In quantum chromodynamics, the confining and strong coupling nature of the theory means that conventional perturbative techniques often fail to apply.
The correlation function of these currents is introduced and treated in the framework of the operator product expansion (OPE), where the short and long-distance quark-gluon interactions are separated.
The former are calculated using QCD perturbation theory, whereas the latter are parametrized in terms of universal vacuum condensates or light-cone distribution amplitudes.
The result of the QCD calculation is then matched, via dispersion relation, to a sum over hadronic states.
Inversely, the parameters of QCD such as quark masses and vacuum condensate densities can be extracted from sum rules which have experimentally known hadronic parts.