Diss. ETH No. 20683 Observation of Diffractive W and Z Boson Production and Standard Model Higgs Boson Search inp the H ! WW ! `ν`ν Channel in s = 7 TeV pp Collisions at CMS A dissertation submitted to ETH Zurich¨ for the degree of Dr. Sc. ETH Z¨urich presented by J¨urgEugster Dipl.-Phys. ETH Z¨urich born October 21th 1981 citizen of Oberergg, AI - Switzerland accepted on the recommendation of Prof. G. Dissertori, examiner Prof. Ch. Anastasiou, co-examiner | 2012 | ii Abstract In this thesis, two analyses using proton-proton (pp) collision data at a center of mass energy of ps = 7 TeV, recorded with the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC), are presented: 1 The first analysis, using an integrated luminosity of 36 pb− of low pile-up data recorded in 2010, uses the forward energy flow and the central charged particle multi- plicity in events with leptonically decaying W and Z bosons to study the effects of the underlying event model. None of the studied Monte Carlo simulations describes the ob- served distributions from data sufficiently. Weak boson events with no significant energy deposits in one of the forward calorimeters are observed. This corresponds to a large pseudorapidity gap (LRG) of at least 1.9 units. The fraction of W(Z) events having a LRG is found to be 1:46 0:09(stat:) 0:38(syst:)% (1:57 0:25(stat:) 0:42(syst:)%). ± ± ± ± The majority of the charged leptons from these W/Z decays are found in the hemisphere opposite to the gap. This gives a strong indication of a diffractive component in the weak boson production, and the fraction of diffractively produced W bosons is found to be 0:73 0:34% which is in agreement with observations from the Tevatron. ± The second analysis presents the search for the SM Higgs boson decaying to a pair of W bosons in the fully leptonic final state. The pp collision data corresponding to a 1 luminosity of 4:9 fb− recorded in 2011 are used. + The W W− event candidates are selected by requiring exactly two oppositely charged leptons and large missing transverse momentum from the escaping neutrinos. The shapes of the distributions of a multivariate discriminator are used to calculate upper limits on the SM Higgs production cross section. Different systematic uncertainties are studied in detail and included in the limit setting procedure using the 2011 dataset. The SM Higgs boson is excluded in the mass range of 134{211 GeV at the 95% confidence level. This is a slightly smaller range than the expected exclusion range of 125{230 GeV. An excess of about 2σ significance is observed in the low mass region below 140 GeV, consistent with a SM Higgs boson with a mass around 125 GeV. iii iv Zusammmenfassung In dieser Dissertation werden zwei Analysen vorgestellt, welche Daten von Proton-Proton (pp) Kollisionen mit einer Schwerpunktsenergie von ps = 7 TeV verwenden, die mit dem Compact Muon Solenoid (CMS) Experiment am Large Hadron Collider (LHC) aufgezei- chnet wurden. Die erste Analyse verwendet Daten aus 2010 mit einer integrierten Luminosit¨atvon 1 ungef¨ahr36 pb− . Sie ben¨utztden vorw¨artsgerichteten Energiefluss und die zentrale Multiplizit¨atder geladenen Teilchen in W und Z Ereignissen mit leptonischem Endzus- tand, um die unterliegende Struktur (underlying event) von pp Kollisionen zu untersuchen. Keines der untersuchten Simulations-Modelle beschreibt diese Verteilungen vollst¨andig. Zus¨atzlich werden W und Z Ereignisse beobachtet, welche keine signifikante vorw¨arts gerichtete Energie aufweisen. Dies entspricht einem Unterbruch im Energiefluss (Large Rapidity Gap, LRG) von mindestens 1.9 Einheiten der Pseudorapidit¨at.Der relative An- teil von W (Z) Ereignissen welche einen solchen LRG aufweisen wird als 1:46 0:09(stat:) ± ± 0:38(syst:)% (1:57 0:25(stat:) 0:42(syst:)%) bestimmt. ± ± Die Mehrzahl der geladenen Leptonen vom W/Z Boson Zerfall werden in der dem LRG gegen¨uberliegenden Hemisph¨aregefunden. Das ist ein starkes Indiz f¨ureine diffraktive Komponente in der Produktion von W und Z Bosonen. Der Anteil der so erzeugten W Bosonen betr¨agt0:73 0:34%, was kompatibel ist mit der Messung welche am Tevatron ± durgef¨uhrtwurde. Die zweite Analyse befasst sich mit der Suche nach dem vom Standard Modell der Teilchenphysik vorhergesagten Higgs Boson, welches unter anderem in zwei W Bosonen + zerf¨allt.Diese W W−-Kandidaten sind charakterisiert durch genau zwei entgegengesetzt geladene Leptonen sowie durch grossen fehlenden transversalen Impuls von den, einer Detektion entgehenden, Neutrinos. Die pp Kollisionen wurden 2011 aufgezeichnet und 1 entsprechen einer integrierten Luminosit¨atvon 4:9 fb− . Die Formen der Verteilungen einer multivariaten Diskriminierenden werden f¨urdie Berech- nung eines oberen Limits f¨ur den Wirkungsquerschnitt der Higgs Produktion ben¨utzt. Verschiedene systematische Unsicherheiten werden im Detail studiert und fliessen in die Limit-Berechnung mit ein. Der beobachtete Bereich in dem das SM Higgs Boson mit 95% Aussagewahrscheinlichkeit ausgeschlossen werden kann ist 134{211 GeV. Dieser ist etwas kleiner als der erwartete Bereich von 125{230 GeV. Unterhalb von etwa 140 GeV kann ein Uberschuss¨ an Daten mit einer Signifikanz von ca. 2σ beobachtet werden. v vi Because it is there. George Mallory [1] vii viii Contents I Introduction 5 1 The Standard Model of Particle Physics 7 1.1 Introduction . .7 1.2 Quantum Electrodynamics . .8 1.3 The Glashow-Salam-Weinberg Model . 10 1.4 Quantum Chromo Dynamics . 16 1.5 Limitations of the Standard Model . 20 2 The Large Hadron Collider 21 2.1 Experimental Needs - Energy and Luminosity . 22 2.2 LHC Design and Injection Chain . 23 2.3 Physics at the LHC . 23 3 The Compact Muon Solenoid Experiment 27 3.1 Overall Design of the CMS Detector . 27 3.2 Tracker System . 28 3.3 Calorimetry . 30 3.4 Superconducting Solenoid . 33 3.5 Muon System . 34 3.6 Data Taking . 35 3.7 Event Reconstruction . 37 II Observation of Diffractive W and Z Boson Production 41 4 Diffractive Production of W and Z Bosons 45 4.1 Diffraction - A General Introduction . 45 4.2 Detour: \Why is Diffraction Called Diffraction?” . 47 4.3 Diffractive Parton Distribution Functions . 47 4.4 Underlying Event and Gap Survival Probability . 48 ix 4.5 Diffractive production of W and Z Bosons . 49 4.6 Analysis Strategy . 50 4.7 Datasets . 51 4.8 Event Reconstruction . 51 4.9 W and Z Event Selection . 56 5 Forward Energy Flow and Central Track Multiplicity in W and Z Events 59 5.1 Forward Energy Flow . 59 5.2 Central Charged Particle Multiplicity . 65 5.3 Correlations between Forward Energy Flow and Central Charged Particle Multiplicity . 65 5.4 Selection of W Events with a LRG . 67 6 Observation of Diffractive Weak Boson Production 71 6.1 LRG Events and Invisible Pile-up Correction . 71 6.2 Forward Energy Flow and Charged Particle Multiplicity in LRG Events . 74 6.3 Size of the LRG . 75 6.4 Hemisphere Correlations in LRG Events . 77 6.5 Diffractive W Boson Production . 79 III Standard Model Higgs Boson Search 81 7 Higgs Production and the H ! WW ! `ν`ν Signature 85 7.1 The Standard Model Higgs Boson at the LHC . 85 7.2 The H WW `ν`ν Signature . 89 ! ! 8 Monte Carlo Simulation of the Gluon-Fusion Higgs Production 93 8.1 Event Generators . 93 8.2 Comparison of Different Event Generators . 95 9 H ! WW ! `ν`ν Analysis 107 9.1 Analysis Strategy . 107 9.2 Datasets . 109 9.3 Event Reconstruction and Pre-Selection . 109 9.4 Background Estimates . 116 9.5 Signal Extraction . 122 x 10 Systematic Uncertainties for the H ! WW ! `ν`ν Analysis 129 10.1 Sources of Systematic Uncertainties . 129 10.2 Theoretical Uncertainties . 130 10.3 Experimental Effects . 132 10.4 Statistical Uncertainties . 139 11 Cross Section Limits and Statistical Significance 141 11.1 Statistical Method . 141 11.2 Results for the SM Higgs Boson Search . 143 12 Higgs Boson Production with a Jet Veto 151 12.1 Simulation of the Jet Veto Efficiency in Parton Shower Monte Carlo . 151 12.2 Ratios of Jet Veto Efficiencies . 155 12.3 Jet Veto Efficiency in Data . 156 IV Summary and Appendices 159 13 Summary and Discussion 161 13.1 Summary and Discussion of Part II . 161 13.2 Summary and Discussion of Part III . 163 A Lepton Definitions 167 A.1 2010 Electron Reconstruction . 167 A.2 2010 Muon Reconstruction . 167 A.3 2011 Muon Reconstruction . 168 B 2010 and 2011 Datasets 171 B.1 2010 Datasets . 171 B.2 2011 Datasets . 171 C 2010 and 2011 Lepton Triggers 175 C.1 2010 Lepton Triggers . 175 C.2 2011 Lepton Triggers . 176 D Higgs Analysis | Detailed Results 179 D.1 Detailed Event Yields at BDT-Level . 179 D.2 Pre- and Post-Fit Normalization . 179 D.3 Channel Compatibility for Signal Injection . 180 xi E Event Display of a Diffractive W Event 185 E.1 Non-Diffractive W Event . 185 E.2 Diffractive W Event . 185 xii Introduction The pursuit to understand and explain the world around us is an enduring one | already two and a half millennia ago the first cornerstones of the modern understanding of nature were set: Democritus atomic hypothesis shaped the idea of basic indivisible building blocks out of which everything is made of. More than thousand years later, Roger Bacon intro- duced the need of empirical methods in order to study nature. Another thousand years later, such experiments led to the discovery of the electron (J. J. Thomson, 1897) and the nucleus (E. Rutherford, 1911), rendering the atom a composite entity with electrons or- biting the nucleus.