The Pauli form factor of the quark induced by instantons

The non-perturbative contribution to the Pauli form factor of the quark, F2(Q 2), is calculated within an instanton model for the QCD vacuum. It is shown that the instantons give a large negative contribution to the form factor. The interaction of photons with hadrons is one of the most powerful tools to investigate the structure of strong interactions. The electromagnetic form factors of hadrons are discussed now widely discussed[1]. This increased interest in the electromagnetic form factors is related with the significant theoretical progress in understanding the connection between form factors and structure functions of hadrons through the generalized parton distributions (GPD) [2], and to by the need to explain the new experimental data from Jefferson Lab [3] for the nucleon and pion form factors at large Q. One of the basic components of the calculation of the form factors within the constituent quark model are the electromagnetic form factors of quarks. It has been shown that the non elementarity of the constituent quarks plays a crucial role for the understanding of the parton distributions and form factors [4, 5]. Moreover quarks form factors have been instrumental in the investigation of the scaling violations in deep-inelastic scattering [6]. We should also mention that in order to explain the high twist effects in Drell-Yan processes [7], various perturbative and non-perturbative gluonic corrections to the photon-quark vertex must be taken into account [8]. The general formula for the photon-quark vertex is Γμ = F1(Q )γμ + iqμσμν 2mq F2(Q ), (1) wheremq is the mass of quark, σμν = i(γμγν−γνγμ)/2 and F1(Q ), F2(Q ) are the Dirac and Pauli form factors, correspondingly. There exist calculations, both perturbative and non-perturbative, of QCD contributions to the F1(Q ) form factor (see [9], [10] and references therein). The usual way to incorporate the non-perturbative QCD dynamics into the calculation of quark form factors is the consideration of the renormalon contributions [10]. However, renormalon contributions amount only to a part of the QCD non-perturbative effects. The instanton model for the QCD vacuum is now one of the most successful models for the description of non-perturbative effects in strong interactions (see the review [11]). Within this model, the QCD vacuum contains complex topological structures related to existence of the instantons, strong fluctuations of the gluon vacuum fields. In QCD the instantons play an important role in describing the realization of chiral symmetry breaking and the origin of the masses of constituent quarks and hadrons. In refs. [12]-[17] it was shown that the instantons produce also specific non-perturbative effects in various hadronic reactions. Recently, the first step in the calculation