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Title:
Instanton vacuum, chiral symmetry breaking, and the gluonic structure of hadrons
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Abstract:
The QCD vacuum is populated by localized nonperturbative fluctuations of the gauge fields(instantons), which carry topological charge and cause the dynamical breaking of the chiralsymmetry of fermions through zero modes of the Dirac operator. An effective description oflow-energy QCD abstracted from this picture (instanton vacuum) describes the basic features ofhadronic correlation functions and has been extensively used in studies of meson and baryonstructure (masses, form factors, etc). The picture also predicts the gluonic structure of hadronsat low scales (mu ~ 1/rho ~ 0.6 GeV). We review a method for evaluating hadronic matrixelements of gluon operators in the instanton vacuum, based on a systematic expansion in theinstanton packing fraction ("diluteness") [1]. We apply this method to several gluonic operatorsof interest: (i) hadronic matrix elements of F^2 and F-Fdua (low-energy theorems from scale andaxial anomaly); (ii) leading-twist gluon density in hadrons (partonic momentum sum rule); (iii)higher-twist quark-gluon operators appearing in DIS processes. As a recent application, wediscuss a calculation of the nucleon matrix element of the dimension-6 CP-odd gluon operatorF-F-Fdual (Weinberg operator), which appears in a scenario of hadronic CP-violation and theneutron electric dipole moment [2].
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