References - Bogoliubov Laboratory of Theoretical Physics - JINR

lz = 0 (along the direction of the fast moving hadron), the asymptotic prediction for F2
leads to a 1/Q 6 fall-off at high Q 2 .
In Fig. 9 the JLab recoil polarization data together with cross section data from
references [9–11] are shown as Q 2 F2p/F1p. The cross section data from Ref. [9] show
flattening above Q 2 of 3 GeV 2 ; the recoil polarization data from JLab do not show yet
the specific pQCD Q 2 dependence, but the new data from GEp-III suggest a decrease in
the slope, hinting that the data might be approaching the pQCD limit.
Figure 9: Q 2 F2p/F1p versus Q 2 for data from GEp-I (filled circles), GEp-II (filled squares), and GEp-III
(filled triangles) experiments.
In another approach, Belitsky, Ji, and Yuan [40] investigated the assumption of quarks
moving collinearly with the proton, underlying the pQCD prediction. It has been shown
in [40] that by including components in the nucleon light-cone wave functions with quark
orbital angular momentum projection lz = 1, one obtains the behavior Q 2 F2/F1 →
ln 2 (Q 2 /Λ 2 ) at large Q 2 , with Λ a non-perturbative mass scale. Choosing Λ around
0.3 GeV, Ref. [40] noticed that the data for Q 2 F2p/F1p support such double-logarithmic
enhancement, as can be seen from Figure 10.
Lattice QCD simulations have the potential to calculate nucleon form factors from
first principles. This is a rapidly developing field and important progress has been made
in the recent past. Nevertheless, lattice calculations are at present still severely limited by
available computing power and in practice are performed for quark masses sizably larger
than their values in nature.
The generalized parton distributions (GPDs) provide a framework to describe the
process of emission and re-absorption of a quark in the non-perturbative region by a
hadron in exclusive reactions. The GPDs can be interpreted as quark correlation functions
and have the property that their first moments exactly coincide with the nucleon form
factors. Precise measurements of elastic nucleon form factors provide stringent constraints
on the parametrization of the GPDs. Early theoretical developments in GPDs indicated
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