Bulletin of the American Physical Society
3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 54, Number 10
Tuesday–Saturday, October 13–17, 2009; Waikoloa, Hawaii
Session BF: Mini-Symposium on Electromagnetic Form Factors - from the Nucleon to Nuclei I |
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Chair: Bogdan Wojtsekhowski, Jefferson Lab Room: Kohala 3 |
Wednesday, October 14, 2009 7:00PM - 7:30PM |
BF.00001: Electromagnetic Form Factors - from Nucleon to Nuclei Invited Speaker: An overview will be given of recent results in both experiment and theory in the field of electromagnetic form factors. The development of polarized electron beams with high polarization at high current and of polarized targets and recoil polarimeters with large figures of merit has resulted in a wealth of new and accurate data for the charge form factor of the proton and for the charge and magnetic form factor of the neutron with a tremendous impact on our understanding of the structure of the nucleon, especially of the importance of the quark angular momentum. An significant side effect has been a renewed study of the influence of two-photon exchange on the form factors. New data on the form factors of nuclei have been scarce, but recent developments at elcctron storage rings have shown the feasibility of future measurements of the charge radii of short-lived isotopes. [Preview Abstract] |
Wednesday, October 14, 2009 7:30PM - 7:45PM |
BF.00002: Generalized Parton Distributions from Nucleon Form Factors and Applications Peter Kroll The extraction of the zero-skewness generalized parton distributions (GPDs) for valence quarks from data on the nucleon form factors will be rewieved. Results for the GPDs, their moments and quark orbital angular momenta as well as for the quark densities in the impact parameter plane will be presented. Applications of the zero-skewness GPDs to wide-angle Compton scattering and to polarization effects in exclusive meson electroproduction will also be discussed. Finally, implications of the recent JLab measurements of the electric and magnetic form factors of the neutron on the GPDs will be mentioned. [Preview Abstract] |
Wednesday, October 14, 2009 7:45PM - 8:00PM |
BF.00003: Baryon resonance electromagnetic transition form factors in a light-cone model Simon Capstick, Bradley Keister Calculations using a constituent quark model of the electromagnetic form factors for transitions between nucleons and excited states of the nucleon with J=1/2 and 3/2, including the Roper resonance, are described. These form factors are calculated with a relativistic model based on light-cone dynamics, fit to the proton and neutron elastic form factors, and using wave functions determined from the solution of a realistic three-quark Hamiltonian. [Preview Abstract] |
Wednesday, October 14, 2009 8:00PM - 8:15PM |
BF.00004: The Gep-III Experiment at Jefferson Lab Hall C Edward Brash Measurements of the elastic electric and magnetic form factors of the proton, $G_{Ep}$ and $G_{Mp}$, respectively, at large momentum transfer, $Q^2$, shed new light on its internal nonperturbative structure. The ratio, $R_p = \mu_p G_{Ep}/G_{Mp}$, where $\mu_p$ is the proton magnetic moment, has been measured extensively over the last decade at the Jefferson Laboratory, using the polarization transfer method, where one measures $R_p$ directly by measuring the ratio of transverse to longitudinal polarizations of the recoiling proton in elastic electron-proton scattering. These data have revealed that the ratio decreases approximately linearly with increasing $Q^2$ above a $Q^2$ $\sim$ 1~GeV$^2$. At the same time, they are in disagreement with previous results obtained using the Rosenbluth method based on cross section measurements. 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. Most recently, the Gep-III Experiment was completed in June of 2008 in Hall C at Jefferson Laboratory. It extends the Q$^2$-range from 5.6 to 8.54 GeV$^2$. In this presentation, I will review the status of the proton elastic electromagnetic form factor data, including the latest results from the Gep-III experiment, and discuss a number of theoretical approaches to describing nucleon form factors. [Preview Abstract] |
Wednesday, October 14, 2009 8:15PM - 8:30PM |
BF.00005: The GEp-2$\gamma$ Experiment at Jefferson Lab Hall-C Mehdi Meziane $\\$Intensive theoretical and experimental efforts have been made over the past decade aiming at explaining the discrepancy between the data for the proton form factor ratio, G$_{Ep}$/G$_{Mp}$, obtained at Jefferson Lab using polarization transfer technique, and the world data obtained by the Rosenbluth method based on cross section measurements. One possible explanation for this difference is a two-photon exchange contribution, where both photons share the momentum transfer about equally. In the Born approximation for a fixed Q$^{2}$, the form factors do not depend upon the energy of the incident electron as they are relativistic invariants. We will report the results of the Jlab Hall-C GEp-2$\gamma$ experiment which was designed to measure a possible kinematical variation of the ratio G$_{Ep}$/G$_{Mp}$ with statistical uncertainties of $\pm$0.01 at Q$^{2}$=2.5 GeV$^{2}$, using the recoil polarization technique. Three kinematics were chosen, corresponding to values of the kinematic factor $\epsilon$=0.15, 0.63 and 0.77. We will describe the new detectors built for both GEp-2$\gamma$ and GEp-III experiments, the electromagnetic calorimeter BigCal which detected the scattered electron, and the focal plane polarimeter (FPP) which measured the polarization of the recoil proton. [Preview Abstract] |
Wednesday, October 14, 2009 8:30PM - 8:45PM |
BF.00006: Transverse Beam Spin Asymmetries at Backward Angles in G$^0$ Juliette Mammei Transverse beam spin asymmetries in elastic electron-nucleon scattering arise due to the interference of the imaginary part of the two-photon exchange amplitude with that of a single photon. Two-photon exchange processes have received renewed interest because the inclusion of the real part of the two-photon exchange amplitude in the electron scattering cross section may account for the difference between polarization transfer measurements and unpolarized cross section measurements of the ratio of $G_E^p/G_M^p$. By measuring the beam-normal single-spin asymmetry, we are testing the theoretical framework used to calculate two-photon exchange effects as well as related $\gamma Z$ and W$^+$W$^-$ box diagrams that are important corrections to precision electroweak measurements. During the G$^{0}$ program, which ran in Hall C at Jefferson Lab, asymmetry data for (quasi-)elastic electron scattering with a transversely polarized electron beam were collected for four target and beam energy combinations at backward ($ \sim 108^{\circ}$) angles - hydrogen and deuterium at 362 and 687 MeV. Results for the asymmetries from hydrogen will be presented and compared with available theoretical models. Results from deuterium, which can be used to extract a transverse asymmetry for the neutron after appropriate corrections, will also be presented. [Preview Abstract] |
Wednesday, October 14, 2009 8:45PM - 9:00PM |
BF.00007: Precision Measurements of the Proton Electromagnetic Form Factors at Low Transferred Momenta Shalev Gilad We shall present recent precision measurements of the proton electromagnetic form factors from Hall A of the Jefferson Laboratory. The measurement, done at transferred momenta between 0.3 and 0.8 (GeV/c)$^{2}$ yielded results with absolute uncertainties of about 1 percent. These new results will be compared to previous measurements and to theoretical model predictions. The implications to theoretical models of nucleon structure, as well as to other fundamental quantities such as the Zemach radius will be discussed. [Preview Abstract] |
Wednesday, October 14, 2009 9:00PM - 9:15PM |
BF.00008: Precise Measurement of the Neutron Magnetic Form Factor in the Few-GeV$^2$ Region Gerard Gilfoyle, Jeffrey Lachniet, William Brooks, Brian Quinn, Michael Vineyard The neutron elastic magnetic form factor $G_M^n$ has been extracted from quasielastic scattering from deuterium in the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The kinematic coverage of the measurement is continuous from 1~$\rm{(GeV/c)^2}$ to nearly 5~$\rm{(GeV/c)^2}$ in four-momentum transfer squared and eclipses the previous data in this region. High precision was achieved with a ratio technique, where many uncertainties cancel, and a simultaneous in-situ calibration of the neutron detection efficiency, the largest correction to the data. Neutrons were detected using the CLAS electromagnetic calorimeters and the time-of-flight scintillators. Data were taken at two different electron beam energies, allowing up to four semi-independent measurements of $G_M^n$ to be made at each value of $Q^2$. The dipole parameterization is found to provide a good description of the data for $Q^2 >$ 1 $(GeV/c)^2$. The impact of these new data on the world data for $G_M^n$ will be presented. [Preview Abstract] |
Wednesday, October 14, 2009 9:15PM - 9:30PM |
BF.00009: Measurements of the Electric Form Factor of the Neutron at High Momentum Transfer Seamus Riordan The electromagnetic form factors of the nucleon provide experimental access to the underlying charge and magnetic moment distributions of quarks. These form factors provide excellent testing grounds for QCD and QCD-inspired models and are of fundamental importance in our understanding of non-perturbative QCD. Of the four nucleon form factors, the electric form factor of the neutron, $G_E^n$, has been measured in the smallest range of momentum transfer. We have measured the electric form factor of the neutron at four $Q^2$ points between 1.2 and 3.5~$\mathrm{GeV}^2$ in Hall A at Jefferson Lab. This more than doubles the momentum transfer region for which this quantity has previously been measured, providing new information on the structure of the neutron. Final results for $G_E^n$ at three $Q^2$ points, 1.7, 2.5 and 3.5~$\mathrm{GeV}^2$, will be presented and compared with QCD-based models and phenomenological approaches. [Preview Abstract] |
Wednesday, October 14, 2009 9:30PM - 9:45PM |
BF.00010: The $^3\mbox{He}(e,e'n)$ Channel in A$_y$ and G$_E$$^n$ Measurements Elena Long Experiments E05-102 and E08-005 involved measurements of electron scattering from polarized $^3$He reactions that have been conducted in Jefferson Lab's Hall A this past year. E08-005 measured the Target Single-Spin Asymmetry A$_y$ in the quasi-elastic $^3\mbox{He}^\uparrow(e,e'n)$ reaction. Plane wave impulse approximation (PWIA) predicts that A$_y$ should be exactly zero. A previous experiment at Q$^2$ of 0.2 (GeV/c)$^2$, where Laget and Nagorny predict A$_y$ to be small, showed a large asymmetry as predicted by Faddeev calculations. The recent experiment measured this asymmetry at Q$^2$ of 0.1 (GeV/c)$^2$, 0.5 (GeV/c)$^2$ and 1.0 (GeV/c)$^2$. This is the first measurement of A$_y$ at large Q$^2$, which is another region where A$_y$ is expected to be small. Any non-zero result is an indication of effects beyond impulse approximation. During E05-102, a parasitic measurement of the electric form factor of the neutron (G$_E$$^n$) was taken using the $^3\vec{\mbox{He}}(\vec{e},e'n)$ channel at Q$^2$ of 0.4 (GeV/c)$^2$, 0.5 (GeV/c)$^2$ and 1.0 (GeV/c)$^2$. An overview of these measurements will be presented. [Preview Abstract] |
Wednesday, October 14, 2009 9:45PM - 10:00PM |
BF.00011: Recent results and future perspectives from the CB@MAMI programme Evangeline Joy Downie The CB@MAMI four-pi spectrometer setup at the A2 Tagged Photon Facility in Mainz, Germany was installed in 2003. Since that time, a series of successful experiments have taken place studying a range of topics from the accurate determination of the eta slope parameter to photon asymmetries in neutral pion threshold photo production. The quality of these results is made possible by the large solid angle coverage of the CB and TAPS spectrometer arrangement in conjunction with the Edinburgh PID detector and two Multi-Wire Proportional Chambers for charged particle identification and tracking. An overview of the Crystal Ball experimental setup will be given and a selection of the physics results that are complete and in production will be shown. The future perspectives for the experiment after the successful MAMI and Glasgow Photon Tagger upgrade to 1.5 GeV and the imminent installation of the Mainz Frozen Spin target will be presented. [Preview Abstract] |
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