Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session J4: Mini-symposium on Partonic Structure beyond Densities: Orbital Motion, Correlations, Fluctuations II |
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Sponsoring Units: DNP Chair: Leonard Gamberg, Penn State University Room: Holiday 3 |
Sunday, April 12, 2015 10:45AM - 11:21AM |
J4.00001: Accessing Fragmentation Functions in e+e- annihilation at Belle and Belle II Invited Speaker: Anselm Vossen Precision measurements of fragmentation functions are necessary for the extraction of most aspects of the nucleon structure from semi-inclusive deep inelastic and proton-proton scattering data. Examples are transverse polarization dependent fragmentation functions that are needed to access the transverse spin structure of the proton and the measurement of the intrinsic transverse momentum dependence of unpolarized fragmentation functions that is needed to access non-collinear aspects of the nucleon structure. Beyond providing crucial input for our studies of the partonic structure of the nucleon, fragmentation functions can also be used to study fundamental aspects of non-perturbative QCD, such as TMD evolution. The cleanest access to fragmentation functions can be gained using e+e- annihilation data. The Belle experiment at KEK collected more than 1 ab$^{-1}$ of data near the $\Upsilon(4S)$ resonance, several orders of magnitude more than was available before the start of the B-factories. This talk will highlight recent results of the Belle fragmentation function measurement program and will give an outlook to future possibilities with the Belle II detector. [Preview Abstract] |
Sunday, April 12, 2015 11:21AM - 11:33AM |
J4.00002: Azimuthal Single-Spin Asymmetries of Charged Pions in Jets in $p^{\uparrow}p$ Collisions at STAR Kevin Adkins The transversity distribution ($h_1\left(x\right)$), which describes the transverse spin structure of quarks inside of transversely polarized protons, is only accessible through channels that couple $h_1\left(x\right)$ to another chiral odd distribution, such as the Collins fragmentation function ($\Delta D\left(z,k_T\right)$). Significant Collins asymmetries of charged pions have been observed in semi-inclusive deep inelastic scattering (SIDIS) data. These SIDIS asymmetries combined with $e^+e^-$ process asymmetries from Belle have allowed for the extraction of $h_1\left(x\right)$ and $\Delta D\left(z,k_T\right)$. Uncertainties on $h_1\left(x\right)$ remain large due to the limited statistics and kinematic reach of the available data. In transversely polarized hadronic collisions, Collins asymmetries may be isolated and extracted by measuring the spin dependent azimuthal distributions of charged pions in jets. This presentation will show the first significant midrapditiy ($\vert\eta\vert < 1$) Collins asymmetries measured in $\sqrt{s}=200$ and $500$ GeV $p^\uparrow p$ collisions. These results access higher momentum scales than the existing SIDIS data and will allow for a comprehensive study of evolution and factorization of the Collins channel. [Preview Abstract] |
Sunday, April 12, 2015 11:33AM - 11:45AM |
J4.00003: Measurement of the Transverse Single-Spin Asymmetries for $\pi^{0}$ and Electromagnetic Jets at forward rapidities at STAR 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. There can also be a spin-dependent hadronization process which can create transverse asymmetries in the twist-3 fragmentation formalism. 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, a Pb-glass electromagnetic calorimeter covering the pseudo-rapidity ($\eta$) range 2.6-4.2 and full azimuth, can detect photons, neutral pions and $\eta$ mesons. For the current run in the year 2015, installation of the Forward Pre-shower Detector opens a new capability of identifying direct photon events. We are measuring $A_N$ for $\pi^{0}$ and electromagnetic(EM) jets reconstructed from photons in the FMS in $p+p$ collisions at $\sqrt{s}$ = 500 GeV that were recorded during the 2011 RHIC run. We report, here, $A_N$ for forward EM-jets and its sensitivity to the event topology of having a coincedence EM-jet at midrapidity. [Preview Abstract] |
Sunday, April 12, 2015 11:45AM - 11:57AM |
J4.00004: Effects of many-body correlations on the Bjorken-x dependence of DIS cross section ratios off nuclei Athanasios Petridis, Allen Barr, Drew Fustin Many-body correlations in nuclei determine the behavior of Deep-Inelastic-Scattering (DIS) cross section ratios off heavy over light nuclei especially for Bjorken-x $>$ 1, obtained at Jefferson Lab. They can be described in terms of quark-cluster formation in nuclei due to wave-function overlapping, manifesting itself when the momentum transfer is high so that the partonic degrees of freedom are resolved. In clusters (correlated nucleons) the quark and gluon momentum distributions are softer than in single nucleons and extend to x $>$ 1. The cluster formation probabilities are computed using a network-defining algorithm in which the initial nucleon density is either standard Woods-Saxon or is input from lower energy data while the critical radius for nucleon merging is an adjustable parameter. The exact choice of critical radius depends on the specific nucleus and it is anti-correlated to the rescaling of the Bjorken-x needed for bound nucleons. The calculations show that there is a strong dependence of the cross section ratios on the Bjorken-x in agreement with the data and that four-body correlations are needed to explain the experimental results even in the range 2 $<$ Bjorken-x $<$ 3. The dependence on the specific exponents of parton distributions in high-order clusters is weak. [Preview Abstract] |
Sunday, April 12, 2015 11:57AM - 12:09PM |
J4.00005: Quantum fluctuations of parton densities in diffractive ep scattering Christian Weiss The nucleon in QCD is a relativistic many-body system, in which configurations with different number of particles (partons) and spatial size exist in a coherent quantum-mechanical superposition. Inclusive deep-inelastic scattering (DIS) measures the quantum average of the parton density at a given light-cone momentum fraction x. The quantum fluctuations of the parton densities can be revealed in diffractive DIS at small x [1]. The fluctuation width of the gluon density is extracted from the ratio of inelastic and elastic diffractive cross sections measured at HERA and related to models of nucleon structure. Detailed studies of quantum fluctuations of gluons in QCD could be performed in next-generation experiments with a future Electron-Ion Collider (EIC).Such fluctuations also affect rapidity gap survival and multiparton interactions in high-energy proton-proton scattering at LHC. \\[4pt] [1] L. Frankfurt, M. Strikman, D. Treleani, C. Weiss, PRL 101 (2008) 202003 [Preview Abstract] |
Sunday, April 12, 2015 12:09PM - 12:21PM |
J4.00006: Intrinsic transverse momentum and QCD vacuum structure P. Schweitzer, M. Strikman, C. Weiss Recent theoretical work has shown the existence of non-perturbative short-range correlations between partons caused by the dynamical breaking of chiral symmetry in QCD [1]. The light sea quarks in the nucleon wave function exist in correlated pairs with transverse size $\ll$ 1 fm, much smaller than the nucleon's valence quark size. This effect broadens the transverse momentum-dependent (TMD) distribution of sea quarks and defines the natural boundary of perturbative QCD evolution. The non-perturbative parton correlations can be studied experimentally in measurements of hadron correlations in semi-inclusive DIS (COMPASS, EIC), and in exclusive meson production processes (JLab 12 GeV). They also influence the rate of multiparton processes in high-energy nucleon-nucleon collisions (LHC). \\[4pt] [1] P.~Schweitzer, M.~Strikman, C.~Weiss, JHEP 1301 (2013) 163. [Preview Abstract] |
Sunday, April 12, 2015 12:21PM - 12:33PM |
J4.00007: Partons within nuclear short range correlations Adam Freese, Misak Sargsian, Mark Strikman Partonic distributions of heavy nuclei differ from partonic distributions of nucleons in several key ways. In addition to Fermi smearing of the parton distributions from nucleonic motion under the mean field, the nuclear momentum distribution has a large high-momentum tail owing to the presence of short range multi-nucleon correlations, which results in enhanced parton densities at Bjorken $x > 1$. Moreover, the quark-gluon degrees of freedom of nucleons are themselves modified by immersion in the nuclear medium, especially at the large momenta dominated by short range correlations. Accordingly, $x > 1$ kinematics are well-primed for the study of the partonic degrees of freedom of short range nuclear correlations. We study here the sensitivity to two- and three-nucleon correlations of deeply inelastic lepton scattering from heavy ions at EIC kinematics, and of dijet production from proton-nucleus collisions at LHC kinematics. [Preview Abstract] |
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