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 GG: Mini-Symposium: 3D View of the Nucleon and Its Spin IV |
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Chair: Mark Jones, Thomas Jefferson National Accelerator Facility Room: Jewett Ballroom C |
Saturday, October 25, 2008 10:30AM - 11:06AM |
GG.00001: The neutron's negative central charge density: the inclusive-exclusive connection Invited Speaker: Models of generalized parton distributions at zero skewness are used to relate the behavior of deep inelastic scattering quark distributions, evaluated at high $x$, to the transverse charge density evaluated at small distances. We obtain an interpretation of the recently obtained negative central charge density of the neutron. The $d$ quarks dominate the neutron structure function for large values of Bjorken $x$, where the large momentum of the struck quark has a significant impact on determining the center of momentum, and thus the ``center'' of the nucleon in the transverse position plane. [Preview Abstract] |
Saturday, October 25, 2008 11:06AM - 11:18AM |
GG.00002: A New Measurement of the Proton Elastic Form Factor Ratio at Low $Q^2$ Xiaohui Zhan A high precision measurement of the proton elastic form factor ratio $\mu G_{\mathrm{E}}/G_{\mathrm{M}}$ in the range of $Q^2$=0.3~-~0.7~$(\mathrm{GeV}/c)^2$ has been made using recoil polarimetry in Jefferson Lab Hall~A. In this low $Q^2$ range, previous data (BLAST: C. B. Crawford {\it et~al.} 2007, {\it Phys. Rev. Lett.} {\bf 98} 052301, LEDEX: G. Ron {\it et~al.} 2007, {\it Phys. Rev. Lett.} {\bf 99} 202002) along with many fits and calculations indicate substantial deviations of the ratio from unity, and continue to suggest that structures might be present in the individual form factors, and in the ratio. In this new measurement, we used the high resolution sepctrometer to detect recoil protons incoincidence with the elastic scattered electrons tagged by BigBite calorimeter. With 80\% polarized electron beam for 24 days, we are able to achieve $\sim 0.5\%$ statistical uncertainty. This high precision result will confirm or refute all existing suggestions of few percent structures in the form factors ratio. Beyond the intrinsic interest in nucleon structure, the improved form factor measurements also have implications for DVCS, determinations of the proton Zemach radius and for parity violation experiments. [Preview Abstract] |
Saturday, October 25, 2008 11:18AM - 11:30AM |
GG.00003: Gluonic Pole Matrix Elements for Fragmentation Function and Universality Leonard Gamberg The non-vanishing of gluonic pole matrix elements can explain the appearance of transverse single spin asymmetries (TSSAs) in high-energy scattering processes. Such matrix elements appear in principle both for transverse momentum dependent (TMD) distribution functions such as the Sivers function and fragmentation functions such as the Collins function. We find that for a specific class of model field theories that the contribution of the gluonic pole matrix element for fragmentation functions vanishes. This outcome is important in the study of universality for fragmentation functions and has impact for QCD theory as it relates to experiments exploring the generalized angular momentum structure of the nucleon. With this result we explore various transverse spin and azimuthal asymmetries for hard process for for various experimental facilities such as JLAB and RHIC-Spin. [Preview Abstract] |
Saturday, October 25, 2008 11:30AM - 11:42AM |
GG.00004: Double Spin Asymmetry for Exclusive $\pi ^{-}$ Electro-production from Deuterium Sharon Careccia At Jefferson Lab, an extensive program of spin structure function experiments is underway in Hall B. We scatter longitudinally polarized electrons with energies of 1.6, 2.5, 4.2, and 5.7 GeV from longitudinally polarized NH$_{3}$ and ND$_{3}$ targets. The large acceptance of the CLAS spectrometer enables the detection of multi-particle final states over a large kinematical range in invariant mass $W$ and momentum transfer $Q^{2}$. In particular, we have studied exclusive $\pi ^{-}$ production in the resonance region from the deuteron target, which is sensitive primarily to neutrons in the target. Preliminary results for the double polarization asymmetry for the 2.5 GeV data will be presented. [Preview Abstract] |
Saturday, October 25, 2008 11:42AM - 11:54AM |
GG.00005: Local Polarimetry for Proton Beams with the STAR Zero Degree Calorimeters David Grosnick A spin physics program using the STAR detector at RHIC is underway that investigates the spin structure of the proton using colliding polarized proton beams at $\sqrt{s} = 200$ GeV, and in the future at 500 GeV. The local polarimeter that uses the beam- beam counters currently works well at $\sqrt{s} = 200$ GeV, but its effectiveness at higher energies may be problematic since the $\sqrt{s}$ dependence of the analyzing power is not known. Data at $\sqrt{s} = 200$ GeV using the Shower Maximum Detectors of the Zero Degree Calorimeters (ZDC) were analyzed to determine the feasibility of using the ZDCs as a second local polarimeter. A six sigma left-right physics asymmetry and an up-down physics asymmetry consistent with zero were measured from a small dedicated data sample with vertical beam polarizations. The physics asymmetry was also calculated as a function of azimuthal angle and displayed a sinusoidal pattern, as expected. These results demonstrate the capability of using the ZDCs as another local polarimeter for STAR at $\sqrt{s} = 200$ GeV, and the performance of both polarimeters will be measured at $\sqrt{s} = 500$ GeV. [Preview Abstract] |
Saturday, October 25, 2008 11:54AM - 12:06PM |
GG.00006: A bottom-up strategy to extract generalized parton distributions from experiment Simonetta Liuti We will present a bottom-up strategy to extract generalized parton distributions in the non-singlet sector from Deeply Virtual Compton Scattering data [1,2]. Our approach makes use of all available experimental information on both the proton and neutron form factors, and on the (inclusive) deep inelastic structure functions within a flexible parameterization based on a Regge-improved diquark model. Additional constraints provided by lattice calculations of the n=2,3 moments of generalized parton distributions are implemented through a reconstruction method based on orthogonal polynomials. $Q^2$ evolution, polynomiality, as well as a new set of constraints provided by dispersion relations [3] are also discussed. [1] S.~Ahmad, H.~Honkanen, S.~Liuti and S.~K.~Taneja, arXiv:0708.0268 [hep-ph]. [2] S.~Ahmad, H.~Honkanen, S.~Liuti and S.~K.~Taneja, Phys.\ Rev.\ D {\bf 75}, 094003 (2007) [3] M.~Diehl and D.~Y.~Ivanov, Eur.\ Phys.\ J.\ C {\bf 52}, 919 (2007) [Preview Abstract] |
Saturday, October 25, 2008 12:06PM - 12:18PM |
GG.00007: Hard Exclusive Processes on $^4$He Simonetta Liuti We investigate both Deeply Virtual Compton Scattering (DVCS) and $\pi^o$ production off nuclear targets in the coherent and incoherent channels. The spin zero of the $4$He target allows on one side a simpler description of its partonic structure characterized at leading twist by two Generalized Parton Distirbutions (GPDs), a chirally-even one, $H^A$ and a chirally-odd one, $H_T^A$. The two GPDs can be studied separately in the DVCS and $\pi^o$ production processes, respectively. Theoretical results using a microscopic approach to nuclear dynamics will be presented on the interpretation of GPDs in nuclei [1.2], as a means to unravel the transverse structure of nuclei in terms of both spatial and momentum degrees of freedom. Calculations of both asymmetries and cross sections for both the Bethe-Heitler and DVCS processes relevant for upcoming measurements at Jefferson Lab [3] will also be discussed. [1] S. Liuti and S.K. Taneja, Phys.Rev. C {\bf 72} :034902,2005. [2] S. Liuti and S.K. Taneja, Phys.\ Rev.\ C {\bf 72}, 032201 (2005) [3] H. Egyian, F.-X. Girod, K. Hafidi, S. Liuti, E. Voutier et al., JLab Experiment E08-024 (2008). [Preview Abstract] |
Saturday, October 25, 2008 12:18PM - 12:30PM |
GG.00008: {\boldmath $\pi^o$} Electroproduction and Transversity Simonetta Liuti Exclusive $\pi^o$ electroproduction from the proton is suggested for extracting the tensor charge and other quantities related to transversity from experimental data [1]. A connection between a description based on partonic degrees of freedom, given in terms of Generalized Parton Distributions (GPDs), and Regge phenomenology is discussed. Pion electroproduction is described in terms of the chiral odd (spin flip) GPDs for both longitudinal and transverse virtual photon polarizations. A mechanism for the $Q^2$-dependence of the $\gamma^* \pi^o$ vertex is proposed that, by treating separately natural and unnatural parity exchanges at this vertex, allows one to separate the transverse and longitudinal virtual photon contributions, the latter being dominated by unnatural exchanges. A study of the sensitivity of different observables in both unpolarized and polarized scattering to both the tensor charge and the transverse anomalous magnetic moment [2], is presented with the aim of providing a practical method for extracting the latter. Future investigations using a variety of targets (proton, deuteron and $^4$He) and probes -- both electron and neutrino scattering as well as hadronic reactions will be discussed. [1] S. Ahmad, G. R. Goldstein and S. Liuti, arXiv:0805.3568 [hep-ph] [2] M. Burkardt, Phys. Lett. B {\bf 639}, 462 (2006). [Preview Abstract] |
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