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Search for Non-Pointing Photons in the Diphoton and Missing Transverse Energy Final State in 7 Tev P P Collisions Using the ATLAS Detector

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Search for Non-Pointing Photons in the Diphoton and Missing Transverse Energy Final State in 7 Tev P P Collisions Using the ATLAS Detector Search for Non-Pointing Photons in the Diphoton and Missing Transverse Energy Final State in 7 TeV p p Collisions Using the ATLAS Detector Nikiforos K. Nikiforou Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2014 c 2014 Nikiforos K. Nikiforou All Rights Reserved ABSTRACT Search for Non-Pointing Photons in the Diphoton and Missing Transverse Energy Final State in 7 TeV p p Collisions Using the ATLAS Detector Nikiforos K. Nikiforou A search for photons originating in the decay of a neutral long-lived particle produced in proton– proton collisions at ps = 7 TeV is presented. The search was performed in the diphoton plus 1 missing transverse energy final state, using the full data sample of 4.8 fb− of 7 TeV proton– proton collisions collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The analysis exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction of photons, and utilizes the excellent time resolution of the calorimeter as an independent cross-check of the results. The search was conducted in the context of Gauge Mediated Supersymmetry Breaking models, where the lightest neutralino is the next- to-lightest supersymmetric particle and has a finite lifetime. In the family of models investigated, supersymmetric particles are produced in pairs due to R-parity conservation, eventually decaying to a pair of neutralinos, each subsequently decaying to a photon and a gravitino. The gravitinos escape the detector, giving rise to missing energy, while the photons can appear not to originate from the primary vertex of the event, and are measured with a delay with respect to the collision time. No excess was observed above the background expected from Standard Model processes. The results were used to set exclusion limits at 95% CL in the two-dimensional parameter space defined by the supersymmetry breaking scale and the lifetime of the lightest neutralino. Table of Contents Foreword xiii 1 Introduction1 2 The Standard Model of Particles and Interactions5 2.1 General Properties of the Standard Model...........................5 2.2 Limitations of the Standard Model...............................8 3 Supersymmetry 11 3.1 Introduction to Supersymmetry................................. 11 3.2 The Minimal Supersymmetric Standard Model....................... 12 3.3 SUSY as a Broken Symmetry................................... 15 3.4 Gauge Mediated Supersymmetry Breaking.......................... 18 3.5 Most Recent Previous Results................................... 23 4 The Large Hadron Collider 24 4.1 Concepts of Accelerator Physics................................. 24 4.2 LHC Design and Operation.................................... 27 4.3 Operating Conditions in 2011 and 2012............................ 30 5 The ATLAS Experiment 32 5.1 Overview of the ATLAS Detector................................ 32 5.2 Inner Detector............................................ 35 5.3 Calorimetry.............................................. 37 i 5.4 Muon Spectrometer......................................... 44 5.5 Trigger and Data Acquisition................................... 46 5.6 Forward Detectors and Luminosity Measurement..................... 48 6 Physics Object Reconstruction and Identification 51 6.1 Electron and Photon Reconstruction in ATLAS...................... 51 6.2 Muons.................................................. 61 6.3 Jets.................................................... 62 6.4 Missing Transverse Energy..................................... 64 6.5 Overlap Removal........................................... 67 7 Calorimeter Pointing and Timing 68 7.1 Pointing Measurement....................................... 68 7.2 Pointing Resolution......................................... 72 7.3 Timing Performance......................................... 73 8 Analysis Strategy 76 8.1 Dataset and Trigger Selection................................... 76 8.2 Tight-Loose Diphoton Selection................................. 77 8.3 Event Cleaning Procedures.................................... 80 8.4 Event Selection............................................ 82 8.5 Signal and Control Region Definitions............................. 82 8.6 Signal Monte Carlo Simulation.................................. 84 9 Signal Efficiencies and Expected Event Yields 88 9.1 Trigger Efficiency.......................................... 88 9.2 Non-Pointing Photon Identification Efficiency....................... 92 9.3 Summary of Signal Efficiencies and Expected Event Yields................ 98 10 Signal and Background Modeling 101 10.1 SPS8 GMSB Signal Modeling................................... 101 10.2 Background Modeling........................................ 105 ii 10.3 Timing Templates.......................................... 115 10.4 Other Backgrounds......................................... 118 11 Systematic Uncertainties 120 11.1 Flat Systematic Uncertainties................................... 120 11.2 Summary of Systematic Uncertainties............................. 123 12 Template Fitting and Limit Setting Procedures 125 12.1 Description of the Template Fitting Procedure........................ 125 12.2 Limit Setting Procedure...................................... 127 12.3 Validation of the Fitting Procedure............................... 127 13 Results and Interpretation 132 13.1 Pointing and Timing Distributions in the Signal Region................. 132 13.2 Limits on SPS8 GMSB Models.................................. 139 14 Conclusions and Outlook 142 Bibliography 144 Appendices 151 A Lifetime Reweighting of Signal Monte Carlo Samples 152 B Isolation Template Fits to the Tight-Loose Control Samples 156 C E miss Systematic Errors per Signal MC Point 160 T D Limit Plots for Different Λ Values 168 iii List of Figures 2.1 Particle Content of the Standard Model............................6 3.1 Spectra of SUSY Particle Masses in SPS8 GMSB Models.................. 19 3.2 Leading Diagrams for Photon and Gravitino Pair Production in GMSB....... 20 3.3 Total Cross Section and Strong-production Fraction versus Λ in SPS8 GMSB.... 21 3.4 Distributions of the NLSP Transverse Momentum and Speed.............. 21 3.5 Fraction of NLSPs Decaying Before the Calorimeter as a Function of τ ....... 22 4.1 The Accelerator Complex at CERN.............................. 29 4.2 Distribution of Mean Number of Interactions Per Bunch Crossing in 2011 and 2012 31 5.1 Three-Dimensional Visualization of the ATLAS Detector................ 34 5.2 The ATLAS Coordinate System................................. 34 5.3 Cut-away View of the ATLAS Inner Detector........................ 35 5.4 The ATLAS Calorimeter System................................ 38 5.5 Accordion Structure of the EM Barrel Calorimeter..................... 39 5.6 LAr Pulse Shape........................................... 40 5.7 Sketch of an EMB Section..................................... 41 5.8 Front End Board Block Diagram................................. 42 5.9 Mechanical Assembly of a Tile Calorimeter Section.................... 44 5.10 Muon Instrumentation of the ATLAS Experiment..................... 45 5.11 Block Diagram of the ATLAS Trigger/DAQ System.................... 47 5.12 Cumulative Integrated Luminosity versus Time....................... 50 iv 6.1 Dielectron Mass Distribution for Z ee in Data and MC................ 54 ! 6.2 Shower Shapes for Photon and Jet Candidates........................ 56 6.3 Means of Photon Discriminating Variables versus η .................... 57 6.4 Example Distributions of Discriminating Variables for Unconverted Photons... 58 6.5 E miss Resolution in 2011 p p Collision Data......................... 66 T 7.1 Schematic Showing the Measurement of the Direction of a Non-Pointing Photon. 69 7.2 Longitudinal Distribution of Primary Vertices Reconstructed Online in the HLT 71 7.3 Vertex z-position Resolution versus Number of Associated Tracks........... 71 7.4 Pointing Resolution versus zDCA for GMSB Signal and Z ee Data and MC... 73 j j ! 7.5 Time Resolution versus Leading Cell Energy in the EMB................. 75 8.1 Distributions of Kinematic Variables for Several SPS8 Λ Values............. 78 8.2 Photon Reconstruction Efficiencies as a Function of zDCA in SPS8 Signal MC.. 79 j j 8.3 E miss Distribution for Events in the Selected Diphoton Sample............. 83 T 9.1 Trigger Efficiency as a Function of z(γ) ........................... 90 j j 9.2 Diphoton Trigger Efficiency as a Function of zPV for Data and MC.......... 91 9.3 L1 Trigger Efficiency in Signal MC for Signal Region Photons versus z(γ) ...... 91 9.4 Loose Identification Efficiency as a Function of zPV in Z ee Tag-and-Probe Studies 93 ! 9.5 Tight Identification Efficiency as a Function of zPV in Z ee Tag-and-Probe Studies 94 ! 9.6 Shower Shape Variables for Photons with Different zDCA Ranges........... 95 j j 9.7 Differences Between Data and MC for w 2 and ws3 versus zPV ............. 97 η j j 9.8 Tight Photon Efficiency With and Without Smearing the ws3 Variable........ 97 9.9 Signal Acceptance Times Efficiency versus τ for Several Λ Values in SPS8 GMSB. 98 10.1 Pileup Dependence of Photon zDCA Distribution for Signal MC............ 103 10.2 Photon Conversion Identification Efficiency in Signal MC, as a Function of zDCA 104 10.3 Pointing Distribution for Different Amounts of Detector Material.......... 104 10.4 Pointing Templates for Signal and Background.......................
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