Bulletin of the American Physical Society
2016 Fall Meeting of the APS Division of Nuclear Physics
Volume 61, Number 13
Thursday–Sunday, October 13–16, 2016; Vancouver, BC, Canada
Session JC: Electromagnetic Interactions |
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Chair: Garth Huber, University of Regina Room: Junior Ballroom B |
Saturday, October 15, 2016 10:30AM - 10:42AM |
JC.00001: Two-photon exchange corrections in elastic lepton-proton scattering Oleksandr Tomalak, Marc Vanderhaeghen In recent years, two experimental approaches, with and without polarized protons, gave strikingly different results for the ratio of the electric to magnetic proton form factors. Even more recently, a mysterious discrepancy ("the proton radius puzzle") has been observed in the extraction of the proton charge radius from the muonic hydrogen versus regular hydrogen and electron-proton scattering. Two-photon exchange (TPE) contributions are the largest source of the hadronic uncertainty in these experiments. To determine TPE corrections to the S level in muonic hydrogen, the forward virtual Compton scattering is calculated within dispersion relation (DR) formalism. Comparing a box graph model with the DRs at fixed low momentum transfer, we develop and test the subtracted DR formalism for TPE in electron-proton scattering. Its relative effect on the cross section is in the $1-2~\%$ range. We include the inelastic states both in the approximation of the {\it near-forward} unpolarized virtual Compton scattering as well as using the empirical information on the $\pi N$ states contribution. We compare the resulting TPE with MAMI, VEPP-3 and CLAS data, and make predictions for the OLYMPUS and the forthcoming MUSE experiments. [Preview Abstract] |
Saturday, October 15, 2016 10:42AM - 10:54AM |
JC.00002: Results from the OLYMPUS Two-Photon Exchange Experiment Colton O'Connor Measurements of the proton's electric-to-magnetic form factor ratio obtained by different methods disagree significantly in a way that depends on $Q^2$. The OLYMPUS experiment was designed to empirically quantify two-photon exchange in lepton-proton scattering, an effect that, in some models, can fully account for this disagreement. This was achieved at the DORIS storage ring at DESY by measuring the ratio of the elastic cross-sections for positron-proton and electron-proton scattering with alternating 2.01 GeV lepton beams incident on an internal hydrogen gas target. Data were collected with an integrated luminosity of over 4.0 fb$^{-1}$ using a large-acceptance toroidal spectrometer and multiple luminosity monitoring systems, allowing for precise results ($<$1\% uncertainty) over the range of 0.6$\leq Q^2\leq $2.2 (GeV/$c$)$^2$. [Preview Abstract] |
Saturday, October 15, 2016 10:54AM - 11:06AM |
JC.00003: A Calculation and Measurement of Radiative Moller Scattering at 100 MeV with DarkLight Charles Epstein A number of current experiments rely on precise knowledge of electron-electron (Moller) and positron-electron (Bhabha) scattering. Some of these experiments, having lepton beams on atomic targets, use these QED processes as normalization. In other cases such as DarkLight (at the Jefferson Lab ERL), with electron beams at relatively low energy (100 MeV) and very high intensity (1 Megawatt), Moller scattering and radiative Moller scattering have such enormous cross-sections that they produce copious amounts of noise that must be understood. In this low-energy regime, the electron mass is also not negligible. As a result, we have developed a new Monte Carlo event generator for the radiative Moller and Bhabha processes, extending existing soft-photon radiative corrections with new, exact single-photon bremsstrahlung calculations, and keeping all terms of the electron mass. DarkLight provides us a unique opportunity to study this process experimentally and compare it with our work. As a result, we are preparing a dedicated apparatus consisting of two magnetic spectrometers as part of the first phase of DarkLight in order to directly measure this process. An overview of the calculation and the status of the experiment’s construction will be presented. [Preview Abstract] |
Saturday, October 15, 2016 11:06AM - 11:18AM |
JC.00004: Coherent electron-positron pair production in ultra-peripheral AuAu collisions at STAR Matthew Rehbein The focus of this study is coherent photoproduction of electron-positron pairs in 200 GeV ultraperipheral AuAu collisions detected by STAR, with an integrated luminosity of 1.9 inverse nanobarns. Because hadronic interactions are suppressed in ultra-peripheral collisions, these events provide an opportunity to study purely electromagnetic interaction in the non-perturbative regime. This presentation will provide a description of the techniques used to select exclusive electron-positron events, as well as the resulting kinematic distributions for pair invariant mass greater than 0.35 GeV, pair transverse momentum less than 0.1 GeV, and absolute value of pair pseudorapidity less than 0.8. Efficiency correction techniques will also be discussed. In previous measurements at the same energy at STAR, the shape of the transverse momentum distribution could not be fully described by the equivalent photon approximation (EPA). Measurements at the LHC indicate that the cross section is reduced by approximately 25 percent compared to the EPA. This study ultimately seeks to examine these effects in more detail at RHIC energies. [Preview Abstract] |
Saturday, October 15, 2016 11:18AM - 11:30AM |
JC.00005: ABSTRACT WITHDRAWN |
Saturday, October 15, 2016 11:30AM - 11:42AM |
JC.00006: Precision Measurement of the $\pi^{0\thinspace }$Lifetime via the Primakoff Effect Liping Gan The neutral pseudoscalar meson$\pi^{\mathrm{0\thinspace }}$plays a fundamental role in QCD at low-energy. The $\pi^{\mathrm{0}}\to \gamma \gamma $decay width offers a sensitive probe for the chiral anomaly and spontaneous chiral symmetry breaking, and the nature of QCD confinement. The theoretical calculations over the last decade have reached 1{\%} precision in the decay amplitude of the $\pi^{\mathrm{0\thinspace }}$into two photons. The experimental measurement of this parameter with a comparable precision will be an important test of QCD. The PrimEx collaboration at Jefferson Lab has developed and performed experiments to measure the$\pi^{\mathrm{0\thinspace }}$radiative decay width via the Primakoff effect. The published result from the first experiment (PrimEx-I) has a 2.8{\%} total uncertainty. The second experiment (PrimEx-II) was performed with the final goal of 1.4{\%} precision. The updated result of PrimEx-II will be presented. [Preview Abstract] |
Saturday, October 15, 2016 11:42AM - 11:54AM |
JC.00007: Semileptonic Decay of $\Lambda_c$ Baryon in Constituent Quark Model Md Mozammel Hussain, Winston Roberts Hadronic form factors for semileptonic decay of heavy $\Lambda_c$ are calculated in non relativistic quark model. The full quark model wavefunctions are employed to numerically calculate the form factors to all order. The form factors are compared with the heavy quark effective theory (HQET) form factors. The differential decay rates, branching fractions are calulcated for $\Lambda_c\to\Lambda^{*} l\nu_l$, $\Lambda_c\to\Lambda^{*} l\nu_l\to\Sigma\pi l\nu_l$ and $\Lambda_c\to\Lambda^{*} l\nu_l\to NK l\nu_l$, for transitions to both ground state and allowed excited states of $\Lambda^{*}$. $\Lambda_c\to\Lambda(1405)$ is found to be the dominant mode other than the ground state, with a significant contribution to the branching fraction of $\Lambda_c\to X_s l\nu_l$. A new estimate for $f=B(\Lambda_c^{+}\to\Lambda l^{+}\nu_l)/B(\Lambda_c^{+}\to X_s l^{+}\nu_l)$ is obtained which is significantly small compared to the particle data group (PDG) estimate ($f=1$). [Preview Abstract] |
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