Instantons and Nucleon Magnetism.pdfVIP

a r X i v : h e p - p h / 0 0 0 9 2 7 4 v 1 2 2 S e p 2 0 0 0 INSTANTONS AND NUCLEON MAGNETISM H. FORKEL Institut fu?r Theoretische Physik, Universita?t Heidelberg, D-69120 Heidelberg, Germany We construct improved QCD sum rules for the nucleon magnetic moments by implementing direct-instanton contributions to the operator product expansion of the nucleon correlator in a magnetic background field. The instanton contributions are found to affect only those sum rules which had previously been considered unstable. The new sum rules show a high degree of stability and reproduce the experimental values of the nucleon magnetic moments for values of the magnetic quark condensate susceptibility which are consistent with other estimates. (Invited talk given at “Hadron Physics 2000”, Caraguatatuba, Sa?o Paulo, Brazil (April 10- 15, 2000). Over the last years, an increasingly multifaceted picture of the role of QCD instantons 1 in hadron physics has begun to emerge from hadron models with more or less instanton-induced structure 2, instanton-liquid vacuum models3, a generalized operator product expansion including instanton contributions (IOPE) 4 and, most recently, lattice calculations 5. One of the major benefits of sharpening this picture by identifying specific hadronic instanton effects will be to end the puzzling elusiveness of explicit glue in hadrons at low energies. Indeed, in most successful models of the classical mesons and baryons, gluons play at best a marginal role. The situation is further obfuscated by the fact that gluons do not directly couple to electroweak probes. Since much of what we know about the classical hadrons derives from their response to such probes, it is particularly desirable to pin down traces of nonperturbative glue in electromagnetic hadron observables. In this talk, I will report on an investigation 6 which takes a step towards uncovering the hidden role of glue in the response to electromagnetic fields by studying the impact of inst