Direct Photon√ Shine: Direct Photon and π0 Production in sNN = 200 GeV Au+Au Collisions Justin Frantz Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2005 c 2005 Justin Frantz All Rights Reserved ABSTRACT 0 √ Direct Photon Shine: Direct Photon and π Production in sNN = 200 GeV Au+Au Collisions Justin E. Frantz With substantial additional statistics due to the inclusion of a new Run2 trig- ger data sample, the PHENIX Collaboration has measured the first positive direct √ photon (γdirect) signal in Au + Au collisions at sNN = 200 GeV and midrapidity (|y| ≤ 0.35). The measurement is made in 10 centrality bins covering 0-92% of the full geometric cross section. Additionally, the new data has extended the previous 0 PHENIX π measurement [69] by 4 GeV/c in its pT range, matching the γdirect mea- surement with a pT range of 1-14 GeV/c which make them the highest pT measure- ments yet in RHIC Au + Au. The γdirect yields are compared amongst themselves, with references, and with the π0The˙ suppression in meson hard scattering previously discovered at RHIC [69] is found to be absent in the direct photons. Specifically, using the NLO perturbative QCD prediction of γdirect as a reference, the nuclear modification factor RAA is found to be consistent with one and pT -independent for pT > 6 GeV/c. Thus, like the d + Au results, these direct photon measurements represent pos- sibly the best available confirmation of the conclusion that the aforementioned sup- pression effect is not due to differences in initial state hard-scattering, but rather, is due to a final state medium which quenches hard quarks and gluons, but not hard direct photons. This conclusion is consistent with final state modifications predicted as indicators of Quark Gluon Plasma (QGP) formation [171]. In addi- tion, the possible model-generated mechanisms for this suppression are constrained 0 further (though only slightly) by our updated π RAA results due to the higher pT values reached. The suppression continues to be independent of pT , signaling strong energy dependence. The direct photon invariant yields in (pT < 6 GeV/c) region are compared to predictions of thermal production and new mechanisms of photon enhancement [89],[178]. Large uncertainties do not allow any definite conclusions about observa- tions or constraints of such enhancements. Contents 1 Introduction 1 2 Particles, The Strong Force, and, QCD 6 2.1 Particle Physics: Quantum Mechanics, Forces, and Fields . 7 2.2 QCD: The Strong Force . 9 2.2.1 Quarks, Gluons, and Color Confinement . 9 2.2.2 QCD, QED, and Standard Model Basics . 13 2.3 Perturbation Theory and Perturbative QCD (pQCD) . 17 2.3.1 Feynman diagrams and Renormalization . 17 2.3.2 The Running Coupling Constant αS . 21 2.3.3 Factorization, PDF’s and Fragmentation . 23 3 The Phase Transition, QGP, and HI Physics 30 3.1 Non-Perturbative and Lattice QCD . 30 3.2 Enter Relativistic Heavy Ions . 37 3.3 The Space-Time Geometry of a RHI Collision . 38 3.3.1 Basics And Static Structure . 38 3.3.2 Spacial Evolution . 39 3.3.3 Time Evolution and Thermal Equilibrium . 41 3.4 4-Momentum Space Evolution: The Energy Density . 43 3.5 QGP Signatures . 44 i 0 4 QGP Signatures in γdirect and Hard π Physics 46 4.1 Hard Scattering and High-pT Suppression . 47 4.2 Quantifying Medium Effects . 48 4.3 Scaling And TheRAB ........................... 49 4.3.1 AB Scaling . 49 4.3.2 Centrality . 51 4.3.3 The Glauber Model and Thickness Functions . 53 4.3.4 Binary and Participant Scaling . 55 4.3.5 Experimental RAB: TAB or Ncoll? . 59 4.4 The Baseline: Review of Hard Scattering in Particle Physics . 60 4.4.1 The Basics: pT xT Scaling, Etc. 61 4.4.2 Nuclear Effects . 67 4.4.3 The kT Smearing Debate . 70 4.5 The Mechanism of High pT Suppression in the QGP . 74 4.5.1 Gluon Radiation . 74 4.5.2 Is it Really a QGP Signature? . 77 4.6 Review of Hard Scattering in A + A ................... 78 4.6.1 pre-RHIC: SPS . 78 4.6.2 RHIC . 79 4.7 Mechanisms for Direct Photon Enhancement and Measurements in A + A ................................... 83 4.7.1 Experimental Results . 83 4.7.2 Thermal Radiation . 84 4.7.3 Jet-Plasma γdirect Production . 90 5 Experiment 93 5.1 RHIC: Trains of Trains . 93 5.1.1 “Runs” and “runs” . 95 5.1.2 RCF and PHONCS . 97 5.2 PHENIX . 97 ii 5.3 Central Magnet and Interaction Region . 98 5.4 Introduction to Particle Energy Loss in Matter . 101 5.5 BBC/ZDC Detectors and Triggering . 102 5.6 Charge Particle Tracking . 105 5.7 EMCal Overview . 107 5.8 Introduction to Showering . 108 5.8.1 PbSc and PbGl . 109 5.8.2 Organization And History . 112 5.8.3 Nominal Resolution . 112 5.8.4 Clustering . 113 5.8.5 Shower Merging . 115 5.8.6 Photon Corrected Energy Ecore . 116 5.9 Data Acquisition and Triggering . 117 6 Analysis 124 6.1 Glauber Model Calculations . 126 6.2 EMCal Calibration . 130 6.2.1 Direct Methods: The Ideal “General Theory” of Calibrations . 130 6.2.2 Averaging Methods of PHENIX . 131 6.2.3 Test Beams, MIP calibrations, Energy Sharing . 132 6.2.4 π0 Corrections . 136 6.2.5 PbGl and the Slope Method . 137 6.2.6 Calibration Quality, Final Resolution . 139 6.2.7 A Problem . 142 6.2.8 Timing Calibrations . 143 6.2.9 Systematic Uncertainties . 147 6.3 Raw Data Extraction . 148 6.3.1 Offline Reconstruction and Software . 148 6.3.2 Data QA . 150 6.3.3 Dead and Hot Towers . 152 iii 6.3.4 Cluster/Single Photon Extraction: Cuts . 156 6.3.5 π0 Yield Extraction . 160 6.4 Level2 High pT Tile Trigger . 170 6.4.1 Event Counting . 170 6.4.2 Trigger Efficiencies . 170 6.4.3 Photon Efficiencies . 172 6.4.4 Photon Matching . 175 6.4.5 PbGl Efficiencies . 175 6.4.6 π0 Efficiencies: The Software Cut . 176 6.4.7 π0 Matching . 180 6.4.8 Systematic Errors . 180 6.5 Introduction to Corrections . 186 6.5.1 Invariant Yields and Cross Sections . 186 6.5.2 Formulae . 187 6.5.3 Bin Shift Correction . 189 6.5.4 Power Law Fitting . 191 6.6 Acceptance and Efficiency Calculations . 195 6.6.1 Geometric Acceptance a .....................195 6.6.2 Efficiency Calculation ......................197 6.6.3 Simulation and Embedding . 198 6.6.4 Smearing . 201 6.6.5 Final Efficiencies . 202 6.6.6 Systematic Uncertainties: Simulation/PID . 202 6.6.7 Systematic Uncertainties: Embedding . 211 6.6.8 Systematic Uncertainties: Calibration . 211 6.6.9 Systematic Uncertainties: Smearing . 213 6.6.10 Calibration and Smearing Correlations . 214 6.6.11 Other Sources of Uncertainty and Error Summary . 216 6.7 γ Conversions and Hadron Contamination . 218 iv 6.7.1 Photon Conversions . 218 6.7.2 Charged Contamination . 220 6.7.3 Jet and Other Correlations Present in Xch . 221 6.7.4 Systematic Errors: Xch ......................223 6.7.5 Neutral Hadron Contamination . 223 6.7.6 Systematic Uncertainty: Xn/n¯ ..................225 6.8 Background Decay Photon Calculation . 227 6.9 Summary of Systematic Errors . 231 7 Results 235 7.1 π0 Spectra . 235 0 7.2 π RAA Values . 236 0 7.2.1 π RAA vs. Centrality (Npart) . 239 7.3 γall Spectra . 240 7.4 Establishment of the Direct Photon Signal: Rγdirect . 241 7.5 Direct Photon Spectra . 243 7.6 Direct Photon RCP ............................243 8 Discussion of Results 245 8.1 The Setting . 246 8.2 Ncollision Scaling and High pT Suppression . 249 8.2.1 Spectral Shapes . 250 8.2.2 Systematic Error Cancellations in Spectral Ratios . 252 8.2.3 The Ratio RCP ..........................254 8.2.4 Absolute Value of RCP ......................258 8.2.5 TAA Scaling . 260 8.2.6 Other Evidence . 266 8.3 Direct Photon RAA and NLO pQCD Performance at RHIC . 268 8.3.1 Modifications or Enhancements of Direct γ? . 273 8.3.2 Suppression of Bremsstrahlung/Fragmentation Photons? . 284 v 0 8.4 High(er?) pT π Suppression . 287 8.5 Au+Au: An Ideal γdirect Laboratory? . 290 8.6 Summary and Restatement of Conclusions . 290 A Run Info 313 B π0 Invariant Yields 315 C Direct Photon Invariant Yields 325 0 D π RAA Values 333 vi List of Figures 2.1 All known elementary particles/fields . 14 2.2 Gluon Compton Scattering ..