Session R15: Electromagnetic Interactions I

Two Photon Exchange (TPE) Experiment at CLAS

The ratio of the Electric (G$_{E})$ and magnetic (G$_{M})$ form factors of the proton measured by Rosenbluth Separation and Polarization Transfer methods differ by a factor of three at Q$^{2}$ of 5.6 GeV$^{2}$. The real part of the Two Photon Exchange (TPE) amplitude in lepton-proton elastic scattering is expected to explain this discrepancy. The ratio of elastic positron-proton to electron-proton cross sections is the only way to measure this real part. We will measure this cross section ratio using a mixed electron-positron beam in CLAS at Jefferson Lab. In this talk I will present how the electron-positron beam is produced, the backgrounds that limit our luminosity and the simulations used to reduce those backgrounds.   

Photodisintegration of Deuterium at Low Energies: Measurements of Cross Section and Fore-Aft Asymmetries Between \textit{E$_{\gamma}$} of 2.44 and 4 MeV at the High Intensity $\gamma$-Ray Source (HI$\gamma$S)

Data were taken recently at HI$\gamma$S to obtain cross section and fore-aft asymmetry measurements in photodisintegration of the deuteron in the energy region of astrophysical importance. Linearly polarized $\gamma$-rays ($\Delta E/E \sim 3 \%$) were incident on a thin D$_2$O target and the outgoing neutrons were detected using three Li-glass detectors placed in the plane of polarization at center-of-mass scattering angles ($\theta$) of 54$^{\circ}$, 88$^{\circ}$, and 125$^{\circ}$. A preliminary analysis indicates small, but non-zero, asymmetries. An outline of the analysis to extract asymmetry and cross sections within an error of $\sim$ 10 \% will be presented. The acquisition and analysis of data used to extract the efficiency of the Li-glass detectors will also be presented. The significance of the asymmetry and cross section measurements will be discussed.   

Measurement of Two Photon Exchange in the ep Elastic Scattering Process using Recoil Polarization

The explanation of the discrepancy between electric form factor measured with recoil polarization and Rosenbluth separation data has been focused on contribution of two photon exchange process which was ignored in the previous radioactive corrections. Experiment E04-109 running at Jefferson Lab Hall C searched for out-of-plane polarization component due to two gamma exchange in the ep elastic scattering process using recoil polarization method. This effect requires small systematical uncertainty to be seen. Two detectors played essential role in this measurement, BigCal and FPP. BigCal is the electromagnetic calorimeter which detects electrons to make coincidence with HMS which is the standard Jefferson Lab Hall C spectrometer to measure protons. The BigCal is used to reject inelastic contribution to get clear elastic scattering events, and FPP is the detector to measure the polarization of protons in HMS focal plane. The preliminary results of BigCal performance and the helicity independence asymmetries measured at 3 kinematics points at Q$^{2}$=2.5Gev$^{2}$ will be reported.   

$\pi^{-}/\pi^{+}$ Ratios of Separated Response Functions in Forward Pion Electroproduction

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 in the $Q^2 = 0.6 - 2.45 $ (GeV/c)$^2$ range. The separated ratio $R_L =\sigma_L^{\pi^-}/\sigma_L^{\pi^+} $ is sensitive to 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. The value of this ratio may also have implications for constraining polarized GPD's with ratios of longitudinal observables. At large $-t$, a separate ratio $R_T = \sigma_T^{\pi^-}/\sigma_T^{\pi^+}\simeq$ 1/4 would suggest a transition between pion and quark knockout mechanisms. Preliminary results on the separate ratios $R_L$ and $R_T$ indicate a dominance of the pion pole diagram at low -t. The results will be compared with a variety of models such as the VGL model.   

Hard Photo-disintegration of proton pairs in $^3$He

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. The current state of the analysis will be shown. 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., and the $\alpha_n$ distribution.   

A new focal plane polarimeter to measure the proton form factor ratio.

One of the two methods available to measure the elastic form factors of the proton, is a measurement of the polarization of the recoil proton in $\vec {e}p\to \,e\vec {p}$; the other is the standard Rosenbluth separation based on cross section. A new experiment in Hall C at JLab is measuring the ratio ${G_{Ep} } \mathord{\left/ {\vphantom {{G_{Ep} } {G_{Mp} }}} \right. \kern-\nulldelimiterspace} {G_{Mp} }$by the recoil polarization method. The polarization of the recoil proton is measured by a new polarimeter (FPP) built and installed near the focal plane of the high momentum spectrometer in Hall C. In this FPP, the protons are scattered in an analyzer and the azimuthal angular distribution of the proton is measured. To maximize the number of interactions in the FPP, two analysers in series are used, each followed by a pair of drift chambers. Performances, resolution and efficiency will be discussed. A preliminary result of the new ratio ${G_{Ep} } \mathord{\left/ {\vphantom {{G_{Ep} } {G_{Mp} }}} \right. \kern-\nulldelimiterspace} {G_{Mp} }$ at Q$^{2}$ = 5.2 GeV$^{2}$ will be shown.   

New Measurements of the Proton Spin-Structure Functions $g_1$ and $g_2$ in and above the Resonance Region

The CLAS (CEBAF Large Acceptance Spectrometer) EG1b experiment in Hall-B at Jefferson Laboratory measured double-spin inclusive and exclusive electron-nucleon scattering asymmetries using longitudinally polarized frozen NH$_3$ and ND$_3$ targets and a longitudinally polarized electron beam at 4 different energies (1.6, 2.5, 4.2, 5.6 GeV). Extraction of the virtual photon asymmetry $A_1$ (for $0.05$ GeV$^2 < Q^2 < 4.0$ GeV$^2$) provides precision measurements of the polarized spin-structure function $g_1$ in and above the resonance region. Linear regression of data between the varying energies yields new constraints on the virtual photon asymmetry $A_2$ (and thus the structure function $g_2$) in the resonance region (for $0.3$ GeV$^2 < Q^2 < 1.0$ GeV$^2$). Measurements of these structure functions and their moments allows testing of QCD models, sum rules, foward-spin polarizability and duality for the proton.   

Photoproduction of the Sigma(1385) resonance at LEPS

The Sigma(1385) hyperon resonance has been measured using the LEPS detector at the SPring-8 facility in Japan. Linearly polarized photons in the range of 1.5-2.4 GeV were incident on a liquid deuterium target, producing a $K^+$ and a $\pi^-$ in the final state. The negative Sigma(1385) was isolated by its decay to ($\Lambda \pi^-$) using the missing mass technique. The same final state particles can also used to identify photoproduction of the $\Sigma^-$ ground state via its decay to ($n\pi^-$). Using simulations to correct for the detector acceptance, it will be possible to link the cross sections for the Sigma(1385) to the previously measured cross sections for photoproduction of $K^+\Sigma^-$. Theoretical calculations for photoproduction of $K^+\Sigma^{*-}$ from the neutron are in progress, and will be discussed along with the data analysis of this reaction.   

Next generation $\Lambda$-hypernuclear spectroscopy via the (e,e$^\prime$K$^+$) reaction at Jefferson Lab

Spectroscopic studies via the (e,e$^\prime$K$^+$) reaction are a very important complementary technique to investigate $\Lambda$-hypernuclear structure; the reaction favorably excites spin-flip states and on light nuclei, compared to the meson-induced reactions, produces mirror hypernuclei. So far, it is the only technique that allows absolute mass determination with accuracies of $\approx$100 keV or better. Two previous experiments that we performed, E89-009 \& E01-011, established the experimental technique, and the latter obtained hypernuclear mass spectra up to $A\approx 30$ with, for reaction spectroscopy, unprecedented energy resolution down to 400 keV (FWHM). Our next experiments will investigate $\Lambda$ hypernuclei up to $^{89}_\Lambda$Sr. A newly constructed electron spectrometer (HES) and splitter magnet will increase the yield by almost one order of magnitude while preserving the achieved energy resolution. Together with the existing Kaon Spectrometer (HKS), they are scheduled for installation in Jefferson Lab's Hall C in 2009. The planned experimental program will for the first time in (e,e$^\prime$K$^+$) reaction spectroscopy also explore $\Lambda$-hypernuclei beyond the p-shell. This presentation will give a status report and outline the experimental program and technique of the E05-115 experiment.