Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session B15: Electromagnetic Interactions I |
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Sponsoring Units: DNP Chair: Joerg Reinhold, Florida International University Room: Hyatt Regency Jacksonville Riverfront City Terrace 11 |
Saturday, April 14, 2007 10:45AM - 10:57AM |
B15.00001: The Double Spin Asymmetry for Exclusive $\pi^{+}$ Production Joshua Pierce The eg1b run was conducted using CLAS (CEBAF Large Acceptance Spectrometer) at the Thomas Jefferson National Accelerator Facility (TJNAF) in 2000 by the CLAS collaboration. A 1.6 GeV - 5.6 GeV polarized electron beam and polarized nuclear targets (composed of NH$_{3}$ and ND$_{3}$) were used, allowing spin asymmetries to be measured. This analysis is of the double spin asymmetry $A_{||}$ in the exclusive production of positive pions from a polarized proton ($ep\rightarrow e\pi^{+}n$). The double spin asymmetry was measured as a function of the four kinematic variables $W$, $Q^{2}$, $\cos\theta ^{*}$ (the angle between the direction of the virtual photon and the produced pion), and $\phi^{*}$ (the angle between the lepton interaction plane and the hadron interaction plane). This asymmetry helps determine the spin structure of the resonances. A brief description of the experimental setup will be given, and preliminary results of the asymmetry as a function of $W$ will be shown. [Preview Abstract] |
Saturday, April 14, 2007 10:57AM - 11:09AM |
B15.00002: Spin Asymmetry on the Nucleon Experiment James Maxwell The Spin Asymmetry on the Nucleon Experiment (SANE) will employ a revolutionary increase in Figure of Merit to obtain precise $g^{p}_{2}$ and $A^{p}_{1}$ results at high \textit{x}. Using the highest available JLab beam energy, a 194 msr electromagnetic calorimeter will view the UVa polarized NH$_{3}$ target at $8.5 \cdot 10^{34}$ proton luminosity. The large Bjorken \textit{x} region provides an important view on proton structure where the sea quarks have been stripped away. Using measurements of these ``naked protons'' is crucial for the understanding of strong QCD and can provide a connection between experimentally measured moments of polarized structure functions and quark matrix elements calculated in lattice QCD. The experiment will take place in 2008, using JLab's 5.7 GeV polarized electron beam, and covering the Bjorken \textit{x} range from 0.3 and 0.8 with an average $Q^2$ of 4.5 GeV$^2$. We will discuss the physics motivation for SANE as well as the proposed experimental arrangement, and expected results. [Preview Abstract] |
Saturday, April 14, 2007 11:09AM - 11:21AM |
B15.00003: ABSTRACT WITHDRAWN |
Saturday, April 14, 2007 11:21AM - 11:33AM |
B15.00004: $K^+ \Lambda$ Photoproduction in the $u$-channel Ken Hicks, Tsutomu Mibe Photoproduction of the $\gamma p \to K^+ \Lambda$ reaction at backward center-of-mass angles has been measured by detecting the $\Lambda$ at forward angles using the LEPS spectrometer. The $K^+$ was identified by the missing mass technique. The data cover center-of-mass angles beyond 145$^\circ$ for six photon energy bins between 1.5 and 2.4 GeV. The results are sensitive to $u$-channel diagrams, especially at the most backward angles, where a neutral hyperon is propagated between the kaon and photon verticies. These data provide the first stringent test of $u$-channel diagrams in hyperon photoproduction. If available, calculations using a full coupled-channels model will be compared with photon energy- dependence of the data. [Preview Abstract] |
Saturday, April 14, 2007 11:33AM - 11:45AM |
B15.00005: The Generalized GDH Sum Rule: Measuring the Spin Structure of $^3$He and the Neutron using Nearly Real Photons Jaideep Singh The generalized Gerasimov-Drell-Hearn (GDH) sum rule is a versatile and important tool for studying nucleon spin structure over the entire $Q^2$ range. Originally derived for real photon absorption ($Q^2$=0), the GDH sum rule has been extended to non-zero $Q^2$. The goals of Jefferson Lab experiment E97-110 are to measure the $Q^2$ dependence of the generalized GDH integral between 0.02 and 0.3 (GeV/c)$^2$, to study its slope near $Q^2 = 0$, and to extrapolate to the real photon point for the neutron and $^3$He. In this low $Q^2$ domain, such measurements serve to test Chiral Perturbation Theory and probe its limits of applicability. Our measurement will also provide new constraints on understanding the neutron and $^3$He spin structure and a better understanding of the effect of nucleon resonances. The experiment utilized the highly polarized electron beam and polarized $^3$He target at Jefferson Lab. Preliminary results will be presented. [Preview Abstract] |
Saturday, April 14, 2007 11:45AM - 11:57AM |
B15.00006: Induced Polarization in $^4$He$(e,e' \vec p)^3$H Steffen Strauch Polarization transfer in the $^4$He($\vec e,e^\prime \vec p\,$)$^3$H reaction up to $Q^2 = 2.6$ (GeV/c)$^2$ has been measured in an attempt to study possible medium modifications of the proton form factors. Indeed, the measured ratio of polarization-transfer coefficients are described by the inclusion of a medium modification of the proton form factors. This interpretation is challenged by a recent calculation including, particularly, a spin-dependent charge exchange in the final-state interaction. The induced polarization in this reaction is a measure of final-state interactions. Precise data on these are key to shed more light on this controversy. In our recent follow-up experiment E03-104 at Jefferson Lab data were taken at a $Q^2$ of 0.8 (GeV/c)$^2$ and 1.3 (GeV/c)$^2$ on $^1$H and $^4$He targets. The extraction of the small induced polarization of the recoiling proton for the $^4$He$(e,e'\vec p)$ reaction 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 extraction procedure and preliminary results will be presented. [Preview Abstract] |
Saturday, April 14, 2007 11:57AM - 12:09PM |
B15.00007: 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 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. Our new data will provide a much more stringent test of these calculations. Preliminary results will be presented. [Preview Abstract] |
Saturday, April 14, 2007 12:09PM - 12:21PM |
B15.00008: Structure In The Proton Form Factor Ratio at Low Q$^2$ Douglas Higinbotham Double polarization asymmetry measurements are allowing the electric to magnetic form factor ratios be determined with unprecedented precision. New results from BLAST and, more recently, from Jefferson Lab indicate that there is an unexpected dip in the proton form factor ratio around a Q$^2$ of 0.3 GeV$^2$. This structure at low Q$^2$ may provide new insight into of the peripheral substructure of the proton. The Jefferson Lab data, which was taken in less than two days as part of a calibration measurement, will be presented along with the expected uncertainties, in the Q$^2$ range from 0.25 to 0.7 GeV$^2$, which would be achieved with a dedicated experiment. [Preview Abstract] |
Saturday, April 14, 2007 12:21PM - 12:33PM |
B15.00009: 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] |
Saturday, April 14, 2007 12:33PM - 12:45PM |
B15.00010: The $\pi^0$ Lifetime: Experimental Probe of the QCD Axial Anomaly Dustin McNulty The $\pi^0 \rightarrow \gamma\gamma$ decay rate is a fundamental prediction of QCD which gives insight into one of its most profound symmetry issues---namely, the Axial or Chiral Anomaly. It is this anomalous symmetry-breaking mechanism by which the $\pi^0 \rightarrow \gamma\gamma$ decay channel primarily proceeds, and thus a measure of its rate or partial width, $\Gamma_{\gamma\gamma}$, represents a direct probe of the anomaly plus chiral corrections. The PrimEx Collaboration at Jefferson Lab has extracted $\Gamma_{\gamma\gamma}$ from precision measurements of $\pi^0$ photo-production cross sections using their Primakoff components. Measurements were made using 5\% X$_{\mathrm{o}}$ nuclear targets of $^{12}$C and $^{208}$Pb with incident photons between 4.9 and 5.5 GeV tagged by the Hall B tagger facility. The $\pi^0$ decay photons were detected by a specially constructed high resolution hybrid calorimeter (HYCAL). In this presentation, recent preliminary results from the PrimEx measurement will be given with emphasis on the cross section analysis and lifetime extraction. [Preview Abstract] |
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