Physics at CEBAF with High Resolution Spectrometers Page: 2 of 9
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2. Structure of the Nucleon
The electric G and magnetic GM form factors of the nucleon are of fundamental
importance in understanding both the nucleon and nuclear structure. They are an essential
ingredient of the electromagnetic response function of nuclei and their Q2 dependence is a
critical testing ground for QC'D based models.
Experimental information on GE is very poor except for the slope at Q2 = 0. Various
models predict different behaviour at large Q2. Experimentally GE has been extracted from
elastic e-d scattering or inclusive quasielastic e-d scattering using Rosenbluth separation
techniques, both of which require the knowledge of the structure of the deuteron in order
to correct for coherent effects and the dominant proton contribution. This introduces large
uncertainties, as at high j the cross section is mainly dominated by GM - the transverse
component. Dombeyl'! and Arnold, Carlson and Gross [+J have shown that G , can be
determined more accurately if one scatters longitudinaly polarized electrons off a polarized
target or measure instead the polarization of the recoiling neutron. Two proposalsilcS!
apply these techniques using the coincidence (e,e'n) reaction to determine G~g. In the
first proposalPCl, polarized electrons are scattered off solid NDs target polarized in the
scattering plane perpendicular to j. Target polarizations between 50% and 70% can be
achieved at 0.25 K in a 5 T magnetic field. In the second proposalcli, a recoil neutron
polarimeter is used to measure the transverse in-plane polarization of the recoiling neutron.
In both cases GE is extracted from the interference product GEGM. The uncertainties in
G obtainable in Proposal C' for Q2 of up to 1.5 (GeV/c)2 is shown in Figure 2. Proposal
C3 proposes to measure-G"k up to 2 (GeV/c)z with nearly similar uncertainties.
0.10 Figure 2. Projected statistical un-
certainties in Gj from the proposed
0.05 experiment 2H(Wef )[cl] permit dis-
tinguishing between G"E = 0 and
0.00 " '"' T ,.." ' " ", E _ 'M.
-0.05 __ __ _ __ _ __ __ _ __ _ __ _
0 0.5 1 1.5 2 2.5
Even though the e-p cross section has been measured precisely up to Q2 ' 31 (GeV/c)2,
G is poorly known up to only 4 (GeV/c)2. A recent SLAC experiment NE11M51 plans to
determine it to 6 (GeV/c)' with uncertainties of up to 21%. As described for the neutron
case, G% can be determined also by performing a coincidence (ee'p) experiment on either
a polarized targetC3l or measuring the transverse in-plane polarization of the recoiling
proton(c2!. Proposal [C2j plans to measure GE to 4.5 (GeV/c)2 with errors of less than
4.6% using a focal plane polarimeter. Measuring the d(i,e'p) reaction in quasifree kine-
matics on a polarized deuteron targetC3 or measuring the outgoing recoiling protonic1l:
provides information on the model for deuteron (which is necessary for the proper extrac-
tion of G E) and also gives us a measure of G in the Impulse Approximation where it
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Saha, Arunava. Physics at CEBAF with High Resolution Spectrometers, report, January 1, 1991; [Newport News, Virginia]. (digital.library.unt.edu/ark:/67531/metadc930860/m1/2/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.