Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session W14: Nucleon Form Factors and Two-Photon Exchange |
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Sponsoring Units: DNP Chair: Andrei Afanasev, The George Washington University Room: Grand Hall East C |
Tuesday, April 3, 2012 10:45AM - 10:57AM |
W14.00001: Overview and Status of the OLYMPUS Experiment Rebecca Russell Analysis via Rosenbluth separation of elastic electron-proton scattering has long yielded the ratio of the electric and magnetic form factors ($G_E/G_M$) to be constant as a function of $Q^2$. More recent studies using polarized beams, extracting $G_E/G_M$ through the polarization transfer technique, show it to decrease linearly with $Q^2$. It is suspected that two-photon exchange processes explain the observed discrepancy, though theoretical calculations vary dramatically. OLYMPUS measures the two-photon contribution by studying the ratio of the cross section for electron-proton scattering to positron-proton scattering using DORIS at DESY. With multi-GEV beam energies and $>100$~mA current on an internal hydrogen gas target, OLYMPUS will be able to precisely measure two-proton exchange for $0.5\leq Q^2\leq 2.5$~GeV$^2$. An update of the status of the experiment, and its 2012 data run, will be presented. [Preview Abstract] |
Tuesday, April 3, 2012 10:57AM - 11:09AM |
W14.00002: The OLYMPUS Target System Brian Henderson The OLYMPUS experiment, ongoing at the DORIS ring at DESY in Hamburg, Germany, seeks to definitively determine the two-photon contribution to lepton-proton scattering through the measurement of the ratio of the cross-sections of electron and positron elastic scattering from a fixed hydrogen target. The experiment utilizes an unpolarized internal hydrogen gas target with an approximate thickness of $3\cdot10^{15}$ atoms$\cdot$cm$^{-2}$. The target chamber consists of a thin walled (100~$\mu$m) aluminum storage cell within an aluminum chamber which is tapered to allow the detection of events at low scattering angles $(\sim12^\circ)$ for the purpose of luminosity monitoring. The gas feed and vacuum systems are designed so as to provide a roughly triangular target density distribution along the beamline with a flow rate into the target of $1.5\cdot10^{17}$ H$_2$~s$^{-1}$. Additionally, the system incorporates a tungsten collimator, wakefield suppressing elements, as well as adjustable beam scrapers for the protection of the target. The design and implementation of the target system and its performance during recent runs will be presented. [Preview Abstract] |
Tuesday, April 3, 2012 11:09AM - 11:21AM |
W14.00003: Track Reconstruction for the OLYMPUS Experiment Colton O'Connor The OLYMPUS Experiment at DESY is currently measuring the ratio between electron-proton and positron-proton elastic scattering cross sections in order to precisely quantify the two-photon-exchange contribution to these processes. Event selection relies on track reconstruction achieved using a large-acceptance magnetic spectrometer consisting of two drift chambers, each with 18 parallel planes of sensitive wires. Track fitting techniques focus on finding the scattering vertex, scattering angles, and particle momenta. For future data-taking periods, a GEM detector will be installed in front of each drift chamber to enhance tracking. The design of these tracking elements will be discussed, along with a description of reconstruction methods. [Preview Abstract] |
Tuesday, April 3, 2012 11:21AM - 11:33AM |
W14.00004: Pion Electroproduction in the OLYPUS Experiment Lauren Ice The OLYMPUS experiment, currently underway at the DESY laboratory, measures the positron-proton to electron-proton scattering cross section ratio to determine the effect of multiple photon exchange on electron-proton elastic scattering. The motivation for this experiment is the discrepancy between measurements of the electric and magnetic form factors of the proton, as found with the Rosenbluth separation technique and polarization transfer methods. The expected culprit of this discrepancy is the two-photon exchange contribution to electron-proton elastic scattering. One of the sources of background for this experiment will be electroproduction processes, which will primarily be pion production from the first resonance of the proton. The study of the pion backgrounds for the OLYMPUS experiment will be presented. The pion backgrounds are being modeled using a pion electroproduction simulation based off of the MAID2007 databases. This is used with a GEANT4 Monte Carlo simulation in order to discriminate between elastic and inelastic electron-proton scattering events during the experiment. [Preview Abstract] |
Tuesday, April 3, 2012 11:33AM - 11:45AM |
W14.00005: Radiative Corrections in the OLYMPUS Experiment Axel Schmidt The OLYMPUS Experiment, underway at DESY, in Hamburg, Germany, is measuring the ratio between the electron-proton and positron-proton elastic scattering cross-sections in order to determine the contribution from two-photon exchange. A deviation in the ratio from unity is caused both by the hard two-photon scattering of interest but also by soft radiative effects which must be accounted for. A simulation has been developed to generate particles according to different radiative models and to propagate these particles via a GEANT4 Monte Carlo through the OLYMPUS detector. This will convolve both the cross-sections with effects such as detector resolution and efficiency. A description of the radiative correction procedure, as well as the latest results from the analysis will be presented. [Preview Abstract] |
Tuesday, April 3, 2012 11:45AM - 11:57AM |
W14.00006: Beyond the Born Approximation: The Two-Photon Exchange Experiment in CLAS Puneet Khetarpal Knowledge of the electromagnetic form factors of the nucleon is crucial both for increasing our understanding of the proton and for interpreting proton knockout measurements from nuclei. However, there is a large discrepancy between proton electric form factor ($G_E(Q^2)$) measurements using the Rosenbluth separation method and ones using polarization transfer. This discrepancy increases to a factor of three at $Q^2 = 6$ GeV$^2$. One of the most plausible proposed explanations is the contribution from two-photon exchange effects to $ep$ elastic scattering. The two-photon exchange contribution is directly proportional to the deviation from unity of the ratio of the $e^{+}p$ to $e^{-}p$ elastic scattering cross sections. This ratio was measured at the Thomas Jefferson National Accelerator Facility using the CLAS detector in Hall-B in 2010-2011. The current status of the experiment is presented. [Preview Abstract] |
Tuesday, April 3, 2012 11:57AM - 12:09PM |
W14.00007: Nucleon elastic form factors in a relativistic quark model Simon Capstick, Bradley Keister Nucleon elastic form factors are described using a light-cone (relativistic) quark model. The constituent quarks have anomalous magnetic moments fit to the nucleon magnetic moments, and simple form factors fit to the higher-$Q^2$ behavior of the nucleon electric and magnetic form factors. This model is able to explain the rapidly decreasing ratio of the proton electric to magnetic form factors, and also predicts a zero in the proton electric form factor at approximately 7 GeV$^2$, which is where the trend of the recent data for $G^p_E$ suggests it should be. [Preview Abstract] |
Tuesday, April 3, 2012 12:09PM - 12:21PM |
W14.00008: Two Photon Exchange for Exclusive Pion Electroproduction Andrei Afanasev, Aleksandrs Aleksejevs, Svetlana Barkanova We perform detailed calculations two-photon-exchange QED corrections to the cross section of pion electroproduction. The results are obtained with and without soft-photon approximation, analytic expressions for the corrections are derived. Relative importance of the two-photon correction is analyzed for kinematics of several experiments at Jefferson Lab. A significant effect due to twophoton exchange is predicted for backward electron scattering angles and large transferred momenta. [Preview Abstract] |
Tuesday, April 3, 2012 12:21PM - 12:33PM |
W14.00009: Luminosity Monitors for the OLYMPUS Experiment J. Diefenbach The OLYMPUS experiment at the storage ring DORIS at DESY (Hamburg, Germany) measures the real part of the two-photon exchange amplitude in elastic electron proton scattering. The real part is directly accessible through a measurement of the ratio of the electron-proton to the positron-proton elastic cross-section. This implies switching the beam species a number of times per day between electrons and positrons. Precise monitoring of the luminosity is required to connect the data sets with each other in the data analysis. The talk will present the three luminosity monitors of OLYMPUS, two tracking telescopes (GEMs and MWPCs) using elastic ep scattering and one totally independent system detecting symmetric M{\o}ller and Bhabha scattering events. The current status and achievements in our first production run from February 2012 will be discussed, together with an outlook on further developments. [Preview Abstract] |
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