Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session K4: Mini-symposium on Partonic Structure beyond Densities: Orbital Motion, Correlations, Fluctuations III |
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Sponsoring Units: DNP Chair: Harut Avakian, Jefferson Lab Room: Holiday 3 |
Sunday, April 12, 2015 1:30PM - 1:42PM |
K4.00001: Electron structure: Shape, size, and generalized parton distributions in QED Gerald A. Miller The shape of the electron is studied. Quantities of interest for the proton: Form factors, generalized parton distributions, transverse densities, Wigner distributions and the angular momentum content--are computed for the electron-photon component of the electron wave function. The influence of longitudinally polarized photons, demanded by the need for infrared regularization via a nonzero photon mass, is included. The appropriate value of this mass depends on experimental conditions, so the size of the electron (defined by the slope of its Dirac form factor) bound in a hydrogen atom is found to be about four times larger than when the electron is in a continuum scattering state. The shape of the electron, as determined from the transverse density and generalized parton distributions, is shown not to be round. An electron distribution function (analogous to the quark distribution function) is defined, and that of the bound electron is shown to be suppressed compared to that of the continuum electron. If the relative transverse momentum of the virtual electron and photon is large, the virtual electron and photon each carry nearly the total angular momentum of the physical electron (1/2), with the orbital angular momentum being nearly (?1/2). [Preview Abstract] |
Sunday, April 12, 2015 1:42PM - 1:54PM |
K4.00002: Extracting GPDs from hard exclusive processes Latifa Elouadrhiri In recent years a set~of non-perturbative functions has been introduced to investigate and describe the structure of hadrons at the quark-gluon level. Transverse spatial distributions of partons, encoded in Generalized Parton Distributions (GPDs) have been widely recognized as key objectives of the JLab 12 GeV upgrade and, and are part of the key physics program of an Electron Ion Collider. Much of the interest in GPDs~has been triggered by their potential to help~unravel the spin structure of the nucleon, as they carry information not only on the longitudinal but also on the transverse position of partons, providing rich and direct information on the orbital motion of quarks. Although the interest in GPDs has grown enormously we are still in need of a consistent theoretical and phenomenological description spanning the full kinematic regime covered by the (un) polarized world-data. One of the main challenges still remaining is the extraction of actual 2$+$1D parton distribution functions from different spin and azimuthal asymmetries with minimal model dependence. In this talk, we present approaches to extract GPDs from existing data on hard exclusive production of photons and pions as well as some future measurements at various facilities worldwide. [Preview Abstract] |
Sunday, April 12, 2015 1:54PM - 2:06PM |
K4.00003: Generalized Parton Distributions Describing Partonic Orbital Angular Momentum Simonetta Liuti, Michael Engelhardt, Abha Rajan, Aurore Courtoy We discuss orbital angular momentum in QCD, in particular, its observability, and its partonic interpretation. Orbital momentum can be defined in QCD using two different decomposition schemes that yield a kinetic and a canonical definition, respectively. We argue that kinetic orbital angular momentum is intrinsically associated with twist three Generalized Parton Distributions (GPDs), and it is therefore readily observable in Deeply Virtual Compton Scattering experiments. On the other hand, canonical angular momentum is defined in terms of a Generalized Transverse Momentum Distribution (GTMD) and it can be therefore observed in scattering processes involving an additional hadronic reaction plane. A comparison between the two definitions can be performed by extending to GTMDs the techniques previously developed for lattice calculations of Transverse Momentum Distributions (TMDs) evaluating the matrix elements of quark bilocal operators containing a staple-shaped Wilson connection. [Preview Abstract] |
Sunday, April 12, 2015 2:06PM - 2:18PM |
K4.00004: Exclusive Measurement of Deeply Virtual Compton Scattering off $^{4}$He Nathan Baltzell Deeply virtual Compton scattering and meson production are proven prime reactions to progress our understanding of partonic structure via Generalized Parton and Transverse Momentum Distribution frameworks. Their extension to nuclei is of particular interest, with the possibility of revealing new information on the modification of partonic structure in nuclear media. An ideal candidate is $^{4}$He due to its simplicity and high density, with spin=0 and isospin=0 resulting in only one contributing GPD at leading twist. We performed an experiment in Hall-B at Jefferson Lab with longitudinally polarized 6 GeV electrons incident on a 6 atm, long, gaseous $^{4}$He target in a 4 T solenoid field. The large acceptance CLAS detector system was augmented by a lead tungstate calorimeter for forward photons. A new radial time projection chamber with cylindrical GEM amplification provided nuclear recoil detection. The first exclusive measurement of DVCS off $^{4}$He will be presented, with $Q^{2}$ and $x_{B}$ in the ranges 1.0-2.3 GeV$^{2}$ and 0.1-0.25. Extractions of beam spin asymmetries and the Compton form factor will be discussed, as well as a simultaneous measurement of $\pi^{0}$ production. [Preview Abstract] |
Sunday, April 12, 2015 2:18PM - 2:30PM |
K4.00005: New Measurements of the Target-normal Single-spin Asymmetry $A_y$ for $^3$He$^\uparrow (e,e^\prime)$ in Quasi-elastic and Deep-inelastic Scattering Todd Averett Results from recent measurements of the target single spin asymmetry (SSA) in $^3$He$^\uparrow (e,e^\prime) $ from $^3$He nuclei polarized normal to the lepton scattering plane. For both elastic scattering and deep-inelastic scattering, this asymmetry, $A_y$, is exactly zero for one-photon exchange. However, it can be non-zero when two-photon exchange is included. For elastic scattering, a large, negative asymmetry with statistical precision 8-10$\sigma$ was recently measured for the first time at Jefferson Lab using an SEOP polarized $^3$He target. At $Q^2\sim1$ GeV$^2$ the asymmetry can be related to moments of the GPDs. Our result is in agreement with a prediction based on a GPD-model and thus provides new, independent constraints on these distributions. In deep-inelastic scattering, a large, positive (neutron) asymmetry was also observed and agreed well with model predictions using the ``Sivers'' distribution obtained from SIDIS transversely measurements. Finally, results will be presented for $^3$He$^\uparrow (e,e^\prime n)$ which at low $Q^2$ is highly dependent on the wave function of the polarized $^3$He nucleus. [Preview Abstract] |
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