ψ> = M²|Ψ> derived from quantization at fixed LF time τ = t+z/c provides a causal, frame-independent, method for computing hadron spectroscopy as well as dynamical observables such as structure functions, transverse momentum distributions, and distribution amplitudes. The LF formalism also leads to novel nuclear phenomena, such as "hidden color", "color transparency", "nuclear-bound quarkonium" and the shadowing and antishadowing of nuclear structure functions. For example, there are five distinct color-singlet Fock state representations of the six color-triplet quarks of the deuteron. The hidden color Fock states become manifest when the deuteron is probed when it has small transverse size, as in measurements of the deuteron form factor at large momentum transfer. The QCD Lagrangian with zero quark mass has no explicit mass scale. However, as shown by de Alfaro, Fubini, and Furlan (dAFF), a mass scale can appear in the equations of motion without affecting the conformal invariance of the Action. When one applies the dAFF procedure to the QCD LF Hamiltonian, it leads to a color confining potential κ⁴ζ² for mesons, where ζ² is the LF radial variable conjugate to the qq-bar invariant mass squared. The same result, including spin terms, is obtained using LF holography (AdS/QCD) - the duality between LF dynamics and AdS₅ - if one modifies the AdS₅ Action by the dilaton exp +κ²z² in the fifth dimension z. If this procedure is generalized using superconformal algebra, the resulting LF eigensolutions provide unified Regge spectroscopy of mesons, baryons, and tetraquarks, including remarkable supersymmetric relations between the masses of mesons, baryons and tetraquarks with a universal Regge slope. One also obtains empirically viable predictions for spacelike and timelike hadronic form factors, structure functions, distribution amplitudes, and transverse momentum distributions. AdS/QCD also predicts the analytic form of the nonperturbative running coupling α_s(Q²) ∝ exp(-Q²/4κ²), in agreement with the effective charge measured from measurements of the Bjorken sum rule. The mass scale κ underlying hadron masses can be connected to the parameter Λ_M-bar_S-bar in the QCD running coupling by matching the nonperturbative dynamics to the perturbative QCD regime. The result is an effective coupling α_s(Q²) defined at all momenta. (author)