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Commissioning of the CMS Tracker and Preparing for Early Physics at the LHC

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Commissioning of the CMS Tracker and Preparing for Early Physics at the LHC Commissioning of the CMS tracker and preparing for early physics at the LHC Matthew Wingham Blackett Laboratory Department of Physics Imperial College London A thesis submitted for the degree of Doctor of Philosophy of Imperial College London and the Diploma of Imperial College. July 2008 Abstract The Compact Muon Solenoid (CMS) experiment is a general purpose detector at the Large Hadron Collider. It has been designed and optimised to discover the Higgs boson and physics beyond the Standard Model. An early discovery of the Higgs boson is the collaboration's top priority and will require a good understanding of both the detector and the physics of the background processes, with a small integrated luminosity. This principle has been the driving force behind the work presented in this thesis. The Silicon Strip Tracker (SST) sits at the heart of the CMS detector. The de- velopment of core algorithms to commission the SST are reviewed and the process of live commissioning at the Tracker Integration Facility is described. A crowning success of this study is the calibration of 1.6 M channels and their synchronisation to a cosmic muon trigger to within 1 ns. The SST is expected to produce ¯ve times more zero-suppressed data than any other CMS sub-detector. As such its e±cient handling within High-Level Trigger algorithms is paramount. The performance of the online hit reconstruction software is pro¯led, the ine±ciencies are characterised and a new scheme to focus on physics regions-of-interest only is proposed. As an example of its success, when running the single ¿ trigger path over H § ¿ §º¿ events, hit reconstruction times were reduced from 838 5 ms to only 5.13! 0.05 ms without any loss in tracking e±ciency. The new soft§ ware is now the trac§ker community's permanent online solution and is expected to become the o²ine solution in the near future. b¹bZ0 production at the LHC is of great interest, primarily due to its status as a background to a supersymmetric Higgs boson production process. The preparation 1 for a cross section measurement with 100 pb¡ of data (expected by the end of 2009) is made. The prominent backgrounds are identi¯ed and a signal selection strategy is developed and optimised using Monte Carlo. This study demonstrates that a cross section measurement with this amount of data is feasible. Finally, a method to estimate background from data is tested. 2 Acknowledgements Working towards my Ph.D has been the most rewarding and enlightening experience of my life. From day one I have found the Imperial College HEP group a welcoming and challenging place to work. I am very proud to have spend the past four years researching here. The thesis presented is the product of four years hard graft and there are many friends and colleagues who have made it possible. I will attempt to list some of the more important ones below. Firstly, I would like the thank STFC for providing me with the necessary funding. Geo® Hall also deserves a special thank you for supervising me and reading through this document. I owe much to Rob Bainbridge who took me under his wing in my second and third years, teaching me about the CMS tracker and C++. In fact Chapters 4 and 5 wouldn't have left the starting blocks without him. Also, Sasha Nikitenko gave much guidance through my physics analysis in Chapter 6 for which I am grateful. Much of this work has been collaborative, especially when I was based at CERN. I owe special thanks to Jonny Fulcher, Nick Cripps, Giacomo Bruno, Domenico Giordano and Puneeth Kalavase for their e®orts. Many other people have o®ered me support throughout the years. There are too many to list in full, but they include all my Ph.D contemporaries; Maiko Takahashi for all her useful analysis tips and being a great friend to share this dusty o±ce with; Seb Greder and Freya Blekman for their ROOT expertise; David Colling and Costas Foudas for their help in my ¯rst year and general physics advice; Mark Raymond for putting up with my complete ineptitude when I ¯rst arrived and anyone who's given me software advice. Finally, I would like to thank Mum, Dad, Elise, Chris and my partner Claire. They have each been an inspiration which has kept me motivated through the highs and lows. This thesis is for you. 3 Contents Abstract 1 Acknowledgements 2 Contents 3 List of Figures 8 List of Tables 14 Chapter 1. The standard model of particle physics 16 1.1 Quantum ¯eld theory 16 1.2 The Standard Model 19 1.2.1 Quantum electrodynamics 20 1.2.2 Electroweak theory 21 1.2.3 Quantum chromodynamics 22 1.2.4 The Higgs mechanism 23 1.2.5 The Standard Model Lagrangian 26 1.2.6 Constraints on the Higgs boson mass 26 1.2.7 Radiative corrections to the Higgs ¯eld 26 Contents 4 1.3 Beyond the Standard Model : supersymmetry 28 1.3.1 The Minimal Supersymmetric Model 29 Chapter 2. Introduction to the LHC and CMS 31 2.1 The LHC 31 2.2 CMS physics motivation 32 2.2.1 Higgs physics searches 32 2.2.2 Further physics studies 34 2.3 The CMS detector 35 2.3.1 The pixel tracker 37 2.3.2 The strip tracker 38 2.3.3 The electromagnetic calorimeter 41 2.3.4 The hadronic calorimeter 42 2.3.5 The magnet 43 2.3.6 The muon tracker and return yoke 43 2.4 SST control, readout and data acquisition 44 2.4.1 The control system 45 2.4.2 Front-end electronics readout 46 2.4.3 The optical links 47 2.4.4 The Front End Driver 47 2.4.5 The event builder 49 2.5 The trigger system 50 2.5.1 Level 1 50 2.5.2 High-level 51 Contents 5 Chapter 3. CMS simulation and reconstruction software 53 3.1 The software framework 53 3.2 Track reconstruction 54 3.2.1 Hit reconstruction 55 3.2.2 Track formation 55 3.2.3 Vertex formation 56 3.2.4 Track reconstruction performance 57 3.3 Electron reconstruction 59 3.4 Muon reconstruction 60 3.5 Jet reconstruction 61 3.5.1 b-jet identi¯cation 62 3.5.2 ¿-jet identi¯cation 63 miss 3.6 ET reconstruction 64 Chapter 4. Commissioning the CMS silicon strip tracker 65 4.1 Integration and commissioning at the TIF 65 4.2 Commissioning procedures 66 4.3 Commissioning software architecture 69 4.3.1 Extracting and summarising commissioning monitorables 72 4.4 Readout connectivity 75 4.5 Internal synchronisation 76 4.6 Optical readout links 80 4.7 APV25 data frame 82 4.7.1 Analogue baseline calibration 84 Contents 6 4.7.2 Pedestals and noise measurement 85 4.8 Synchronisation to the LHC 87 4.9 APV25 pulse shape tuning 88 4.10 Summary 89 Chapter 5. Local reconstruction software for the CMS silicon strip tracker 90 5.1 CMS event data 90 5.1.1 The HLT time budget 90 5.2 Local reconstruction software architecture 91 5.2.1 Software performance 93 5.3 Regional reconstruction 95 5.3.1 Performance in global mode 98 5.3.2 Performance in regional mode 101 5.3.3 Performance in layers mode 106 5.4 on-demand reconstruction 107 5.5 Extension to other sub-detectors 110 5.6 Summary 111 1 0 Chapter 6. Toward a 100 pb¡ bbZ¹ cross section measurement at the LHC 112 6.1 b¹bH0 and b¹bZ0 production 112 6.1.1 gg b¹bZ0 matrix element calculation 114 ! 6.1.2 Z0 decay modes 115 6.1.3 Previous studies 116 Contents 7 6.2 Background processes 116 6.3 Sample production and preselection 120 6.4 Cross section measurement 122 6.5 A generator level signal selection strategy 123 6.6 Detector simulation and event reconstruction 129 6.6.1 Particle identi¯cation 129 6.6.2 Electron identi¯cation 131 6.6.3 Muon identi¯cation 133 6.6.4 b-jet tagging 134 6.6.5 Energy scaling 136 miss 6.6.6 ET and dilepton invariant mass reconstruction 138 6.7 Signal selection e±ciency measurement 139 6.7.1 Tuning the selection parameters 139 6.7.2 Monte Carlo signal and background selection e±ciencies 144 6.7.3 Systematic uncertainty in event selection e±ciencies 148 6.8 Background estimation 148 6.9 Contributions from the ccZ¹ 0 background 151 6.10 Summary 153 Chapter 7. Conclusions 155 Glossary 157 References 160 8 List of Figures 1.1 Feynman diagrams showing dominant loop corrections to free W § and Z0 boson ¯elds. 27 1.2 Indirect SM Higgs boson mass measurement from W § and t masses. 28 1.3 A ¢Â2 ¯t of the SM to existing electroweak data as a function of Higgs boson mass. 29 1.4 Feynman diagrams for second order corrections to the Higgs boson mass. 30 2.1 Higgs boson production channel cross sections as a function of mass. 33 2.2 Higgs boson decay channel branching ratios as a function of mass. 34 2.3 Higgs boson decay width as a function of mass. 35 2.4 The CMS detector and its co-ordinate system. 36 2.5 A slice of the CMS barrel in the x-y plane. 36 2.6 A quadrant of CMS in the x-z plane.
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