### Session Q8: Electromagnetic Interactions

 Monday, April 24, 2006 1:30PM - 1:42PM Q8.00001: K*$^{0}$ photoproduction off the proton at CLAS Ishaq Hleiqawi , Ken Hicks The photoproduction of vector mesons has previously focused on the nonstrange sector, with $\rho$, $\omega$ or $\phi$ mesons in the final state. The lightest vector meson with nonzero strangeness is the K* of which little is known for photoproduction. The large acceptance of the CLAS detector makes it possible to capture both K* decay products, the pion and the kaon. In this talk we will show differential cross sections for the K*$^{0}\Sigma ^{+}$ final state over photon energies ranging from about 1.8 to 3.0 GeV. These data are compared with a theoretical model by Zhao \textit{et al.} using a quark-model for the K*-baryon couplings. Our data show that the forward-angle data are well described by the t-channel, hence providing constraints for the K*$\Sigma$N coupling constant At larger angles, the s-channel is well described by the model of Zhao \textit{et al.} over a range of angles and photon energies. The K* couplings determined from our data will more tightly constrain calculations for scalar kaon production, where K* exchange occurs as a virtual particle in the t-channel. Monday, April 24, 2006 1:42PM - 1:54PM Q8.00002: The Neutron Charge Form Factor at Low Q$^{2}$ Eugene Geis , Ricardo Alarcon , Michael Kohl , Richard Milner , Vitaly Ziskin At the MIT-Bates Linear Accelerator Center, the neutron charge form factor G$_{E}^{n}$ has been measured by means of (e,e'n) quasielastic scattering of polarized electrons from vector-polarized deuterium. The experiment used the longitudinally polarized stored electron beam of the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic deuterium target provided by an atomic beam source. The measurements have been carried out with the symmetric Bates Large Acceptance Spectrometer Toroid (BLAST) with enhanced neutron detection capability. From the beam-target double polarization asymmetry with the target spin oriented perpendicular to the momentum transfer the form factor G$_{E}^{n}$ is extracted over a range of four-momentum transfer Q$^{2}$ between 0.12 and 0.60 (GeV/c)$^{2}$ with minimized systematic dependencies. Monday, April 24, 2006 1:54PM - 2:06PM Q8.00003: Inclusive Electron-Nucleus Scattering at $x>1$ and High $Q^2$ Donal Day Inclusive electron scattering from nuclei provides a rich, yet complicated mixture of physics that has yet to be fully exploited. The cross section at high momentum transfer arises from several reactions including quasi--elastic scattering from nucleons and deep inelastic scattering from the quark consitituents of the nucleons. Inclusive electron scattering presents the opportunity to investigate the transition from a regime where nucleon degrees of freedom hold sway to one where the more fundamental QCD interactions, involving quarks and gluons, are expected to dominate. Data at large values of $x$ over a range of $Q^2$ can be employed to study a wide variety of topics: nuclear momentum distributions, medium modifications of nucleon properties, the influence of final state interactions and the approach to $y$-scaling, the strength of nucleon-nucleon correlations at large $x$, and the approach to $x$- scaling. We recently completed such an experiment in Jefferson Lab's Hall C in which \mbox{6 GeV} electrons were scattered from both few-body nuclei and heavy nuclei at high momentum transfers. This experiment significantly extended the kinematic region at high momentum transfer and large (negative) $y$ (corresponding to large Bjorken $x$), explored in previous experiments. After a brief statement of the physics motivation for this experiment, we will present preliminary results. Monday, April 24, 2006 2:06PM - 2:18PM Q8.00004: CLAS g10 Analysis on Single Photopion Productions from Deuterium Wei Chen Photopion productions from nucleons are essential probes of the transition from meson-nucleon degrees of freedom to quark-gluon degrees of freedom in exclusive processes. The cross sections of these processes are also advantageous, for the investigation of the oscillatory behavior around the quark counting prediction, since they decrease relatively slower with energy compared with other photon-induced processes. Recent data from JLab experiment E94-104~[1,2] show dramatic change in the scaled differential cross-section from the $\gamma n \rightarrow \pi^- p$ and $\gamma p \rightarrow \pi^+ n$ processes in the center of mass energy between 1.8 GeV to about 2.4 GeV. We are carrying out a CLAS approved analysis~[3] of the JLab CLAS g10 data on the $\gamma n \rightarrow \pi^{-} p$ to investigate this dramatic behavior in much finer photon energy bins. Furthermore, the angular dependence of the scaling behavior for this process will also be studied in detail. We will report the status of the analysis in this presentation. \newline \newline References: \newline [1] L.Y. Zhu {\it et al.}, Phys. Rev. Lett. {\bf 91}, 022003 (2003).\newline [2] L.Y. Zhu {\it et al.}, Phys. Rev. C {\bf 71}, 044603 (2005); nucl-ex/0409018.\newline [3] http://www.tunl.duke.edu/$\sim$mep/clas/caa\_pion.pdf Monday, April 24, 2006 2:18PM - 2:30PM Q8.00005: Search for the Onset of Color Transparency : JLab E02-110 experiment Lamiaa El Fassi The JLab E02-110 experiment was carried out in 2003-2004 using the CEBAF Large Acceptence Spectrometer (CLAS) located in Hall B at Jefferson Laboratory. The main goal of this experiment is to look for the onset of Color Transparency (CT) in the exclusive incoherent $\rho^{0}$ electroproduction on deuterium, carbon and iron. These measurements study the $Q^{2}$ dependence of the nuclear transparency $T_{A}$, the ratio of the nuclear per-nucleon ($\sigma _{A}/A$) to the free nucleon ($\sigma _{N}$) cross sections, at fixed coherence length $l_{c}$ of quark-antiquark fluctuations of the virtual photon. A significant increase of $T_{A}$ with $Q^{2}$ would be a signature of CT. A brief description to the experiment and the ongoing analysis work will be presented in this talk. Monday, April 24, 2006 2:30PM - 2:42PM Q8.00006: Precise Measurement of the Nuclear Dependence of the EMC Effect Aji Daniel Since the original observation of the modification of nuclear structure functions by the European Muon Collaboration, there has been intense experimental and theoretical efforts aimed at understanding nuclear effects in parton distribution functions. However, the experimental focus has been mainly on heavy nuclei. I will present preliminary results from Jefferson Lab experiment E03-103, a high precision measurement of the EMC effect with emphasis on the large $x_{Bj}$ region and few-body nuclei. Data on light nuclei ,$^3$He and $^4$He, will allow direct comparison to exact'', few-body calculations of the EMC effect and will allow an unambiguous determination of the functional form of the $A$- dependence of the EMC effect. The large $x_{Bj}$ data are particularly sensitive to conventional nuclear physics effects such as binding and Fermi motion and will constrain models that incorporates these effects. This, in turn, will provide information on the role of these conventional nuclear physics effects at lower $x_{Bj}$ where, more exotic effects are thought to be important. Monday, April 24, 2006 2:42PM - 2:54PM Q8.00007: Double and target asymmetries for the $e p \rightarrow e' p \pi^0$ production Angela Biselli An extensive experimental program to measure the spin structure of the nucleons is carried out in Hall B with the CLAS detector at Jefferson Lab using a polarized electron beam incident on a polarized target. Spin degrees of freedom offer the possibility to test, in an independent way, existing models of resonance electroproduction. The present analysis selects the exclusive channel $\vec{p}(\vec{e},e',p)\pi^{0}$ from data taken in 2000-2001, to extract single and double asymmetries in a $Q^2$ range from 0.2 to 0.75 $\rm{GeV}^2$ and a $W$ range from 1.1 to 1.6 GeV. Results of the asymmetries will be presented as a function of the center of mass decay angles of the $\pi^0$ and compared with the unitary isobar model MAID [1], the dynamic model by Sato and Lee [2] and the dynamic model DMT [3]. \newline \newline [1] D. Drechsel et al., Nucl. Phys. {\bf A}645 (1999) 145-174 \newline [2] H. Lee, Nucl. Phys. {\bf A}513 (1990) 511 \newline [3] S.~S. Kamalov, Phys. Lett. {\bf B} 522 (2001) 522 Monday, April 24, 2006 2:54PM - 3:06PM Q8.00008: Exploration of Short Range Nucleon-Nucleon Correlations at JLab Douglas Higinbotham Measuring the probability of a nucleon-nucleon short range correlation inside a nucleus has historically been problematic. Competing mechanisms, such as final state re-scattering and meson exchange currents, can produce the same final state as one would expect from an initially correlated pair. Preliminary results of recent Jefferson Lab measurements, performed at kinematics which minimize the effects of such competing mechanisms, will be presented along with a quantitative picture of clustering in the nucleus. Monday, April 24, 2006 3:06PM - 3:18PM Q8.00009: Study of the Exclusive d(e,e$^\prime$p)n Reaction Mechanism at High Q$^2$ Natalia Dashyan , Kim Egiyan High Q$^2$ d(e,e$^\prime$p)n scattering is one of the simplest and best ways to investigate the short range properties of the deuterium wave function (WF), possible modifications of the bound nucleon structure, as well as of the structure and nature of short range nucleon correlations (SRC). For these investigation, the mechanisms of this reaction should be understood properly. Although the d(e,e$^\prime$p)n reaction is the simplest one, its mechanism remains to be checked at high Q$^2$: short distances are involved and, therefore, it is unclear what degrees of freedom (nucleonic or quark-gluonic) play a more important role in electron-nucleon interaction. There are also subtle interplays between the interaction of different intermediate exited states of the struck fast nucleon with the second nucleon. To solve these problems the reaction should be investigated as completely as possible, at different kinematic conditions. We investigated the d(e,e$^\prime$p)n reaction mechanism in the Q$^2$ = (2 - 6) GeV$^2$ interval using the CLAS detector at Jefferson Lab. The angular and momentum distributions of recoil neutrons were studied in detail at momenta of p$_n$ = (0 - 2) GeV/c and in angular (relative to the transfered momentum direction) range 20$^o$ - 160$^o$. The experimental data are compared with the theoretical calculations [1] based on the diagrammatic approach of , and reasonable agreement was obtained at least in the momentum range p$_n \leq$ 1 GeV/c. [1]J.-M. Laget, Phys. Lett. {\bf B609}, 49 (2005).