Baryon form factors at high Q 2 and the transition to perturbative QCD

Abstract

The status of electromagnetic form-factors of nucleons and non-strange nucleon resonances is rewiewed. Of special interest is the question of what regions in Q 2 correspond to the breakdown of the validity of constituent quark models, and the region of Q 2 corresponding to the onset of the validity of perturbative QCD. Theoretical models based on perturbative QCD are reviewed and results of calculations compared to available experimental data. It is observed that relativistic constituent quark models become invalid for Q 2 greater than about 2 GeV 2/ c 2, compared with available experimental data. The Q 2 dependence of experimental form-factors appears to begin to follow perturbative QCD scaling rules for Q 2 as low as 5 or 6 GeV 2/ c 2. A notable exception is the δ(1232), whose form-f actor decreases faster than other states. However the normalized magnitudes of form-factors at experimentally accessible values of Q 2 obtained with perturbative QCD sum rule techniques is controversial and currently unsettled. The diquark model is discussed as a possible bridge between low and high- Q 2 kinematic regions.