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Measurement of the Mass of the Higgs Boson in the Two Photon Decay Channel with the CMS Experiment

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Measurement of the Mass of the Higgs Boson in the Two Photon Decay Channel with the CMS Experiment Facolt`adi Scienze Matematiche Fisiche e Naturali Ph.D. Thesis Measurement of the mass of the Higgs Boson in the two photon decay channel with the CMS experiment. Candidate Thesis advisors Shervin Nourbakhsh Dott. Riccardo Paramatti Dott. Paolo Meridiani Matricola 1106418 Year 2012/2013 Contents Introduction i 1 Standard Model Higgs and LHC physics 1 1.1 StandardModelHiggsboson.......................... 1 1.1.1 Overview ................................ 2 1.1.2 Spontaneous symmetry breaking and the Higgs boson . 3 1.1.3 Higgsbosonproduction ........................ 5 1.1.4 Higgsbosondecays ........................... 5 1.1.5 Higgs boson property study in the two photon final state . 6 1.2 LHC ....................................... 11 1.2.1 The LHC layout............................. 12 1.2.2 Machineoperation ........................... 13 1.2.3 LHC physics (proton-proton collision) . 15 2 Compact Muon Solenoid Experiment 17 2.1 Detectoroverview................................ 17 2.1.1 Thecoordinatesystem . 18 2.1.2 Themagnet ............................... 18 2.1.3 Trackersystem ............................. 19 2.1.4 HadronCalorimeter. 21 2.1.5 Themuonsystem............................ 22 2.2 Electromagnetic Calorimeter . 24 2.2.1 Physics requirements and design goals . 24 I Contents II 2.2.2 ECAL design............................... 24 2.3 Triggeranddataacquisition . 26 2.3.1 CalorimetricTrigger . 27 2.4 CMS simulation ................................. 28 3 Electrons and photons reconstruction and identification 32 3.1 Energy measurement in ECAL ......................... 32 3.2 Energycalibration ............................... 33 3.2.1 Responsetimevariation . 34 3.2.2 Intercalibrations. 36 3.3 Clusteringandenergycorrections . 38 3.4 Algorithmic corrections to electron and photon energies ........... 39 3.4.1 Parametric electron and photon energy corrections . ....... 40 3.4.2 MultiVariate (MVA) electron and photon energy corrections . 41 3.5 Photonreconstruction ............................. 43 3.5.1 Reconstruction of conversions . 43 3.6 Electronreconstruction. 43 3.7 ECAL noiseandsimulation ........................... 46 3.7.1 Time dependent simulation . 48 4 Measurement of the energy scale and energy resolution 50 4.1 Intrinsic Electromagnetic Calorimeter (ECAL) energy resolution . 51 4.2 Measurement of the in situ energyresolution. 51 4.2.1 Contributions to the in situ energy resolution . 51 4.2.2 Energy scale and resolution with Z e+e− events . 54 → 4.2.3 Definition of electron energy for ECAL energy scale and resolution studies 57 4.2.4 Z e+e− eventselection ....................... 58 → 4.2.5 Simulation................................ 61 4.2.6 Comparison between data and Monte Carlo (MC) samples . 61 4.2.7 Pile-upre-weighting . 63 4.2.8 FitMethod ............................... 65 Contents III 4.2.9 Energy scale correction and experimental resolution estimation . 67 4.2.10 Uncertainties on peak position and experimental resolution . 67 4.3 Smearingmethod ................................ 69 4.3.1 Mitigation of the likelihood fluctuations . ..... 72 4.3.2 ET dependentenergyscale. 74 4.3.3 Minimizationalgorithm . 75 4.4 Energy scale corrections and additional smearing derivation......... 76 4.4.1 Energyscalecorrections . 77 4.4.2 Additionalsmearings . 84 4.4.3 ValidationwithtoyMCstudy . 90 4.4.4 Systematic uncertainties on additional smearings . ...... 90 5 Search for a Higgs boson in the H γγ channel 94 → 5.1 Introduction................................... 94 5.2 Trigger...................................... 95 5.3 Simulatedsamples ............................... 96 5.4 Diphoton vertex identification . 101 5.4.1 Basealgorithms.............................101 5.4.2 Per-event probability of correct diphoton vertex choice . 102 5.4.3 Preselection ...............................103 5.5 Cut-based selection and categorization: untagged categories . 105 5.5.1 Single photon identification . 105 5.5.2 Di-photon event selection . 105 5.5.3 Event classification . 106 5.6 MVA-based selection and categorization: untagged categories . 107 5.6.1 Single photon identification . 107 5.6.2 Di-photon event selection . 108 5.6.3 Event classification . 108 5.7 Exclusivemodes.................................112 5.7.1 Leptontag................................112 5.7.2 METtag ................................115 Contents IV 5.7.3 VBF...................................117 5.8 Statisticalanalysis ............................... 122 5.8.1 Exclusionlimits.............................122 5.8.2 Quantification of an excess . 123 5.9 Signalextraction ................................ 123 5.9.1 Background modelling (fb).......................124 5.9.2 Signalmodelling ............................125 5.10 Systematicuncertainties . 131 5.11Results......................................135 6 Higgs Mass measurement 143 6.1 Uncertainty on the photon energy scale . 144 6.1.1 Extrapolation from electrons to photons . 144 6.1.2 Extrapolation from Z to H (125) energies . 145 6.1.3 Summary of the systematic errors on the photon energy scale . 146 6.2 Propagation of energy scale and resolution uncertainties to the signal parametric model146 6.3 Results......................................146 Conclusion 149 Bibliography 150 List of acronyms 159 Introduction The Standard Model (SM) of particle physics describes elementary particles and their interactions in the contest of the Quantum Field Theory. It has been very successful in describing high energy measurements at the actual experimental limits. The electro-weak sector of the theory is spontaneously broken by an additional scalar field (the Higgs field) with a non void expectation value for the ground state. The mass of the particles is given by their interaction with the Higgs field, whose quantum is the Higgs boson. The Higgs boson of the SM is a scalar particle and with spin 0 with a coupling to the other particles proportional to their masses. The Higgs boson mass is instead a free parameter of the theory. The 4th July 2012, at the Large Hadron Collider (LHC) the A Toroidal LHC Appa- ratuS experiment (ATLAS) and the Compact Muon Solenoid (CMS) collaborations have announced the discovery of a new boson. In this thesis, the measurement of the properties of the new boson in the di-photon decay channel, with the CMS data, is presented. The objective of the analysis is the study of the new boson properties in order to assess the compatibility with the SM predictions. The objective of this thesis is to present the measurement of the parameters of the boson in the di-photon final state, with a special highlight to the close relation between the analysis sensitivity and the Electromagnetic Calorimeter (ECAL) performance, and illustrate in details the mass measurement and its uncertainties. The data collected in 2011 at 7 TeV center-of-mass energy and in 2012 at 8 TeV by the CMS experiment are used in this analysis. A theoretical introduction to the Higgs boson, its production mechanism at LHC and decay modes is given in the first chapter. An overview of the Higgs boson properties that i Contents ii is possible to study in the di-photon final state is presented. Notions about the LHC and the proton-proton physics environment are also given. Chapter 2 is focused on the CMS detector. Particular attention is devoted to the ECAL description given its central role in the H γγ analysis sensitivity. → The Higgs signal is expected as a resonance in the di-photon invariant mass over a smoothly falling background. According to the SM predictions, the natural width of the Higgs boson is negligible with respect to the experimental resolution. An excellent energy and direction resolution of the photons is therefore crucial. sensitivity. The electron and photon reconstruction is described in Chapter 3, starting from the ECAL energy measurement algorithms, the ECAL in situ calibration and the energy cor- rections for electrons and photons. My personal contribution to the ECAL calibration activities in the LHC Run1 is to validate, through the evaluation of the resolution improvements in Z e+e− events, → each step of the calibration procedure. The ECAL conditions for the legacy reprocessing of 7 and 8 TeV data have been validated with the tools I developed during this thesis. Chapter 4 is focused on the measurement of the energy scale and resolution for elec- trons and photons. The tools I developed are described and the results I obtained with the legacy reprocessing for the paper in preparation are reported. The origin of discrepancies between the data and the simulation are also discussed, and the corrections needed in the H γγ analysis to compensate such discrepancies are shown. → The strategy and the main details of the H γγ analysis are presented in Chapter 5 → with the corresponding results. Chapter 6 is dedicated to the mass measurement and the discussion of the results and uncertainties. I’ve contributed to estimate the uncertainties due to the energy scale and resolution and to reduced them improving the correction derivation described in Chapter 4. In this thesis, the most recent CMS public results on the H γγ analysis are shown, → whilst at the moment of writing, the reprocessing of the data with final calibration is being used in the H γγ analysis for final results to be published. The systematic uncertainties → presented in this thesis are however the one related to the most recent public results. Chapter 1 Standard Model Higgs and LHC physics 1.1 Standard Model Higgs boson The Standard Model (SM) [1, 2, 3] has been very successful in
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