References - Bogoliubov Laboratory of Theoretical Physics - JINR

Figure 10: The test of the modified scaling prediction for Q 2 F2p/F1p versus Q 2 [40] for data from GEp-I
(filled circles), GEp-II (filled squares), and GEp-III (filled triangles) experiments.
that measurements of the separated elastic form factors of the nucleon to high Q 2 may
shed light on the problem of nucleon spin. The first moment of the GPDs taken in the
forward limit yields, according to the Angular Momentum Sum Rule [41], a contribution
to the nucleon spin from the quarks and gluons, including both the quark spin and orbital
angular momentum. The t-dependence of the GPDs has been modeled using a factor
corresponding to the relativistic Gaussian dependence of both Dirac [42] and Pauli [43]
form factors of the proton. Extrapolation of these GPDs to t = 0 leads to the functions
entering into the Angular Momentum Sum Rule, and an estimate of the contribution of
the valence quarks to the proton spin can then be obtained.
In a recent development of these ideas Guidal et al [44] have shown that the difficulties
encountered in the Gaussian parametrization used in earlier work could be surmounted
with a Regge parametrization. In one version of their parametrization, these authors
obtain excellent fits to all 4 nucleon form factors. In a different approach Diehl et al [45]
use theoretically motivated parametrization of the relevant GPDs in the very small and
very large x-domains, and interpolate by fitting the nucleon Dirac form factors F p
1 and
F n 1
. They derive the valence contribution to Ji’s sum rule [41]. In a related approach,
Ji [46] has shown that the GPDs provide a classical visualization of the quark orbital
motion.
7 Conclusions
Form factor data are of great interest as a testing ground for QCD, as results from
lattice QCD calculations become increasingly accurate and realistic. Phenomenological
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