Modified approach for calculating vacuum susceptibility

By differentiating the dressed quark propagator with respect to a constant external field, the linear response of the nonperturbative dressed quark propagator in the presence of the constant external field can be obtained. From this general method, taking the vector vacuum susceptibility as an illustration, we one extract a model-independent expression for the vacuum susceptibility in the quantum chromodynamical (QCD) sum rule two-point external field formula. This expression for the vacuum susceptibility is quite different from that given in the previous literature. The numerical values of the vector vacuum susceptibility are calculated within the framework of the rainbow-ladder approximation of the Dyson-Schwinger approach. A comparison with the results of the previous approaches is given.