Thesis Submitted to Mcgill University in Partial Fulfillment of the Requirements for the Degree of MASTER of SCIENCE
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Measurement of Associated Z Boson and Charmp Quark Production in Proton-Proton Collisions at s = 13 TeV Luis Pinto Cabrera Department of Physics McGill University, Montreal August, 2020 A thesis submitted to McGill University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE c Luis Pinto Cabrera 2020 Abstract This thesis presents the measurements of the differential production cross-sections of a Z boson in association with at least one charm-quark-initiated jet, where the Z boson de- cays leptonically into a muon-antimuon pair. Data recorded in proton-proton collisions by the ATLAS detector at the Large Hadron Collider (LHC) during the years of 2015 to 2018 is utilised, when the LHC was running at a center of mass energy of 13 TeV. The studied data corresponds to a total integrated luminosity of 139 fb−1. The measurements are com- pared to simulated data produced at next-to-leading-order and normalized to theoretical predictions at next-to-next-leading order. The results show that predictions underestimate the total number of events by 50% but display a good modelling of all distribution shapes of the chosen observables in data. The discrepancy can be attributed to a problem in the normalisation but further studies need to be made to settle the issue. iii Résumé Cette thèse présente les mesures de la section efficace de production différentielle d’un boson Z en association avec au moins un jet de saveur charme, ou le boson Z se désin- tègre leptoniquement en une paire muon-antimuon. Les données mesurées des collisions proton-proton par le détecteur ATLAS au grand collisionneur à Hadrons (GCH) pendant les années 2015 à 2018 sont utilisées, lorsque le GCH fonctionnait à un centre d’énergie de masse de 13 TeV. Les données étudiées correspondent à un total de luminosité intégrée de 139 fb−1 . Les mesures sont comparées aux données simulées produites au deuxième ordre et normalisées aux prédictions théoriques du troisième ordre. Les résultats montrent que les prédictions sous-estiment le nombre total d’évènements par 50% mais représentent un bon modèle pour toutes les formes de distribution des observables choisies des données. La contradiction peut être attribuée à un problème dans la normalisation, cependant des études plus poussées sont nécessaires afin de résoudre ce problème. iv Acknowledgements This thesis is dedicated to my parents and my brother for all the love and support I have received from them. I would like to thank my supervisor Prof. François Corriveau for the incredible op- portunity to work on the ATLAS experiment, for helping me choose such an interesting research topic and for all the feedback I have received on the analysis and the thesis during this period. I would also like to thank Dr. Jonathan Bossio for everything that he has taught me, for all the long calls explaining me details about the analysis and for all the support during these past two years. I would like to thank my friends, new ones and old ones, and the ATLAS group at McGill, that made the last two years a great experience and Montreal a place I can call home. v Author’s Contribution The author of this thesis contributed to the development of the analysis framework used to obtain all the results reported in this thesis. This framework will ultimately be used by the Z + HF group to investigate the Z + b(b) and Z + c(c) production. All chapters in this thesis are written by the author and contributions of the author to the analysis start from Chapter 4. vi Contents Introduction2 1 Theoretical introduction4 1.1 The Standard Model................................ 4 1.1.1 Particle content and interactions...................... 5 1.1.2 Quantum Chromodynamics......................... 6 1.2 Monte Carlo simulations.............................. 7 1.3 Z boson production ................................ 8 2 Experimental setup 11 2.1 Accelerator complex at CERN........................... 11 2.2 The Large Hadron Collider ............................ 12 2.2.1 Luminosity.................................. 13 2.3 The ATLAS detector................................ 14 2.3.1 ATLAS Geometry .............................. 14 2.3.2 Inner Detector................................ 15 2.3.3 Calorimeter.................................. 16 2.3.4 Muon Spectrometer ............................. 17 2.3.5 Trigger and Data Acquisition........................ 17 3 ATLAS object reconstruction and identification 19 3.1 Vertex definition and reconstruction ....................... 19 3.2 Muon definition and reconstruction........................ 20 3.2.1 Muon identification ............................. 20 3.2.2 Muon isolation................................ 21 3.3 Jet definition and reconstruction ......................... 22 3.3.1 Tagging.................................... 24 3.4 Missing energy definition and reconstruction................... 25 4 Description of data sets and selections 26 4.1 Data and simulated samples............................ 26 vii 4.1.1 Data set description............................. 26 4.1.2 Simulated data description ......................... 27 4.2 Event and object selections ............................ 29 4.2.1 General selection............................... 29 4.2.2 Muon selection................................ 30 4.2.3 Jet selection ................................. 30 4.2.4 Overlap removal ............................... 31 4.2.5 Z final state selection ............................ 31 4.3 Cutflows....................................... 31 4.4 Detector-level control distributions........................ 37 4.4.1 Run 2 data and Monte Carlo comparison ................. 37 4.4.2 Comparison between data-taking periods ................. 44 5 Flavour fit 50 5.1 Closure test..................................... 67 6 Measurement of Z boson production in association with at least one c-jet 71 6.1 Cross-section definitions.............................. 71 6.2 Fiducial phase space................................ 72 6.3 Unfolding...................................... 72 6.3.1 Closure Test ................................. 76 6.4 Uncertainties in the cross-section measurements................. 78 6.5 Results ....................................... 83 7 Discussion and conclusion 88 Appendices 90 A Light-jets rejection study 91 B Additional plots 93 B.1 Run 2 data MC - data detector-level distributions................ 93 B.2 Comparison between data-taking periods at detector-level ........... 95 B.3 Unfolding migration matrices ........................... 96 B.4 Closure test on unfolding ............................. 101 Abbreviations 108 Bibliography 109 viii 1 Introduction Particle physics is a branch of physics that studies the elementary particles and their interactions at the smallest scale. The Standard Model (SM) is one of the most successful theories within particle physics, validated by many experiments since its formulation. In recent years, scientists have been using experimental data produced by colliders to probe this theory. The Large Hadron Collider (LHC) is the world’s largest and most powerful particle collider and it has produced more data than ever produced before at energy scales only seen in the early universe. Particles produced in proton-proton collisions at the LHC are recorded at four main interaction points along the accelerator rings. The ATLAS (A Toroidal LHC ApparatuS) detector is located at one of them. The goal of this thesis is to use the data collected by the ATLAS detector to measure the production rate of the Z boson in conjunction with a charm quark. The understanding of this process is essential to probe one of the underlying theories of the SM called Quantum Chromodynamics (QCD), particularly to probe perturbative QCD and test the effects of non-perturbative corrections. The thesis is organised as follows: Chapter1 presents the theory of the Standard Model, a description of Monte Carlo simulations and details about the production of Z bosons in colliders. The Large Hadron Collider (LHC) and the ATLAS experiment are described in Chapter2. Chapter3 discusses how particles and events are reconstructed offline from data recorded by the ATLAS detector. Chapter4 describes the experimental data and simulated samples used in this thesis, along with the selection criteria chosen to extract the signal of interest while maintaining a high background rejection. Moreover, this chapter also displays a comparison between data and Monte Carlo simulation (MC) of relevant physical quantities at detector-level to validate the predictions which will be used in the following chapters. Chapter5 describes the methodology to extract only events containing charm- quark-initiated jets in the experimental data. In a nutshell, the contribution of each quark flavour to the total number of jets is estimated through a data-driven likelihood fit for each variable of the analysis. The obtained normalization is then used to disentangle Z+c-jet events from events containing other quarks produced at the same time as the Z. Chapter 2 6 presents the cross-section strategy and displays the results. Lastly, the conclusion and outlook are given in Chapter7. 3 Chapter 1 Theoretical introduction 1.1 The Standard Model The universe and all the matter around us is made of fundamental components of matter called elementary particles. These basic building blocks and the forces that govern them are described by the quantum field theory of the Standard Model (SM). The Standard Model was developed in the early 1970s and it has become the accepted theory