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Using gauge invariant quark Green's functions, defined with path-orderedgluon field phase factors along polygonal lines, and functional relations amongthem, two compatible bound state equations of the Dirac type are establishedfor quark-antiquark systems, each relative to the quark or to the antiquark ofthe system. The kernels of the bound state equations are defined through aseries of Wilson loop averages along closed polygonal contours and theirfunctional derivatives on them. A sufficient criterion for spontaneous chiralsymmetry breaking is derived, relating the Goldstone boson wave function in thezero total momentum limit with the scalar part of the gauge invariant quarktwo-point Green's function.

Gauge invariant bound state equations for quark-antiquark systems in QCD