Bulletin of the American Physical Society
2017 Fall Meeting of the APS Division of Nuclear Physics
Volume 62, Number 11
Wednesday–Saturday, October 25–28, 2017; Pittsburgh, Pennsylvania
Session FK: Electromagnetic Interactions |
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Chair: Fatiha Benmokhtar, Duquesne University Room: City Center B |
Thursday, October 26, 2017 4:00PM - 4:12PM |
FK.00001: Compton Scattering and Nucleon Polarizabilities at HI$\gamma$S Xiaqing Li The electromagnetic polarizabilities of the nucleon are fundamental quantities related to nucleon structure. In the past decade, effective field theories have successfully established a bridge between the QCD and low-energy description of the nucleon and made predictions of the polarizabilities. Lattice QCD calculations are also eminent on electromagnetic polarizabilities. High precision data are now needed to benchmark these calculations. To this end, the Compton scattering program at the High Intensity $\gamma$-Ray Source (HI$\gamma$S) has measured Compton scattering cross sections on the deuteron at 65 and 85 MeV to extract neutron polarizabilities. Recently, data has also been collected at HI$\gamma$S to measure the beam asymmetry of linearly polarized Compton scattering on the proton at 84 MeV to extract proton polarizabilities independently from the Baldin sum rule. In addition, the Compton scattering cross sections on $^4$He was measured using a circularly polarized photon beam at 84 MeV. The scattered photons were detected by eight large-volume NaI(Tl) detectors located both in and out of the scattering plane. In this talk, I will present the details of the experimental method and preliminary analysis of the 84 MeV Compton scattering data on $^4$He and the proton. [Preview Abstract] |
Thursday, October 26, 2017 4:12PM - 4:24PM |
FK.00002: Calculation and Measurement of Low-Energy Radiative Moller Scattering Charles Epstein A number of current nuclear physics experiments have come to rely on precise knowledge of electron-electron (Moller) and positron-electron (Bhabha) scattering. Some of these experiments, having lepton beams on targets containing atomic electrons, use these purely-QED processes as normalization. In other scenarios, with electron beams at low energy and very high intensity, Moller scattering and radiative Moller scattering have such enormous cross-sections that the backgrounds they produce must be understood. In this low-energy regime, the electron mass is also not negligible in the calculation of the cross section. This is important, for example, in the DarkLight experiment (100 MeV). As a result, we have developed a new event generator for the radiative Moller and Bhabha processes, with new calculations that keep all terms of the electron mass. The MIT High Voltage Research Laboratory provides us a unique opportunity to study this process experimentally and compare it with our work, at a low beam energy of 2.5 MeV where the effects of the electron mass are significant. We are preparing a dedicated apparatus consisting of a magnetic spectrometer in order to directly measure this process. An overview of the calculation and the status of the experiment will be presented. [Preview Abstract] |
Thursday, October 26, 2017 4:24PM - 4:36PM |
FK.00003: Abstract Withdrawn
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Thursday, October 26, 2017 4:36PM - 4:48PM |
FK.00004: Abstract Withdrawn
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Thursday, October 26, 2017 4:48PM - 5:00PM |
FK.00005: Measurement of Compton scattering cross section during PrimEx-II Experiment at Jefferson Lab Li Ye The electron Compton scattering is the most known fundamental QED process, however, a precision measurement of its cross section for the beam energy above 1 GeV has been lacking up to now. An updated high precision measurement of the neutral pion lifetime via the Primakoff effect (PrimEx-II) experiment was performed in Hall B of Jefferson Lab in 2010. The experiment used small angle coherent photoproduction of $\pi^0$'s in the Coulomb field of a nucleus, $i.e.$ the Primakoff effect, to determine the lifetime with a precision of less than $1.5\%$ . It therefore requires thorough understanding of the underlying systematic uncertainties. To facilitate that data for well known electromagnetic processes were taken concurrently with the photoproduction data. This analysis pertains to measuring the Compton scattering cross section, which occurs at similar kinematics as the primary process. The combination of the well established theory of this process with large collected statistics allowed to extract this cross section with high precision in an energy region of 4-5 GeV for $^{12}C$ and $^{28}Si$ targets. The results of this analysis will be presented. [Preview Abstract] |
Thursday, October 26, 2017 5:00PM - 5:12PM |
FK.00006: The Neutral Pion Radiative Width: Results from PrimEx II Experiment at Jefferson Lab Ilya Larin The $\pi^{\mathrm{0}}\to \gamma \gamma $ decay represents one of the key processes in anomaly sector of QCD. Its amplitude is determined by the chiral anomaly resulting from the coupling of quarks to the electromagnetic field. Recent theoretical calculations have been made within Chiral Perturbation Theory. Including well-known parameters, they predict decay width at a percent precision level. Therefore, precise measurement of $\pi^{\mathrm{0}}\to \gamma \gamma $ decay width gives a unique possibility to check theory predictions. Primakoff effect is a powerful tool for measuring radiative width of particles. In this talk, preliminary results of the PrimEx-II experiment performed at Jefferson Lab will be presented. The PrimEx-I experiment performed in 2004 achieved precision of 2.8{\%}. The purpose of the second experiment was to reach the 1.5{\%} precision level. PrimEx-II was performed in 2010 with collection of five times more statistics. Future Primakoff experiments planned at Jefferson Lab will also be discussed. [Preview Abstract] |
Thursday, October 26, 2017 5:12PM - 5:24PM |
FK.00007: Electron Scattering from a Bound Nucleon on the Light-Front. Frank Vera, Misak Sargsian We calculate the cross section of the electron scattering from a bound nucleon within light-front approximation. The advantage of this approximation is the possibility of systematic account for the off-shell effects which become essential in high energy electro-nuclear processes aimed at probing the nuclear structure at small distances. The derived cross section is compared with the results of other approaches treating the off-shell effects in electron-nucleon scattering. [Preview Abstract] |
Thursday, October 26, 2017 5:24PM - 5:36PM |
FK.00008: Accessing the real part of the forward elastic J/Psi-p and Phi-p threshold scattering amplitudes Oleksii Gryniuk, Marc Vanderhaeghen We provide an updated analysis of the forward $J/\psi$-p scattering amplitude, relating its imaginary part to $\gamma p\to J/\psi p$ and $\gamma p\to c\bar cX$ cross section data, and calculating its real part through a once-subtracted dispersion relation. From a global fit to both differential and total cross section data, we extract a value for the spin-averaged $J/\psi$-p s-wave scattering length $a_{\psi p}=0.046\pm0.005$~fm, which can be translated into a $J/\psi$ binding energy in nuclear matter $B_\psi=2.7\pm0.3$~MeV. We estimate the forward-backward asymmetry to the $\gamma p\to e^-e^+p$ process around the $J/\psi$ resonance, which results from interchanging the leptons in the interference between the $J/\psi$ production and the Bethe-Heitler mechanisms. We show that this asymmetry can reach values around -25\%. Its measurement can thus provide a very sensitive observable for a refined extraction of $a_{\psi p}$. A preliminary analysis of the $\phi$-p forward scattering led to the estimated forward-backward asymmetry reaching values of about -20\% for the beam energies $E=2.2,4.4$ GeV. The suitable kinematics were investigated for the specific setup of HMS and SHMS detectors of Hall C at JLab. Studies for the other JLab experiments (e.g. GlueX, SOLID) are ongoing. [Preview Abstract] |
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