Nucleon electromagnetic form factors Page: 4 of 12
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VMD models form a subset of models using dispersion relations, which relate form
factors to spectral functions. These spectral functions can also be thought of as a
superposition of vector meson poles, but include contributions from n-particle
production continua. This framework allows then a model-independent fit13 to all
available EMFF data in the space- and the time-like region.
Many attempts have been made to enlarge the domain of applicability of pQCD
calculations to moderate Q2-values. Kroll et al." have generalized the hard-scattering
scheme by assuming nucleons to consist of quarks and diquarks. The diquarks are
used to approximate the effects of correlations in the nucleon wave function. This
model is equivalent to the hard-scattering formalism of pQCD in the limit Q2 -> .
Chung and Coester6 have developed a relativistic constituent quark model with
effective quark masses and a confinement scale as free parameters.
Lu et al." have recently expanded the cloudy bag model, whereby the nucleon is
described as a bag containing three quarks, but including an elementary pion field
coupled to them, in such a way that chiral symmetry is restored. Finally, recent
developments18 within the Skewed Parton Distribution formalism indicate a relation
between the EMFF behaviour at larger Q2-values and the nucleon spin.
NUCLEON FORM FACTORS
Over 20 years ago Akhiezer and Rekalot9 showed that the accuracy of EMFF
measurements could be increased significantly by scattering polarized electrons off a
polarized target (or by equivalently measuring the polarization of the recoiling
nucleon). In the early nineties a series of measurements2a-" at the MIT-Bates facility
showed the feasibility of that measurement principle.
Neutron Magnetic Form Factor
Significant progress has been made in measurements of G" at low QZvalues by
measuring the ratio of quasi-elastic neutron and proton knock-out from a deuterium
target. This method is practically insensitive to nuclear binding effects and to
fluctuations in the luminosity and detector acceptance. The basic set-up used in all
such measurements was very similar: the electron was detected in a magnetic
spectrometer with coincident neutron/proton detection in a large scintillator array. The
main technical difficulty in such a ratio measurement is the absolute determination of
the neutron detection efficiency. For the measurements at Bates25 and ELSA26 the
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Jager, Kees de. Nucleon electromagnetic form factors, article, January 1, 2000; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc710306/m1/4/: accessed December 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.