Gauge-invariant field-strength correlators for QCD in a magnetic background

We consider the properties of the gauge-invariant two-point correlation functions of the gauge-field strengths for QCD in the presence of a magnetic background field at zero temperature. We discuss the general structure of the correlators in this case and provide the results of an exploratory lattice study for N_f = 2 QCD discretized with unimproved staggered fermions. Our analysis provides evidence for the emergence of anisotropies in the nonperturbative part of the correlators and for an increase of the gluon condensate as a function of the external magnetic field.