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Measurement of Jet Substructure in Boosted Top Quark Decays at CMS

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Measurement of Jet Substructure in Boosted Top Quark Decays at CMS vorgelegt von Jan Skottke geboren am 3. Januar 1995 Masterarbeit im Studiengang Physik Universität Hamburg November 2019 1. Gutachter: Prof. Dr. Johannes Haller 2. Gutachter: Dr. Roman Kogler Abstract A measurement is presented of the differential t¯t production cross section as a function of the N-subjettiness ratio τ32 in fully merged hadronic top quark de- 0 cays. These decays of the type t ! bW ! bqq¯ are reconstructed using anti-kT jets with a distance parameter of 0:8 and transverse momentum greater than 400 GeV. The measurement is performed with data collected with the CMS detector at a center-of-mass energy of 13 TeV in 2016 corresponding to an integrated lumi- nosity of 35:9 fb−1. The data is unfolded to the particle level in order to correct for detector effects. This makes it possible to compare the data to predictions from event generators using different tunes and varied model parameters. It is found that the measurement is sensitive to the simulation of final state radia- tion, allowing for the possibility to constrain model parameters associated with it. This is important for achieving higher precision in the identification of jets originating from fully merged hadronic top quark decays. iii Zusammenfassung Die vorliegende Arbeit stellt eine Messung des differentiellen t¯t Produktion- swirkungsquerschnittes als Funktion des N-subjettiness Verhältnisses τ32 in vol- lkommen kollimierten Top Quark Zerfällen vor. Zerfälle der Art t ! bW ! bqq¯0 werden mit anti-kT Jets mit einem Radiusparameter von 0:8 und einem transver- salen Impuls von mehr als 400 GeV rekonstruiert. Die Messung benutzt Daten, welche vom CMS Detektor, bei einer Schwer- punktsenergie von 13 TeV aufgenommen wurden und einer integrierten Lumi- nosität von 35:9 fb−1 entsprechen. Die gemessenen Daten werden um Detek- toreffekte korrigiert und mit Monte Carlo simulierten Verteilungen verglichen. Durch diese Entfaltung ist es möglich, die Daten mit verschiedenen Vorhersagen von Ereignisgeneratoren mit verschiedenen Einstellungen und variierten Mod- ellparametern zu vergleichen. Es zeigt sich, dass die Messung sensitiv auf die Modellierung der Abstrahlung von Endzustandsteilchen ist, was es ermöglicht entsprechende Modellparameter einzuschränken. Dies ist vor allem für eine höhere Präzision bei der Identifikation von Jets aus vollkommen kollimierten Top Quark Zerfällen entscheidend. v Contents 1 Introduction 1 2 Theory 3 2.1 The Standard Model of Particle Physics . 3 2.1.1 Quantum Electro Dynamics . 4 2.1.2 Quantum Chromo Dynamics . 5 2.1.3 Charged-Current Weak Interaction . 6 2.1.4 Electroweak Unification . 7 2.1.5 Higgs Mechanism & Spontaneous Symmetry Breaking . 9 2.1.6 Shortcomings of the Standard Model . 10 2.2 The Top Quark . 11 2.2.1 Production and Decay at the LHC . 11 2.3 Physics of Proton-Proton Collisions . 13 2.4 Jet Substructure . 14 2.5 Simulation of Proton-Proton Collisions . 17 3 Measurement of Jet Substructure 19 3.1 Jet Mass . 19 3.2 N-subjettiness . 19 4 Experiment 23 4.1 The Large Hadron Collider . 23 4.2 The Compact Muon Solenoid . 25 4.2.1 Coordinate System . 26 4.2.2 Magnet . 27 4.2.3 Tracking System . 27 4.2.4 Calorimeters . 28 4.2.5 Muon System . 30 4.2.6 Trigger . 30 5 Reconstruction and Identification of Objects 33 5.1 Signature of Particles in the CMS Detector . 33 5.2 Particle Flow Algorithm . 34 vii Contents Contents 5.3 Muon Identification . 35 5.4 Electron Identification . 36 5.5 Reconstruction of Jets . 36 5.5.1 The anti-kT Jet Clustering Algorithm . 36 5.5.2 Pileup Mitigation Techniques . 38 5.5.3 Jet Energy Corrections . 39 5.5.4 b tagging . 39 5.6 Missing Transverse Momentum . 40 6 Analysis 41 6.1 Data Sets and Event Simulation . 41 6.1.1 Data Sets . 42 6.1.2 Monte Carlo Samples . 42 6.2 Analysis Strategy . 45 6.3 Studies on Particle Level . 45 6.4 Studies on Reconstruction Level . 47 6.5 Unfolding . 55 6.5.1 Regularized Unfolding . 55 6.5.2 Determination of Bin Sizes . 57 6.5.3 Migration Matrix . 61 6.5.4 Validation Tests . 62 6.6 Uncertainties . 66 6.6.1 Statistical Uncertainties . 66 6.6.2 Experimental Uncertainties . 66 6.6.3 Model Uncertainties . 68 6.6.4 Total Uncertainty and Correlation . 69 6.7 Unfolding of Data . 69 7 Summary and Outlook 75 viii 1 Introduction The standard model of particle physics (SM) is a theory, that describes three of the four know fundamental forces. Despite the successful description of ex- perimental data, there are still unanswered phenomena which the SM can not describe. For this reason particle collider like the Large Hadron Collider (LHC) are built to further test the standard model and search for new physics. The top quark is the heaviest particle in the standard model. Due to its high mass, it has a high Yukawa coupling to the Higgs boson and therefore plays an important role in the electroweak sector. Many new physics models predict new heavy particles which also have high Yukawa couplings to the top quark. Since those hypothetical new particles are often excluded at low masses, searches for new physics often aim at heavy particles. Because of the large mass of those new particles the decay products, e.g. top quarks, are highly Lorentz boosted. As a consequence their decay products can be reconstructed with a single jet. To distinguish jets originating from top quarks and jets originating from other particles, information regarding their substructure is being used in the recon- struction of collision events at the LHC. Therefore, a good understanding of the jet substructure is important. This analysis presents a first measurement of the differential t¯t production cross section as a function of the N-subjettiness ratio τ32 in the boosted regime with the Compact Muon Solenoid (CMS) detector. It is a variable to characterize the substructure of hadronic jets and plays an important role in the identification of jets containing a top quark decay since it discriminates three-prong decays of the top quark from two-prong and one-prong topologies. This analysis uses data collected with the CMS detector in proton-proton collisions at the LHC with a center-of-mass energy of 13 TeV in 2016 corresponding to an integrated luminosity of 35:9 fb−1. For this analysis, data is unfolded to particle level using regularized unfolding within the TUnfold framework. The result can then be used to constrain different tunes of the simulation to achieve higher precisions in the identification of jets originating from fully merged top quark decays. The thesis is structured in the following way. The theoretical foundations of the standard model, a more detailed look at the top quark, and an introduction to jet substructure are provided in Chapter 2. An overview of already performed 1 studies on jet substructure is given in Chapter 3. The experimental setup con- sisting of the LHC and the CMS experiment are introduced in Chapter 4. This is followed by a description of the algorithms that are used to reconstruct and identify physical objects measured inside the detector in Chapter 5. In Chapter 6 the analysis is presented, discussing the used data and simulation samples, the phase space definition, further selection requirements, treatment of the uncer- tainties, and presenting the measurement of the differential t¯t production cross section as a function of τ32. The thesis is closed by a conclusion and outlook in Chapter 7. 2 2 Theory The following chapter gives an introduction to the theoretical background of the presented analysis. It starts with an overview of the standard model of particles in 2.1, followed by a more detailed look into the top quark and its properties in 2.2. Physics of proton-proton collisions are described in 2.3 and jet substructure is discussed in 2.4. The chapter closes with a description of simulations in high-energy physics. 2.1 The Standard Model of Particle Physics The Standard Model of particle physics is a quantum field theory that describes all known elementary particles and their interactions through three of the four known fundamental forces. Those forces are the strong, weak and electromag- netic interaction. The fundamental particles can be grouped into fermions and 1 bosons with 2 and integer spin, respectively, as shown in Fig. 2.1. Fermions are further divided into quarks and leptons. Quarks are grouped into two different types, up-type and down-type. Up-type quarks carry a charge of 2 1 3 e and down-type quarks a charge of − 3 e, where e denotes the charge of the positron. Both types also carry a color charge with three different states. Lep- tons can either be charged or neutral. Charged leptons have an electric charge of −e while neutral leptons, called neutrinos, carry no electric charge. In addition, fermions also have a weak isospin which is for up-type quarks and neutrinos 1 1 T3 = 3 and for down-type quarks and charged leptons T3 = − 3 . Fermions come in three generations. Each fermion has an anti-fermion with inverted electrical- and color-charge. The gauge bosons are the mediators of the fundamental forces. The electromag- netic and strong force are mediated through the massless photons and gluons, respectively. The massive W± and Z0 bosons mediate the weak interaction. Those bosons carry a spin of one, while the Higgs boson is scalar. The Higgs boson is a result of the spontaneous symmetry breaking in the electroweak sec- tor and provides an explanation of how elementary particles get their masses.
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