Session HC: Generalized Parton Distributions

Deeply Virtual Compton Scattering Cross-Sections with CLAS

The exclusive electroproduction of a photon off a nucleon provides three-dimensional information on the nucleon structure. This reaction proceeds via the Bethe-Heitler (BH) process (photon emitted by electron), and the Deeply Virtual Compton Scattering (DVCS) process (photon emitted by proton). BH and DVCS interfere at the amplitude level. In the Bjorken regime of large $Q^2$ at fixed $x_B$, and for $-t/Q^2<1$, the amplitude factorizes, the non-perturbative part described by Generalized Parton Distributions (GPDs). GPDs are the Fourier transform of the spatial distributions of partons in the transverse plane at fixed longitudinal momentum fraction $x_B$. The BH and DVCS contributions create harmonic dependencies of observables as functions of the angle between the leptonic and hadronic planes, which are then used to extract GPDs. The BH/DVCS cross-sections on unpolarized hydrogen target have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) in a dedicated experiment, at a polarized beam of energy 5.75 GeV, in the valence region $0.1 < x_B < 0.6$, $1 < Q^2 < 5$, and for $0.09 < -t < 2$ GeV$^2$. We will present results for the unpolarized cross-sections as well as the beam-polarized cross-section differences,and discuss their impact on the GPD extraction strategies.   

Energy Dependent DVCS Cross Sections from JLab Hall A

In 2010, in experiments E07-007 (hydrogen target) and E08-025 (deuterium target), the Jefferson Lab Hall A collaboration measured the helicity-dependent and helicity-independent cross sections at fixed $x_B = 0.36$, at $Q^2 = 1.5$, $1.75$, and $2.0$ GeV$^2$, and at two beam energies, 4.45 and 5.55 GeV. We detected the scattered electron in the Hall A High Resolution Spectrometer (HRS-L), and the coincidence photon in an upgraded 208 element PbF$_2$ calorimeter. Exclusivity is inferred by missing mass in the $(e,e'\gamma)X$ reaction. In the unpolarized cross sections, the $|\mathrm{DVCS}|^2$ and $\Re\mathrm{e}[\mathrm{DVCS}^\dagger\mathrm{BH}]$ terms have different kinematic dependencies on the incident beam energy. I present preliminary results on the energy-dependence of the cross sections, and discuss their sensitivity to the Generalized Parton Distributions (GPDs).   

Near Threshold Electroproduction of J/psi with SoLID at Jefferson Lab

An experiment of measuring the electroproduction of J/psi meson cross section off a proton target near threshold is under preparation at Jefferson Lab. The high luminosity at the energy upgraded JLab together with the large acceptance of the SoLID spectrometer in Hall A allows a fully exclusive measurement of this process with an 11 GeV electron beam. The measurement is sensitive to the non-perturbative gluonic interaction between the J/psi and nucleon and might reveal an enhancement of the cross section just above threshold. Such a measurement would open a new window to study QCD in the non-perturbative region using charmonium.   

Deeply Virtual Compton Scattering on the Proton

DVCS on the proton was measured at Jefferson Lab with CLAS at Hall B with a polarized 5.88 GeV electron beam on an unpolarized hydrogen target. A preliminary measurement of unpolarized and polarized cross sections was made over wide kinematics, from $1\ \textrm{GeV}^{2} < Q^{2} < 5\ \textrm{GeV}^{2}$, $0.1 < x_{B} < 0.6$ and $0.1\ \textrm{GeV}^{2} < -t < 2\ \textrm{GeV}^{2}$. In the handbag model, this reaction is sensitive to the GPD $H$. The Compton form factor $\mathcal{H}_{\textrm{Im}}$, which is proportional to $H$, was extracted. In addition, we have made a comparison of measured cross sections with predictions from several different handbag based models. This measurement allows for further constraints to be placed on the various models, especially on $H$. Preliminary results for the extraction of the GPD $H$ will be presented and discussed.   

Spin Asymmetry Measurements for Deeply Virtual Compton Scattering on Polarized Protons

Generalized Parton Distributions (GPDs) have emerged as a universal tool to describe hadrons, particularly nucleons, in terms of their elementary constituents, quarks and gluons. Spin asymmetry measurements in the reaction $\vec{e}\vec{p}\rightarrow$ep$\gamma$, such as the proton target spin asymmetry which is directly proportional to the imaginary part of the Deeply Virtual Compton Scattering amplitude, give access to different combinations of GPDs. Combined measurements of proton target spin asymmetry (TSA), electron beam helicity asymmetry (BSA), and electron-proton double spin asymmetry (DSA) at the same kinematic points allows access to GPDs through a semi-model independent extraction of Compton Form Factors (CFFs). Preliminary TSA, BSA, and DSA studies for the reaction $\vec{e}\vec{p}\rightarrow$ep$\gamma$ and extracted CFFs will be presented from a dedicated experiment at Jefferson Lab using the CEBAF 6 GeV polarized electron beam, a polarized solid state \(^{14}NH_{3}\) target, and the CEBAF Large Acceptance Spectrometer (CLAS) equipped with an additional Inner Calorimeter (IC). The accessible kinematic range for these measurements covers 1 $<$ Q\(^2\) $<$ 4.5 GeV\(^2\), 0.1 $<$ x\(_B\) $<$ 0.58, and 0.08 $<$ -t $<$ 1.8 GeV\(^2\).   

The gluon contributions to timelike and spacelike DVCS

We study $O(\alpha_s)$ corrections to timelike and spacelike virtual Compton scattering amplitudes, and their influence on the predictions for the observables which will be measured at JLab12, COMPASS-II experiment at CERN and future Electron Ion Collider. We emphasize the importance of the gluonic contributions, even in the medium energy range. We stress that the timelike and spacelike cases are complementary and that their difference deserves much special attention.   

Nucleon Structure Studies through Timelike Compton Scattering

Hard exclusive processes have emerged as a class of reactions providing novel information on the quark and gluon distributions in hadrons. Factorization theorems allow one to express amplitudes of these processes in terms of Generalized Parton Distributions (GPDs). Deeply Virtual Compton Scattering (DVCS) has been the focus of interest as it provides the cleanest tool for accessing the quark GPDs of the nucleon. Time-like DVCS, also known as Time-like Compton Scattering (TCS), is the inverse process to space-like DVCS that can be probed through the photoproduction of lepton pairs. TCS can be an effective tool for studying the real part of the Compton amplitude. Combining space-like and time-like data thus makes it possible to test the universality of GPDs. The first studies of TCS using real tagged and quasi-real untagged photons were performed at Jefferson Lab 6 GeV. In this talk we will present preliminary results on angular asymmetries and extraction of the real part of Compton form-factors. We will also discuss future plans for di-lepton production in the Jefferson Lab 12-GeV era.   

Exclusive $\pi^0$ and $\eta$ Electroproduction at Multi GeV Energy and Momentum Transfer

Exclusive $\pi^0$ and $\eta$ electroproduction were measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections $d^4\sigma/dtdQ^2dx_Bd\phi$ and structure functions $\sigma_T+\epsilon\sigma_L, \sigma_{TT}$ and $\sigma_{LT}$ as functions of $t$ were obtained over a wide range of $Q^2$ and $x_B$. In addition, the ratios of $\pi^0$ to $\eta$ electroproduction were obtained. The data appear to confirm that the reactions are predominately due to transverse photons. The data are compared with Regge and handbag theoretical calculations. In the handbag framework the transverse reactions involve helicity flip of the active quarks, and transversity GPDs in which the helicity of the recoil nucleon can be either flipped or non-flipped. The data are compared with two handbag based calculations which have different combinations of GPDs.   

Physics Opportunities with a Neutral Particle Spectrometer in Hall C

Hard scattering processes can probe the transverse momentum and spatial distributions of quarks, anti-quarks, and gluons in hadrons. This information is more complete than what can be obtained from studies of inclusive and elastic scattering alone. Measurements of reactions with neutral final states play an important role in this by, for instance, allowing one to probe universal features of parton distributions and to verify the hard scattering formalism in thus far unexplored regimes. At the 12 GeV Jefferson Lab Hall C with its heavily-shielded detector setup in a highly-focusing magnetic spectrometer with large momentum reach will be the optimal Hall for certain classes of deep exclusive and semi-inclusive measurements, and in particular those requiring precision measurements, for instance, high quality L/T separations. The addition of a neutral particle detection facility would significantly augment these scientific capabilities. In this talk we will give an overview of the scientific program enabled by such a facility, and discuss its requirements and plans.