Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session CC: Electromagnetic Interactions I |
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Chair: Cynthia Keppel, Hampton University/Jefferson Lab Room: Newport News Marriott at City Center Pearl Salon I |
Friday, October 12, 2007 9:00AM - 9:12AM |
CC.00001: Exchange currents in light-front quantum models of elastic electron-deuteron scattering Yunfei Huang, Wayne Polyzou We present calculations of exchange current contributions to elastic electron-deuteron scattering in a Poincar\'e invariant quantum model with a light-front kinematic symmetry. Current conservation, current covariance and discrete symmetries can be used to express all of the elastic current matrix elements in terms of three independent matrix elements of the $+$ component of the current. Invariant ``impulse approximations'' are defined by assuming that there are no two-body contributions to the independent current matrix elements [1]. While the covariance and current conservation constraints generate implicit exchange currents, it is also possible to add explicit exchange current contributions to the independent current matrix elements. We calculate the contribution of model ``pair currents'' that have an operator structure motivated by exchange currents contributions generated by the Blankenbecler-Sugar reduction of the Bethe-Salpeter equation [2]. \newline [1.] P.L.Chung, et.al., Phys. Rev. C37, 2000(1988). \newline [2.] F. Coester and D.O.Riska, Annals of Physics 234(1994)141. [Preview Abstract] |
Friday, October 12, 2007 9:12AM - 9:24AM |
CC.00002: Deuteron structure studies from electron scattering from vector and tensor polarized deuterium with BLAST Michael Kohl The electromagnetic structure of the deuteron is manifest in many polarization observables accessible by electron scattering in elastic, quasielastic, and pion production kinematics which have been measured simultaneously with the BLAST experiment at MIT-Bates. The deuteron's quadrupole moment and associated nonspherical shape give rise to elastic tensor analyzing powers and vector correlation parameters. Electrodisintegration in the quasielastic regime allows for a systematic study of final state interaction, meson exchange and relativistic effects. Pion electroproduction from tensor-polarized deuterium is well suited to probe effects due to the two-nucleon singlet-S state. This talk will discuss the current status of the data analysis and present preliminary results. [Preview Abstract] |
Friday, October 12, 2007 9:24AM - 9:36AM |
CC.00003: New Measurement of Elastic $ed$ Scattering Byungwuek Lee, Seonho Choi, Douglas Higinbotham, Ronald Gilman Differences between previous measurements of low momentum transfer electron-deuteron elastic scattering prevent a clean determination of even the sign of the leading low momentum transfer relativistic corrections, or of the convergence of chiral perturbation theory. We have attempted to resolve this issue with new high-precision measurements in Jefferson Lab Hall A, with $<$1\% statistical and 2 -- 3 \% systematic uncertainties. Elastic electron scattering was measured on targets of tantalum, carbon, hydrogen, and deuterium at beam energies of 362 and 687 MeV. The additional targets provide both kinematic calibration information, and tests of the precision of determining cross sections in better measured reactions. The four-momentum transfer covered the range 0.01 -- 0.5 GeV$\,^2$. The experiment included a new beam calorimeter, to better calibrate the low currents used in the experiment, and new collimators to better define the spectrometer solid angles. We will present preliminary results, including the deuteron $A(Q)$ structure function. [Preview Abstract] |
Friday, October 12, 2007 9:36AM - 9:48AM |
CC.00004: Nucleon Polarisabilities from Deuteron Compton Scattering, and Its Lessons for Chiral Counting Harald W. Griesshammer Chiral Effective Field Theory with explicit $\Delta(1232)$ degree of freedom is for photon energies up to $300$ MeV the tool to accurately determine the proton and neutron polarisabilities from Compton scattering in a model-independent and systematic way. It proves in particular indispensable to understand deuteron Compton scattering at $95$ MeV as measured at SAL. Simple consistency arguments derived from nuclear phenomenology lead for the deuteron to the correct Thomson limit, demonstrating gauge-invariance and shedding new light on Weinberg264s proposed power-counting of nuclear forces. In our global analysis of all elastic proton and deuteron Compton scattering up to $150\;\mathrm{MeV}$, we find for the static scalar dipole polarisabilities $\bar{\alpha}^p=(11.0\pm1.4_{\mathrm{stat}}\pm0.4_{\mathrm{sys}})\times10^{-4} \;\mathrm{fm}^3$, $\bar{\beta}^p=(2.8\mp1.4_{\mathrm{stat}}\pm0.4_{\mathrm{syst}})\times 10^{-4}\;\mathrm{fm}^3$ for the proton and $\bar{\alpha}^n=(11.6\pm1.5_{\mathrm{stat}}\pm0.6_{\mathrm{syst}})\times 10^{-4}\;\mathrm{fm}^3$ $\bar{\beta}^n=(3.6\mp1.5_{\mathrm{stat}}\pm0.6_{\mathrm{syst}})\times 10^{-4}\;\mathrm{fm}^3$ for the neutron. Thus, proton and neutron polarisabilities are identical within the accuracy of available data. New experiments e.g.~at MAXlab (Lund) will improve the statistical error-bar. [1] R.~P.~Hildebrandt, H.~W.~Grie{\ss}hammer and T.~R.~Hemmert, submitted to Phys.~Rev.~C~[nucl-th/0512063]. [2] H.~W.~Grie{\ss}hammer: \emph{Power-Counting in Chiral EFT from a Minimum of Phenomenology}, in preparation. [Preview Abstract] |
Friday, October 12, 2007 9:48AM - 10:00AM |
CC.00005: The D(e,e$'$p) reaction at GeV energies Sabine Jeschonnek, J. Wallace Van Orden Currently, several data sets on $D(e,e'p)n$ reactions, taken at Jefferson Lab, are analyzed or have been published recently. A solid theoretical description is necessary in order to understand these data and extract all possible information, both on the reaction mechanism and the nuclear ground state. Final state interactions and relativistic treatment of the current operator are essential. We present the results of a new calculation with a relativistic wave function for the initial deuteron state. We will discuss the sensitivity of various observables to the employed parametrization of the nucleon-nucleon scattering amplitude in the final state, and investigate several observables at high missing momentum. [Preview Abstract] |
Friday, October 12, 2007 10:00AM - 10:12AM |
CC.00006: Induced Polarization in $^4$He$(e,e' \vec p)^3$H Simona Malace The transferred polarization in the $^4$He($\vec e,e^\prime \vec p\,$)$^3$H reaction at $Q^2$ values of 0.4, 0.5, 1.0, 1.6, and 2.6 GeV$^2$ have been measured to study possible medium modifications of the proton form factors. The measured ratio of polarization-transfer coefficients was described by the inclusion of in-medium proton form factors. This interpretation was recently challenged by a calculation by Schiavilla {\it et al} including, particularly, a spin-dependent charge exchange in the final-state interaction (FSI). The induced polarization of the recoiling proton in this reaction is a measure of FSI. Precise data on these are key to shed more light on this controversy. Even more, a precise knowledge of the induced polarization will allow improvement of the FSI treatment by providing additional experimental constraints to existing theoretical calculations. In our follow-up experiment E03-104 at JLab high statistics data were taken at a $Q^2$ of 0.8 GeV$^2$ and 1.3 GeV$^2$ on $^1$H and $^4$He targets. The extraction of the small induced polarization is complicated by the possible presence of instrumental asymmetries in the focal-plane polarimeter. The induced polarization in the elastic $^1$H$(e,e'\vec p)$ reaction is (in one-photon approximation) zero and provides crucial information about these false asymmetries. Our preliminary results indicate an induced polarization of about $-0.03$ and seem to be in reasonable agreement with the RDWIA calculation of Udias {\it et al}. [Preview Abstract] |
Friday, October 12, 2007 10:12AM - 10:24AM |
CC.00007: Polarization Transfer in $^4$He$(\vec e,e' \vec p)^3$H Michael Paolone Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium, including possible modification of the nucleon form factor and/or spinor. In our recently completed experiment E03-104 at Jefferson Lab we measured the proton recoil polarization in the $^4$He($\vec e,e^\prime \vec p\,$)$^3$H reaction at a $Q^2$ of 0.8 (GeV/$c$)$^2$ and 1.3 (GeV/$c$)$^2$ with unprecedented precision. These data complement earlier data between 0.4 and 2.6 (GeV/$c$)$^2$ from both Mainz and Jefferson Lab, in which the measured ratio of polarization-transfer coefficients differs from a fully relativistic DWIA calculation. The earlier polarization-transfer data are equally well described by the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model and by a recent calculation including, particularly, a spin-dependent charge exchange in the final-state interaction. However, the preliminary analysis of our present data possibly implies an unexpected Q$^2$ dependence of the ratio of polarization-transfer coefficients. Due to its high statistical precision the new data allow for a detailed study of the missing momentum dependence of individual polarization-transfer coefficients in the $^4$He($\vec e,e^\prime \vec p\,$)$^3$H reaction. Final results will be discussed. [Preview Abstract] |
Friday, October 12, 2007 10:24AM - 10:36AM |
CC.00008: Double-Coincidence $^{12}{\rm C}(e,e'p)$ in a Correlations Dominant Regime Peter Monaghan We performed an experiment to investigate short-range correlations in carbon via a {\it triple-coincidence} $(e,e'pN)$ reaction in Hall A at Jefferson Lab. As a natural consequence of studying the three-body reaction, we collected high-quality {\it double-coincidence} $(e,e'p)$ data, which are presented here. Our kinematics were chosen with $Q^{2} = 2$ (GeV/c)$^{2}$ and $x_{B} > 1$ to provide a regime in which short-range correlations are expected to dominate the initial state; thus, the electrons were scattering primarily off nucleon pairs. The resulting $(e,e'p)$ data were obtained over a high missing-momentum region, $P_{m} \sim 200 - 600$ MeV/c. We present the cross-section data for the bound-state reaction $^{12}$C$(e,e'p)^{11}$B and also for scattering to the continuum from carbon, and compare our results to relativistic theoretical calculations. [Preview Abstract] |
Friday, October 12, 2007 10:36AM - 10:48AM |
CC.00009: Short-Range Correlations in $^{12}$C(e,e$^{\prime}$pn) Ramesh Subedi Correlations in nuclei, i.e. deviations from independent-particle behaviour, are generally classified into two types: long-range correlations due to the long-range, attractive part of the nucleon-nucleon interaction, and short-range correlations dominated by the short-range, repulsive part of the nucleon-nucleon interaction. We made direct observation of short-range correlated NN-pairs using the exclusive $^{12}$C$(e,e^{\prime}pN)$ reaction in a triple-coincidence measurement in the experiment E01-015 in Hall A at Jefferson Lab. We will present results from our analysis of the $^{12}$C$(e,e^{\prime}pn)$ reaction. From this analysis we conclude that there are nearly 20 times more n-p short-range correlated pairs than p-p short-range correlated pairs. [Preview Abstract] |
Friday, October 12, 2007 10:48AM - 11:00AM |
CC.00010: Cross sections for the $(e,e'p)$ reactions for $^{208}$Pb and $^{209}$Bi at high momentum transfer measured at Jefferson Lab Juan Carlos Cornejo, Joaquin Lopez Herraiz The reactions $^{208}$Pb$(e,e'p)^{207}$Tl and $^{209}$Bi$(e,e'p)^{208}$Pb have recently been measured at Jefferson Lab in fixed quasielastic kinematics, q=1 GeV/c,~ $\omega$ = 0.433 GeV, Q$^2$ = 0.81 GeV$^2$. This is the first time that these reactions have been measured for $x_B=1$, a condition which allows for a relatively unambiguous identification of long range correlations affecting the occupancy probability of the valence single nucleon states. Cross sections were measured symmetrically on both sides of the three momentum transfer from 0 to 500 MeV/c in missing momentum for $^{208}$Pb and from 100 MeV/c to 300 MeV/c in $^{209}$Bi. The low lying states in $^{207}$Tl are the focus of this study using the high resolution spectrometers of Hall A. Results for the $^{209}$Bi(e,e'p)$^{208}$Pb will also be shown. In the range of missing momenta $<$ 300 MeV/c ~a complex structure in the cross section asymmetry, $A_{TL}$, is predicted within the impulse approximation. [Preview Abstract] |
Friday, October 12, 2007 11:00AM - 11:12AM |
CC.00011: Relativistic models for the (e,e'p) reaction on $^{208}$Pb Joaquin Lopez Herraiz, Juan Carlos Cornejo $^{208}$Pb is the nucleus that one would pick as the best existing test of the shell model. That's why it has been extensively explored in the past, for example, by means of the (e,e'p) reaction. Jefferson Lab is ideally suited to perform (e,e'p) reactions and recently the (e,e'p) reaction on lead has been measured for the first time at this facility. We attempt to determine spectroscopic factors by comparing high statistics, quasielastic data for cross-sections at several q values to relativistic mean field predictions, over a wide range of missing momentum. The A$_{TL}$ cross section asymmetry predictions from these relativistic models will also be compared to the data measured in fixed quasielastic kinematics, q=1 GeV/c, $\omega$= 0.433 GeV, Q$^2$ = 0.81 GeV$^2$ at Jefferson Lab. Relativistic mean field calculations predict values of $A_{TL}$ that deviate substantially from the predictions that do not include the enhancement of the lower component of the wave function due to dynamical relativistic effects. The role played by correlations in the high missing momentum region of the (e,e'p) reaction is expected to be disentangled. [Preview Abstract] |
Friday, October 12, 2007 11:12AM - 11:24AM |
CC.00012: Spectrometer optics studies and target development for the $^{208}$Pb(e,e'p) experiment in Hall A at Jefferson Lab Guido M. Urciuoli, Juan Carlos Cornejo, Joaquin Lopez Herraiz The reactions $^{208}$Pb$(e,e'p)^{207}$Tl and $^{209}$Bi$(e,e'p)^{208}$Pb have recently been measured at Jefferson Lab using the high resolution spectrometers of Hall A. Monte Carlo simulations of these reactions showed that with a missing energy resolution of 1 MeV, spectrum fitting techniques could extract the cross sections for the states of $^{207}$Tl up to 3.5 MeV excitation. Extensive spectrometer optics studies were required to attempt to achieve the requisite missing energy resolution. In addition, the low melting temperature of the heavy metal targets required rastering the electron beam spot on the target and a special target holder to be employed. The measurement was possible because of the unique electron beam characteristics of CEBAF, the high resolution spectrometers of Hall A and the use of a novel target design which allowed for up to 80 $\mu$A of beam on target. The experimental challenges and procedures will be discussed. [Preview Abstract] |
Friday, October 12, 2007 11:24AM - 11:36AM |
CC.00013: Neutron-Proton Bremsstrahlung; with explicit correction terms at 225 MeV Virginia Brown, Jerrold Franklin, Perry Anthony Neutron-proton bremsstrahlung ($np\gamma$) with explicit correction terms including, higher partial waves, higher rescattering effects, longer-range integrations, out-of-plane contributions, neutron and proton relativistic spin effects, charge form- factor contributions, and meson-exchange effects included to order K in the photon momentum [1] are calculated with the Argonne nucleon-nucleon potential to compare to experimental results at 225 MeV obtained by Y. Safkan et al. [2] at LANCE. The data include various coplanar nucleon exit angles. These are the first experimental ($np\gamma$) data to explicitly measure the photon angulardistribution. Finite-size detector effects are determined with the out-of-plane calculations. \begin{thebibliography}{2} \bibitem{\bf} V. R. Brown and J. Franklin, Phys. Rev. C {\bf 8}, 1706 (1973). \bibitem{\sa} Y. Safkan et al.,Phys. Rev. C{\bf 75}, 031001 (2007). \end{thebibliography} [Preview Abstract] |
Friday, October 12, 2007 11:36AM - 11:48AM |
CC.00014: Nucleon form factors and charge densities from the BLAST Experiment Chris Crawford The BLAST experiment was designed to study in a systematic manner the spin-dependent electromagnetic interaction. Utilizing the polarized electron beam in the MIT-Bates South Hall Storage Ring, highly-polarized isotopically pure targets of hydrogen and deuterium, and the symmetric general purpose BLAST detector; precise measurements have been made which permit the extraction of the proton and neutron charge and magnetic form factors. The neutron electric form factor especially is now known to a precision comparable to that of the other nucleon form factors. These results, together with previously existing data, will constrain theoretical models constructed to explain the detailed structure of nucleon form factors. [Preview Abstract] |
Friday, October 12, 2007 11:48AM - 12:00PM |
CC.00015: A measurement of two-photon exchange in unpolarized elastic electron-proton scattering James Johnson Inconsistency between the measurements of the proton elastic form factors using recoil polarization and precision Rosenbluth separations suggests the presence of a two-photon exchange term. We look for the effects of two-photon exchange on electron-proton elastic scattering through both precision comparisons of cross section and recoil polarization measurements, and mapping nonlinearities in the reduced cross section with respect to the virtual photon longitudinal polarization. We achieve the necessary sensitivity using a modified Rosenbluth method. By detecting protons instead of electrons, we reduce the variation in the cross section as well as the magnitude of the radiative corrections. We have taken measurements between 0.6 and 5.74 $Q^2$, sensitive to the nonlinear portion of two-photon exchange at the low end and the full two photon effect at the high end. We will present initial results, and show the sensitivity we hope to obtain in the final measurement. [Preview Abstract] |
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