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Strategy for an Early Observation of the ZZ Diboson Production in the Four Lepton final States at 10 Tev with the CMS Experiment at CERN

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Strategy for an Early Observation of the ZZ Diboson Production in the Four Lepton final States at 10 Tev with the CMS Experiment at CERN UNIVERSITA` DEGLI STUDI DI TORINO FACOLTA` DI SCIENZE MATEMATICHE, FISICHE E NATURALI CORSO DI LAUREA MAGISTRALE IN FISICA DELLE INTERAZIONI FONDAMENTALI Strategy for an early observation of the ZZ diboson production in the four lepton final states at 10 TeV with the CMS experiment at CERN Relatore Dott. Nicola Amapane Co-Relatore Dott.ssa Chiara Mariotti Candidato Laura Nervo Anno Accademico 2008=2009 \You have not truly understood something until when you are not able to explain it to your grandmother." Albert Einstein \The joy of physics isn't in the results, but in the search itself" Dennis Overbye Alla mia famiglia, in particolare mia madre, che mi stimola e mi aiuta sempre nelle piccole e nelle grandi imprese quotidiane, e mio fratello, che mi ha guidato e supportato nel cammino della scienza . ♦ To my family, in particular my mother, who always encourages and helps me in the small and large-size daily businesses, and my brother, who advised to me and supported me moving down the road of science . v Contents Contents vii Introduction 1 1 The Standard Model Higgs boson3 1.1 Higgs boson mass.....................................3 1.1.1 Theoretical constraints..............................3 1.1.2 Experimental constraints............................4 1.2 Higgs boson search at the LHC.............................6 1.2.1 Higgs boson production.............................6 1.2.1.1 Gluon-gluon fusion...........................7 1.2.1.2 Vector boson fusion..........................8 1.2.1.3 Associated production.........................8 1.2.2 Higgs boson decay................................8 1.2.2.1 Low mass region............................9 1.2.2.2 Intermediate mass region....................... 10 1.2.2.3 High mass region............................ 10 1.2.2.4 Higgs boson total decay width.................... 11 1.3 Electroweak measurements at LHC........................... 12 1.3.1 Measurement of the W boson and the top quark masses........... 12 1.3.2 Measurement of the Z boson mass....................... 14 1.3.3 Drell-Yan production of lepton pairs...................... 15 1.3.4 The dibosons WW , WZ and ZZ ........................ 16 2 The Large Hadron Collider 19 2.1 The Large Hadron Collider............................... 19 2.1.1 Purpose of LHC................................. 20 2.1.2 The accelerator.................................. 21 2.1.3 Phenomenology of proton-proton collisions.................. 23 3 The CMS experiment 27 3.1 The Tracker........................................ 27 3.2 The Electromagnetic Calorimeter............................ 29 3.3 The Hadronic Calorimeter................................ 30 3.4 The magnet........................................ 31 3.5 The Muon System.................................... 33 vii viii Contents p 4 The H ! ZZ(∗) ! 4l channel at s = 10 TeV 35 4.1 Physics processes and their simulation......................... 35 4.1.1 Signal: H ! ZZ(∗) ! 4l ............................. 36 4.2 Backgrounds....................................... 37 4.2.1 qq¯ ! ZZ(∗) ! 4l ................................. 37 4.2.2 gg ! ZZ(∗) ! 4l ................................. 39 4.2.3 qq=gg¯ ! Zb¯b ! 4l ................................ 40 4.2.4 qq=gg¯ ! tt¯ ! 4l ................................. 42 4.2.5 Other backgrounds................................ 43 4.3 Weights.......................................... 43 4.3.1 Weights for ZZ as a function of m4l ...................... 44 4.3.2 Weight for Z+jets................................ 44 4.4 Trigger, skimming and pre-selection.......................... 45 4.4.1 The CMS trigger................................. 45 4.4.2 Event skimming................................. 46 4.4.3 Event preselection................................ 47 4.4.4 Discriminating observables........................... 51 4.4.4.1 Lepton isolation............................ 52 4.4.4.2 Impact parameter........................... 55 4.4.4.3 Kinematics............................... 58 4.4.5 Baseline event selection and results....................... 58 4.4.5.1 Baseline event selection........................ 58 4.4.5.2 Analysis results with L = 1 fb−1: a simple counting experiment approach................................ 61 p 5 The ZZ(∗) ! 4l analysis strategy at s = 10 TeV 67 5.1 Removing Zb¯b from Z+jets samples.......................... 68 5.2 The selection steps.................................... 69 5.2.1 Z and Z∗ boson invariant mass constraints.................. 69 5.2.2 Cuts on the isolation and the 3D impact parameter significance....... 71 5.2.2.1 The new isolation definition...................... 71 5.2.2.2 The impact parameter significance cut................ 74 5.2.3 The 2D cut .................................... 77 rd 5.2.4 Cut on the pT of the 3 isolated lepton.................... 78 th 5.2.5 Cut on the pT of the 4 isolated lepton.................... 80 5.3 Results after the full selection.............................. 81 5.3.1 Combined significance for ZZ(∗) ! 4l ..................... 83 6 Control of Zb¯b and tt¯ backgrounds from data 85 6.1 Definition of the control region ............................. 85 6.1.1 The 3µ1e channel................................. 87 6.2 Fitting the mZ1 distribution............................... 88 Conclusions 91 A Isolation algorithm 95 Contents ix A.1 Electron isolation..................................... 95 A.2 Muon isolation...................................... 97 B Lepton reconstruction and identification 99 B.1 Electrons......................................... 99 B.1.0.1 Electron reconstruction........................ 99 B.1.0.2 Electron identification......................... 102 B.2 Muons........................................... 102 B.2.1 The \tracker muon" selection.......................... 104 C The Standard Model and the Higgs mechanism 107 C.1 The Standard Model of elementary particles...................... 107 C.2 The electroweak theory................................. 108 C.3 The Higgs mechanism.................................. 111 C.3.1 Vector boson masses and couplings....................... 113 C.3.2 Fermion masses and couplings.......................... 114 List of Figures 116 List of Tables 121 Abbreviations 123 Symbols 125 Bibliography 127 Ringraziamenti 135 Introduction The Standard Model (SM) of elementary particles is today one of the best theories of modern physics. It is a simple and comprehensive theory that explains the hundreds of known particles that compose matter with only six quarks and six leptons and their complex interactions with the exchange of three types of force carriers (more details in AppendixC). The Standard Model is a succesful theory. Experiments have verified its predictions to incredible precision. But it does not explain everything. For example, gravity is not included in the Standard Model. While the SM provides a very good description of phenomena observed by experiments, it is still an incomplete theory. The problem is that the Standard Model cannot explain why some particles exist as they do. For example, even though physicists knew the masses of all the quarks except for the top quark for many years, they were simply unable to accurately predict the top quark's mass without experimental evidence because the Standard Model cannot explain why a particle has a certain mass. Also, both the photon and the W boson are force carrier bosons: why is the photon massless and the W boson massive? In the Standard Model, particle masses arise from a breakdown of the electroweak symmetry group SU(2)I ⊗ U(1)Y (see AppendixC for further details). The simplest way to realize such \spontaneous" ElectroWeak Symmetry Breaking (EWSB) is the so called Higgs mechanism, which gives rise in the SM to a massive scalar particle, the Higgs boson, never found experimentally so far. Both gauge bosons and fermions acquire mass by interacting with the Higgs boson. Even if the Higgs boson has never been found experimentally, direct and indirect searches have been carried out at the Large Electron-Positron Collider (LEP-2): a fit of experimental data indicates ∼ 76 GeV=c2 as most probable value, with an upper bound of 157 GeV=c2 (95% C.L.). Direct searches have fixed a lower bound of 114:4 GeV=c2 (95% C.L.). At Tevatron experiments have demonstrated a robust Higgs boson program in their first 2 fb−1 of recorded data. Observed limits of the ratio σ95CL/σSM are 7.8 (1.4) at mH = 115 (160) GeV. All these experimental results suggest a low value for the Higgs boson mass. Theoretical considerations extend the Higgs boson discovery range up to ∼ 1 TeV/c2. Therefore, also high mass values must be taken into account. 1 2 Introduction The search for the Higgs boson and for evidence of new Physics beyond the Standard Model are among the main goals of the Large Hadron Collider (LHC) at CERN and of the Compact Muon Solenoid (CMS) experiment in particular. The LHC is a proton-proton collider with a nominal energy of 14 TeV in the centre of mass and a nominal luminosity of 1034 cm−2s−1 and will allow the Higgs boson to be searched in the entire expected mass range. The work presented in this thesis has been carried out within the Torino CMS group between October 2009 and April 2010. Its aim is to develop a study of the process ZZ(∗) ! 4l, which is an irreducible background for the Higgs boson search in the channel H ! ZZ(∗) ! 4l. This is expected to be one of the most important channels for the discovery of a \heavy" Higgs boson at the LHC, because of its clear signature over the hadronic background and of the high branching
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