References - Bogoliubov Laboratory of Theoretical Physics - JINR

MEASUREMENTS OF GEp/GMp TO HIGH Q 2 AND
SEARCH FOR 2γ CONTRIBUTION IN ELASTIC ep AT JEFFERSON LAB
Vina Punjabi 1 † 2 ‡
and Charles Perdrisat
(1) Norfolk State University, Norfolk, Virgina 23504, USA
(2) The College of William and Mary, Williamsburg, Virginia 23187, USA
† E-mail: punjabi@jlab.org ‡ E-mail: perdrisa@jlab.org
Abstract
The ratio, μpGEp/GMp, whereμp is the proton magnetic moment, has been
measured extensively over the last decade at the Jefferson Laboratory, using the
polarization transfer method. This ratio is extracted directly from the measured
ratio of transverse to longitudinal polarizations components of the recoiling proton
in elastic electron-proton scattering. The polarization transfer results are of unprecedented
high precision and accuracy, due in large part to the small systematic
uncertainties associated with the experimental technique. Prior to these measurements,
the form factors were empirically observed to exhibit dipole forms, such that
μpGEp/GMp ≈ 1 over all regions of momentum transfer studied. With the Hall A
results confirming that the ratio μpGEp/GMp shows a steady decrease below unity
as a function of Q 2 , beginning around Q 2 ≈ 1GeV 2 , discussions revolving around
the implication of this deviation from dipole behavior for the structure of the proton
have been accompanied by renewed experimental interest in these elastic form
factors.
Starting in the fall of 2007, two new experiments, GEp-III and GEp-2γ in Hall C
at JLab, measured the form factor ratio, GEp/GMp; the GEp-III experiment pushed
the highest Q 2 limit from 5.6 to 8.49 GeV 2 , with intermediate points at 5.2 and 6.8
GeV 2 ,andtheGEp-2γ experiment measured the ratio in three different kinematics
at the constant value Q 2 =2.5 GeV 2 , by changing beam energy and detector angles.
Preliminary results from both experiments are reported.
1 Introduction
The structure of the proton has been investigated experimentally and theoretically over
the past 50 years using elastic electron scattering. The two Sachs form factors, GEp and
GMp, describe the nucleon charge- and magnetization distribution; these form factors
have been traditionally obtained by Rosenbluth separation method [1]. The GMp-data
obtained by this method have shown good internal consistency up to 30 GeV 2 . However,
the characterization of GEp has suffered from large inconsistencies in the data base, which
are now understood to be the result of the fast decrease of the contribution of GEp to the
cross section.
Indeed, the study of the electromagnetic form factors has become quite central to the
rapid progress that is being made in hadronic physics. The results of the GEp-I and GEp-
II experiments [2–4] have stimulated a huge amount of theoretical activity, as evidenced
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