References - Bogoliubov Laboratory of Theoretical Physics - JINR

the light-front nucleonic wave function of Schlumpf [34]. Under such a transformation,
the spins of the constituent quarks undergo Melosh rotations. These rotations, by mixing
spin states, play an important role in the calculation of the form factors.
Cardarelli et al. [35] calculated
the ratio with light-front dynamics
and investigated the effects of
SU(6) symmetry breaking. They
showed that the decrease in the ratio
with increasing Q
Figure 8: Theoretical calculations with RCQM models,
compared to the data from GEp-I (filled circles), GEp-II
(filled squares), and GEp-III (filled triangles) experiments.
2 is due to
the relativistic effects generated by
Melosh rotations of the constituent
quark’s spin. In Ref. [36], they
pointed out that within the framework
of the RCQM with the lightfront
formalism, an effective onebody
electromagnetic current, with
a proper choice of constituent quark
form factors, can give a reasonable
description of pion and nucleon
form factors. The chiral constituent
quark model based on Goldstoneboson-exchange
dynamics was used
by Boffi et al. [37] to describe the elastic electromagnetic and weak form factors.
More recently Gross and Agbakpe [38] revisited the RCQM imposing that the constituent
quarks become point particles as Q 2 →∞as required by QCD; using a covariant
spectator model which allows exact handling of all Poincaré transformations, and
monopole form factors for the constituent quarks, they obtain excellent ten parameter
fits to all four nucleon form factors; they conclude that the recoil polarization data can
be fitted with a spherically symmetric state of 3 constituent quarks. Figure 8 shows
predictions from References [32, 33, 35–38]; in all predictions the ratio decrease with Q 2 ,
showing the same trend as the data, with the exception of the predictions of Boffi et
al. [37] and Frank et al. [33].
The nucleon electromagnetic form factors provide a famous test for perturbative QCD.
Brodsky and Farrar [39] derived scaling rules for dominant helicity amplitudes which are
expected to be valid at sufficiently high momentum transfers Q 2 . A photon of sufficient
high virtuality will interact with a nucleon consisting of three mass less quarks moving
collinearly with the nucleon. When measuring an elastic nucleon form factor, the final
state consists again of three mass less collinear quarks. In order for this process to happen,
the large momentum of the virtual photon has to be transferred among the three quarks
through two hard gluon exchanges. Because each gluon in such a hard scattering process
carries a virtuality proportional to Q 2 , this leads to the pQCD prediction that the helicity
conserving nucleon Dirac form factor F1 should fall as 1/Q 4 at sufficiently high Q 2 . In
contrast to the helicity conserving form factor F1, the Pauli form factor F2 involves a
helicity flip between the initial and final nucleons. Hence it requires one helicity flip at
the quark level, which is suppressed at large Q 2 . Therefore, for collinear quarks, i.e.
moving in a light-cone wave function state with orbital angular momentum projection
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