Session C7: Electromagnetic Interactions I

Phenomenological Hints To The Cause Of The EMC Effect

Deep-inelastic scattering cross section ratios plotted as a function of xB show that quark structure in nuclei is different then free nucleons. Recent Hall C data showed that the slope in the 0.3 $<$ xB $<$ 0.7 region of the EMC effect scales as the local nuclear density and not the average nuclear density. This result lead to the comparison of xB $>$ 1 short-range correlation plateaus to the magnitude of the EMC effect slope and a clear linear relation between the two effects has now been shown. In this talk, I will discuss the EMC effect and the short-range correlation plateaus and what the phenomenological relationship between the two implies about the cause of the EMC effect.   

The CLAS Two Photon Exchange Experiment: Experimental Methods

There is a large discrepancy between the proton electric form factor (${G_E}^p$) measured using the Rosenbluth separation and polarization transfer methods. The most likely explanation is two-photon exchange (TPE). A precise measurement of the ratio of the cross sections of electron-proton and positron-proton elastic scattering is a model-independent way to measure the TPE amplitude. The TPE experiment recently took data at Jefferson Lab Hall B. The primary electron beam is used to create an intense bremsstrahlung photon beam. Some of the photons are then converted to a mixed identical $e^+/e^-$ beam which then interacts with the liquid hydrogen target. The $e^+p$ and $e^-p$ events are detected by the CLAS. This talk will present the experimental techniques for producing the mixed identical $e^+/e^-$ beam and the methods for controlling systematic uncertainties.   

The CLAS Two Photon Exchange Experiment: Analysis Methods

The two photon exchange contribution to the elastic electron proton scattering has been suggested as the most likely explanation for the discrepancy observed between Rosenbluth and the polarization transfer methods in the measurement of proton electric-to-magnetic form factor ratio of the proton ($G_E^p/G_M^p$). The Two Photon Exchange (TPE) experiment recently completed data taking in Jefferson Lab's Hall-B and will directly measure, with high precision, the TPE contribution to elastic scattering by comparing the ratio of cross sections for electron-proton to positron-proton elastic scattering. The data were collected using a primary electron beam of 5.6 GeV to produce a photon beam, which in turn was used to produce a mixed $e^+/e^-$ beam that was incident upon an LH$_2$ target. The CEBAF Large Acceptance Spectrometer (CLAS) was used to detect both the scattered leptons and protons. The resulting data span $Q^2$ up to 2.5 GeV$^2$ and nearly the entire epsilon range. The analysis techniques will be discussed along with some preliminary results.   

Electroproduction of $\pi^0$ in the resonance region at high $Q^2$ with CLAS

An extensive program is underway at Jefferson Lab to study the eletromagnetic excitations of baryon states. We report the analysis of exclusive single $\pi^0$ and $\eta$ electroproduction in the resonance region at Jefferson Lab in the $Q^2$ range of 2 to 6 $GeV^2/c^2$. A longitudinally polarized $5.75$ $GeV$ electron beam was incident on a 5 cm long liquid Hydrogen target. The CLAS spectrometer at Jefferson Lab was used to detect the final state particles. The average beam polarization was $70\%$. The data was taken between October 2001 and January 2002. Preliminary results for differential cross sections and beam spin asymmetries over the entire $4\pi$ $c.m.$ solid angle will be presented. This high precision measurement will allow us to access the structure and dynamics of nucleon excitations with masses up to 2 $GeV$.   

Triple Coincidence Beam Spin Asymmetry Measurements in Deeply Virtual Compton Scattering

The Generalized Parton Distributions (GPDs) provides hitherto the most complete information about the quark structure of hadron. GPDs are accessible through hard-exclusive reactions, among which Deeply Virtual Compton Scattering (DVCS) is the cleanest reaction. A dedicated DVCS experiment on Hydrogen (E00-110) ran in the Hall A at Jefferson Laboratory in Fall 2004. I present here Beam Spin Asymmetry (BSA) results for the $ep \rightarrow ep\gamma$ reaction studied in the E00-110 experiment with fully exclusive triple coincidence $H(e,e'\gamma p)$ detection.   

A$_y$ Measurement from $^3\mbox{He}^\uparrow(e,e'n)$ Scattering at Jefferson Lab

Recently A$_y$ asymmetry measurements have been conducted in Jefferson Lab's Hall A through electron scattering from a vertically polarized $^3$He target. Experiment E08-005 measured the target single-spin asymmetry A$_y$ in the quasi-elastic $^3\mbox{He}^\uparrow(e,e'n)$ reaction. Plane wave impulse approximation (PWIA) predicts that A$_y$ should be exactly zero. A previous experiment at Q$^2$ of 0.2 (GeV/c)$^2$, where full calculations of Laget and Nagorny indicated A$_y$ to be small, showed a large asymmetry as calculated by the Bochum group using Faddeev calculations to solve the three-body problem exactly. The recent experiment measured this asymmetry at Q$^2$ of 0.1 (GeV/c)$^2$, 0.5 (GeV/c)$^2$ and 1.0 (GeV/c)$^2$. This is the first measurement of A$_y$ at large Q$^2$, which is another region where A$_y$ is expected to be small. Any non-zero result is an indication of effects beyond simple impulse approximation. This measurement will test the models used to extract neutron form factor from polarized $^3$He. Details of the measurement will be presented.   

Hard breakup of the deuteron into two $\Delta$ -isobars

Photodisintegration of the deuteron into two $\Delta$-isobars at large center of mass angles is studied within the QCD hard rescattering model (HRM). According to the HRM, the reaction proceeds in three main steps: the photon knocks the quark from one of the nucleons in the deuteron; the struck quark rescatters off a quark from the other nucleon sharing the high energy of the photon; then the energetic quarks recombine into two outgoing baryons emerging at large transverse momenta. Within the HRM, the cross section is expressed through the amplitude of $pn\rightarrow \Delta\Delta$ scattering which we evaluated based on the quark-interchange model of hard hadronic scattering. We predict that the cross section of the deuteron breakup to $\Delta^{++}\Delta^{-}$ is 4-5 times larger than that of the breakup to the $ \Delta^{+}\Delta^{0}$ channel. Also, the angular distributions for these two channels are markedly different. These can be compared with the predictions based on the assumption that two hard $\Delta$-isobars are the result of the disintegration of initial $\Delta\Delta$ components of the deuteron wave function. In this case, the angular distributions and cross sections of the breakup in both $ \Delta^{++}\Delta^{-}$ and $\Delta^{+}\Delta^{0}$ channels are expected to be similar.   

Kinematic Issue in DVCS and GPDs

Whether the kinematics includes the hard transverse photon momenta or not makes a dramatic difference in computing deeply virtual Compton scattering (DVCS) in terms of the widely used reduced operators that define generalized parton distributions (GPDs). We present a tree-level complete DVCS amplitude including the lepton current which plays the role of spin filter to analyze such kinematic dependence on the contribution of longitudinally polarized virtual photon as well as the conservation of angular momentum. We show that the contribution of the longitudinally polarized virtual photon in the kinematics with the hard transverse photon momenta should not be neglected in the analysis of DVCS amplitudes. It is also found that in such kinematics angular momentum is not conserved if the amplitudes are calculated in terms of the reduced operators that define GPDs.