Abstract
The existing theory of hard exclusive QCD processes is based on two assumptions: (i) factorization into a hard block convoluted with the light front distribution amplitudes; (ii) use of perturbative gluon exchanges within the hard block. However, unlike deep inelastic scattering and jet physics, the characteristic momentum transfer $Q$ involved in the factorized block is not large enough for this theory to be phenomenologically successful. In this work, we revisit the latter assumption (ii), by explicitly calculating the instanton-induced contributions to the hard block, and show that they contribute substantially to the vector, scalar, and gravitational form factors of the pseudoscalar, scalar, and vector mesons, over a wide range of momentum transfer.
Highlights
One approach is focused on the nontrivial vacuum properties, with the central aspects of chiral symmetry breaking and confinement
Brief introduction to instanton effects So far we focused on the nonperturbative approach to physics of chiral symmetry breaking, namely the Nambu and Jona-Lasinio (NJL) model
We include in the distribution functions several possible Dirac/chiral structures allowed by parity
Summary
The field of hadronic physics going back to the pioneering theoretical and experimental works of the 1960s, continues to be a field of active development till today. It remains still deeply divided along two conceptually different approaches. One approach is focused on the nontrivial vacuum properties, with the central aspects of chiral symmetry breaking and confinement. The discovery of instantons and the development of numerical lattice gauge theory have put the Euclidean formulation of QCD at the center stage. The theory and phenomenology of multiple Euclidean correlation functions became the primary source of information about quark-quark interactions. The discussion has shifted to the properties of operators made of 4-, 5-, and 6-quarks and their mixture with gluons
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