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Abstract: In this article we address the physical basis of the deviation of hadronshapes from spherical symmetry non-spherical amplitudes with focus on thenucleon and $\Delta$. An overview of both the experimental methods and resultsand the current theoretical understanding of the issue is presented. At thepresent time the most quantitative method is the $\gamma^{*} p \to \Delta$reaction for which significant non-spherical electric E2 and Coulombquadrupole C2 amplitudes have been observed with good precision as a functionof Q^{2} from the photon point through 6 GeV^{2}. Quark model calculations forthese quadrupole amplitudes are at least an order of magnitude too small andeven have the wrong sign. Lattice QCD, chiral effective field theory, anddynamic model calculations which include the effects of the pion-cloud are inapproximate agreement with experiment. This is expected due to the spontaneousbreaking of chiral symmetry in QCD and the resulting, long range low Q^{2}effects of the pion-cloud. Other observables such as nucleon form factors andvirtual Compton scattering experiments indicate that the pion-cloud is playinga significant role in nucleon structure. Semi-inclusive deep inelasticscattering experiments with transverse polarized beam and target also show theeffect of non-zero quark angular momentum.



Autor: A.M. Bernstein, C.N. Papanicolas

Fuente: https://arxiv.org/







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