Búsqueda Del Bosón De Higgs Del Modelo Standard En El Canal De
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Instituto de F´ısicade Cantabria (CSIC-Universidad de Cantabria) y Departamento de F´ısicaModerna (Universidad de Cantabria) B´usquedadel Bos´onde Higgs del Modelo Standard en el canal de desintegraci´on H ! WW ∗ ! 2µ2ν en el experimento CMS del LHC Memoria de tesis presentada por Rebeca Gonz´alezSu´arez para optar al grado de Doctor Dirigida por Dr. Javier Cuevas y Dr. Teresa Rodrigo Anoro Santander, Junio de 2010 Instituto de F´ısicade Cantabria (CSIC-Universidad de Cantabria) y Departamento de F´ısicaModerna (Universidad de Cantabria) Search for a SM Higgs Boson in the LHC with the CMS experiment using the H ! WW ∗ ! 2µ2ν decay channel Memoria de tesis presentada por Rebeca Gonz´alezSu´arez para optar al grado de Doctor Dirigida por Dr. Javier Cuevas y Dr. Teresa Rodrigo Anoro Santander, Junio de 2010 Dr. Javier Cuevas, profesor titular F´ısicaAt´omicadel departamento de F´ısicade la Universidad de Oviedo, y Dr. Teresa Rodrigo Anoro, Catedr´aticade Universidad del ´areade F´ısica At´omica,Molecular y Nuclear de la Facultad de Ciencias de la Universidad de Cantabria. Certifican: Que la presente memoria: B´usquedadel Bos´onde Higgs del Modelo Stan- dard en el canal de desintegraci´on H ! WW ∗ ! 2µ2ν en el experimento CMS del LHC, ha sido realizada bajo nuestra direcci´onen el Departamento de F´ısicaMod- erna de la Facultad de Ciencias de la Universidad de Cantabria por Rebeca Gonz´alez Su´arez, para optar al grado de Doctor en Ciencias F´ısicas. Y para que as´ıconste, en cumplimiento de la legislaci´onvigente, firmamos el pre- sente certificado: Santander, Junio de 2010 Contents 1 Introduction 1 2 Standard Model Higgs Boson 5 2.1 The Standard Model . .5 2.1.1 Elementary particles and interactions . .5 2.2 Gauge Field Theories . .8 2.2.1 Abelian Gauge Invariance (QED) . .8 2.2.2 Gauge Theory of Strong Interactions (QCD) . 10 2.2.3 The Gauge Theory of Electroweak Interactions . 10 2.2.4 The Standard Model Lagrangian . 11 2.3 The Higgs Mechanism . 12 2.3.1 The SM Lagrangian with Higgs . 14 2.4 The search for the SM Higgs Boson . 15 2.4.1 Theoretical bounds on mH ..................... 16 2.4.2 Experimental bounds on mH .................... 18 2.4.3 Higgs Physics at LEP . 18 2.4.4 Higgs Physics at Tevatron . 21 2.5 Beyond the Standard Model . 23 2.5.1 Supersymmetry (SUSY) . 24 2.5.2 The Minimal Supersymmetric Extension of the Standard Model (MSSM) . 24 2.5.3 Higgs Bosons in the MSSM . 25 3 The CMS experiment at LHC 27 3.1 The Large Hadron Collider . 27 3.2 The CMS experiment . 29 3.3 Detector Overview . 30 3.3.1 Magnet . 31 3.3.2 Inner Tracking System . 32 3.3.3 Electromagnetic Calorimeter . 34 3.3.4 Hadron Calorimeter . 36 3.3.5 Muon System . 38 3.3.5.1 Muon Alignment . 40 vii CONTENTS 3.3.5.2 Alignment and Calibration scenarios . 42 3.3.6 Trigger and data acquisition . 43 3.4 CMS Software and Computing . 45 3.4.1 CMS Remote Analysis Builder . 48 3.4.2 ROOT . 48 3.5 Workflow of the Analysis . 49 4 Search for a SM Higgs in the WW ∗ decay channel 51 4.1 Higgs Phenomenology at the LHC . 51 4.2 Higgs searches in CMS . 54 4.3 The WW ∗ channel . 56 4.3.1 Signature of Signal and Backgrounds . 57 4.3.2 Description of the Search Process . 59 4.4 LHC Center of mass Energy and Integrated Luminosity Scenarios . 63 4.5 Monte Carlo Datasets . 64 4.5.1 Monte Carlo Generators . 64 4.5.2 Monte Carlo 7 TeV Production . 65 4.6 Signal and Background Samples and Cross-Sections . 68 5 Physics Observables 73 5.1 Muons . 73 5.1.1 Muon Reconstruction . 73 5.1.2 Muon Identification . 77 5.1.3 Muon Isolation . 81 5.1.4 Muon selection . 83 5.2 Electrons . 84 5.2.1 Electron Reconstruction . 84 5.2.2 Electron identification . 85 5.2.3 Electron selection . 86 5.3 Jets . 87 5.3.1 Jet Flavors . 88 5.3.1.1 Calo Jets . 88 5.3.1.2 Particle Flow Jets . 88 5.3.1.3 Track Jets . 89 5.3.2 Jet Algorithms . 89 5.3.2.1 Iterative Cone . 90 5.3.2.2 Midpoint Cone . 90 5.3.2.3 Seedless Infrared-Safe Cone . 91 5.3.2.4 kt .............................. 91 5.3.2.5 Anti-kt ........................... 91 5.3.3 Jet Energy Corrections . 91 5.3.4 Jet Plus Tracks . 93 5.3.5 Jets in the analysis . 93 5.3.6 Jet Veto . 96 viii CONTENTS 5.4 missing ET .................................. 98 5.4.1 Calorimeter missing ET ....................... 99 5.4.2 Track Corrected missing ET ..................... 99 5.4.3 Particle Flow missing ET ...................... 100 5.4.4 PT of the di-µ system . 100 5.4.5 missing ET in the analysis . 101 6 Event Selection and Kinematics 109 6.1 Trigger Selection and Datasets . 109 6.1.1 Skim and Pre-selection . 112 6.2 Muon Selection . 113 6.3 Central Jet Veto . 113 6.4 Definition of the Variables . 114 6.5 Sequential cuts . 124 6.5.1 Mass dependent Optimization . 124 6.6 Final Event Selection . 125 6.6.1 Study at a centre of mass energy of 14 TeV . 125 6.6.2 Study at a centre of mass energy of 10 TeV . 127 6.6.3 Study at a centre of mass energy of 7 TeV . 128 6.7 Signal and Background production rates . 129 6.8 µµ, eµ and ee final states . 132 6.9 Complementary Strategies for the Analysis . 132 6.9.1 Use of track Jets and b-tagging . 133 6.9.1.1 Track Jets . 133 6.9.1.2 Soft-muon and Life time based b-tagging . 134 6.9.2 Multivariate Methods . 136 7 Methods to estimate the Background from Data and Systematic Un- certainties 141 7.1 Control Regions to estimate the Background . 141 7.1.1 tt¯ Background Normalization . 142 7.1.2 WW Background normalization . 145 7.1.3 Z + jets Background normalization . 145 7.2 Control of the Muon Fake Rate in W + jets events . 146 7.3 missing ET studies from data . 148 7.4 WW Cross-section measurement . 149 7.5 Systematic Uncertainties . 154 7.5.1 Luminosity . 154 7.5.2 Muon identification and Trigger efficiencies . 154 7.5.3 Misalignment and Miscalibration scenarios . 155 7.5.4 Missing ET modeling . 155 7.5.5 Jet reconstruction efficiency . 155 7.5.6 Jet energy scale . 156 7.5.7 ISR/FSR . 158 ix CONTENTS 7.5.8 PDF estimation . 158 7.5.9 Normalization for backgrounds . 159 7.5.10 Monte Carlo statistics . 159 8 Results 161 8.1 Limits and Significance . 161 8.2 Results at a centre of mass energy of 14 TeV . 162 8.3 Projections at 10 TeV . 164 8.4 Projections at 7 TeV . 166 9 Conclusions 171 10 Resumen del trabajo realizado y Conclusiones obtenidas 177 A Signal and Background yields 185 p A.1 Tables for 1 fb−1 at s = 14 TeV . 185 p A.2 Tables for 200 pb−1 at s = 10 TeV . 188 p A.3 Tables for 1 fb−1 at s =7TeV ...................... 190 B Results for the µµ final state at a centre of mass energy of 7 TeV 195 C First LHC data analysis 199 C.1 Events with muons in the final state . 200 C.2 Current LHC run conditions . 201 C.3 Early data analysis . 202 C.3.1 Muon Fake Rate . ..