The structure of hadrons
Structure of the nucleon from electromagnetic form factors
ECT*, I-38050 Villazzano (Trento), Italy
2 Center for Theoretical Physics, Sloane Physics Laboratory, Yale University, New Haven, Connecticut 06520-8120, USA
Published online: 12 February 2004
Recent experimental data on the ratio of electric to magnetic elastic form factors are reviewed in light of a model of the nucleon with an intrinsic (quark-like) structure and a meson cloud. The analysis points to the astonishing result that the proton electric form factor vanishes at Q 2~ 8 (GeV/c)2 and becomes negative beyond that point. The intrinsic structure is estimated to have a r.m.s. radius of ~ 0.34 fm, much smaller than the proton r.m.s. radius ~ 0.87 fm. The calculations are in perfect agreement with the proton data, but deviate drastically from neutron data at Q 2>1 (GeV/c)2. Relativistic invariance is a crucial ingredient responsible for the vanishing of . Symmetry, rather than detailed dynamics, appears to be a determining factor in the structure of the nucleon. Scaling appears to occur at much larger values, Q 2≥ 30 (GeV/c)2, than previously thought.
PACS: codes. – 25.30.Bf - Elastic electron scattering / codes. – 13.40.Gp - Electromagnetic form factors / codes. – 14.20.Dh - Protons and neutrons / codes. – 24.85.+p - Quarks, gluons, and QCD in nuclei and nuclear processes
© Società Italiana di Fisica, Springer-Verlag, 2004