Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session HC: Electromagnetic Interactions II |
Hide Abstracts |
Chair: Sebastian Kuhn, Old Dominion University Room: Newport News Marriott at City Center Pearl Salon I |
Saturday, October 13, 2007 9:00AM - 9:12AM |
HC.00001: Scaling study of the pion electroproduction cross sections and the pion form factor Tanja Horn One of the main objectives of Jefferson Lab is to understand the structure of the nucleon in terms of the quark-gluon degrees of freedom. Measurements of inclusive processes, such as deep inelastic scattering (DIS), have shown that in the limit of large $Q^2$, at fixed values of $x_B$, such processes can be viewed as scattering from individual partons within the hadronic system. A similar factorization of scales, allowing perturbative QCD concepts to be used in the description of hadrons, may be expected in hard exclusive processes. The $Q^2$ dependence of the separated p($e,e^\prime \pi^+$)n cross sections provides one of the best ways to test the factorization of long-distance from short-distance physics, and at which values of $Q^2$ it applies. Such tests will lay the foundation for a reliable interpretation of the results from the GPD program at Jefferson Lab, and for GPD studies with a future electron-ion collider. In this talk I will present a QCD scaling study using published $^{1}$H($e,e^\prime \pi^+$)n cross sections and new results from the $\pi$CT experiment (E01-107), which was carried out in Hall C at Jefferson Lab. [Preview Abstract] |
Saturday, October 13, 2007 9:12AM - 9:24AM |
HC.00002: Truncated Moment Analysis of Nucleon Structure Functions Ales Psaker, Eric Christy, Cynthia Keppel, Wally Melnitchouk The understanding of quark-hadron duality in nucleon structure functions (namely, the similarity between the scaling and resonance averaged functions) within QCD is currently incomplete. While moments of structure functions can be analyzed within the operator product expansion in terms of leading and higher twist contributions, the description of duality as a function of Bjorken $x$ requires phenomenological models. We employ a novel new approach using ``truncated'' moments, or integrals of structure functions over restricted regions of $x$, to study the degree to which individual resonance regions are dominated by leading twists. Because truncated moments obey the same $Q^2$ evolution equations as the leading twist parton distributions, our approach makes possible for the first time a description of resonance region data and the phenomenon of quark-hadron duality directly from pQCD. [Preview Abstract] |
Saturday, October 13, 2007 9:24AM - 9:36AM |
HC.00003: Hard Photo-disintegration of proton pairs in $^3$He Ronald Gilman, Eli Piasetzky, Ishay Pomerantz Hard deuteron photo-disintegration has been investigated for 20 years, as its cross sections follow the constituent counting rules and it provides insight into the interplay between hadronic and quark-gluon degrees of freedom in high-momentum transfer exclusive reactions. We have now measured for the first time hard $pp$-pair disintegration in the reaction $\gamma \, ^3 {\rm He} \to $pp$ + n$, using kinematics corresponding to a spectator neutron. Clues to the underlying physics can be found in the comparison of our measurements with deuteron photo-disintegration, the energy dependence of the cross sections at 90$^{\circ}$ c.m., the $\alpha_n$ distribution, and the angular distribution. [Preview Abstract] |
Saturday, October 13, 2007 9:36AM - 9:48AM |
HC.00004: Quark-Hadron Duality on the Neutron (${\rm ^3}$He) Spin Structure Patricia Solvignon In 1970, Bloom and Gilman made a surprising observation that the spin-independent structure function $F_2$ measured in the resonance region averages on the curve determined by the deep inelastic scattering data when a proper scaling variable is used. This phenomenon is called quark-hadron duality and links the non-pertubative and perturbative regimes of QCD. Recently, quark-hadron duality has been quantitatively established for the spin-independent structure function $F_2$ of the proton in Jefferson Lab Hall C. New results are also coming out for the spin structure function $g_1$ on the proton and the deuteron from Halls B and C. Jefferson Lab Hall A experiment E01-012 used the polarized $^3$He target for a measurement of the spin structure function $g^{^3{\rm He}}_1$ and the virtual photon asymmetry $A^{^3{\rm He}}_1$ in the resonance region over a $Q^2$ range from 1.0 to 4.0 $(GeV/c)^2$. Data from E01-012 compared with deep inelastic scattering data will provide a test of quark-hadron duality for the spin structure functions of the neutron. This will be one of the first tests of the spin and flavor dependence of quark-hadron duality. The demonstration of duality for spin structure functions will help to study the transition from partonic to hadronic degrees of freedom and to quantifie the size of higher twist effects. Details of the experiment and final results will be presented. [Preview Abstract] |
Saturday, October 13, 2007 9:48AM - 10:00AM |
HC.00005: Measuring the Neutron and $^{3}$He Spin Structure at Low $Q^{2}$ Vincent Sulkosky The study of the nucleon spin structure has been an active field for the past few decades. The ultimate goal is to understand the structure and interactions of protons and neutrons in terms of quarks and gluons. The Gerasimov-Drell-Hearn (GDH) sum rule is an important tool available to study nucleon spin structure. Originally derived for real photon absorption, the sum rule was first extended to non-zero $Q^{2}$ in 1989. The extension of the sum rule provides a $Q^{2}$-dependent relation that can be used to study the nucleon spin structure and make comparisons between theoretical predictions and the experimental data. Jefferson Lab experiment E97-110 performed a precise measurement of the $Q^{2}$ dependence of the extended GDH integral and of other moments of the neutron and $^{3}$He spin scructure functions between 0.02 and 0.3 GeV$^{2}$. These data allow us to test chiral perturbation theory calculations and check the GDH sum rule by extrapolating to the real photon point. The data were taken in Hall A using the Jefferson Lab highly polarized continuous electron beam and a polarized $^{3}$He target. The experimental details will be discussed, and preliminary results will be presented. [Preview Abstract] |
Saturday, October 13, 2007 10:00AM - 10:12AM |
HC.00006: Inclusive Scattering from Nuclei at $x>$1 and high Q$^2$ with a 6 GeV beam Nadia Fomin Inclusive electron scattering from nuclei at large $x$ and $Q^2$ is the result of a reaction mechanism that includes both quasi--elastic scattering from nucleons and deep inelastic scattering from the quark consitituents of the nucleons. Consequently, it provides an opportunity to investigate the transition from a regime where nucleon degrees of freedom dominate to one where the more fundamental QCD interactions are exposed. Data in this regime can be used to study a wide variety of topics, including the extraction of nuclear momentum distributions, the influence of final state interactions and the approach to $y$-scaling, the strength of nucleon-nucleon correlations, and the approach to $x$- scaling, to name a few. We recently peformed an experiment in Jefferson Lab's Hall C using a 6 GeV beam and a range of both light and heavy nuclei which was designed to significantly extend the kinematic region at high momentum transfer and large (negative) $y$, previously explored in SLAC experiment NE3 and Jefferson Lab experiment E89-008. After a brief statement of the physics goals of this experiment, we will present results and discuss possible interpretations. [Preview Abstract] |
Saturday, October 13, 2007 10:12AM - 10:24AM |
HC.00007: A Precision Measurement of the Neutral Pion Lifetime Via the Primakoff Effect Eric Clinton The neutral pion lifetime is arguably the most precise theoretical calculation possible in low energy QCD, but the current world data are not commensurate with theory. Recent calculations predict a neutral pion radiative width of 8.1 eV $\pm$ 1\%, while the PDG average stands at 7.84 eV $\pm$ 7\%. The Primakoff Experiment (PrimEx) collaboration has utilized the Primakoff effect, photo-meson production in the Coulomb field of nuclei, to generate neutral pions. PrimEx collected data in Hall B at the Thomas Jefferson National Accelerator Facility with the expectation of measuring the neutral pion lifetime to an accuracy of 1.5\%. Results of this measurement will be presented. This result is a stringent test of the U(1) axial anomaly, and thus fills an important gap in our knowledge of low energy QCD. [Preview Abstract] |
Saturday, October 13, 2007 10:24AM - 10:36AM |
HC.00008: Semi-Inclusive, Single Pion Electro-production from a Deuterium Target. Narbe Kalantarians Relative to the case for the proton, very little is known of
Semi-Inclusive Deep Inelastic Scattering (SIDIS) physics
resulting from a deuteron target. Electrons with energy of 5.3
GeV are used on both deuteron and proton targets to study single
pion production in the SIDIS framework for all 3 pion charge
states. What makes this specific experiment unique is that it
offers a virtually free neutron target by tagging protons with
momentum 70-150 MeV and scattering angle greater than 90$^{0}$
called `spectators' that are detected by a radial time projection
chamber that uses gaseous electron multipliers. The scattered
electrons and resulting pions are then detected in the CEBAF
Large Acceptance Spectrometer. The kinematic range covered is
$W>2$GeV, $Q^2>1$GeV$^{2}$, $0.1 |
Saturday, October 13, 2007 10:36AM - 10:48AM |
HC.00009: Barely Off-Shell Nucleon Structure Nathan Baillie The Barely Off-Shell Nucleon Structure (BoNuS) experiment measured electron scattering from neutrons bound in deuterium nuclei at Jefferson Lab's Hall B with the intent of obtaining the ratio F$_{2}^{n}$/ F$_{2}^{p}$ at high Bjorken x. The F$_{2}^{n}$ structure function is difficult to obtain due to nature's lack of a free neutron target. Previous experiments have measured inclusive quasi-elastic scattering in atomic nuclei, corrected for scattering from the protons in the nucleus, and relied upon models for the neutron binding and motion to obtain F$_{2}^{n}$. In BoNuS we restrict our analysis to neutron scattering events tagged by a backward-going low-energy recoil proton. This selects loosely bound neutrons that are nearly on their mass-shell and have few final-state interactions with the recoil proton. The recoil protons were detected in a Radial Time Projection Chamber (RTPC) using gas electron multipliers (GEMs) in a cylindrical geometry around the target. We will present the first preliminary results for F$_{2}^{n}$ in the resonance and deep-inelastic regions. [Preview Abstract] |
Saturday, October 13, 2007 10:48AM - 11:00AM |
HC.00010: A Measurement of the Electron Compton Scattering Cross Section in the Jefferson Lab PrimEx Experiment Yelena Prok The PrimEx experiment at Jefferson Lab has been designed to perform a high precision ($\sim$ 1.5 \%) measurement of the neutral pion lifetime using the small angle coherent photoproduction of $\pi^0$s in the Coulomb field of a nucleus, i.e., the Primakoff effect. This measurement is a rigorous test of axial anomaly, which is a fundamental prediction of low energy QCD. In view of the required precision for this experiment, periodic measurements of the well known cross section in Compton scattering off atomic electrons were performed. The data were collected in Hall B of Jefferson Lab, using a high resolution hybrid calorimeter (HyCal), photon tagger, and carbon and beryllium targets. Analysis techniques and preliminary results will be presented in this talk. [Preview Abstract] |
Saturday, October 13, 2007 11:00AM - 11:12AM |
HC.00011: Target Normal Single-Spin Asymmetry in Inclusive DIS Tim Holmstrom A proposal (P07--013) to measure the target normal single spin asymmetry $A^n_N$ in inclusive deep-inelastic $n^{\uparrow}(e,e')$ reaction with a vertically polarized $^3$He target has been conditionally approved to run during Jefferson Lab's Hall A neutron transversity experiments (E06--010) and (E06--011) in summer 2008. The expected accuracy of this measurement is $\delta A^n_N = 2$--$4\times 10^{-4}$ at four different values of the invariant mass. The normal spin asymmetry in DIS probes helicity--flip amplitudes at the quark level related to effects beyond the leading-twist picture of DIS. Preparations for this experiment will be discussed including the development of a secondary luminosity monitor for Hall A. [Preview Abstract] |
Saturday, October 13, 2007 11:12AM - 11:24AM |
HC.00012: New Measurements of the Nuclear Dependence of the EMC Effect Aji Daniel The European Muon Collaboration discovered that structure functions in nuclei are modified compared to those for a free nucleon. Even after twenty five years of experimental and theoretical investigations, the origin of the effect is still not well explained. I will present preliminary results from Jefferson Lab experiment E03-103 $\--$ a precision measurement of the EMC effect on light to medium-heavy nuclei with emphasis on the large $x_{Bj}$ region. Our data will directly measure the $A$ - dependence of the EMC effect. Data on light nuclei, $^3$He and $^4$He, allow direct comparison to `exact' few-body calculations of the EMC effect, and will also provide guidance for calculations of nuclear effects in deuterium. The large $x_{Bj}$ data are particularly sensitive to the details of nuclear structure, and our data will provide a reliable base-line to constrain models that incorporate conventional nuclear effects such as binding and Fermi motion. Conventional nuclear effects lead to modifications of the structure functions at all $x_{Bj}$ values, so that a quantitative understanding is important before the addition of more exotic effects that might be required to explain the observed nuclear dependence. [Preview Abstract] |
Saturday, October 13, 2007 11:24AM - 11:36AM |
HC.00013: Deep Exclusive process of $\gamma^* p \to \pi^+ n$ channel from the CLAS data Kijun Park Exclusive $\gamma^* N \to \pi N$ processes are essential probes to study the transition from meson-nucleon degree of freedom to quark-gluon degree of freedom. One of the simplest signatures for this transition is the scaling of the cross section with center-of-mass energy. Cross sections of these processes are also helpful to investigate the oscillatory behavior around the quark counting rule predictions, since they decrease slower with energy than other photon-induced processes. We have analyzed the $\gamma^* p \to \pi^+ n$ channel within $6\;\rm{GeV}$ electro-production data. CLAS has the distinct advantage of permitting a much finer energy scan and simultaneous coverage of a large angular range, which will help investigate the dramatic behavior observed in other experiments. We could verify the counting rule in the scaled differential cross section for different $Q^2$ at large hadron angle in center-of-mass system. The preliminary differential cross section will be presented and discussed. [Preview Abstract] |
Saturday, October 13, 2007 11:36AM - 11:48AM |
HC.00014: Separated Response Function Ratios in Forward Pion Electroproduction Cornel Butuceanu The first complete separation of the four unpolarized electromagnetic response functions above the dominant resonances has been made for forward, exclusive $\pi^{\pm}$ electroproduction on the nucleon. Measurements were made above the resonance region at fixed invariant mass $W$ = 1.95 GeV in the $Q^2 = 0.6 - 1.6 $ (GeV/c)$^2$ range. The separated ratio $R_L = \sigma_L^{\pi^-}/\sigma_L^{\pi^+} $ is sensitive to any small isoscalar contamination to the dominant isovector pion exchange amplitude which is the basis for the determination of the charged pion form factor, $F_{\pi}(Q^2)$ from electroproduction data. A favorable value of this ratio may also have implications for constraining polarized GPD's with ratios of longitudinal observables. At large $-t$, a separated ratio $R_T = \sigma_T^{\pi^-}/\sigma_T^{\pi^+}\simeq$ 1/4 would suggest a transition between pion knockout and quark knockout mechanisms. Preliminary results on the separate ratios $R_L$ and $R_T$ and their evolution with $-t$ will be presented and discussed. [Preview Abstract] |
Saturday, October 13, 2007 11:48AM - 12:00PM |
HC.00015: Time-resolved propagation of electromagnetically interacting fermions Athanasios Petridis, Zachary Kertzman, Khinlay Win The propagation of electromagnetically interacting fermion- antifermion states representing mesons in free space and in medium is studied by means of the coupled time-dependent Dirac equation. This is solved numerically using the staggered leap- frog method on a spatial lattice. The stability of the equation is challenged by the presence of time-retarded potentials generated by the 4-vector, fermionic currents. The known effect of high-frequency oscillations that is inherent in the Dirac equation becomes less pronounced in the presence of mutual interactions. The time-development of the system is evaluated to address its dissociation. Self-interaction that amounts to emission and subsequent absorption of photons is also studied and the effects on wavefunction and mass renormalization are evaluated. [Preview Abstract] |
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