Bulletin of the American Physical Society
2015 Fall Meeting of the APS Division of Nuclear Physics
Volume 60, Number 13
Wednesday–Saturday, October 28–31, 2015; Santa Fe, New Mexico
Session CH: Ultrarelativistic Heavy Ions I |
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Chair: Megan Connors, Yale University Room: General Kearny |
Thursday, October 29, 2015 8:30AM - 8:42AM |
CH.00001: Observation of the critical end point in the phase diagram for hot and dense nuclear matter Roy Lacey Excitation functions for the Gaussian emission source radii difference ($R^2_{\mathrm{out}} - R^2_{\mathrm{side}}$) obtained from two-pion interferometry measurements in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions, are studied for a broad range of collision centralities. The observed non-monotonic excitation functions validate the finite-size scaling patterns expected for the deconfinement phase transition and the critical end point (CEP), in the temperature vs. baryon chemical potential ($T,\mu_B$) plane of the nuclear matter phase diagram. A Dynamic Finite-Size Scaling (DFSS) analysis of these data suggests a second order phase transition with the estimates $T^{\mathrm{cep}} \sim 165$~MeV and $\mu_B^{\mathrm{cep}} \sim 95$~MeV for the location of the critical end point. The critical exponents ($\nu \approx 0.66$ and $\gamma \approx 1.2$) extracted via the same DFSS analysis, places this CEP in the 3D Ising model universality class. [Preview Abstract] |
Thursday, October 29, 2015 8:42AM - 8:54AM |
CH.00002: Low Momentum Direct Photons in $Au+Au$ collisions at $\sqrt{s} = 39$ GeV and $62.4$ GeV measured by the PHENIX Experiment at RHIC Vladimir Khachatryan Direct photons, which are produced during all stages of a heavy-ion collision, directly probe the conditions of their production environment. The large yield and large anisotropy of low momentum direct photons observed in 200 GeV $Au+Au$ collisions pose a significant challenge to theoretical models. Measurements at lower collision energy may provide new insight on the origin of the low momentum direct photons. Direct photons are difficult to measure with electromagnetic calorimeters, in particular at low momentum, because of neutral hadron and minimal-ionizing particle contamination, large decay photon backgrounds, and worsening calorimeter resolution. Therefore PHENIX has measured the direct photons at $\sqrt{s} = 200$ GeV via their external conversion to di-electron pairs. This method virtually eliminates the hadron contamination due to a very clean photon identification based on di-electron pair. The same method is also used in our current analysis of the direct photons at two lower energies. We will present the current status on the measurements of the low momentum direct photons at $\sqrt{s} = 39$ GeV and $62.4$ GeV. [Preview Abstract] |
Thursday, October 29, 2015 8:54AM - 9:06AM |
CH.00003: The iTPC upgrade for BES-II Flemming Videbaek STAR has proposed to upgrade the inner sectors of the STAR TPC to increase the segmentation on the inner padplane and to renew the inner sector wires. The upgrade will provide better momentum resolution, better \textit{dE/dx} resolution and, most importantly, it will provide improved acceptance at high rapidity to \textbar $\eta $\textbar $\le $ 1.5 compared to the current TPC configuration of \textbar $\eta $\textbar $\le $ 1 and to extend the pt coverage towards lower pt. The enhanced measurement capabilities of STAR after the iTPC upgrade are a vital part of the BES-II effort for 2019-2020. The expanded rapidity coverage provides a major benefit for many analyses, especially those sensitive to changes in correlation lengths near a critical point, like the net-proton Kurtosis which exhibits interesting energy trends that only appear near the edge of the current STAR acceptance. In the area of dielectron measurements it reduces hadron contamination from a dominant source of uncertainty to an expected statistical uncertainty of only 10{\%}, and will enable significantly improved understanding of in-medium modifications. In this talk I will discuss the physics impact and give a technical overview of the detector upgrade. [Preview Abstract] |
Thursday, October 29, 2015 9:06AM - 9:18AM |
CH.00004: Crossover Equation of State Compared to Lattice QCD and to Baryon Fluctuations in the RHIC Beam Energy Scan Joseph Kapusta, Michael Albright, Clint Young We match hadronic equations of state at low energy densities to a perturbatively computed equation of state of quarks and gluons at high energy densities. The hadronic equations of state include all known hadrons; repulsive interactions are taken into account via two versions of the excluded volume approximation. A switching function is employed to make the crossover transition from one phase to another without introducing a thermodynamic phase transition. A fit to accurate lattice calculations of the pressure and trace anomaly, with temperature $100 < T < 1000$ MeV and $\mu = 0$, determines the parameters. These parameters quantify the behavior of the QCD running gauge coupling and the hard core radius of the nucleon. With no new parameters, the pressure and trace anomaly from lattice calculations for $\mu = 400$ MeV are equally well reproduced, as is the speed of sound. We then compute the skewness and kurtosis and compare to measurements of the fluctuations of the proton number distribution in central Au-Au collisions as measured by the STAR collaboration in a beam energy scan at RHIC. The crossover equations of state can reproduce the data if the fluctuations are frozen at a temperature significantly lower than the average chemical freeze-out. [Preview Abstract] |
Thursday, October 29, 2015 9:18AM - 9:30AM |
CH.00005: STAR Au $+$ Au Fixed Target Results Kathryn Meehan The RHIC Beam Energy Scan (BES) program was proposed to look for the turn-off of signatures of the quark gluon plasma (QGP), search for a possible QCD critical point, and study the nature of the phase transition between hadronic and partonic matter. The results from the NA49 experiment at CERN have been used to claim that the onset of deconfinement occurs at a collision energy around a center-of-mass energy of 7 GeV, the low end of the BES range [1]. Data from lower energies are needed to test if this onset occurs. The goal of the STAR Fixed-Target Program is to extend the collision energy range in BES II with the same detector to energies that are likely below the onset of deconfinement. Currently, STAR has inserted a gold target into the beam pipe and conducted test runs at center-of-mass energies 3.9 and 4.5 GeV. Tests have been done with both Au and Al beams. First physics results from a Coulomb analysis of Au $+$ Au fixed-target collisions, which are found to be consistent with previous experiments, will be presented. These results demonstrate that STAR has good particle identification capabilities in this novel detector setup. Furthermore, the Coulomb potential, which is sensitive to the Z of the projectile and degree of baryonic stopping, will be compared with published results from the AGS. \\[4pt] [1] Xi (Omega) production in Pb$+$Pb Collisions at 158 GeV/c, G. Odyniec for the NA49 Collaboration, J. Phys. G, 23, 1827 (1997) [Preview Abstract] |
Thursday, October 29, 2015 9:30AM - 9:42AM |
CH.00006: A Tungsten Powder Epoxy Scintillating Fiber EMCAL for sPHENIX Vera Loggins The sPHENIX detector is a proposed new detector at the Relativistic Heavy Ion Collider (RHIC). The sPHENIX physics program focuses on jets and hard probes of the quark gluon plasma (QGP). The proposed design of the electromagnetic calorimeter (EMCAL), made of a tungsten powder and epoxy composite with embedded scintillating fibers, is designed to have a small Moliere radius and short radiation length, and will be located at a radius of about 90 cm from the interaction region. It will have an energy resolution $12\%/\sqrt{E}$ and will be used in conjunction with a new hadronic calorimeter (HCAL) to provide a jet energy resolution $\sigma_{E}/E=120\%/\sqrt{E}$ to resolve single photons and electrons, as well as photon jets, in the high multiplicity environment of central heavy ion collisions. The $\eta$ and $\phi$ segmentation of the EMCAL is 0.024 x 0.024. Preliminary tests of the calorimeter design have already taken place. In this talk, I will focus on the process of building these prototype modules and the preparation of the modules for the test beam at Fermilab in 2016. [Preview Abstract] |
Thursday, October 29, 2015 9:42AM - 9:54AM |
CH.00007: R\&D Studies of a Lead-Scintillating Fiber Calorimeter as a STAR Forward Detector Prashanth Shanmuganathan A forward upgrade of the STAR detector will achieve several physics goals. Examples are studying the internal structure of nucleons and nuclei through measurement of di-jets and Drell-Yan and improvements in the resolution of energy weighted event plane determination for study of more central and more peripheral events in heavy-ion collisions. The AGS E864 lead-scintillating fiber calorimeter cells ($(10\,cm)^2 \times 117\,cm$) were repurposed by pixelizing their readout into a three by three array of $(3.3\,cm)^2$ pixels. A prototype six by six array of these cells (324 pixels) was mounted on the west side of the STAR detector during Run14 and events from $^{3}$He+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV were recorded. The detector response was simulated by a GEANT model using HIJING particle production. Further tests of the pixelized cells were conducted at the Fermilab Test Beam Facility. In this talk, we will present the calorimeter response in $^3$He+Au collisions using reconstructed $\pi^0$ from clusters formed from energy deposition by $\pi^0$ decay gammas. Energy resolution and shower shapes from pixelization are also discussed using test beam data and simulations. [Preview Abstract] |
Thursday, October 29, 2015 9:54AM - 10:06AM |
CH.00008: Precision Charged Particle Tracking with sPHENIX Michael McCumber The PHENIX Collaboration is pursuing a series of aggressive upgrades aimed at excellent jet reconstruction capabilities to make use of the enhanced luminosity at the Relativistic Heavy Ion Collider, complement measurements being made at the Large Hadron Collider, and shed new light on the microscopic structure of the quark-gluon plasma. With a new detector, sPHENIX, offering large coverage electromagnetic and hadronic calorimetry and precision charged particle tracking, we will be well positioned to provide a broad and exciting program of jet probe and upsilon measurements. This talk will present the role that a precision charged particle tracking and vertex detector will play in the sPHENIX program. Details will be given on the tracking design and performance for reconstructing charged particles. The capabilities for bottom jet identification, upsilon reconstruction, and fragmentation function measurements in heavy ion collisions will be covered. [Preview Abstract] |
Thursday, October 29, 2015 10:06AM - 10:18AM |
CH.00009: Hadronic Calorimeter Development for a New Jet Detector at RHIC Edouard Kistenev Brookhaven National Laboratory is planning a new jet, large-acceptance jet detector at RHIC, optimized to make best use of the beams delivered by RHIC to study the properties of the Quark Gluon Plasma via a comprehensive program of jet probes, direct photon tagged jets, and heavy flavor. The calorimetry in this new jet detector is the most important single tool for electron, photon, and neutral pion measurements and drives overall detector dimensions and cost. We report here on a development of a new novel and compact calorimeter system which is comprised of a full acceptance electromagnetic calorimeter complemented by a coarse and relatively thin ($\sim$1 Labs) front section of hadronic calorimetry inside superconducting solenoid, and a deep ($\sim$4Labs) outer calorimeter which also serves as a flux return for the detector solenoid. Priority in this talk will be given to the hadronic sections and sensing elements (scintillating tiles), which penetrate the whole depth of iron and varying in size from $\sim$20cm in the inner to more then 100cm in outer sections. In-situ detector calibration and triggering will also be discussed. [Preview Abstract] |
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