Bulletin of the American Physical Society
APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017; Washington, DC
Session M13: Ultra-relativistic Heavy-Ions |
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Sponsoring Units: DNP Chair: Christine Aidala, University of Michigan Room: Roosevelt 5 |
Sunday, January 29, 2017 3:30PM - 3:42PM |
M13.00001: Centrality and Transverse Momentum Dependence of HBT Radii in Au+Au Collisions Benjamin Schweid The expansion dynamics of relativistic heavy ion collisions is influenced by the transport properties of the created medium, as well as the path of the reaction trajectory in the ($T, \mu_B$)-plane. Such an influence can manifest as quantifiable changes in the magnitude of the space-time extent of the emission source, characterized by the so-called HBT radii $R_{out}$, $R_{side}$ and $R_{long}$. We will present and discuss recent HBT measurements which extend the upper momentum range of measurements that have been made in the STAR detector for Au+Au collisions at several collision centralities and beam energies. [Preview Abstract] |
Sunday, January 29, 2017 3:42PM - 3:54PM |
M13.00002: Forward calorimetry for heavy-ion physics at the STAR experiment Daniel Brown A forward calorimeter utilizing hadronic and electromagnetic calorimetry at the STAR experiment of RHIC will achieve a variety of physics goals. These goals include studying long-range rapidity correlations, event plane correlations in heavy-ion interactions, and studying the gluon contribution to the proton spin. Upgrades to the AGS E864 lead-scintillating fiber calorimeter have increased spatial resolution by utilizing cell pixelization. Light collection has been optimized and fringe field effects have been minimized by the introduction of Fresnel lenses and mu-metal shielding. A prototype consisting of a 2x3 cell stack was installed into the forward region of STAR for the end of run16. This prototype investigated the introduction of these new techniques as well as a trial of Silicon Photomultipliers (SiPMs) as an alternate to traditional Photomultiplier Tubes (PMTs). SiPMs do not suffer from fringe field effects, but are susceptible to radiation damage by neutrons, so their performance during the prototype operation was analyzed. This talk will discuss the effects of Fresnel lenses on light collection, mu-metal shielding effects on PMTs, and radiation effects on SiPMs. [Preview Abstract] |
Sunday, January 29, 2017 3:54PM - 4:06PM |
M13.00003: System Size and Shape Dependence of the Viscous Damping of Anisotropic Flow Niseem Abdelrahman We present recent STAR measurements of the anisotropic flow coefficients $v_n$ ($n = 1-6$) in Au+Au, Cu+Cu, Cu+Au collisions at $\sqrt{s_{NN}}=200$~GeV and U+U collisions at $\sqrt{s_{NN}}=193$~GeV. For a given system, the differential $v_n$ measurements indicate acoustic scaling patterns which reflect the detailed dependence of $v_n$ on collision-system size and eccentricity ($\varepsilon_n$). These measurements constrain the viscous coefficient which encodes the specific shear viscosity $\eta/s$. Our measurements show that all the collision-systems give the same viscous coefficient after scaling-out the collision-system size and eccentricity. [Preview Abstract] |
Sunday, January 29, 2017 4:06PM - 4:18PM |
M13.00004: An Improved Event Plane Detector for the STAR Experiment Justin Ewigleben The BES program at RHIC has shown hints of a critical point and first order phase transition at the BES energies. Key measurements for locating the critical point and determining the first order phase transition are limited by poor event plane resolution, limited statistics and a TPC-only centrality determination. Therefore, phase II of the BES program was proposed to take data with upgraded detectors and increased statistics for the further investigation. A new event plane and collision centrality detector is planned to replace the existing detector, the BBC, with higher granularity and acceptance. The design of the EPD consists of two scintillator discs at z= $\pm$ 3.75m from the center of STAR, covering 2.2 $< \eta <$ 5.1, the same as the BBC. The detector will be read out by silicon photomultipliers - an inexpensive and magnetic field insensitive replacement for the traditional phototube. A prototype of the detector, consisting of a single sector was integrated into STAR during the 2016 run, which will be shown. The optimized segmentation, size and shape of the final design was decided in order to maximize event plane resolution, , centrality estimation and flow harmonic measurements. We will discuss the plans to install one quarter of a disc into STAR for the 2017 run. [Preview Abstract] |
Sunday, January 29, 2017 4:18PM - 4:30PM |
M13.00005: Background subtraction from $X_{J\gamma}$ and photon+jets in PYTHIA simulated p+p collisions at $\sqrt{S_{nn}}$ =200 GeV Elias Mansbach Heavy-ion collisions create a new form of matter called the Quark-Gluon Plasma (QGP), in which quarks and gluons (partons) are not confined into nucleons and form a nearly ideal fluid. The QGP is a densely colored medium and understanding its features is one of the key subjects in the study of Quantum Chromodynamics. Sometimes in these collisions, partons will "hard scatter," fragment, and hadronize into jets, which are a columnated spray of particles. Photon+jet pairs have been lauded as "golden probes" of the QGP as photons do not have a QCD color charge and therefore do not interact with the QGP. Thus, photons can give us a good idea of the in-medium energy loss by partons traversing the QGP that fragment into jets. This talk analyzes photon+jet data taken from PYTHIA simulations of p+p collisions at $\sqrt{S_{nn}}$ =200 GeV and discusses $X_{J\gamma}$ data, where $X_{J\gamma}$ is the ratio of the jet's and photon's transverse momenta. A detailed look at the usefulness of unfolding and FastJet's background subtraction to reduce the effects of an azimuthally isotropic background modeled with a Boltzmann distribution on an embedded PYTHIA event is presented for data at varying jet radii and momenta transferred. [Preview Abstract] |
(Author Not Attending)
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M13.00006: Azimuthally differential pion femtoscopy relative to the second and third harmonic event planes in Pb-Pb 2.76 TeV collisions from ALICE Mohammad Saleh Azimuthally differential femtoscopic measurements, being sensitive to spatio-temporal characteristics of the source as well as to the collective velocity fields at freeze-out, provide very important information on the nature and dynamics of the system evolution. While the HBT radii modulations with respect to the second harmonic event plane reflect mostly the spatial geometry of the source, the third harmonic results are mostly defined by the system dynamics. In this talk, we present azimuthally differential pion femtoscopy with respect to second and third harmonic event planes as a function of the pion transverse momentum for different collision centralities. Our results on the dependence of the side-, out-, and long-radii on the pion emission angle with respect to the second harmonic event plane qualitatively agree with theoretical calculations, but the details show significant deviations. The final-state source eccentricity is found to be significantly smaller than the initial state source eccentricity. While the final-state source eccentricity for the second harmonic event plane remains positive in all centralities, the third harmonic event plane eccentricity becomes negative. All these results are compared to existing models. [Preview Abstract] |
Sunday, January 29, 2017 4:42PM - 4:54PM |
M13.00007: Chiral transport effects in charged anisotropic plasma within a magnetic field at strong coupling Matthias Kaminski, Martin Ammon, Roshan Koirala, Julian Leiber, Jackson Wu We compute the quasi-normal mode frequencies of gauge field and metric perturbations around black branes which are electrically and magnetically charged. By use of the gauge/gravity correspondence, these fluctuations are dual to conserved current operators of a particular class of strongly coupled field theories with a chiral anomaly. Within such a theory, we consider a thermal charged plasma state subjected to an external magnetic field. Quasi-normal mode frequencies are dual to the poles in the two-point functions of these conserved currents, encoding information about transport and dissipation in the plasma. For comparison, we also compute the same two-point functions in the hydrodynamic limit with field-theoretic methods. Together, these two approaches reveal various effects of the magnetic field and chiral transport coefficients on the location of the hydrodynamic poles (e.g. analogous to the chiral magnetic and chiral vortical effects), as well as transport effects beyond the hydrodynamic approximation. We conjecture qualitative conclusions for heavy-ion collision experiments. [Preview Abstract] |
Sunday, January 29, 2017 4:54PM - 5:06PM |
M13.00008: Constituent quarks and systematic errors in mid-rapidity charged multiplicity ($dN_{\rm ch}/d\eta$ distributions Michael Tannenbaum Although it was demonstrated more than 13 years ago that the increase in midrapidity $dN_{\rm ch}/d\eta$ with increasing centrality of Au+Au collisions at RHIC was linearly proportional to the number of constituent quark participants (or ``wounded quarks'', $Q_W$) in the collision, it was only in the last few years that generating the spatial positions of the three quarks in a nucleon according to the Fourier transform of the measured electric charge form factor of the proton could be used to connect $dN_{\rm ch}/d\eta/Q_W$ as a function of centrality in p(d)$+$A and A$+$A collisions with the same value of $dN_{\rm ch}/d\eta/Q_W$ determined in p$+$p collisions. One calculation, which only compared its calculated $dN_{\rm ch}/d\eta/Q_W$ in p$+$p at $\sqrt{s_{NN}}=200$ GeV to the least central of 12 centrality bin measurements in Au$+$Au by PHENIX, claimed that the p$+$p value was higher by ``about 30\%'' from the band of measurements vs. centrality. However the clearly quoted systematic errors were ignored for which a 1 standard deviation systematic shift would move all the 12 Au$+$Au data points to within 1.3 standard deviations of the p$+$p value, or if the statistical and systematic errors are added in quadrature a difference of 35 $\pm$ 21\% . [Preview Abstract] |
Sunday, January 29, 2017 5:06PM - 5:18PM |
M13.00009: Constraining the Polarized Gluon Distribution Through Di-jet Measurements at $\sqrt{s} = 510$ GeV at STAR Daniel Olvitt, Jr The production of jets from polarized p+p collisions at STAR is dominated by quark-gluon and gluon-gluon scattering. The di-jet double spin asymmetry ($A_{LL}$) is sensitive to the polarized gluon distribution ($\Delta G$). Di-jets are also advantageous because the parton momentum fraction, x, of initial partons may be reconstructed to first order from the final state measurements. Both jet and di-jet $A_{LL}$ measurements at $\sqrt{s}$ = 200 GeV have helped to constrain $\Delta G$ to values of x $\approx$ 0.05. In 2012, data were collected at $\sqrt{s}$ = 510 GeV in order to probe lower values of x. Jet and di-jet preliminary $A_{LL}$ results have been released and will soon be incorporated into theoretical fits. In 2013, data were also collected at $\sqrt{s}$ = 510 GeV. An update on the di-jet $A_{LL}$ measurement using polarized p+p data collected at STAR during 2013, with an estimated 250 pb$^{-1}$ of integrated luminosity will be presented. [Preview Abstract] |
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