Bulletin of the American Physical Society
APS April Meeting 2014
Volume 59, Number 5
Saturday–Tuesday, April 5–8, 2014; Savannah, Georgia
Session H14: Spin Structure |
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Sponsoring Units: GHP Chair: Harut Avakian, Thomas Jefferson National Accelerator Facility Room: 102 |
Sunday, April 6, 2014 8:30AM - 8:42AM |
H14.00001: Sivers Function in the Quasi-Classical Approximation Matthew Sievert, Yuri Kovchegov We study the origin of the Sivers function in the quasi-classical limit (McLerran-Venugopalan model), applicable when the density of color charges is large. The classical limit can be achieved by a heavy nucleus, which already possesses a large number of color charges in its rest frame, or by boosting any hadron to sufficiently high energy that gluon bremsstrahlung drives up the charge density. The large charge density in the classical limit allows us to resum multiple rescatterings and permits a mean-field description, such as a hadron made up of a large number of independent low-$x$ partons. This allows us to decompose the TMD's of the hadron in terms of the TMD's of its partons, the Wigner distributions of the partons within the hadron, and Wilson lines due to multiple rescattering. We find that the Sivers function of the hadron receives one contribution that simply aggregates the Sivers functions of the partons, and another due to the combination of orbital angular momentum and screening due to multiple rescattering. This channel is fundamentally different from the known ``lensing mechanism,'' and it provides a simple interpretation for the process dependence of the Sivers function. This method can be readily extended to study other TMD's and to include quantum evolution. [Preview Abstract] |
Sunday, April 6, 2014 8:42AM - 8:54AM |
H14.00002: Transverse Single-Spin Asymmetries for Jet-like Events at Forward Rapidities in $p+p$ Collisions at $\sqrt{s}$ = 500 GeV with the STAR Experiment Mriganka Mouli Mondal Large transverse single-spin asymmetries ($A_N$) have been observed for forward inclusive hadron production in $p+p$ collisions at various experiments. In the collinear perturbative scattering picture, twist-3 multi-parton correlations can give rise to such an asymmetry. A transversely polarized quark can also give rise to a spin-dependent distribution of its hadron fragments via the Collins mechanism. The observed $A_N$ may involve contributions from both processes. These can be disentangled by studying asymmetries for jets, direct photons and jet-fragments. The STAR Forward Meson Spectrometer (FMS), a Pb-glass electromagnetic calorimeter covering the pseudo-rapidity range 2.6-4.2 and full azimuth, can detect photons, neutral pions and eta mesons. We are measuring $A_N$ for jet-like events reconstructed from photons in the FMS in $p+p$ collisions at $\sqrt{s}$ = 500 GeV that were recorded during the 2011 RHIC run. We study $A_N$ as a function of the number of observed photons, thereby exploring asymmetries for a range of event classes. The current status of the analysis will be discussed. [Preview Abstract] |
Sunday, April 6, 2014 8:54AM - 9:06AM |
H14.00003: Measurement of transverse spin dependent fragmentation of $\pi^0$/$\eta$ mesons in $e^+e^-$ Annihilation at Belle Hairong Li Large transverse single spin asymmetries $A_N$ have been observed in polarized proton-proton collisions over a wide range of energies. The mechanism behind this effect is still not understood. One possible contribution is the so-called Collins effect, which describes the polarization dependent fragmentation of transversely polarized quarks. In addition on shedding light on the mechanism behind $A_N$ in polarized p+p collisions a precise knowledge of the spin dependent fragmentation function is also needed for the extraction of the so-called transversity parton distribution function (PDF), one of the three leading twist PDFs that is needed to describe the proton in a collinear picture. Recently, the Collins effect has been measured for charged pions in e+e- annihilation at the Belle and BaBar experiments. This talk will focus on the measurement of the Collins effect for the neutral $\pi^0$ and $\eta$ mesons in e+e- annihilation near the Y(4S) resonance at the Belle experiment. This channel is of interest to study the flavor dependence of the Collins effect and to investigate the mechanism behind the observed difference of $A_N$ for $\eta$ and $\pi^0$ mesons. [Preview Abstract] |
Sunday, April 6, 2014 9:06AM - 9:18AM |
H14.00004: Extracting $W$ Single Spin Asymmetry in Longitudinally Polarized $pp$ Collisions at PHENIX forward arms Abraham Meles The parity-violating asymmetry $A_L$ in the production of $W$ bosons in $p+p$ collisions at $\sqrt{s}$ = 510 GeV is sensitive to the polarization of light quarks and anti-quarks in the proton. However, identifying the lepton from the decay of the $W$ is challenging due to a great background of hadronic processes. In the forward and backward hemispheres of PHENIX at RHIC, the muon spectrometers have been recently upgraded in order to provide additional trigger and tracking information to suppress those backgrounds. One of those upgrades is the Forward Vertex (FVTX) detector, a silicon-strip tracker. In 2013, PHENIX collected approximately 240 pb$^{-1}$ of polarized $p+p$ collisions at $\sqrt{s}$ = 510 GeV with a beam polarization of 52$\%$. The ability of the FVTX to improve the $W$ signal will be reviewed, and progress on analysis of real data in the RHIC 2013 run will be discussed. [Preview Abstract] |
Sunday, April 6, 2014 9:18AM - 9:30AM |
H14.00005: Neutron structure via forward tagging of the $eD\to e^{\prime}NX$ reaction at the Electron-Ion Collider Kijun Park I report about the status of a Jefferson Lab 2014 LDRD project exploring the physics potential of deep-inelastic scattering on polarized light ions with forward spectator nucleon tagging with the proposed Electron-Ion Collider (EIC). Such measurements offer unique capabilities for precision studies of neutron spin structure, nuclear modifications of partonic structure, and multiple scattering effects at high energies. We simulate the tagged processes on the $D$ as a function of the $Q^2$, $x_{\rm Bj}$, and the spectator momenta. We quantify the effects of the intrinsic motion of beam particles, as well as the EIC detector acceptance and resolution, on the projected observables. We present results of a model-independent extraction of the free neutron structure function $F_2^n$ through on-shell extrapolation in the spectator momentum~[MSargsian]. Comparison of the bound proton structure function $F_2^p$ with the free proton result provides a crucial test of the method and allows for an unambiguous identification of nuclear binding effects. Future work will extend these studies to polarized deuterons (spin structure functions $g_1^n$ and $g_1^p$) and hard exclusive processes (GPDs), and model the final-state detection through a full GEANT4-based simulation. [Preview Abstract] |
Sunday, April 6, 2014 9:30AM - 9:42AM |
H14.00006: Deeply virtual Compton scattering on longitudinally polarized protons at CLAS Angela Biselli The Generalized Parton Distributions (GPDs) have emerged as a universal tool to describe hadrons in terms of their elementary constituents, the quarks and the gluons. Deeply Virtual Compton Scattering (DVCS) ($e p \rightarrow e' p' \gamma$) is one of the simplest processes that can be described in terms of GPDs. The DVCS-Bethe-Heitler (BH) interference gives rise to spin asymmetries, which can be connected to combinations of Compton Form Factors (CFFs), which are integrals of GPDs. The longitudinal target single-spin asymmetry (SSA) is directly proportional to the imaginary part of the DVCS amplitude, and gives access to a combination of the CFFs $Im(\tilde {\mathcal{H}})$ and $Im(\mathcal{H})$, whereas the double-spin asymmetry (DSA) is proportional to a combination of the CFFs of $Re(\tilde{\mathcal{H}})$ and $Re(\mathcal{H})$. These asymmetries were measured in a dedicated experiment at Jefferson Lab using the CEBAF 6-GeV polarized-electron beam, a longitudinally polarized solid-state ${}^{14}{\rm NH}_3$ target, and the CEBAF Large Acceptance Spectrometer, together with the Inner Calorimeter. DVCS/BH events were selected over the following kinematic ranges: 1 $< Q^{2} <$ 4.5 GeV${}^{2}$, 0.1 $< x_{B} <$ 0.58, 0.08 $< -t <$ 1.8 GeV${}^{2}$ and, the target-SSA and DSA were [Preview Abstract] |
Sunday, April 6, 2014 9:42AM - 9:54AM |
H14.00007: Quark Angular Momentum Distribution in the Transverse Plane Lekha Adhikari, Matthias Burkardt Several possibilities to relate the t-dependence of generalized parton distributions (GPDs) to the distribution of angular momentum in the transverse plane are discussed. None of them turns out to correctly describe the orbital angular momentum distribution that one would identify for a longitudinally polarized target. [Preview Abstract] |
Sunday, April 6, 2014 9:54AM - 10:06AM |
H14.00008: The D-term of Exploding Q-Balls Michael Cantara, Peter Schweitzer Form factors of the energy momentum tensor contain essential information on the considered particle such as mass, spin, and a property called D-term. Although it is experimentally unknown, the D-term is a particle property as fundamental as mass, spin, electric charge or magnetic moment. Only very recently it became clear how the D-term of the nucleon can in principle be studied in hard exclusive reactions like deeply virtual Compton scattering or hard meson production. But present data do not yet allow an unambiguous determination of the D-term of nucleons or nuclei. Meanwhile the only source of information are theoretical studies. Interestingly, in all calculations (in models, effective theories) the D-terms of various particles (pions, nucleons, nuclei, photons) were always found to be negative. The deeper reason for this was recently elucidated in a study of Q-balls in scalar field theories with U(1) symmetry: stable Q-balls must have a negative D-term. However, also meta-stable and even unstable Q-balls have negative D-terms. The emerging question is whether one can ever encounter a positive D-term in a physical situation. We show that this can indeed happen in the Q-ball system in the limit when unstable Q-balls dissociate into a Q-cloud, i.e. free and unbound Q-quanta. [Preview Abstract] |
Sunday, April 6, 2014 10:06AM - 10:18AM |
H14.00009: $W^{\pm}$ production measurement at mid-rapidity in 510 GeV polarized $p+p$ collisions at PHENIX Nerangika Bandara, David Kawall Measurement of parity violating longitudinal single spin asymmetries of $W$ production is a complementary approach, free from fragmentation uncertainties compared to Semi-inclusive Deep Inelastic Scattering measurements, probing the flavor-separated polarized sea quark distributions in the proton. At mid-rapidity range of $|\eta|<0.35$, candidate $W^{\pm}/Z$ events are identified through their $e^{\pm}$ decay channels. In 2013, PHENIX at the Relativistic Heavy Ion Collider recorded data with an integrated luminosity of $\sim$146 pb$^{-1}$ in longitudinally polarized $p+p$ collisions at $\sqrt{s} = 510$ GeV, approximately three times the statistics from 2009, 2011 and 2012 combined and average beam polarization of 52$\%$. The analysis status on the single spin asymmetry of the run 2013 will be presented. [Preview Abstract] |
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