Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session Q10: Electromagnetic Interactions |
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Sponsoring Units: DNP Chair: Zein-Eddine Meziani, Temple University Room: Governor's Square 10 |
Monday, May 4, 2009 10:45AM - 10:57AM |
Q10.00001: Measurement of the $^{4}He$ Charge Form Factor at Jefferson Lab Elena Khrosinkova Preliminary results from a Jefferson Lab experiment that measured the charge form factor of $^{4}He$ will be presented. The experiment was performed in the Hall A Facility using a cryogenic target, and the two High Resolution Spectrometers, which detected scattered electrons and recoil nuclei in coincidence. The results will be compared to theoretical calculations based on the impulse approximation with inclusion of meson exchange currents, and on quark dimensional scaling. [Preview Abstract] |
Monday, May 4, 2009 10:57AM - 11:09AM |
Q10.00002: ABSTRACT WITHDRAWN |
Monday, May 4, 2009 11:09AM - 11:21AM |
Q10.00003: Impulse Approximation limitations to the $(e,e'p)$ reaction on $^{209}$Bi: cross sections at quasielastic kinematics Juan Carlos Cornejo, Joaquin Lopez Herraiz Experiment E06007 at Jefferson Lab measured cross sections for the $(e,e'p)$ reaction at constant $(\textbf{q},\omega)$ for Q$^2$ = 0.81 GeV$^2$ over a range of missing momenta from 100 to 300 MeV/c on both sides of the three momentum transfer $\textbf{q}$=1.0 GeV/c. This is the first measurement of the $^{209}Bi(e,e'p)^{208}Pb$ reaction reported at quasielastic kinematics. Cross sections for the knockout of the 1h9/2 proton to the ground state of $^{208}Pb$ will be presented and compared to theoretical predictions. [Preview Abstract] |
Monday, May 4, 2009 11:21AM - 11:33AM |
Q10.00004: Photoproduction and Decay Modes of the x(1280) Meson Ryan Dickson, Reinhard Schumacher A meson of mass $m_x=1281$ MeV and a FWHM of $\Gamma_x=18$ MeV is seen at Jefferson Lab with CLAS in photoproduction off the proton using real photons in the energy range between 1.9 GeV and 3.4 GeV. Both the $f_1(1285)$ and the poorly-known $\eta(1295)$ are candidates for this observed state. The decay modes seen are $x\rightarrow\eta\pi^{+}\pi^{-}$, $K^+\bar{K^0}\pi^-$, $K^-K^0\pi^+$, and $K^+K^-\pi^0$ with a substantial fraction going through $a_0(980)\pi$. No signal is seen in $x\rightarrow\rho^0\gamma$. The relative branching fraction $\Gamma_{KK\pi}/\Gamma_{\eta\pi\pi}$ is consistent with world data for the $f_1(1285)$ state. The unseen $\rho^0\gamma$ decay mode is not consistent with the $f_1(1285)$ state, however, and may be more consistent with the $\eta(1295)$. Angle and energy dependencies of the measured cross-sections are in fair agreement among the observed decay modes. [Preview Abstract] |
Monday, May 4, 2009 11:33AM - 11:45AM |
Q10.00005: The Near-Threshold Pion Photoproduction Program at MAX-lab G.V. O'Rielly Pion photoproduction near threshold is a fundamental process that can be used to test the predictions of chiral effective-field theories, dispersion theoretical approaches and other quark-based models of the nucleon. Despite the importance of having data on the energy and angular dependence for all four pion channels in the near-threshold region, only the $\gamma p \rightarrow \pi^{\circ} p$ channel has been studied extensively. The new Photon Tagging Facility at MAX-lab is uniquely suited to study pion photoproduction between threshold and the $\Delta$-resonance. A comprehensive program to investigate both charged channels, $\pi^{+} n$ and $\pi^{-} p$, as well as the neutral channel $\gamma n \rightarrow \pi^{\circ} n$ is underway at MAX-lab. This includes measurements of the angular distributions for $\pi^{+}$ photoproduction from the proton and heavy nuclei in order to investigate the $p$-wave contributions to this process, as well as a measurement of the total cross section for $\gamma n \rightarrow \pi^{-} p$ to enable the determination of the threshold $E_{0^{+}}\left(\pi^{-}n\right)$ amplitude. Complementing these measurements of the charged pion channels and the existing data on the $\pi^{\circ} p$ channel, an experiment to measure neutral pion photoproduction off the neutron is being developed. An overview of the planned MAX-lab pion program, together with preliminary results from measurements already performed will be presented. [Preview Abstract] |
Monday, May 4, 2009 11:45AM - 11:57AM |
Q10.00006: Precision Measurement of $\pi^0$ Electroproduction Cross Section Near Threshold Mitra Shabestari Experiment E04-007, a high precision measurement of the reaction $H\left({e,e'p}\right)\pi^0$ near threshold, which was performed at Hall A of Jefferson Laboratory. Measurements were made in a fine grid of $Q^2$, $0.045\left( {{{{GeV}} \mathord{\left/ {\vphantom {{{GeV}} c}} \right. \kern- \nulldelimiterspace} c}} \right)^2 \leq Q^2 \leq 0.15\left( {{{{GeV}} \mathord{\left/ {\vphantom {{ {GeV}} c}} \right. \kern-\nulldelimiterspace} c}} \right)^2 $, and $\Delta W$ range of $0 MeV \leq \Delta W \leq 30 MeV$. Polarized electron beams at energies of 1194 and 2232 MeV were used to bombard a liquid hydrogen target. The pion was identified by detecting the electron in one of the high-resolution spectrometers in coincidence with the recoiled proton, detected in the large acceptance BigBite spectrometer. These coincidence data allow us to reexamine the aforementioned disagreement and test chiral QCD dynamics. The experimental details will be discussed, and some preliminary results will be presented. [Preview Abstract] |
Monday, May 4, 2009 11:57AM - 12:09PM |
Q10.00007: First Measurement of Differential Photoproduction Cross Sections for the $\Lambda(1405)$ Using CLAS Kei Moriya, Reinhard Schumacher The $\Lambda(1405)$ is a well-established hyperon state just below $\mathrm{N}\overline{\mathrm{K}}$ threshold. Although some properties were established in the early hadronic beam experiments of the 1960s, many experimental properties remain undetermined. In this talk we present preliminary results for the first measurements of the photoproduction differential cross section of the $\Lambda(1405)$ using CLAS at Jefferson Lab. The event sample consisted of $\sim 1.8 \times 10^5$ reconstructed $\Lambda(1405)$ hyperons photoproduced off the proton, with photon energies between 1.5 and 3.9 GeV. The differential cross section for the two charged $\Sigma \pi$ decay modes will be presented. For comparison, the $\Lambda(1520)$ differential cross sections derived from the same data set will be shown. Comparison of these hyperons may reveal clues to the internal structure of the $\Lambda(1405)$. [Preview Abstract] |
Monday, May 4, 2009 12:09PM - 12:21PM |
Q10.00008: Measurement of tMeasurement of the Proton Elastic Form Factor Ratio at Low $Q^2$ Xiaohui Zhan A high precision measurement of the proton elastic form factor ratio $\mu G_{\mathrm{E}}/G_{\mathrm{M}}$ in the range of $Q^2$=0.3 - 0.7~GeV/$c^2$ has been made using recoil polarimetry method in Jefferson Lab Hall A. In this low $Q^2$ range, previous data (Bates, LEDEX) along with many fits and calculations indicate substantial deviations of the ratio from unity, and continue to suggest that structures might be present in the individual form factors, and in the ratio. In E08-007, with 80\% polarized electron beam for 24 days, we are able to achieve $\sim 0.5\%$ statistical uncertainty. This high precision result will confirm or reute all existing suggestions of few percent structures in the form factors ratio. Beyond the intrinsic interest in nucleon structure, the improved form factor measurements also have implications for DVCS, determinations of the proton Zemach radius and for parity violation experiments. [Preview Abstract] |
Monday, May 4, 2009 12:21PM - 12:33PM |
Q10.00009: Recoil polarization measurements of the proton form factor ratio $G_E^p/G_M^p$ to high $Q^2$ in Hall C at Jefferson Lab Andrew Puckett Experiment E04-108 in Hall C at Jefferson Lab measured the ratio of the proton's electric ($G_E$) and magnetic ($G_M$) form factors using the recoil polarization technique at three different values of squared four-momentum transfer $Q^2$--5.2, 6.8, and 8.5 GeV$^2$. Data taking was completed in June 2008. Two new detectors were built by the collaboration to carry out this experiment. A large solid-angle electromagnetic calorimeter was used to detect elastically scattered electrons in coincidence with scattered protons detected by the Hall C High Momentum Spectrometer (HMS). The calorimeter allowed a clean rejection of the significant inelastic backgrounds present at such high $Q^2$. A new Focal Plane Polarimeter (FPP) was installed in the HMS detector hut to measure the polarization of the scattered proton. Following a discussion of the data analysis method, preliminary results will be reported. [Preview Abstract] |
Monday, May 4, 2009 12:33PM - 12:45PM |
Q10.00010: Trace Anomaly in Strong Electromagnetic Fields Lance Labun, Johann Rafelski Violation of the superposition principle in the Maxwell field could arise from intrinsic nonlinearity in the fundamental theory of the photon, such as seen in Born-Infeld (BI) electrodynamics, but certainly occurs due to charged particle vacuum fluctuations, i.e. one-loop quantum electrodynamics (QED). The necessary presence of a dimensioned scale in a nonlinear theory induces a nonvanishing trace in the energy-momentum tensor $$ T_{\mu}^{\mu} = -M\frac{d L_{\rm eff}}{dM} = -4\left(L_{\rm eff}-\mathcal{S} \frac{\it \partial L_{\rm eff}}{\partial S} - P \frac{\it \partial L_{\rm eff}}{\partial P}\right), \quad $$ where $\mathcal{S,P}$ are the scalar and pseudo scalar field invariants. $T_{\mu}^{\mu}$ has the form of the Einstein cosmological constant, and hence intense electromagnetic fields generate a localized, dark energy-like concentration.\\[0.2cm] References: \\ {\it Trace Anomaly of Nonlinear Electrodynamics and its (Anti)\ Gravitational Effect} arXiv:0811.4467 [hep-th] and \\ {\it QED Conformal Anomaly in External Fields} arXiv:0810.1323 [hep-ph] [Preview Abstract] |
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