On-shell expansion of effective action and quark-line diagrams in quantum chromodynamics.

The formalism of the on-shell expansion of the effective action allows us to obtain an exact expression for the scattering amplitude among the bound states which are excited above the nonperturbatively condensed vacuum. The on-shell expansion scheme is first generalized to the Grassmannian variables and then applied to get the two-body hadronic scattering amplitude within the framework of quantum chromodynamics. It automatically separates the diagrams which contribute to the condensation, wave function, and scattering parts. An exact expression for the quark-line diagrams are thus obtained.