FIZIKA B 13 (2004) 1, 13 - 26
STRUCTURE OF THE NUCLEON FROM ELECTROMAGNETIC FORM FACTORS
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 ³
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
Keywords: nucleon intrinsic structure, form factors, r.m.s. radii, symmetry, scaling