References - Bogoliubov Laboratory of Theoretical Physics - JINR

5 Results of GEp-2γ Experiment
So what are the causes for the different results for μGEp/GMp, from cross section and from
polarization measurements? No experimental explanation has been found [13]. Part of
the answer may be that radiative corrections at large Q 2 are both large and strongly
ɛ dependent, and these corrections are important for cross section data, but not for
polarization data.
The JLab polarization results have led to a reexamination of the two-photon contribution,
which is included in standard radiative corrections only in the limit of one of the photon
energies being small. Recent calculations of the contribution of two-photon exchange
with both photons approximately sharing the momentum transfer, first by Guichon et
al [21], then Afanasev et al [16], and Blunden, Melnitchouk and Tjon [15], have shown that
it might contribute a correction to the standard radiative correction of several % at ɛ =1.
The preliminary results from the
GEp-2γ experiment for the proton
form factor ratio as extracted from
the polarization ratio are shown in
Figure 6. The error bars shown are
statistical only. The data points lay
almost on the same constant line thus
showing no deviations from the Born
approximation at a percent level. On
the same plot several theoretical predictions
are shown. These are corrections
to the Born approximation. In
[16] the two-photon exchange contributions
are calculated at the partonic
level assuming factorization of the
soft nucleon-quark part, and the hard
electron-quark interaction where twophoton
exchange takes place via box
diagram. In another approach [15,22,
23], the two-photon exchange effects
are calculated at the hadronic level.
Figure 6: Form factor ratio as measured in GEp-2γ experiment.
Theoretical predictions labeled as GPD are from
Afanasev et al. [16]; labeled as hadronic are from Blunden
et al. [15], and labeled as BLW and COZ are calculations
done by Kivel and Vanderhaeghen using proton distribution
amplitudes from references [25] and [26].
Similar two-photon exchange box diagrams are used where the quarks are replaced by
nucleons. Kivel and Vanderhaeghen [24] estimate the two photon exchange contribution
to elastic electron-proton scattering, using several models for the nucleon distribution
amplitudes [25, 26].
The key idea of the GEp-2γ experiment, is to study the ɛ dependence of the recoil
proton polarization at a fixed value of Q 2 , hence, (1) the proton momentum is same and
as a consequence the analyzing power, Ay, of the reaction used to measure the proton
polarization, is the same for all the kinematics; and (2) since the setting of the HMS
is fixed, the transport of the spin from the focal plane where it was measured, back
to the target is the same for all the data points. This is a significant advantage in the
measurement of the polarization ratio Pt/Pl. For these reasons the systematic uncertainty
will be quite small for the final GEp-2γ results.
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