Magnetic susceptibility of the QCD vacuum

We investigate the magnetic susceptibility of the QCD vacuum, based on the instanton vacuum. Starting from the instanton liquid model for the instanton vacuum, we derive the light-quark partition function $Z[V,T,\hat{m}]$ in the presence of the current quark mass $\hat{m}$ as well as the external Abelian vector and tensor fields. We calculate a two-point correlation function relevant for the magnetic susceptibility and derive it beyond the chiral limit. We obtain for the different flavors the following magnetic susceptibility: $\chi_{u,d}< i\psi_{u,d}^\dagger \psi_{u,d}>_0 \sim 40\sim45 {\rm MeV}$, while $\chi_{s}< i\psi_s^\dagger \psi_s >_0 \simeq 6\sim 10 {\rm MeV}$ with the quark condensate $<i\psi^\dagger \psi>_0$.