Session B10: Sub-Nucleonic Degrees of Freedom

Deeply Virtual Compton Scattering (DVCS) on a Longitudinally Polarized Proton Target Using CLAS

DVCS has been identified as the cleanest process to access the Generalized Parton Distributions (GPDs) at medium energies through the so-called handbag mechanism. In electron scattering, scaling of the cross section has been measured at Jefferson Lab at photon a virtuality Q$^{2}$=2 GeV$^{2}$ [1], and DVCS has been measured in a large kinematic range using a polarized electron beam with CLAS [2]. This allowed access to a combination of GPD \textbf{\textit{H}} an \textbf{\textit{H}}-tilde with \textbf{\textit{H}} giving the dominant contribution. Using a longitudinally (along the beam line) polarized proton target the target asymmetry is dominantly described by the same two GPDs, but with \textbf{\textit{H}}-tilde giving the dominant contribution [3]. The combination of the two measurements will allow a precise separation of the two GPDs. The experiment (eg1-dvcs) ran during 3 periods in 2009 and collected high statistics data with the CLAS detector in Hall B. The quality of the data, and the status of the analysis will be described. The relevance of this data set and its kinematic extension to larger Q$^{2}$ with CLAS12 after the JLab energy upgrade will be discussed as well. [1] C. Munoz-Camacho, Phys. Rev. Lett.97, 262002, 2006. [2] F.X. Girod et al., Phys.Rev.Lett.100:162002, 2008. [3] S. Chen et al., Phys. Rev. Lett.97:072002, 2006.   

Measurement of the anti-quark distributions in the proton with Drell-Yan process

The Drell-Yan process is an excellent tool to explore the partonic structure of hadrons. This process, which occurs in high energy hadron-hadron scattering, proceeds when a quark and an antiquark of the interacting hadrons annihilate producing a virtual photon which decays into a lepton-antilepton pair. Since the interaction requires an antiquark, it is well suited for probing the sea distributions of hadrons. In particular, fixed-target kinematics favor the annihilation of a beam quark with target antiquark. By interchanging proton and neutron (deuteron) targets, the Drell-Yan process can be used to measure the ratio of anti-up to anti-down quarks in the proton. A new experiment, Fermilab E-906/SeaQuest, will improve our knowledge of the nucleon structure by extending the range of previous measurements to larger Bjorken-x. This talk will present an overview of the previous anti-down/anti-up measurements and highlight the features and expected results of the upcoming E-906/SeaQuest experiment, which will start data collection in summer 2010.   

Report on the Spin Asymmetries of the Nucleon Experiment

The Spin Asymmetries of the Nucleon Experiment (SANE) measured the proton spin structure function $g_2$ in a range of Bjorken $x$, $0.3 < x < 0.8$, where extraction of the matrix element $d_2^p $ (an integral of $g_2$ weighted by $x^2$) is most sensitive. The data was taken from $Q^2$ equal to $2.5 GeV^2$ up to $6.5 GeV^2$. In this polarized electron scattering off a polarized hydrogen target experiment, two double spin asymmetries, $A_{\parallel}$ and $A_{\perp}$ were measured using the BETA (Big Electron Telescope Array) detector. BETA consists of a scintillator hodoscope, gas Cherenkov, lucite hodoscope and a large array of lead glass detectors. With a unique open geometry, a threshold gas Cherenkov detector allowed BETA to cleanly identify electrons for this inclusive experiment. We will discuss the performance of BETA with an emphasis on the gas Cherenkov and present the current status of experimental analysis.   

A Precision Measurement of Neutron d2

The asymmetries $A_{\perp}$ and $A_{\parallel}$ as well as the absolute cross section $\sigma_0$ for polarized $^3He$, were measured in the deep inelastic quark region, $0.2\leq x\leq0.7$ and $2\leq Q^2\leq6 GeV^2$, to access information on the neutron. The experiment was preformed at Jefferson Laboratory located in Newport News, Va. From these asymmetries and absolute cross sections, the spin structure functions $g_1$ and $g_2$ will be extracted. Using the spin structure functions the higher twist matrix element $d{^n_2}$ will then be evaluated. The quantity $d{^n_2}$ is a probe into quark-gluon correlations, and currently\footnote{Matthias Burkardt. Parton Distributions in the Impact Parameter Space. 2009.} can be thought of effectively as the averaged Lorentz color force acting on the quarks just after they are struck by a virtual photon. Alternatively it was suggested earlier\footnote{B.W. Filippone and Xiangdong Ji. The spin structure of the nucleon. ADV.NUCL.PHYS.,26:1,2001.1},that the gluon field and nucleon polarization interactions lead to effective magnetic and electric color polarizabilities $\chi_B$ and $\chi_E$. The matrix element $d{^n_2}$ can be expressed as a linear combination of the electric and magnetic color polarizabilites, which can be calculated using Lattice QCD.   

The helicity asymmetry measurements for pi0 photoproduction with FROST at Jlab

One of the long standing unsolved problem in the nuclear physics is the nucleon resonances N* and $/Delta$*. The lifetime of these intermediate states are very short since they decay strongly. Their parameters, mass, width, and coupling constants to various decay modes are poorly known. To solve these problems, double polarization experiments are considered to be a very effective tool since spin is a good quantum number. I will present the result of one pion photo-production from the first double polarization experiment at Jlab, comparing the prediction of theoretical models.   

Semi-Inclusive Pion Electroproduction in Deep Inelastic Scattering

Measurements of pion electro-production in semi-inclusive deep inelastic scattering (SIDIS) have been performed. Data were taken with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 5.498 GeV longitudinally polarized electron beam and an unpolarized liquid hydrogen target during the E1-f run period in 2003. All three pion channels ($\pi^+$, $\pi^0$ and $\pi^-$) were measured simultaneously over a large range of kinematics($Q^2\approx$ 1-4 $GeV^2$ and $x\approx$0.1-0.5). Preliminary results from our study of single-spin azimuthal asymmetries from all three pion channels as functions of x, z, and $P_{T}$, from which $A_{LU}^{sin\phi}$ is extracted, will be presented, as will preliminary measurements of $A_{UU}^{cos\phi}$ and $A_{UU}^{cos2\phi}$ in the charged pion channels. This new high statistical data could provide access to transverse-momentum dependent parton distribution functions (TMD's), which are thought to be important in understanding of the physics underlying the spin structure of the nucleon.   

The Measurement of Transverse Single Spin Asymmetry of Forward Charged Hadrons in the PHENIX experiment at RHIC

The measurement of transverse single spin asymmetries provides an opportunity to probe the parton structure of transversely polarized nucleons. We present PHENIX preliminary results of transverse single spin asymmetries of non-identified charged hadrons measured in the muon spectrometers ($1.2 < \eta < 2.5$) from transversely polarized p+p collisions at $\sqrt{s}=200$GeV as a function of $x_{F}$ and $p_{T}$. PHENIX has lower $x_{F}$ and higher $p_{T}$ coverage than the Brahms experiment, which has made these measurements in the past. At lower $x_{F}$ we can study the turn-on of the asymmetry as a function of $x_{F}$, and the crossover region between pQCD and TMD factorization is at higher $p_{T}$. Perturbative QCD predicts that the asymmetry should decrease as 1/$p_{T}$. For this purpose we also show the $p_{T}$ dependent asymmetry in a very narrow $x_{F}$ range around the turn-on region.   

Neutron Electric Polarizability from Lattice QCD

The background field method has been used extensively in lattice QCD simulations to compute electromagnetic polarizabilities for hadrons. The extraction of electric polarizability is easier for neutral particles than for charged ones, and the best testing ground is the neutron. Previous lattice simulations produced puzzling results for its polarizability: the values seemed to agree with the experimental result, but they did not exhibit the expected chiral behavior. Recent theoretical developments coupled with new simulations show that the story is more complex. The background field method extracts the polarizabilities from the tiny shifts in the energy of the hadron placed in a static electromagnetic field. As such, it is very sensitive to the choice of boundary conditions, fitting procedure and the method used to include the electric field on the lattice. We will discuss these new developments and present our results for the neutron electric polarizability.   

The CLAS12 Detector and Its Physics Program at the JLAB 12 GEV Energy Upgrade

The Jefferson Lab 12 GeV upgrade provides the kinematic reach to access generalized parton distributions and transverse momentum dependent parton distributions in a large kinematic range in the nucleon's valence quark regime. The CLAS12 detector has been developed as an upgraded version of CLAS with high luminosity capabilities, and improved particle identification. The detector construction has started in 2009. A broad program has been developed to map the nucleon's 3-dimensional spin and flavor content through the measurement of deeply virtual exclusive and semi-inclusive processes. Other programs to study nucleon elastic and transition form factors, as well as nuclear phenomena have also been approved. An overview is presented of the CLAS12 detector and the initial physics program after the energy doubling of the JLab electron accelerator.