Session LF: Mini-Symposium on Hadron Structure and QCD in High Energy Processes IV

Quark Structure of the Nucleon and Angular Asymmetry of Proton-Neutron Hard Elastic Scattering

We investigate the asymmetry in angular distribution of hard elastic proton-neutron scattering with respect to 90$^{\circ}$ center of mass scattering angle. This asymmetry on quark level is generated due to the mixture of quark scatterings with and without flavor interchange in the isoscalar $pn$ state. We demonstrate that the magnitude of the angular asymmetry is related to the helicity-isospin symmetry of the quark wave function. Our estimate of the asymmetry within the quark-interchange model of hard scattering demonstrates that the quark wave function of a nucleon based on the exact SU(6) symmetry predicts an angular asymmetry opposite to that of experimental observations. On the other hand the quark wave function within diquark picture of the nucleon produces an asymmetry consistent with the data. Comparison with the data allowed us to extract the relative sign and the magnitude of the vector and scalar diquark components of the quark wave function of nucleon. Overall, our conclusion is that the angular asymmetry of hard elastic scattering of baryons provides a new venue in probing quark-gluon structure of baryons and should be considered as an important observable in constraining the theoretical models.   

Basis function approach to Hamiltonian light front gauge theory

Hamiltonian light-front quantum field theory constitutes a framework for the non-perturbative solution of invariant masses and correlated parton amplitudes of self-bound systems. By choosing the light-front gauge and adopting a basis function representation, we obtain a large, sparse, Hamiltonian matrix for mass eigenstates of gauge theories that is solvable by adapting the {\it ab initio} no-core methods of nuclear many-body theory. Full covariance is recovered in the continuum limit, the infinite matrix limit. There is considerable freedom in the choice of the orthonormal and complete set of basis functions with convenience and convergence rates providing key considerations. Here, we use a two-dimensional harmonic oscillator basis for transverse modes that corresponds with eigensolutions of the soft-wall AdS/QCD model obtained from light-front holography. We outline our approach, present illustrative features of some systems in a cavity.   

Polarized structure functions from AdS/CFT

We investigate deep inelastic and elastic scattering on a polarized spin-$\frac{1}{2}$ hadron using gauge/string duality. AdS/CFT correspondence provides us new insights into gauge theories in strong coupling regime. This spin-$\frac{1}{2}$ hadron corresponds to a supergravity mode of the dilatino. The polarized deep inelastic structure functions are computed in supergravity approximation at large t' Hooft coupling $\lambda$ and finite $x$ with $\lambda^{-1/2}\ll x<1$. Furthermore, we discuss the moments of all structure functions, and propose an interesting sum rule $\int_{0}^{1} \textrm{d}x g_2\left(x, q^2\right) =0$ for $g_2$ structure function which is known as the Burkhardt-Cottingham sum rule in QCD. In addition, we discuss the possible small-x contributions for $g_1$ due to the consideration of the angular momentum sum rule. In the end, the elastic scattering is studied and elastic form factors of the spin-$\frac{1}{2}$ hadron are calculated within the same framework.   

The STAR Forward GEM Tracker

The STAR collaboration is preparing a tracking detector upgrade, the Forward GEM Tracker (FGT), which will focus on novel spin physics measurements in high-energy polarized proton-proton collisions at a center-of-mass energy of $500\,$GeV, determining the flavor dependence ($\Delta \bar{u}$ versus $\Delta \bar{d}$) of the polarized sea. STAR plans to probe these polarized distribution functions using parity violating W production in the electron/positron decay mode. This upgrade will consist of six triple-GEM detectors with two dimensional readout arranged in disks along the beam axis. The FGT project has completed an extensive R$\&$D program of industrially produced GEM foils at Tech-Etch Inc. in comparison to GEM foils produced at CERN based on optical measurements, testbeam and $^{55}$Fe source measurements of a triple-GEM prototype detector using $10\times 10$cm$^{2}$ GEM foils. The FGT project requires large GEM foils ($\sim 40\times 40$cm$^{2}$) which are currently being tested. The FGT design, the status of large GEM foil tests, the performance of triple-GEM prototype detectors based on industrially produced GEM foils along with the status of the FGT construction and the installation schedule will be presented.   

Trigger Upgrade of PHENIX Muon Arms for Polarized Sea Quak Measurement and Background Study at $\sqrt{s}$=500 GeV

Parity-violating production of the W boson with longitudinally polarized protons at RHIC provides a direct measure of the individual polarizations of the quarks and anti-quarks in the colliding protons. The high energy scale set by the W-mass makes it possible to extract quark and anti-quark polarizations from inclusive lepton spin asymmetries in W-production with minimal theoretical uncertainties. This program thus will break new ground in our detailed understanding of the proton's structure. The program was initiated by the first operation of RHIC polarized proton beams at its highest operational energy $\sqrt{s}=500$ GeV. A new trigger on forward muons in PHENIX identifies and triggers on high momentum muons from W decay suppressing a large number of background low momentum muons coming from hadronic decays. In this talk, I will update the installation and commissioning status of the new trigger electronics and discuss about the observed background conditions at 500 GeV from Run09 data.   

Silicon Pixel Detector for Vertex Tracker in RHIC-PHENIX experiment

The PHENIX experiment at the RHIC will be upgraded with a Silicon Vertex Tracker (VTX) in 2010 to enhance the physics capability in both of spin and heavy ion program. The VTX will be placed near the collision point with large geometrical acceptance ($\phi \quad \sim $ 2$\pi $, $\vert \eta $ $\vert >$1.2). It is able to identify the heavy quark productions by measuring the displaced vertex and identify the Jet production by measuring momentum of charged tracks. The VTX is required fine spatial resolutions with low material budget and high speed readout up to 20 KHz trigger rate. The VTX is consisted from 2 inner pixel layers and 2 outer stripixel layers. The pixel detectors are 30 of ladder modules and their readout electronics. The ladder module is made from 4 pixel sensors modules (pixel size 50*425 $\mu $m$^{2})$, carbon fiber support structure including cooling pipe, and fin pitch low radiation length readout bus. We archived 1.26{\%} X/X$_{0}$ in total to minimize the multiple scattering. The spatial resolution of the prototype ladders was measured as 14 um in $\phi $ direction and 150$\mu $m in Z by using 120GeV proton beam. The production of pixel ladders and readout electronics has been started in spring of 2009. We will report their performance and status of the production.   

Nuclear modification factor $R_{CP}$ for $\phi$ meson production in $d$+$Au$ collisions at $\sqrt{S_{NN}}=$200~GeV measured by the PHENIX experiment at RHIC

In $d$+$Au$ collisions, vector mesons produced in hard scattering are sensitive to various nuclear effects such as parton shadowing/saturation in the small x region (forward rapidity) leading to suppression, and antishadowing (large x region, backward rapidity) or the Cronin effect which both can produce enhancement. Since approaches such as the Color Glass Condensate (CGC) and pQCD-based Glauber-Eikonal models do not agree on the nature of these nuclear effects on particle production at large rapidity, it is essential that they be tested with experimental data in this kinematic regime. Knowledge of the difference between the forward and backward rapidity regions, in $d$+$Au$ collisions, could also be used to separate the initial-state nuclear wave function modifications and final state in-medium effects in $Au$+$Au$ collisions. In addition, the relative ratio for the production of $\rho$, $\omega$ and $\phi$ can provide information on the production mechanisms of light vector mesons. The PHENIX collaboration at RHIC has recently collected data in $d$+$Au$ collisions at $\sqrt{s}$=200~GeV during the 2008 run. The latest work on the $R_{CP}$ measurements of $\phi$, through the di-muon decays at forward and backward rapidities (1.2<$\eta$<2.2), will be discussed.   

Jet Reconstruction in d+Au collisions at RHIC-PHENIX

Collisions between protons (deuterons) and nuclei provide a testing ground to understanding nuclear effects, e.g. shadowing and the EMC effect in nuclear parton distribution functions, nuclear $k_T$ effects from multiple scattering, etc. Fully reconstructed jets offer a new experimental handle on probing this physics. It has the advantage over single particle and di-hadron measurements since the underlying kinematics of the hard scattering process is better known. In spite of the seemingly small acceptance of the PHENIX detector at RHIC, measurements of fully reconstructed jets using the Anti-$k_T$ algorithm are possible. In this contribution we will present the current status of measurements using fully reconstructed jets in $d$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using the PHENIX detector at RHIC. We discuss the current and future physics that are provided with such measurements focusing on the centrality dependence of nuclear $k_T$ and jet $p_T$ cross-sections related to the nuclear parton distribution functions.   

Reduced helicity amplitudes for deuteron photodisintegration

We apply the reduced nuclear amplitude analysis to the helicity amplitudes of deuteron photodisintegration. This combines covariant, point-like amplitudes for the nucleons with the electric and magnetic form factors of the nucleons. The point-like amplitudes are modeled on the QCD one-gluon exchange amplitudes for $\gamma M\rightarrow q\bar q$, where $M$ is a $q\bar q$ meson. The form factors take into account the internal structure of the nucleons. We compare the resulting cross section and polarization observables to recent data.   

The Effect of Recent Pion Photoproduction Data on the SAID and MAID Analyses

Major experimental contributions to the SAID Pion Photoproduction data base from JLab, Mainz, Bonn, LNS, and Spring-8 have recently been publish or are about to be published. These data significantly constrain the SAID and MAID Partial-Wave Analyses. Yet there are still disagreements between the two analyses, especially in their predictions for double scattering channels. Three of these laboratories (JLab, Mainz, and Bonn) are in a position to perform ``complete'' experiments on complementary pion channels and in overlapping energy ranges. Latest Partial-Wave Analysis results will be compared to the latest published data and similarities and differences between SAID and MAID will be discussed.