Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session DF: Electromagnetic Interactions |
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Sponsoring Units: DNP Chair: Cynthia Keppel, Hampton University/Thomas Jefferson National Accelerator Facility Room: Gaylord Opryland Hermitage C |
Friday, October 27, 2006 2:00PM - 2:12PM |
DF.00001: Search for the onset of Color Transparency in $\rho ^{0}$ electroproduction. Lorenzo Zana, Maurik Holtrop The nuclear transparency for the coherent production of $\rho ^{0}$ mesons was measured on $^{2}$H, $^{12}$C and $^{56}$Fe in the $Q^{2}$ range of 1.-2.5 GeV$^{2}$/c$^{2}$ with the CLAS detector at Jefferson Laboratory. The nuclear transparency is extracted for a number of bins in $Q^{2}$ as the ratio of $\rho_0$ production on a nuclear target over the production on deuterium. Systematic errors were reduced by measuring on these two targets simultaneously. A rise in the nuclear transparency for increasing $Q^{2}$ would indicate the onset of Color Transparency. We will discuss the experimental setup, the data analysis, preliminary results, and outlook for this experiment. [Preview Abstract] |
Friday, October 27, 2006 2:12PM - 2:24PM |
DF.00002: Search for medium effects on light vector mesons Rakhsha Nasseripour, Chaden Djalali, Dennis Weygand, Michael Wood Theoretical calculations predict the modification of properties of vector mesons, such as a shift in their masses and/or broadening of their widths in dense nuclear matter. These effects can be related to partial restoration of chiral symmetry at high density or temperature. To explore these, we performed an experiment at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). The data were taken with a beam of tagged photons with energies up to 4 GeV on various nuclear targets. The properties of the $\rho$ vector mesons were investigated via their rare leptonic decay to $e^+e^-$ . This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The combinatorial background in the mass spectrum was removed by a self-normalizing mixed-event technique. The $\rho$ mass distributions were extracted for each of the targets. We obtained statistically significant results regarding medium modification of the $\rho$ in the nuclear medium that rule out large medium effects (mass shift parameter $\alpha > 0.1$) within the 99\% conincidence level. [Preview Abstract] |
Friday, October 27, 2006 2:24PM - 2:36PM |
DF.00003: Rosenbluth Separation of the pion electroproduction cross section from Hydrogen, Deuterium, Carbon and Copper targets Xin Qian, Ben Clasie, Dipangkar Dutta, Haiyan Gao Pion electroproduction data was collected from six targets, including hydrogen, deuterium, carbon and copper, in Jefferson Lab experiment E01-107. The primary motivation of this experiment is to search for signatures of a phenomenon predicted by perturbative quantum chromodynamics (pQCD) known as Color Transparency (CT). In this experiment the nuclear transparency of pions is extracted by using the ratio of semi-exclusive pion electroproduction from nuclear targets to the same from a hydrogen target. This method relies on the assumption that the reaction mechanism of electropion production from hydrogen is similar to the quasi-free eletropion production from nuclear targets. This assumption can tested by performing a Longitudinal-Transverse (L-T) separation of the pion electroproduction cross section and comparing the separated cross sections from hydrogen to that from heavier targets. In order to perform an L-T separation, data were collected at forward and backward electron angles at fixed momentum transfer squared ($Q^2$), for $Q^2$=2.15 and 4.0 $GeV^2/c^2$, which were chosen to fall within the $Q^2$ range over which the nuclear transparency is extracted. The extracted longitudinal and transverse cross sections at the two $Q^2$ settings from hydrogen, deuterium, carbon and copper targets will be presented. [Preview Abstract] |
Friday, October 27, 2006 2:36PM - 2:48PM |
DF.00004: Status of the Jefferson Lab BONUS Experiment, an Effective Free Neutron Target Vladas Tvaskis To understand the structure of the nucleon is one of the fundamental goals of nuclear and high-energy physics. Deep-inelastic lepton scattering off proton and nuclear targets has produced a large amount of accurate data on the proton structure function. However, due to the unavailability of free neutrons, the neutron structure function must be extracted from measurements on nuclear targets. The precision of such extractions is limited because of the theoretical uncertainties introduced by the nuclear models needed to deduce information from the bound nucleons in the nuclei. To alleviate this problem the $Barely$ $Of$-$shell$ $NUcleon$ $Structure$ $(BONUS)$ $Collaboration$ has constructed a novel radial time projection chamber (RTPC) that uses a gas electron multiplier readout to detect slow, backward - going spectator protons resulting from electron-deuteron interactions. Spectator protons in the RTPC are detected in coincidence with electrons in the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at Jefferson Lab, thereby ensuring an inclusive electron - neutron scattering event. Two months of data (roughly 900 million triggers) were collected in late 2005 at beam energies from 1.1 to 5.3 GeV. Preliminary results will be discussed, focusing on the performance of the RTPC in CLAS. [Preview Abstract] |
Friday, October 27, 2006 2:48PM - 3:00PM |
DF.00005: Measurement of the Compton scattering cross section during PrimEx Experiment at Jefferson Lab Pawel Ambrozewicz A precision experiment to extract the neutral $\pi^0$ lifetime was performed in Hall B of Jefferson Lab in the Fall of 2004. The experiment used Primakoff effect, small angle coherent photoproduction of $\pi^0$'s in the Coulomb field of various nuclei, to determine the radiative decay width of the $\pi^0$. This measurement constitutes one of the most precise tools in investigating fundamental symmetry predictions of low energy QCD. The projected experimental accuracy of this lifetime determination is 1.5\%, it therefore requires thorough understanding of the underlying systematic uncertainties. To facilitate that Compton scattering data were taken along with the photoproduction data. This allowed to measure the Compton scattering cross section with high precision in a few GeV region. The results of this analysis will be presented. [Preview Abstract] |
Friday, October 27, 2006 3:00PM - 3:12PM |
DF.00006: A Precision Measurement of the Pair Production Cross Section in the Jefferson Lab PrimEx Experiment Aram Teymurazyan The Jefferson Lab Hall B PrimEx Collaboration, is performing a $1.4\%$ level measurement of the absolute cross section for the photo-production of neutral pions in the Coulomb field of a nucleus. Two key elements of the PrimEx experimental setup are the Jefferson Lab Hall B photon tagger, and the new 1728 channel hybrid calorimeter (HyCal) for detecting the two decay photons from the neutral pions. In the view of the stringent requirements on the required precision of the photon flux for this experiment, periodic measurements of the pair production cross section were performed throughout the run. In these measurements, both the photon energy and flux were determined by the tagger, and the electron-positron pairs were swept by a magnetic field and detected in the calorimeter. The experimental setup and the analysis of these measurements will be described. In addition, the pair production cross sections so obtained will be compared to those expected by theory. [Preview Abstract] |
Friday, October 27, 2006 3:12PM - 3:24PM |
DF.00007: $\pi^{0}$ meson radiative width results on $^{12}$C from the PrimEx collaboration at Jefferson Lab Eric Clinton The $\pi^{0}$ lifetime is arguably the most precise theoretical calculation possible in low energy QCD, but current world's data is not commensurate with current theory. The next-to-leading order Chiral Perterbation Theory calculation [hep-ph/0206007] calls for a $\pi^{0}$ radiative width of 8.1 eV $\pm$ 1\%. The PDG average stands at 7.84 eV $\pm$ 7\%. The Primakoff Experiment (PrimEx) collaboration has utilized the Primakoff effect, photo-meson production in the Coulomb field of nuclei, to measure the $pi^{0}$ radiative width. The final sensitivity of this $\pi^{0}$ lifetime measurement is expected to approach 1.5\%. The PrimEx collaboration recorded data in the Fall of 2004 in Hall B of the Thomas Jefferson National National Accelerator Facility. Preliminary results for this run will be presented. This is expected to be a stringent test of the U(1) axial anomaly and thus fill an important gap in our knowledge of low energy QCD. [Preview Abstract] |
Friday, October 27, 2006 3:24PM - 3:36PM |
DF.00008: Pion-Nucleon Single Charge Exchange at $T_{\pi^-}$ = 10.6, 20.6, and 39.4 MeV Donald Isenhower Measurements will be presented for the differential cross sections for $\pi^- p \rightarrow \pi^o n$ near $0^o, 90^o,$ and $180^o$ at $T_{\pi^-}$ = 10.6, 20.6, and 39.4 MeV ($P_{\pi^{-}}$ = 55.4, 78.6, and 112.0 MeV/c) from LAMPF Experiment 882. These data include the lowest energies ever measured for this interaction and are the only low-energy data to cover the entire angular region from $0^{o}$ to $180^{o}$. The results are compared with the partial wave analyses and potential models. The goal of determination of the differential cross sections to better than 10{\%} has been obtained by these measurements. [Preview Abstract] |
Friday, October 27, 2006 3:36PM - 3:48PM |
DF.00009: Theoretical Investigation of $A_{TL'}$ in Electron Scattering from the Deuteron Sabine Jeschonnek Currently, several data sets on $D(e,e'p)n$ reactions, taken at Jefferson Lab, are analyzed. 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. In order to gain a full understanding of this important reaction, we need to consider several observables: cross section, response functions, and asymmetries. Final state interactions and relativistic treatment of the current operator are very important at the relevant high energies. The asymmetry $A_{TL'}$, which is non-zero only for out-of-plane kinematics, has been measured in Jefferson Lab's Hall B. This observable is very interesting because it is highly sensitive not just to central final state interactions, but also to spin-orbit final state interactions. I will briefly discuss the employed theoretical model, and focus on the sensitivity of the results to the various final state interactions and wave functions. [Preview Abstract] |
Friday, October 27, 2006 3:48PM - 4:00PM |
DF.00010: Out-of-Plane Measurements of the Fifth Structure Function of the Deuteron Gerard Gilfoyle We have measured the asymmetry $A_{LT}^\prime$ associated with the fifth structure function in quasi-elastic kinematics at beam energies of $\rm 2.56 ~GeV$ and $\rm 4.23 ~GeV$ over a $Q^2$ range $\rm 0.1 - 2.0~ (GeV/c)^2$ with the CLAS detector at Jefferson Lab. The differential cross section of the $D(\vec e,e^\prime p)n$ reaction with a polarized beam and unpolarized target has a component that is the imaginary part of the interference term between the longitudinal and transverse parts of the nuclear current. This fifth structure function is non-zero only for protons ejected out of the scattering plane defined by the incoming and outgoing electron and is sensitive to final-state interactions. Only limited measurements have been made of this quantity before now. We extract $A_{LT}^\prime$ using quasi-elastic, missing momentum ($p_m$) spectra weighted by $\sin(\phi_{pq})$ where $\phi_{pq}$ is the angle between the scattering plane and the plane defined by the ejected proton and neutron. We will present event selection criteria, calibrations, and consistency checks of the analysis. We will show results for measurements of $A_{LT}^\prime$ that explore different $Q^2$ regions and different $W$ ranges near the quasi-elastic peak. The data agree with theoretical calculations at low $p_m$, but diverge at higher missing momenta. [Preview Abstract] |
Friday, October 27, 2006 4:00PM - 4:12PM |
DF.00011: Nucleon Polarisabilities from Deuteron Compton Scattering, and Its Lessons for Chiral Power 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 polarisabilities of the proton and neutron from Compton scattering experiments 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 case to the correct Thomson limit, demonstrating gauge-invariance and shedding new light on Weinberg´s proposed power-counting of nuclear forces. In our global analysis of all elastic proton and deuteron Compton scattering up to 150 $MeV$, 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. Therefore, 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. \newline [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 27, 2006 4:12PM - 4:24PM |
DF.00012: A(Q) at Low Q in ed Elastic Scattering Douglas Higinbotham Using the Jefferson Lab Hall A high resolution spectrometers, data have been taken to resolve a discrepancy between low Q elastic deuteron cross section measurements. This new data will provide a test of models of deuteron structure including chiral perturbation theory, conventional non-relativistic models, and relativistic models. An overview of the new data will be presented along with the expected uncertainties. [Preview Abstract] |
Friday, October 27, 2006 4:24PM - 4:36PM |
DF.00013: Measuring the Neutron and $^{3}$He Spin Structure at Low $Q^{2}$ Vincent Sulkosky Originally derived for real photon absorption, the Gerasimov-Drell-Hearn (GDH) sum rule was first extended to non-zero $Q^{2}$ in 1989. The extension of the sum rule provides a unique relation, valid at any $Q^{2}$, that can be used to study the nucleon spin structure. The goal of Jefferson Lab experiment E97-110 is to perform a precise measurement of the $Q^{2}$ dependence of the generalized GDH integral and of the moments of the neutron and $^{3}$He spin structure functions between 0.02 and 0.3 GeV$^{2}$. This $Q^{2}$ range will allow us to test predictions of Chiral Perturbation Theory, and verify the GDH sum rule by extrapolating the integral to the real photon point. This measurement also provides a better understanding of the nucleon resonances. The data have been taken in Hall A using the Jefferson Lab high polarization continuous electron beam and a polarized $^{3}$He target. The status and perspectives of the data analysis will be discussed, and preliminary results will be shown. [Preview Abstract] |
Friday, October 27, 2006 4:36PM - 4:48PM |
DF.00014: A Measurement of $G_{E}^n$ at High Momentum Transfer in Hall A Robert J. Feuerbach, Bogdan Wojtsekhowski A precision measurement of the electric form-factor of the neutron, $G_E^n$, at $Q^2$ up to 3.5 $\mathrm{GeV}^2$ was recently completed in Hall A at the Thomas Jefferson National Accelerator Facility(Jefferson Lab). The ratio $G_E^n/G_M^n$ was measured through the beam-target asymmetry $A_\perp$ of electrons quasi-elastically scattered off neutrons in the reaction ${}^{3}\overrightarrow{He}(\overrightarrow{e},e' n)$. The experiment took advantage of recent developments of the electron beam and target, as well as two detectors new to Jefferson Lab. The measurement used the accelerator's 100\% duty-cycle high-polarization (typically 84\%) electron beam and a new, hybrid optically-pumped polarized ${}^{3}\overrightarrow{He}$ target which achieved polarizations above 50\%. A medium acceptance (80msr) open-geometry magnetic spectrometer (BigBite) detected the scattered electron, while a new neutron detector was constructed to observe the released neutron. An overview of the experiment and the experimental motivation will be discussed, in particular the large range of predictions from modern calculations for $G_E^n$ at this relatively high $Q^2$. Finally, the analysis progress and preliminary results will be presented. [Preview Abstract] |
Friday, October 27, 2006 4:48PM - 5:00PM |
DF.00015: N-$\Delta$ transition form factors Mandar Bhagwat Nucleon and $\Delta$ amplitudes have been obtained by solving a Poincar$\acute{e}$-covariant Faddeev equation, which describes baryons as composites of confined-quarks and confined-nonpointlike-diquarks. The amplitudes were used to calculate the nucleon form factors. The calculation predicts a ratio $\mu_{p}G^{p}_{E}/G^{p}_{M}$ that agrees with extracted JLab data and also predicts that this ratio will pass through zero at $Q^2 = 6.5 \rm{GeV^2}$. This prediction will be tested in forthcoming JLab experiments. We have extended the framework to study the electromagnetic N-$\Delta$ transition form factors. Results for the ratios $G_{E}(p^2)/G_{M}(p^2)$ and $G_{C}(p^2)/G_{M}(p^2)$ are compared with observations. Effects of pion loops, which in dynamical coupled-channel models contribute substantially to $G_{C}(p^2)/G_{M}(p^2)$ at low $Q^2$, are also considered. [Preview Abstract] |
Friday, October 27, 2006 5:00PM - 5:12PM |
DF.00016: Data consistency checks for Jefferson Lab Experiment E00-002 John Telfeyan, Gabriel Niculescu, Ioana Niculescu Jefferson Lab experiment E00-002 aims to measure inclusive electron-proton and electron-deuteron scattering cross section at low Q squared and moderately low Bjorken x. Data in this kinematic region will further our understanding of the transition between the perturbative and non-perturbative regimes of Quantum Chromodynamics (QCD). As part of the data analysis effort underway at James Madison University (JMU) a comprehensive set of checks and tests was implemented. These tests ensure the quality and consistency of the experimental data, as well as providing, where appropriate, correction factors between the experimental apparatus as used and its idealized computer-simulated representation. This contribution will outline this testing procedure as implemented in the JMU analysis, highlighting the most important features/results. [Preview Abstract] |
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