Light-Front-Quantized QCD in Light-Cone Gauge: The Doubly Transverse Gauge Propagator
The light-front (LF) quantization of QCD in light-cone gauge has a number of remarkable advantages, including explicit unitarity, a physical Fock expansion, the absence of ghost degrees of freedom, and the decoupling properties needed to prove factorization theorems in high momentum transfer inclusive and exclusive reactions. We present a systematic study of LF-quantized gauge theory following the Dirac method and construct the Dyson-Wick S-matrix expansion based on LF-time-ordered products. The free theory gauge field is shown to satisfy the Lorentz condition as an operator equation as well as the light-cone gauge condition. Its propagator is found to be transverse with respect to both its four-momentum and the gauge direction. The interaction Hamiltonian of QCD can be expressed in a form resembling that of covariant theory, except for additional instantaneous interactions which can be treated systematically. The renormalization constants in YM theory are shown to satisfy the identity $Z_1=Z_3$ at one loop order. The QCD $\beta$ function computed in the noncovariant light-cone gauge agrees with that known in the conventional framework. Some comments on the relationship of our LF framework, with the doubly transverse gauge propagator, to the analytic effective charge and renormalization scheme defined by the pinch technique, the unitarity relations and the spectral representation are also made. LF quantization thus provides a consistent formulation of gauge theory, despite the fact that the hyperplanes $x^{\pm}=0$ used to impose boundary conditions constitute characteristic surfaces of a hyperbolic partial differential equation.