FIZIKA B 13 (2004)  1, 13 - 26

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ECT*, I-38050 Villazzano (Trento), Italy, and
Center for Theoretical Physics, Sloane Physics Laboratory,
Yale University, New Haven, Connecticut 06520-8120, USA

Received 13 August 2003;    Accepted 26 April 2004
Online 16 July 2004

Recent TJNAF 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 (quark-antiquark) cloud. The analysis points to the remarkable result that the proton electric form factor vanishes at Q2 ~ 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 Q2 > 1 (GeV/c)2. Relativistic invariance is a crucial ingredient responsible for the vanishing of GEp. Symmetry, rather than detailed dynamics, appears to be a determining factor in the structure of the nucleon. Scaling appears to occur at Q2 30 (GeV/c)2, i.e. at much larger values than previously thought.

PACS numbers: 25.30.Bf, 13.40.Gp, 14.20.Dh, 24.85.+p
UDC 539.17

Keywords: nucleon intrinsic structure, form factors, r.m.s. radii, symmetry, scaling

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