Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session HG: Electromagnetic Interactions II |
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Chair: Jian-Ping Chen, Thomas Jefferson National Accelerator Facility Room: Jewett Ballroom C |
Saturday, October 25, 2008 2:00PM - 2:12PM |
HG.00001: Measurement of two-photon exchange effect with CLAS Maryam Moteabbed, Brian Raue The two-photon exchange phenomenon is believed to be responsible for the discrepancy observed between the ratio of proton electric and magnetic form factors, measured by the Rosenbluth and polarization transfer methods. This disagreement is about a factor of three at $Q^2$ of 5.6 GeV$^2$. The two-photon exchange (TPE) radiative correction can be directly measured by taking the ratio of the electron-proton and positron-proton elastic scattering cross sections, as it changes sign with respect to the charge of the incident particle. A test run of a modified beamline has been conducted with the CLAS detector at Thomas Jefferson National Accelerator Facility. This test run demonstrated the feasibility of producing a mixed electron/positron beam of good quality. In addition, $e^-+p$ and $e^++p$ elastic scattering data were obtained for $Q^2$ up to nearly 1.0 GeV$^2$ and preliminary results will be presented. [Preview Abstract] |
Saturday, October 25, 2008 2:12PM - 2:24PM |
HG.00002: Diffractive Slope Extraction of Exclusive $\rho^{0}_{L}$ and $\rho^{0}_{T}$ at HERMES Brian Ball Hard exclusive $\rho^0$ electroproduction from a $^1$H target has been studied at the HERMES experiment. In particular, the meson helicity dependence of the cross-section slope parameter $b$ as a function of the squared four-momentum of the virtual photon $Q^{2}$ is presented. The parameter $b$ is related to the meson transverse size, and at higher $Q^{2}$ accesses color transparency. While $b$ is well studied for the helicity averaged case, there exists no previous experimental data regarding the $\rho^{0}$ helicity dependence. The HERMES data suggests a sizeable difference in the values of $b$ (and thus the transverse size of the $\rho^{0}$) for different helicities and a sizeable decrease of $b$ with increasing $Q^{2}$, an effect known as shrinkage. [Preview Abstract] |
Saturday, October 25, 2008 2:24PM - 2:36PM |
HG.00003: Simulations of Lepton-Hadron Discrimination in pp Collisions at Forward Rapidity with the STAR Detector at RHIC B.S. Page In the coming years STAR will be measuring flavor separated polarized anti-quark distribution functions by studying the reaction $ q+\overline{q} \Rightarrow W^{+(-)} \Rightarrow e^{+(-)}$ in polarized pp collisions at \mbox{$\sqrt{s} =$ 500 GeV.} Detection of high p$_T$ charged leptons with the Endcap Electromagnetic Calorimeter (EEMC) in the presence of a large hadronic background presents a significant challenge. To develop a hadron background rejection algorithm, we produced a series of Pythia simulations of $W$ signal and QCD background events using GEANT and the full detector model. Pre-selection of those Pythia events likely to pass our triggers allowed for the generation of a background sample with an integrated luminosity comparable to that of the expected data. The rejection algorithm itself is based on three principles: isolation conditions on energy and tracks around the candidate electron, vetoes on energy and tracks opposite in azimuth from the candidate electron, and the properties of longitudinal and transverse shower development in the EEMC. This talk will detail the rejection algorithm which achieves a signal to background ratio of greater than one to one over a large fraction of the detected lepton energy spectrum. [Preview Abstract] |
Saturday, October 25, 2008 2:36PM - 2:48PM |
HG.00004: Cross-section measurement of the $\gamma n \rightarrow \pi^- p$ process from deuterium Wei Chen Photopion production from nucleons are essential probes of the transition from meson-nucleon degrees of freedom to quark-gluon degrees of freedom in exclusive processes. There are few previous measurements of $\pi^{-}$ photo-production above photon energies of 2 GeV. Data taken during the CLAS g10 running period cover a significantly extended range of kinematics. We carried out an analysis of the $\gamma n \rightarrow \pi^- p$ process for photon energies between 1 to 3.5 GeV and pion center-of-mass angles between $50^{\circ}$ to $150^{\circ}$. Preliminary results are extracted and will be presented in this talk. [Preview Abstract] |
Saturday, October 25, 2008 2:48PM - 3:00PM |
HG.00005: Investigating Neutron Polarizabilities Through Compton Scattering on Light Nuclei Deepshikha Shukla This talk will focus on elastic Compton scattering on deuteron and He-3 as an instrument to extract the neutron polarizabilities. The calculations for this process have been performed for photon energies comparable to the pion mass within the framework of Chiral effective field theory. The results show that these computations, when used in concert with future data from MAXLab and HIGS, should give significant new information about the neutron polarizabilities. [Preview Abstract] |
Saturday, October 25, 2008 3:00PM - 3:12PM |
HG.00006: Isospin Mixing of Quark Cluster States in the Bag Model Thomas Ward, John Rasmussen, Guy Emery The isospin mixing of quark cluster dibaryon states interpreted within the multi-quark bag model of Maulders, Aerts and de Swart (MAS) will be studied. Dibaryon resonances composed of color-magnetic quark cluster states are predicted to be strongly isospin mixed via the off-diagonal quark cluster color-magnetic interaction. For example, the isospin mixing of two dibaryon ${ }^3P_{0,1,2} $ multiplets identified at 2016- and 2052-MeV are calculated to be strongly mixed. The degeneracy of the two multiplets is completely removed by the off-diagonal interaction whose extracted and calculated strength varies between 60-120 MeV depending on the spin of the multiplet member and reproduces well the identified spectrum of states. [Preview Abstract] |
Saturday, October 25, 2008 3:12PM - 3:24PM |
HG.00007: Time-dependent solutions to the Dirac equation for interacting systems Athanasios Petridis, Zachary Kertzman, Khinlay Win Time-dependent solutions to the minimally-coupled Dirac equation are obtained numerically using the staggered leap-frog method. The systems studied include: (1) Electromagnetically mutually interacting fermion-antifermion pairs propagating in free space and in medium representing mesons that undergo electromagnetic dissociation. The stability of the algorithm is challenged by the presence of time-retarded potentials generated by the 4-dimentional, fermionic currents. (2) Fermions interacting with fixed solenoidal magnetic vector potentials giving rise to a time-dependent Aharonov-Bohm effect especially inside quantum rings. The resulting interference patterns are evaluated to study the induced action. (3) Self-interacting fermions, emitting and re-absorbing photons. Their dynamic mass renormalization is studied and the ensuing problem of numerical stability and convergence is discussed. [Preview Abstract] |
Saturday, October 25, 2008 3:24PM - 3:36PM |
HG.00008: Experiment E06-009: Measurement of Longitudinal and Transverse Structure Functions on Deuterium in the Nucleon Resonance Region Ibrahim Albayrak This experiment ran in Hall C at Jefferson Lab to measure L/T separated structure functions from deuterium from the quasielastic region through the nucleon resonance region and beyond (up to W$^{2}$=4.5 GeV$^{2})$, spanning the four-momentum transfer range 0.25 $<$ Q$^{2 }<$ 4.0 (GeV/c)$^{2 }$ which is full Q$^{2}$ range for two running periods. ROSEN07, however, being the second period, focused on higher Q$^{2}$ range above 2 (GeV/c)$^{2}$. The Rosenbluth separation technique will be used to separate longitudinal and transverse cross sections to extract structure functions F$_{1}$, F$_{2}$, F$_{L}$ and R. This experiment is the second part of the first global survey of these fundamental quantities on deuterium which began with the Hall C experiment E02-109. The measurement of these fundamental quantities allows a variety of physics issues to be addressed, including: an evaluation of QCD moments of the deuteron and neutron structure functions, and quark-hadron duality in deuteron. This experiment was completed in july 2007 using the High Momentum Spectrometer to detect electrons from a 4 cm deuterium target. An overview will be presented of the experiment and analysis, along with preliminary results. [Preview Abstract] |
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