Session D12: Hadronic Physics

$\gamma p \rightarrow K^{+}\Lambda$ Differential Cross Section and Recoil Polarization Measurements from the CLAS \textit{g11a} Dataset

We present measurements of $\gamma p \rightarrow K^{+}\Lambda$ differential cross section and $\Lambda$ recoil polarization from the CLAS \textit{g11a} dataset. The measurements cover the center-of-mass energy range from 1.62~GeV to 2.84~GeV and a wide range of center-of-mass angles ($-0.90\leq\cos\theta_{CM}^{K}\leq0.90$). We have analyzed this reaction via both the $K^{+}p\pi^{-}$ and $K^{+}p$ (missing $\pi^{-}$) final-state topologies independently and found the results to be in excellent agreement. Previous $\gamma p \rightarrow K^{+}\Lambda$ differential cross section results from the CLAS (Bradford, \textit{et al.} 2005) and SAPHIR (Glander, \textit{et al.}, 2004) Collaborations show discrepancies in magnitude at most energies and a discrepancy in scale and shape at ${\sqrt{s}\approx 1.9~\textrm{GeV}}$. These discrepancies have been problematic for interpretations of $K^{+}\Lambda$ photoproduction mechanisms. These new \textit{g11a} differential cross section results show excellent agreement with the previous CLAS result. The recoil polarization results show agreement with previous data from the CLAS, SAPHIR, and GRAAL experiments, and are a $\approx 500$~MeV extension of the observed energy range.   

$\eta$ and $\eta\prime$ photoproduction off the proton

The photoproduction of $\eta$ and $\eta\prime$ mesons (I=1/2) serves as an isospin filter facilitating the study of $N^*$ resonances. Total and differential cross sections for these mesons have been determined using the Crystal Barrel CsI(Tl) calorimeter at ELSA, University of Bonn in Germany, in the energy range $E_{\gamma}$ = 850 to 2550 MeV by analyzing the neutral decay modes: $\eta\rightarrow3\pi^0$, $\eta\rightarrow2\gamma$, and $\eta\prime\rightarrow2\pi^0\eta$. In this experiment, the $\rm BaF_2$ spectrometer TAPS was placed in the forward direction increasing the solid angle coverage to nearly $4\pi$. For the first time, these measurements cover the full angular range in $\cos{\theta^{\rm cms}_{\rm meson}}$. In separate beam time using the same detector setup, linearly polarized photons were produced by coherent bremsstrahlung off a diamond radiator to allow the extraction of the photon beam asymmetry, $\Sigma$. Preliminary results for pseudoscalar mesons in the range $E_{\gamma}$ = 1100 to 1700 MeV are discussed.   

$K^{+}\Sigma^0$ Photo-production at CLAS, Jefferson Laboratory

Preliminary differential cross sections and recoil polarizations in $K^{+}\Sigma^{0}$ photo-production in the $\emph{g11a}$ dataset measured by the CLAS detector at Jefferson Laboratory will be presented. The measurements represent both a wider energy range (1.8 GeV $\leq \sqrt{s} \leq 2.84$ GeV) and higher statistics than previous CLAS and SAPHIR measurements. Prospects of a Partial Wave Analysis on these new measurements searching for missing baryon resonances will also be mentioned.   

Measurement of the Beam Asymmetry $\Sigma$ in $\pi^+$ $\pi^-$ Photoproduction using the CLAS Spectrometer at Jefferson Laboratory

For an unambiguous extraction of resonance parameters via partial-wave analysis, not only unpolarized differential cross sections are needed, but also polarization observables. The beam asymmetry in double-pion production will provide important model constraints for the determination of resonance contributions to this reaction. The CLAS g8b experiment, as part of the $N^*$ spectroscopy program at Jefferson Lab in Newport News, VA, has accumulated photoproduction data using a linearly-polarized tagged-photon beam incident on an unpolarized hydrogen target. The excellent degree of polarization reached values exceeding 90\% at different positions of the coherent bremsstrahlung peak between 1.3 and 2.1 GeV. The g8b $\pi^+$$\pi^-$ data sample comprises of roughly 100M events and thus provides sufficient statistics to study the 5-dimensional observable $\Sigma$ as a function of various masses and angles. In addition to $\Sigma$, preliminary results on the extraction of the additional observable $I^s$ will be discussed, which arises for two pseudoscalar mesons in the final state.   

Kaon Electroproduction at low Q$^{2}$ and W=2.2 GeV off hydrogen

A measurement of the H(e,e'K) reaction was performed at Hall A, TJNAF as part of the hypernuclear experiment E94-107. One important ingredient to the hypernuclear cross section calculation is the elementary cross section for production of hyperons, $\Lambda $ and $\Sigma ^{0}$. This was measured using a hydrogen (i.e. a proton) target. Data was taken at very low Q$^{2}$ ($\sim $0.07 GeV$^{2})$ and W$\sim $2.2 GeV. Kaons were detected along the direction of \textbf{q}, the momentum transferred by the incident electron ($\Theta _{CM}\sim $6$^{o})$. In addition, there is not much data available for electroproduction of hyperons at low Q$^{2}$ and $\Theta _{CM}$ and the available theoretical models differ a lot in this kinematical region of W. The measurement of the elementary cross section will help not only in the hypernuclear spectroscopy studies but also in constraining existing theoretical models for the elementary reaction. Measurements of the differential cross sections and the $\Sigma ^{0}$/$\Lambda $ production ratio will be reported as well as their results binned in Q$^{2}$ ,W and $\Theta _{CM}$ to understand the dependence on these variables. Details of the calculations and results will be shown.   

Nucleon resonance electrocouplings from the CLAS data on charged double pion electroproduction.

Measurements of charged double pion (2$\pi$) electroproduction off protons with CLAS [1-3] provided the most extensive experimental data set of nine 1-fold differential cross sections. Phenomenological analysis of these data was carried out within the framework of model [4] in the kinematic range: 1.3 $<$ $W$ $<$ 1.8 GeV and 0.25 $<$ $Q^{2}$ $<$ 1.5 GeV$^2$. A successful description of all observables was achieved, allowing us to establish all essential contributing mechanisms, consisting of various meson-baryon isobar channels, direct 2$\pi$ production, and to isolate the resonant parts of the cross sections. For the first time electrocouplings of the $P_{11}(1440)$ and $D_{13}(1520)$ states were obtained in studies of 2$\pi$ electroproduction. Good agreement with the previous results [5] obtained from studies of $\pi^{+}n$ and $\pi^{0}p$ electroproduction channels show that these electrocouplings can be evaluated reliably. The CLAS 2$\pi$ data allowed us to determine electrocouplings for several high lying $N^{*}$ states (M $>$ 1.65 GeV), that have major hadronic decays with 2$\pi$ emission. The information on amplitudes of contributing non-resonant mechanisms will be used in future $N^*$ studies in a global multi-channel analysis under development at EBAC [6]. [1] M.~Ripani et al., Phys. Rev. Lett. {\bf 91}, 022002 (2003). [2] G.~V. Fedotov et al., (CLAS Collaboration), arXiv:0809.1562 [nucl-ex] accepted by Phys. Rev. C. [3] http://clasweb.jlab.org/physicsdb/ (CLAS Physics Data Base). [4] V.~I. Mokeev et al., arXiv: 0809.4158[hep-ph]. [5] I.G.Aznauryan, et. al., Phys. Rev.,C71, 015201 (2005). [6] A.Matsuyama, et. al., Phys.Rep., 439, 193 (2007)$;$ B.Julia-Diaz, et.al., Phys. Rev., C76, 065201 (2007).   

Measurement of the Neutron ($^3$He) Spin Structure Functions at low $Q^{2}$

Experiment E97-110 was performed at Jefferson Lab in Hall A to examine the neutron and $^{3}$He spin structure. A longitudinally-polarized electron beam (1.1 $\leq E \leq$ 4.4 GeV) was scattered from a longitudinally or transversely polarized $^{3}$He target. Polarized cross-section differences were measured to extract the spin structure functions $g_{1}(x,Q^{2})$ and $g_{2}(x,Q^{2})$. The extended Gerasimov-Drell-Hearn integral and the moments of the neutron and $^{3}$He spin structure functions were extracted at very low momentum transfers (0.02 $< Q^{2} <$ 0.3 GeV$^{2}$). This $Q^{2}$ range allows us to make a benchmark-check of chiral perturbation theory calculations. The experimental details will be discussed and results on moments of the spin structure functions will be presented.   

Possibility for a New Measurement of the Proton Elastic Form Factor Ratio at Very Low $Q2$

The proton form factors at low $Q^2$ encode information about the peripheral structure of the proton as well as the interplay between the magnetic and electric charge distributions [1]. Furthermore, low $Q^2$ form factor measurements impact high precision experiments, for example, the measurement of the hydrogen hyperfine splitting [2]. Polarization transfer and beam target asymmetry measurements [3,4] allow determination of the electric to magnetic form factor ratio with unprecedented precision down to $Q^2 \simeq 0.15$ GeV$^2$. A new approved experiment at JLab will extend the $Q^2$ range down to 0.015 GeV$^2$. At even lower $Q^2$ the beam-target asymmetry method is impeded by the need to detect either a very forward electron or to use a low energy beam which must traverse the high magnetic field of a polarized target. We suggest an alternative measurement using colliding proton and electron beams which will allow a measurement of the form factor ratio to extremely low $Q^2$ ($\sim 10^{-4}$ GeV$^2$). The opportunity for this measurement will be discussed. \newline \textbf{References:} [1] G. A.~Miller, E.~Piasetzky and G.~Ron, Phys. Rev. Lett. {\bf 101}, 082002 (2008). [2] A. C. Zemach, Phys. Rev. {\bf 104}, 1771 (1956). [3] C.~B.~Crawford {\it et al.}, Phys. Rev. Lett. \textbf{98}, 052301 (2007). [4] G.~Ron {\it et al.}, Phys. Rev. Lett. {\bf 99}, 202002 (2007).   

Self-Energy Effect to Two-Body Bound-State in Light-Front Dynamics

Solving the relativistic bound-state problem is an important task in hadronic physics. Even the two-body bound-state problem has been solved only under a certain approximation due to the nonperturbative nature. The two-body Bethe-Salpeter equation in the Wick-Cutkosky model was often solved in the ladder approximation without including the self-energy, although many different and more accurate treatments of the numerical method to solve the bound-state problem have been developed nowadays. In this presentation, we use the light-front dynamics (LFD) to solve the two-body bound-state problem and extend the light-front ladder approximation to include the self-energy correction and associate counter-terms. Using the variational principle, we present the numerical result of the binding energy versus the coupling constant of the ladder graph with or without the self-energy correction and compare with the available previous results obtained either analytically or numerically. We also discuss the full LFD kernel including ladder, cross-ladder, stretched-box and self-energy graphs.