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Constraints of Leptoquark Models with CMS Data

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Constraints of Leptoquark Models with CMS Data Constraints of Leptoquark Models with CMS Data Masterarbeit vorgelegt von Henrik Jabusch November 2019 Eingereicht am 27. November 2019 1. Gutachter: Prof. Dr. Johannes Haller 2. Gutachter: Dr. Roman Kogler Abstract Recently observed deviations from the Standard Model prediction in B meson decays could be explained by existence of leptoquarks (LQs). LQs are new hypothetical bosons mediating transitions between leptons and quarks, and are predicted by several different theories beyond the Standard Model. Many direct searches for LQs have been performed at the Large Hadron Collider at CERN, but no evidence for their existence was found so far. In this master’s thesis, a remodeling of the CMS search for the pair production of scalar leptoquarks decaying exclusively into top quarks and muons is presented using a parameterized description of the CMS detector. The results are validated against those obtained with a full detector simulation. Good agreement is found. A reinterpretation of CMS data is performed to test a novel LQ flavor model motivated by recent hints towards lepton flavor universality violation. It includes sophisticated production processes of scalar leptoquarks beyond those of pair production only, which depend on the strength of the Yukawa coupling λ between the LQs and the quarks and leptons. The impact on the LQ production cross section and the kinematics of such decays is studied. For the scalar leptoquark S3, different couplings to third-generation quarks and second-generation leptons are studied and limits on the mass and the production cross section are derived. The LQ coupling to a b quark and a muon is constrained for the first time and found to yield the most stringent exclusion limits on the lower mass of the S3. For λ = 1, it is MLQ = 1520 GeV. Kurzfassung Kürzlich beobachtete Abweichungen von der Standardmodellvorhersage in Zerfällen von B- Mesonen könnten durch die Existenz von Leptoquarks (LQs) erklärt werden. LQs sind neue hypothetische Bosonen, die Lepton-Quark-Übergänge vermitteln und von mehreren verschie- denen Theorien jenseits des Standardmodells vorhergesagt werden. Viele direkte Suchen nach LQs wurden am Large Hadron Collider am CERN durchgeführt, ohne dass derzeit Beweise für deren Existenz gefunden wurden. In dieser Masterarbeit wird eine Neumodellierung einer CMS-Analyse mithilfe einer para- metrisierten Beschreibung des CMS-Detektors präsentiert. In der CMS-Analyse wurde nach Paarproduktion von skalaren Leptoquarks gesucht, welche ausschließlich in Top-Quarks und Myonen zerfallen. Die Ergebnisse der Neumodellierung werden mit den Ergebnissen der voll- ständigen Detektorsimulation verglichen. Eine gute Übereinstimmung wird gefunden. Eine Neuinterpretation von CMS-Daten wird durchgeführt, um ein neuartiges LQ-Flavor- Modell zu testen, welches durch jüngste Hinweise bezüglich einer Verletzung der Leptonflavor- universalität motiviert ist. Dieses beinhaltet erweiterte Produktionsprozesse von skalaren LQs jenseits der Paarproduktion, welche von der Stärke der Yukawa-Kopplung λ zwischen den LQs und den Leptonen und Quarks abhängen. Die Auswirkungen auf die Produktionswirkungsquer- schnitte der LQs und auf die Kinematik solcher Zerfälle wird untersucht. Für das skalare Lep- toquark S3 werden verschiedene Kopplungen an Quarks der zweiten Generation und Leptonen der dritten Generation untersucht und Grenzen auf die Masse sowie den Produktionswirkungs- querschnitt abgeleitet. Zum ersten Mal wird die LQ-Kopplung zu einem Bottom-Quark und einem Muon eingeschränkt. Diese stellen zudem die stärksten unteren Ausschlussgrenzen der Masse des S3 dar. Für λ = 1 beträgt diese MLQ = 1520 GeV. Contents 1. Introduction1 2. The Standard Model and Leptoquarks in BSM Physics3 2.1. The Standard Model of Particle Physics.....................3 2.1.1. Quantum Chromodynamics: The Strong Interaction...........5 2.1.2. The Proton: Structure and Collisions...................6 2.1.3. Electroweak Symmetry Breaking and the Higgs Mechanism......7 2.2. Shortcomings of the SM............................. 12 2.3. Beyond the Standard Model........................... 13 3. Leptoquark Phenomenology 16 3.1. Experimental Motivation............................. 16 3.2. Production of Leptoquarks in pp Collisions................... 19 3.3. Current Status of Leptoquark Searches at the LHC................ 20 3.4. Indirect Constraints on Leptoquarks....................... 21 3.5. Flavorful Leptoquark Model........................... 21 4. Experimental Setup 27 4.1. The Large Hadron Collider............................ 27 4.2. The Compact Muon Solenoid Experiment.................... 30 4.2.1. The Coordinate System.......................... 30 4.2.2. Detector Components........................... 32 iv 5. Leptoquark Analysis by CMS 37 5.1. Data Set and Event Generation.......................... 37 5.2. Event Selection.................................. 38 5.3. Leptoquark Mass Reconstruction......................... 39 5.3.1. Reconstruction of the Neutrino...................... 40 5.3.2. Reconstruction of the Top Quarks.................... 41 5.3.3. Reconstruction of the Leptoquarks.................... 41 5.4. Results....................................... 42 6. Remodeling the CMS Analysis 45 6.1. Signal Event Generation............................. 45 6.2. Detector Simulation with DELPHES ........................ 46 6.2.1. Tuning DELPHES ............................. 48 6.3. Results....................................... 54 7. Search for Scalar Leptoquarks 60 7.1. Signal Event Generation............................. 60 7.2. Results....................................... 61 −1/3 − 7.2.1. Decay Scenario: S3 → tµ ...................... 61 −1/3 − 7.2.2. Decay Scenario: S3 → tµ , bν .................... 65 −4/3 − 7.2.3. Decay Scenario: S3 → bµ ...................... 68 +2/3 7.2.4. Decay Scenario: S3 → tν ....................... 71 8. Conclusion and Outlook 74 A. Additional Tables 76 Bibliography 81 v 1. Introduction In particle physics, the Standard Model (SM) is a theory that describes three of the four known fundamental forces (the electromagnetic, weak and strong interactions) and classifies all known elementary particles. Formulated in the 1970’s as a gauge quantum field theory, the SM is extremely successful to date, tested in numerous experiments with unimagined precision. Al- though the SM is renormalizable and mathematically self-consistent, it leaves some questions unanswered. Neither does it contain gravity, explain neutrino masses, have a candidate for (cold) dark matter, nor allows for a unification of gauge couplings. In fact, the SM at the lat- est breaks down at energy scales where gravitation comes into play. This energy scale, called Planck scale, is of order 1019 GeV. Compared to the electroweak scale of about 100 GeV, this gives rise to the hierarchy problem that asks for a reason of this huge discrepancy. Conse- quently, the mass of the Higgs boson is unstable with respect to large quantum corrections, as quadratically divergent terms are added to the Higgs mass in higher order corrections. Many different theories were developed addressing the shortcomings of the SM, e.g. Grand Unified Theories (GUTs), Supersymmetry (SUSY) and compositeness models. In such beyond the Standard Model (BSM) approaches, often new particles such as leptoquarks (LQs) appear. Leptoquarks are hypothetical bosons that simultaneously couple to leptons and quarks. There- fore, they carry lepton and baryon number, and could violate the SM property of lepton flavor universality (LFU). Hints for such BSM effects have recently been observed in the decay of B mesons. Since LQs could explain all observed deviations simultaneously, they are promising candidates for new physics. Searches for LQs have been performed in many experiments, where many decay modes have been investigated, but no evidence for their existence was found so far. Hence, constraints on the LQ parameter space were provided. These constraints were generally obtained by simplified models considering leading-order diagrams for LQ pair production cross section only. A deeper investigation with more sophisticated models is needed to identify the regions of the LQ parameter space that are already excluded by current measurements, and in particular those that are not ruled out yet. This thesis aims at an exploration of the LQ parameter space by performing a more physically complete reinterpretation of published measurements conducted at the Large Hadron Collider 1 1. Introduction (LHC) at CERN. First, a search for pair-produced LQs coupled to third-generation quarks per- formed by the CMS collaboration [1] is remodeled using the fast simulation approach of the DELPHES framework [2] for the detector simulation. In addition, DELPHES is validated by com- paring this remodeling to the original analysis. The second step constitutes the transition to a more sophisticated LQ model that includes additional LQ production processes, which addi- tionally depend on the Yukawa coupling λ between LQs and quarks and leptons. Using data collected from the Compact Muon Solenoid (CMS) detector, constraints on the LQ mass, the LQ production cross section and also the Yukawa coupling are set. This thesis is organized as follows. Chapter2 discusses the theoretical foundations introduc- ing the SM and several BSM theories. The phenomenology of LQs is presented in Chapter3, including a novel LQ flavor model. A brief introduction to the Large Hadron Collider (LHC) and the CMS experiment is given in Chapter4. Chapter5 summarizes
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