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Direct Measurement of the Top Quark Decay Width in the Muon+Jets Channel Using the CMS Experiment at the LHC

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Vrije Universiteit Brussel Faculteit Wetenschappen en Bio-ingenieurswetenschappen Departement Natuurkunde Direct measurement of the top quark decay width in the muon+jets channel using the CMS experiment at the LHC Lieselotte Moreels Promotor Prof. Dr. Petra Van Mulders Co-promotor Prof. Dr. Jorgen D’Hondt Proefschrift ingediend met het oog op het behalen van de academische graad Doctor in de Wetenschappen April 2018 Doctoral examination commission: Prof. Dr. Nick van Eijndhoven (Vrije Universiteit Brussel, chair ) Prof. Dr. Alberto Mariotti (Vrije Universiteit Brussel, secretary ) Prof. Dr. Petra Van Mulders (Vrije Universiteit Brussel, supervisor ) Prof. Dr. Jorgen D’Hondt (Vrije Universtiteit Brussel, advisor ) Prof. Dr. Dirk Ryckbosch (Universiteit Gent) Dr. Ivan Marchesini (Vrije Universiteit Brussel) Dr. Rebeca Gonzalez Suarez (University of Nebraska, USA) Prof. Dr. Philippe Claeys (Vrije Universiteit Brussel) Cover illustration: Fisheye view of the CMS detector. The barrel (front) and one of the endcaps (back) are connected by the beam pipe. Picture taken from CMS-doc-13419-v1. Cover kindly designed by Isis Van Parijs. ISBN: 978 94 923 1271 6 NUR: 924, 926 c 2018 Lieselotte Moreels All rights reserved. No parts of this book may be reproduced or transmitted in any form or by any means, electronic, mechanical, pho- tocopying, recording, or otherwise, without the prior written permission of the author. Contents Introduction 1 1 The Particle Universe 3 1.1 The Standard Model . .4 1.1.1 Particles and forces . .4 1.1.2 Mathematical framework . .7 1.1.3 Open questions . 10 1.2 The Top Quark Sector . 12 1.2.1 Production and decay . 13 1.2.2 Top quark mass . 15 1.2.3 Top quark decay width . 15 2 The CMS Experiment and the LHC 23 2.1 The Large Hadron Collider . 23 2.2 The CMS Experiment . 26 2.2.1 The silicon tracker . 28 2.2.2 The electromagnetic calorimeter . 29 2.2.3 The hadron calorimeter . 31 2.2.4 The muon detectors . 33 2.2.5 Trigger and data acquisition . 34 2.2.6 The CMS experiment during the 2016 proton run . 38 3 Event Generation and Simulation 41 3.1 Simulation of Events at Proton Colliders . 41 3.1.1 The hard-scattering process . 42 3.1.2 Parton shower . 44 3.1.3 Hadronisation and decay . 46 3.1.4 Underlying event . 47 3.1.5 Simulation of the CMS detector . 49 3.2 Simulated Events for Top Quark Studies . 49 3.3 Reweighting Procedure for Simulated Samples . 53 3.3.1 Reweighting of the simulated top quark width . 53 3.3.2 Reweighting of the simulated top quark mass . 56 iii iv CONTENTS 4 Reconstruction of Proton Collisions 61 4.1 Track Reconstruction . 62 4.2 Calorimetry . 64 4.3 Reconstruction and Identification of Particles and Jets . 64 4.3.1 Muon reconstruction . 65 4.3.2 Electron reconstruction . 69 4.3.3 Jet reconstruction . 70 4.3.4 Identification of jets originating from b quarks . 74 4.3.5 Missing transverse energy reconstruction . 76 5 Event Selection & Reconstruction 79 5.1 Trigger and Basic Event Requirements . 79 5.2 Event Selection . 80 5.3 Event Reconstruction . 86 5.3.1 Reconstruction of generated top quark events . 86 5.3.1.1 Jet-parton matching . 87 5.3.1.2 Resolution functions . 87 5.3.2 Reconstruction of the top quark pair system . 89 5.3.2.1 Categorisation of events . 91 5.3.2.2 Kinematic fit . 91 5.3.2.3 Further selection requirements . 95 6 Measurement of the Top Quark Width 101 6.1 Measurement Procedure using One Variable . 101 6.1.1 Construction of a likelihood function . 102 6.1.2 Evaluation of the likelihood function . 107 6.1.3 Calibration curve . 108 6.1.4 Pull . 110 6.2 Combination of Several Sensitive Variables . 111 6.2.1 Template construction for another variable . 111 6.2.2 Combination of both variables . 113 6.2.2.1 Calibration curve . 114 6.2.2.2 Pull . 114 6.3 Systematic Uncertainties . 116 6.3.1 Theoretical uncertainties . 116 6.3.2 Experimental uncertainties . 118 6.3.3 Summary of systematic uncertainties . 120 6.4 Measurement of the Top Quark Decay Width . 123 7 Conclusions & Prospects 125 Summary 131 Samenvatting 133 Bibliography 135 CONTENTS v Appendix A Breakdown of Systematic Uncertainties 151 Appendix B Effect of the Systematic Uncertainties on the Sensitive Variable Distributions 157 Author’s Contributions 161 List of Abbreviations 163 Glossary 167 Acknowledgements 169 vi CONTENTS Introduction Complete knowledge about everything in the universe is the dream of physicists and evil masterminds trying to take over the world. The standard model (SM) of particle physics embodies much of our current understanding of what matter consists of and how it interacts. Yet, some puzzles remain. It is unclear how gravity can be incorporated into the SM and none of the known particles can account for the dark matter that is observed in the universe. Being the heaviest particle in the SM, the top quark is expected to be most sensitive to beyond the SM effects, such as interactions with as yet unknown particles. Besides direct searches for new physics, precise measurements of the properties of SM particles and interactions can be compared to their predictions, thus favouring or excluding certain beyond the SM theories. Further, precision measurements allow to perform consistency checks of the SM itself. Using the measurements of certain parameters as input, the values of other quantities can be predicted and compared to their direct measurements. It is therefore important that the uncertainties on all measurements are as small as possible, since this leads to more stringent tests of the SM. This thesis investigates the decay properties of the top quark. In the SM the top quark decays almost exclusively into a W boson and a b quark. The probability for this process to happen is reflected in the top quark decay width, which is predicted to have a value around 1.33 GeV. If the top quark is able to decay into other particles as well, as is possible in several extensions of the SM, the top quark decay width will be larger than the SM prediction. This is investigated by performing a direct measurement of the top quark decay width using proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of 13 TeV. The data were recorded by the CMS experiment in 2016 and correspond to an integrated luminosity of 35.9 fb−1. At first, a concise overview of the standard model is presented in Chapter 1. The production and decay of the top quark is described in more detail and the current status of the top quark mass and decay width measurements is given. Top quarks are typically studied by producing them in particle colliders. Chapter 2 describes the Large Hadron Collider and the Compact Muon Solenoid experiment, which were used to produce and record the top quark events analysed in this thesis. In order to understand how the observations in a particle detector correspond to the theoretical interactions between colliding particles, collision events are simulated. A step-by-step overview is given in Chapter 3. In addition, the simulated samples used in this thesis are summarised and a reweighting technique is presented to acquire simulated top quark pair events with a distribution characterised by an arbitrary top quark mass 1 2 Introduction and decay width. Chapter 4 expands on how electronic detector signals are processed and combined to reconstruct particles. These are used in Chapter 5 to set up a basic event selection and reconstruct higher-order objects, such as the top quark. Based on the reconstructed event topology, additional requirements are posed on the selected events. The measurement procedure itself is based on a maximum likelihood technique and is described in Chapter 6. It is set up using simulated events only, so as not to let the result of the measurement influence the procedure. Probability density functions are constructed using the distributions of variables that are sensitive to changes in the top quark decay width. These changes are simulated using the reweighting technique introduced in Chapter 3. Further, the measurement procedure is calibrated and the systematic uncertainties affecting the measurement are discussed. In addition, the likelihood functions of several sensitive variables are combined to get a more precise measurement. The results of the two top quark decay width measurements presented in this thesis are summarised in Chapter 7 and possible improvements to obtain an even more precise measurement are discussed. As is common in high-energy physics, particle masses and momenta will be expressed in GeV/c2 and GeV/c, resp., where 1 eV ' 1.6 × 10−19 J is equal to the energy an electron acquires when it is accelerated over a potential difference of 1 V. Further, natural units are used, i.e. ~ = 1 = c, which allows to express both mass and momentum in units of GeV. The Particle Universe 1 All throughout history people have been intrigued by nature. Beholding its awe-inspiring beauty spawned a great number of questions, such as how life originated and what is the meaning of life. These questions are beyond the scope of this thesis. Other questions enquired about the fundamental building blocks that everything that can be seen consists of. Over the centuries, many theories have been put forward, evolving from a simple classification such as the elements water, earth, wind, and fire to the standard model of particle physics. Only the latter will be discussed in this chapter. Deconstructing matter, the concept of molecules and atoms is well-known to most.
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