ABSTRACT a Measurement of Charged Particle
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ABSTRACT A Measurement of Charged Particle Ratios at High Transverse Momentum in an Ultra-Relativistic Heavy Ion Collision Matthew Allen Horsley Yale University December 2002 Ultra-relativistic heavy ion collisions taking place at RHIC are thought to create conditions favorable for the creation of a quark gluon plasma (QGP). It is the main goal of the Relativistic Heavy Ion Collider (RHIC) to create and provide a definitive characterization of the quark-gluon plasma believed to be created in high energy heavy ion collisions. A determination of the initial conditions leading to the formation of a QGP is an important part of understanding its properties. Information about the evolution of the system formed during a heavy ion collision can be obtained by investigating charged particle ratios. The charged pion and kaon particle ratios as well as the anti-proton-to-proton ratio have been measured at high transverse momentum using a RICH detector. Comparisons have been made to previous measurements made with smaller collision systems and are found to be consistent with expectations derived from these smaller systems. The transverse momentum dependence of the charged particle ratios is consistent with being constant over the range measured, 0.75 <p⊥[GeV/c] < 2.5 and can be described within a thermodynamical model of the collision and is an indication that chemical equilibrium was achieved over the course of the collision. A Measurement of Charged Particle Ratios at High Transverse Momentum in an Ultra-Relativistic Heavy Ion Collision A Dissertation Presented to the Faculty of the Graduate School of Yale University in Candidacy for the Degree of Doctor of Philosophy By Matthew Allen Horsley Dissertation Directors: John Harris, Gerd Kunde December 2002 c Copyright 2003 by Matthew Allen Horsley All Rights Reserved Acknowledgements I would like to thank my wife, Gabel, for her support and encouragement over the course of my research, for without her this work would not have been possible. Like- wise I owe a debt of gratitude to my family, as their support has been constant and unflagging for the time spent here at Yale. My advisor John Harris and co-advisor Gerd Kunde have played principle roles in my growth and development during my research at Yale and I would like to express my deep felt gratitude to both of them. A number of people have contributed to the RICH detector and without their efforts none of this would have been possible. Along with Gerd, I would like to recognize and thank Nikolai Smirnov, Brian Lasiuk and Jamie Dunlop. Lastly, I would like to thank Betty, Manuel, Helen, Jon, Christina, Mike, and Sevil. The time spent with them here at Yale was great! iii Contents Acknowledgements iii 1 Introduction 1 2 Experimental Facilities 6 2.1RHIC................................... 7 2.2ZDC.................................... 9 2.3STAR................................... 10 2.3.1 STARMagnet........................... 12 2.3.2 TimeProjectionChamber.................... 14 2.3.3 CentralTriggerBarrel...................... 23 2.4RICH................................... 25 2.4.1 OperatingPrinciples....................... 25 2.4.2 Detector Placement, Design and Construction . 27 3 Event Reconstruction 41 3.1TPCEventReconstruction........................ 41 3.1.1 TPCSpacePointReconstruction................ 42 3.1.2 tptTrackModel......................... 43 3.1.3 GlobalTrackModel....................... 46 3.1.4 EventVertexDetermination .................. 47 3.1.5 PrimaryTrackModel....................... 48 3.2RICHEventReconstruction....................... 50 3.2.1 Cluster Finding and Position Resolution . 50 iv 4 RICH Data Analysis 54 4.1EventSelection.............................. 55 4.1.1 CentralEvents.......................... 56 4.1.2 Vertex Acceptance . 57 4.2PrimaryTrackMatchingwithRICH.................. 58 4.2.1 ProximityMatchingofRICHClusters............. 59 4.2.2 RICHDetectorAlignment.................... 60 4.2.3 RICHResiduals.......................... 61 4.3 Description of Analytical Solution for Pattern Recognition . 70 4.3.1 Cherenkov Fiducial Area Determination . 71 4.3.2 Cherenkov Fiducial Area Properties . 77 4.4PatternRecognition........................... 83 4.4.1 Parameters used in Pattern Recognition . 84 4.4.2 Characterization of Charge Clusters using Pattern Recognition 87 4.4.3 PhotonMultiplicities....................... 95 4.5ParticleRatio’s.............................. 98 4.5.1 FittingProcedure......................... 99 5 Results and Discussion 102 5.1Overview.................................. 102 5.2ChargedParticleRatios......................... 103 π− 5.2.1 π+ vs p⊥ .............................. 104 K− 5.2.2 K+ vs p⊥ ............................. 107 p¯ 5.2.3 p vs p⊥ .............................. 110 5.3 Nucleon-Nucleon and Nucleus-Nucleus Comparisons . 114 5.3.1 TransverseMomentumDependence............... 120 5.4ComparisonwithModels......................... 128 5.4.1 MonteCarloModels....................... 129 5.4.2 StatisticalModels......................... 130 5.4.3 P⊥ Dependence of Charged Particle Ratios: Model Predictions 132 6 Conclusions 142 v Bibliography 152 vi List of Figures 1.1 QCD Phase diagram. Boundary separating hadronic matter from de- confined matter is indicated by curved line in diagram. 2 2.1 View of the RHIC collider complex. Facilities used in the formation and transportation of ions to the RHIC collider are shown as well. 6 2.2 Tandem Van de Graaff at Brookhaven National Laboratory. 8 2.3 Plan view of the collision region and (section A-A) ”beam’s eye” view of the ZDC location indicating deflection of protons and charged frag- ments (with Z/A ∼ 1) downstream of the ’DX’ Dipole magnet. 9 2.4 Cut-away view of the STAR detector. RICH detector subsystem not shown in this view (see Figure 2.19 for more detail). 11 2.5STARMagnet............................... 12 2.6STARMagnetPoleTip.......................... 12 2.7STARTPC................................. 15 2.8Outerfieldcagestructure......................... 17 2.9InnerfieldcageStructure......................... 17 2.10 Aluminum gas containment vessel cross section. Dimensions in mm. 17 2.11 Electron drift velocity measured in East TPC for a period of 1 month. 19 2.12TPCsectorlayout............................. 20 2.13 Detail of a single super-sector showing inner and outer sectors. 21 2.14 Depiction of a charged particle travelling through the TPC, leaving in its wake a trail of ionized gas molecules. The drift direction for the secondary electrons is shown, along with a close-up of the MWPC used todetectthem............................... 22 i ii 2.15 Central Trigger Barrel. The middle left figure shows the individual trays arranged in a cylinder. The top figure shows close-up of a single tray housing a pair of plastic scintillating slats. Middle right figure shows the cross section of a single tray seen from the end, while the lowerfigureshowsthecrosssectionfromtheside............ 24 2.16Cherenkovlightcone............................ 26 2.17 Projection of a Cherenkov light cone onto an imaging plane. 26 2.18 Installation of the RICH detector into the STAR experimental set-up. The detector is shown housed in the aluminum safety box. A large frame is seen attached to the detector for the purposes of installation andisnotpresentduringdatataking................... 27 2.19 Placement of RICH detector relative to the STAR TPC (STAR mag- net not shown in this figure). Charged particle tracking for tracks intersecting RICH is accomplished using TPC sectors 5 and 19. 28 2.20 Schematic diagram of the RICH detector showing the main components oftheRICHdesign. ........................... 29 2.21 C6F14 index of refraction. Figure obtained from ref. [1]. 31 2.22 C6F14 transmission.Figureobtainedfromref.[1]............ 31 2.23Quartzindexofrefraction......................... 32 2.24Quartztransmission.[1]......................... 32 2.25 Angles made at refractive boundaries by the Cherenkov light cone emit- tedbyachargedparticleofnormalincidence.............. 33 2.26 CsI quantum efficiency as a function of wavelength. Error bars cor- respond to an uncertainty of ±2% See Ref. [2] and references cited therein for details on determination of CsI quantum efficiency. 34 2.27 Closeup of a RICH pad segment showing dimensions of pad along with thelocationoftheanodewiresabovethepad.[3]........... 35 2.28 Exponential pulse height distribution for single electrons (class 1) de- tected with MWPC. Chamber gain can be calculated from the inverse slope obtained from fit (∼ 40)....................... 36 iii 2.29 Schematics of the functional blocks composing the GASSIPLEX chip used in RICH detector electronics. Picture taken from reference [4]. 38 2.30 Typical distribution of pixel ADC values taken during a pedestal run. 39 2.31 Typical RMS distribution of the pixel ADC values taken during a pedestalrun................................ 39 3.1 Example of 2 clusters found by TPC cluster finder. The cluster on the right has multiple hits, reconstructed using a simple deconvolution algorithm (see Figure 3.2). Figure obtained from ref. [5]. 42 3.2 TPC cluster RMS, plotted padrow vs time bucket. Black points repre- sent clusters having a single hit, red points are clusters having multiple hits.Figureobtainedfromref.[5].................... 43 3.3STARTPCtptflowchart........................ 44 3.4Segmentformation............................. 45 3.5 Helix projected onto x-y plane. Figure obtained from ref. [6]. 50 3.6 Helix projected onto s-z plane. Figure obtained from ref. [6]. 50 3.7 Display of