Authors: A. J. Buchmann
Title: Excited nucleon electromagnetic form factors from broken spin-flavor symmetry
Keywords: SU(6) symmetry, 1/N expansion, form factors
Abstract:
Information on the spatial and spin-flavor structure of nucleon ground and excited states is encoded in various electromagnetic form factors, which are measured via elastic and inelastic electron-proton scattering. Recent analyses have made it possible to extract the small N->Delta(1232) charge quadrupole over magnetic dipole transition form factor ratio C2/M1 from the inelastic electron-proton scattering cross section [1]. This observable is important for determining the geometric shape of the nucleon.
Based on a quark model calculation including two-body exchange currents, we proposed a relation between the inelastic N->Delta(1232) charge quadrupole transition and the elastic neutron charge form factors [2].Together with a corresponding relation between inelastic and elastic magnetic form factors we were able to predict the inelastic C2/M1 ratio from the elastic neutron form factor ratio G_C^n/G_M^n. It turned out that the proposed relation is in good agreement with experiment from low to high momentum transfers [1].
In this talk, I show that the N->Delta(1232) and the neutron charge form factors are related as a result of the underlying SU(6) spin-flavor symmetry of QCD. First, I review some important results concerning the connection between broken SU(6) spin-flavor symmetry and the 1/N_c expansion of QCD. In particular, we will see that QCD features a well defined method for classifying the importance of various SU(6) symmetry breaking operators [3]. Then, I apply this method to the problem at hand and clarify the range of validity of the proposed relation. I emphasize the usefulness of this method for a wide variety of problems in the physics of excited nucleons.
References
[1] D. Drechsel, S. S. Kamalov, L. Tiator,
Eur. Phys. J. A 34, 69 (2007).
[2] A. J. Buchmann, Phys. Rev. Lett. 93, 212301 (2004);
AIP Conf. Proc. 904, 110 (2007), arXiv:0712.4270v1 [hep-ph].
[3] A. J. Buchmann, J. A. Hester, and R. F. Lebed, Phys. Rev. D 66, 056002
(2002). |