DISS. ETH NO. 26007 SEARCH FOR SUPERSYMMETRY IN OPPOSITE SIGN SAME FLAVOR LEPTON FINAL STATES WITH THE CMS DETECTOR A THESIS SUBMITTED TO ATTAIN THE DEGREE OF DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zuric¨ h) PRESENTED BY Minna Leonora Vesterbacka Olsson Master of Science, Lund University Born on 14 July 1989 Citizen of Sweden CERN-THESIS-2019-113 ACCEPTED ON THE RECOMMENDATION OF Prof. Dr. Rainer Wallny, examiner Dr. Else Lytken, Senior Lecturer, co-examiner 2019 Abstract This thesis presents searches for physics beyond the Standard Model (SM) of particle physics. The searches 1 are performed using 35.9 fb− of proton–proton collision data collected with the CMS detector during 2016 at a center-of-mass energy of √s = 13 TeV, at the CERN Large Hadron Collider (LHC). The new physics phenomena targeted assume strong and electroweak production of Supersymmetric (SUSY) particles in the framework of mimimally supersymmetric standard model. A powerful tool for searching for SUSY is by using final state scenarios containing two electrons or two muons of opposite electric charge. The document contains a summary of the theoretical framework of the SM and SUSY, along with a comprehensive description of the CMS experiment at the LHC accelerator complex. This thesis is based on two analyses, the first targeting strong and electroweak production of superpartners that lead to the leptonic final state along with many jets and large missing transverse momentum. The other analysis is targeting the direct production of the superpartners of the SM electrons and muon, using a final state of only leptons and large missing transverse momentum. Common to these searches is the use of the missing transverse momentum, which is a crucial variable for searches for R-parity conserving SUSY. The thesis contains a chapter on the performance of the missing transverse momentum reconstruction algorithm. The remainder of the thesis contains an overview of the search strategies and the estimation of the SM background processes specific to each SUSY production mode. Since no excess of collision data is observed with respect to the predicted SM backgrounds in neither of the searches, a statistical interpretation of the results is performed to set upper limits in the production cross sections on the SUSY particles. These limits greatly extend the limits set using 8 TeV collision data during the LHC Run 1. 2 Zusammenfassung In dieser Dissertation werden physikalische Prozesse jenseits des Standardmodells der Teilchenphysik (SM) 1 gesucht. Die Analysen werden mit 35.9 fb− Proton Proton Kollisionsdaten durchgefu¨hrt, die 2016 mit dem CMS Detektor bei einer Schwerpunktsenergie von 13 TeV im LHC am CERN gesammelt wurden. Die neuen Physikphanomene¨ zielen auf starke und elektroschwache Produktion von supersymmetrischen (SUSY) Teilchen im Rahmen des minimal SUSY Standardmodells ab. Leistungsf¨ahige Suchstrategien nach SUSY Teilchen verwenden Endzustandsszenarien mit zwei Elektronen oder zwei Myonen entgegengesetzter elektrischer Ladung. Dieses Manuskript enth¨alt eine Zusammenfassung der theoretischen Modelle von SM und SUSY sowie eine umfassende Beschreibung des CMS Experiments am Large Hadron Collider (LHC) Beschleunigerkomplex. Die Arbeit basiert auf zwei Analysen: Die erste zielt auf die starke und elektroschwache Produktion von Superpartnern ab, die zu einem leptonischen Endzustand zusammen mit mehreren Jets und großem fehlendem transversalen Impuls fuhren.¨ Die zweite Analyse untersucht die direkte Produktion der Superpartner von SM-Elektronen und -Myonen, wobei ein rein leptonischer Endzustand mit großem fehlendem transversalen Impuls verwendet wird. Beide Suchen haben die Verwendung des fehlenden transversalen Impulses gemein, der eine entscheidende Variable fur¨ die Suche nach R-Parity erhaltender SUSY ist. Diese Dissertation enth¨alt ein Kapitel ub¨ er die Leistung des Rekonstruktionsalgorithmus des fehlenden transversalen Impulses sowie einen Ub¨ erblick ub¨ er die Suchstrategien und die Absch¨atzung der SUSY Produktionsmodus spezifischen SM Hintergrundprozesse. Da bei keiner der Suchen ein Ub¨ erschuss an Kollisionsdaten in Bezug auf die vorhergesagten SM Hintergrundpr- ozesse beobachtet wird, wird eine statistische Interpretation der Ergebnisse durchgefuhrt,¨ um Obergrenzen fur¨ die Produktionsquerschnitte der SUSY Teilchen festzulegen. Diese neuen Grenzwerte erweitern die w¨ahrend des LHC Run 1 unter Verwendung von 8 TeV Kollisionsdaten gemesssenen Grenzwerte erheblich. 3 Acknowledgements A great number of people deserve recognition for directly or indirectly providing me the support needed to complete this thesis. First and foremost, I am grateful for the mentorship of my advisor at ETH Zuric¨ h, Professor Rainer Wallny. I would like to thank Rainer for his support and for all the opportunities he has provided me during the past four years, and for pairing me up with the best post-doc I could ask for, Pablo Martinez Ruiz Del Arbol. Pablo has provided me the guidance and creativity needed to become a great physicist. I consider myself extremely fortunate to have learnt from someone as knowledgable as Pablo, and his constant support and inspiration has improved many workdays. I owe an immense amount of gratitude to Zeynep Demiragli, for her leadership, her mentorship, and later on, also our friendship. There has been times during my PhD when I lacked motivation and energy. Without the tireless support and encouragement of Lesya Shchutska, Loukas Gouskos and Mariarosaria D’Alfonso the MET paper and Slepton paper would not have been published. I was lucky to be surrounded by a great group of friends during my time in Geneva. I would especially like to thank Rickard, Sophie, Linnea and Andrew for all the great adventures, R1 breaks and late nights. This place would not have been the same without you, and I will cherish our friendships for life. To Stany, you are a great inspiration and have always been the best support. I am proud to call you my friend. Finally, I want to express my endless gratitude to my family. To my older brother, Adam, I am thankful for your brotherly advice and your love. I know I always have your support, even though I might not always follow the path you would have wanted. To his wife, Linn´ea, I am thankful for our common passion for science and our long friendship. Your energy and motivation has always been an inspiration to me, and I am looking forward to having a third doctor in the family :). To my father, Martin, the first doctor in the family, I am thankful that you early on sparked my interest for science. Even though I did not become the doctor you had intended me to, I know how extremely proud you are of me today. To my mother, Lillemor, I am thankful for your unconditional love and the close relationship we have had for 30 years, and will have for many more. Our weekly phonecalls have been the greatest support I could imagine during these past four years. I dedicate this work to my parents, as their undivided love and encouragement is the reason I can call myself a doctor today. 4 TABLE OF CONTENTS page 1 INTRODUCTION ............................................ 9 I THEORETICAL FOUNDATIONS 11 2 THE STANDARD MODEL ....................................... 13 2.1 Particles and interactions ..................................... 14 2.2 The SM Lagrangian ........................................ 16 2.3 The success and shortcomings of the SM ............................. 24 3 SUPERSYMMETRY ........................................... 26 3.1 Problems of the SM ........................................ 27 3.2 Minimal Supersymmetric Standard Model............................ 28 3.3 SUSY breaking mechanisms .................................... 32 4 LEPTONIC SUSY SEARCHES ..................................... 34 4.1 SUSY at the LHC ......................................... 35 4.2 Simplified models .......................................... 36 4.3 SUSY with opposite sign same flavor leptons .......................... 36 II EXPERIMENTAL SETUP 42 5 THE LARGE HADRON COLLIDER ................................. 43 5.1 The accelerator chain ....................................... 44 5.2 Beam parameters .......................................... 45 5.3 Coordinate system and kinematic variables ........................... 46 6 THE CMS EXPERIMENT ....................................... 48 6.1 The CMS detector ......................................... 49 6.2 The Magnet ............................................. 50 6.3 The Tracker ............................................. 50 6.4 The ECAL ............................................. 51 6.5 The HCAL ............................................. 53 6.6 The muon system .......................................... 54 7 EVENT RECONSTRUCTION ..................................... 56 5 7.1 The trigger system ......................................... 57 7.2 Data reconstruction ........................................ 59 7.3 Simulated events .......................................... 59 8 PHYSICS OBJECTS .......................................... 65 8.1 The Particle Flow Algorithm ................................... 66 8.2 Electrons .............................................. 68 8.3 Muons ................................................ 71 8.4 Jets ................................................. 74 8.5 b-tagged jets ............................................ 77 8.6 Isolated tracks ........................................... 79 III SEARCH METHODOLOGY 80 9 MISSING TRANSVERSE MOMENTUM ..............................