References - Bogoliubov Laboratory of Theoretical Physics - JINR

y the nearly 1000 combined citations of these papers and several review papers of nucleon
form factors [5–8]. An entirely new picture of the structure of the proton has emerged
after the GEp-I and GEp-II experiments showed that the ratio GEp/GMp was in fact not
constant, and decreased by a factor of 3.7 over the Q 2 range of 1 to 5.6 GeV 2 . These
results are illustrated in Fig. 1, where they are also compared with Rosenbluth separation
data [9–12].
The meaning of the results seen in Fig.
1 is that the spatial distribution of the
electric charge of the proton is “softer”,
i.e. larger in extent (in the Breit frame),
than its magnetization currents distribution,
which is definitively not intuitive.
However, the relativistic boost required to
transform these spatial distributions back
to the laboratory frame are not trivial and
only the form factors themselves are relativistic
invariants. Recently G.A. Miller
[14] has shown that an invariant charge distribution
can only be defined on the wave
front; the two-dimensional charge density
on the wave front is the Fourier transform
of the Dirac form factor, F1.
It is well known by now that GEp is
difficult to obtain from Rosenbluth separation,
a technique which is also especially
Figure 1: Comparison of μpGEp/GMp from the two
JLab polarization data (filled circle and square) [3,4],
and Rosenbluth separation data (empty triangle) [9–
12]; dashed curve is a re-fit of Rosenbluth data [13];
solid curve is an updated form of the fit in ref. [4].
sensitive to systematics errors and subject to large, ɛ-dependent radiative corrections.
The two-photon exchange contribution, neglected in the past, has been shown to be an
important term to add to the standard radiative corrections for cross section data; it has
a strong ɛ-dependence and brings the Rosenbluth form factor ratio closer to the recoil
polarization results [15, 16]. Two-photon contributions are expected to affect the recoil
polarization results only very weakly [16].
Following the unexpected results from the polarization experiments, a new experiment,
GEp-III [17], was approved to extend the Q 2 -range to 9 GeV 2 in Hall C; and to check
the hypotheses of two-photon exchange contribution in ep elastic scattering, the GEp-2γ
experiment [18], using recoil polarization, was approved to measure the ratio at Q 2 of 2.5
GeV 2 for three different kinematics. Two new detectors were built by the collaboration
to carry out these experiments; a large solid-angle electromagnetic calorimeter and a
double focal plane polarimeter. In both experiments the recoil protons were detected in
the high momentum spectrometer (HMS) equipped with the new focal plane polarimeter.
The scattered electrons were detected in a new lead glass calorimeter (BigCal) built for
this purpose out of 1744 glass bars, 4x4 cm 2 each, giving a total frontal area of 2.6 m 2 ,
which provides complete kinematical matching. This experiment finished taking data in
the spring of 2008. The data analysis is in progress. In this paper, we will describe the
recoil polarization method, the experimental results, and discuss the status of the proton
elastic electromagnetic form factor data, including the latest results from the GEp-III and
GEp-2γ experiments, and compare them to a number of theoretical predictions.
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