References - Bogoliubov Laboratory of Theoretical Physics - JINR

models have recently been challenged by the elastic form factors obtained at Jefferson Lab,
resulting in an intense discussion of questions related to the shape of the proton [47, 48],
and the contribution of the quark orbital angular momentum to its spin, for example see
Ref. [41].
The new data from the GEp-III experiment at higher Q 2 , although still preliminary,
show a slowing decrease of G p
E /Gp
M with Q2 relative to the linear decrease observed in
the Hall A data for Q 2 ≤ 5.6 GeV 2 . Although the statistical significance of this change
in behavior is somewhat marginal, its physical implications are interesting to consider.
A constant ratio G p
E /Gp
M at asymptotically large Q2 is a signature of the onset of the
dimensional scaling expected from perturbative QCD for a nucleon consisting of three
weakly interacting quarks.
The final answer to the question of the origin of the discrepancy between the results
from Rosenbluth separation and polarization transfer will have to come from experiments.
There are several directly- and indirectly related experiments designed to provide answer
to the question. For example, (1) the GEp-2γ experiment measured ɛ-dependence of
the GEp/GMp ratio at constant Q 2 , obtained from recoil polarization in Hall C [18].
The preliminary results for the μpGEp/GMp ratio at Q 2 =2.49 GeV 2 , at the 3 values of
ɛ 0.14, 0.63 and 0.79, were shown in Fig. 6; no deviation from constancy is seen at
the level of 0.01 absolute, showing that the recoil polarization results follow the Born
approximation at this Q 2 . (2) Cross section difference in e + and e − proton scattering
in Hall B [49] (also approved experiment OLYMPUS at DESY [50]) ; this difference is
twice the contribution of a two-photon exchange; and (3) Measuring non-linearity of the
Rosenbluth plot; experiment E05-017 measured the cross sections with high precision at
several Q 2 values as a function of ɛ to extract the ratio; data taking for this experiment
was completed in July 2007 [51].
The measurement of nucleon form factors to the highest possible Q 2 is one of the prime
tenets of the JLab 12 GeV upgrade. Two new proposals, GEp-IV [52] and GEp-V [53],
were submitted to JLab PAC 34 and 32, respectively to extend the proton form factor
ratio measurements, when a 12 GeV beam becomes available. In Experiment GEp-V the
ratio will be measured in Hall A to 14.8 GeV 2 , and in the GEp-IV experiment in Hall
C, it will be measured to 13 GeV 2 . These planned experiments covering the Q 2 range
from 10-15 GeV 2 following the 12 GeV upgrade of the CEBAF accelerator, will answer
the question of whether the flattening hinted at by the GEp-III experimental results is
continues to decrease and eventually cross zero.
real or whether G p
E /Gp
M
Acknowledgments
The authors wish to thank the organizers of the Dubna Dspin-09 workshop for their
invitation to present this paper. The authors are supported by grants from the NSF(USA),
PHY0753777 (CFP), and DOE(USA), DE-FG02-89ER40525 (VP).
References
[1] M.N. Rosenbluth, Phys. Rev. 79, 615 (1950).
[2] M.K. Jones et al., Phys. Rev. Lett. 84, 1398 (2000).
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