The field strength correlator from QCD sum rules

where the field strength F a μν = ∂μA a ν − ∂νA a μ + gfAμA c ν , z = y− x and P denotes path ordering of the exponential. In general, the gauge invariant field strength correlator could be defined with an arbitrary gauge string connecting the end points x and y, but in this work we shall restrict ourselves to a straight line. It is the basic ingredient in the stochastic model of the QCD vacuum [1,2] and in the description of high energy hadron-hadron scattering [3–6]. In the spectrum of heavy quark bound states it governs the effect of the gluon condensate on the level splittings [7–10] and it is useful for the determination of the spin dependent parts in the heavy quark potential [11,12]. The correlator can be related to a correlator of a colour singlet current composed of a (fictitious) infinitely heavy octet quark and the gluon field