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Search for New Physics with Top Quark Pairs in the Fully Hadronic Final State at the ATLAS Experiment

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Search for New Physics with Top Quark Pairs in the Fully Hadronic Final State at the ATLAS Experiment Dissertation submitted to the Combined Faculties of the Natural Sciences and Mathematics of the Ruperto-Carola-University of Heidelberg, Germany for the degree of Doctor of Natural Sciences Put forward by Mathis Kolb born in Heidelberg Oral examination: November 21, 2018 Search for new physics with top quark pairs in the fully hadronic final state at the ATLAS experiment Referees: Prof. Dr. André Schöning Prof. Dr. Tilman Plehn Search for new physics with top quark pairs in the fully hadronic final state at the ATLAS experiment Fully hadronic final states containing top quark pairs (tt) are investigated using proton- proton collision data at a center of mass energy of 13 TeV recorded in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider at CERN. The bucket algorithm suppresses the large combinatorial background and is used to identify and reconstruct the tt system. It is applied in three analyses. A model independent search for new heavy particles decaying to tt using 36 fb−1 of data is presented. The analysis concentrates on an optimization of sensitivity at tt masses below 1:3 TeV. No excess from the Standard Model prediction is observed. Thus, upper limits at 95% C.L. are set on the production cross section times branching ratio of 0 benchmark signal models excluding e.g. a topcolor assisted technicolor ZTC2 in the mass range from 0:59 TeV to 1:25 TeV. The prospects of a search for the production of the Higgs boson in association with tt using 33 fb−1 of data recorded solely in 2016 are studied. The expected sensitivity based on statistical uncertainties implies that at least 950 fb−1 of data would be required for an observation. From the additionally conducted trigger study it is concluded that developments at trigger level are required to use the analysis strategy presented in future searches for new physics. Suche nach neuer Physik mit Top Quark Paaren im vollhadronischen Endzu- stand am ATLAS Experiment Vollhadronische Endzustände mit Top Quark Paaren (tt) werden mit Proton-Proton Kollisionsdaten, die 2015 und 2016 bei einer Schwerpunktenergie von 13 TeV am ATLAS Experiment am Large Hadron Collider am CERN aufgenommen wurden, untersucht. Der Bucket Algorithmus unterdrückt den großen kombinatorischen Untergrund und identifiziert und rekonstruiert das tt System. Er wird in drei Analysen angewendet. Eine modellunabhängige Suche nach schweren Teilchen, die in tt zerfallen, wird unter Verwendung von 36 fb−1 an Daten präsentiert. Die Analyse konzentriert sich auf ei- ne Verbesserung der Sensitivität bei niedrigen Ditop Massen unterhalb von 1:3 TeV. Es lassen sich keine Abweichungen von der Standardmodellvorhersage beobachten. Da- her werden Grenzwerte mit 95% C.L. auf das Produkt aus Wirkungsquerschnitt und Verzweigungsverhältnis für Referenzsignalmodelle bestimmt, die z.B. erlauben das “top- 0 color assisted technicolor” ZTC2 zwischen 0:59 TeV und 1:25 TeV auszuschließen. Die Perspektiven der Suche nach der assoziierten Produktion eines Higgs Bosons mit tt werden anhand von 33 fb−1 ausschließlich 2016 genommener Daten ermittelt. Die erwartete Sensitivität, basierend auf statistischen Unsicherheiten, würde mindestens 950 fb−1 an Daten für eine Beobachtung erforderlich machen. Aus der zusätzlich durchgeführten Triggerstudie wird geschlussfolgert, dass Entwick- lungen im Triggerbereich benötigt werden, um die präsentierte Analysestrategie in zu- künftigen Suchen nach neuer Physik verwenden zu können. Contents 1. Motivation and outline 11 2. Theory 14 2.1. Standard Model of particle physics .................. 14 2.1.1. Electroweak symmetry breaking ................ 16 2.1.2. Quantum Chromodynamics .................. 19 2.2. Hadron collider physics ......................... 21 2.2.1. Proton-proton collisions and Monte Carlo .......... 22 2.2.2. Hadron collider kinematics ................... 24 2.2.3. Jet clustering .......................... 25 2.3. Top quark physics ........................... 30 2.4. Beyond the Standard Model physics ................. 33 2.4.1. Topcolor assisted technicolor ................. 35 2.4.2. Warped extra dimensions ................... 37 2.4.3. Dark matter ........................... 38 2.5. Statistics ................................ 40 2.5.1. Test of the SM-only hypothesis ................ 40 2.5.2. Exclusion limits ......................... 41 3. Experimental setup 42 3.1. Large Hadron Collider ......................... 42 3.2. ATLAS detector ............................ 48 3.2.1. Magnet system ......................... 49 3.2.2. Inner Detector ......................... 49 3.2.3. The calorimeters ........................ 51 3.2.4. The muon system ........................ 54 3.2.5. Data acquisition and trigger system .............. 54 3.3. Event simulation ............................ 57 3.3.1. Monte Carlo generation .................... 58 3.3.2. Detector simulation ...................... 61 3.4. Reconstruction and selection of physics objects and events in ATLAS 61 3.4.1. Tracks and vertices ....................... 61 3.4.2. Muons .............................. 62 3.4.3. Topological clusters ...................... 64 3.4.4. Electrons ............................ 64 3.4.5. Jets ............................... 65 7 3.4.6. Flavor tagging of jets ...................... 69 3.4.7. Missing transverse momentum ................. 70 3.4.8. Overlap removal ........................ 70 4. Description of the bucket algorithm 71 5. Performance of the bucket algorithm 79 5.1. Introduction ............................... 79 5.2. Data and simulation .......................... 79 5.3. Event preselection ........................... 80 5.4. Performance on simulated events ................... 81 5.5. Data–MC comparison ......................... 84 5.6. Conclusion ................................ 85 6. Search for new heavy particles decaying to a top quark pair in the fully hadronic final state 89 6.1. Introduction ............................... 89 6.2. Event selection ............................. 90 6.2.1. Object reconstruction ..................... 90 6.2.2. Event preselection and categorization ............. 92 6.3. Signal and background modeling ................... 95 6.3.1. Signal modeling of narrow spin-1 resonances ......... 95 6.3.2. Signal modeling of spin-2 resonances ............. 95 6.3.3. Signal modeling of broad spin-1 resonances .......... 96 6.3.4. Top quark pair modeling .................... 96 6.3.5. General treatment of simulated events ............ 96 6.4. Performance of top quark pair reconstruction ............ 96 6.4.1. Mass resolution ......................... 97 6.4.2. Reconstruction efficiency .................... 99 6.5. Multijet trigger ............................. 103 6.6. QCD multijet background estimation ................. 106 6.7. ABCD-Likelihood ............................ 107 6.8. Systematic uncertainties ........................ 110 6.8.1. Integrated luminosity ...................... 111 6.8.2. Pileup simulation ........................ 113 6.8.3. Flavor tagging of the jets ................... 113 6.8.4. Multijet shape uncertainty ................... 114 6.8.5. ABCD correlation ....................... 115 6.8.6. Jet vertex tagger ........................ 116 6.8.7. Jet energy resolution ...................... 116 6.8.8. Jet energy scale ......................... 117 6.8.9. Cross-section of SM top quark pair .............. 117 6.8.10. Hard scatter generation .................... 117 6.8.11. Fragmentation and hadronization model ........... 119 8 6.8.12. Scales and additional radiation ................ 121 6.8.13. Parton distribution functions ................. 122 6.8.14. Trigger scale factor ....................... 123 6.8.15. Electroweak corrections .................... 123 6.8.16. NNLO effect on SM top quark pairs ............. 123 6.9. Control variables ............................ 124 6.10. Results .................................. 130 6.10.1. Expected performance with the Asimov dataset ....... 130 6.10.2. Observed limit based on 2015 and 2016 data ......... 134 6.11. Summary and Outlook ......................... 143 7. Search for the Standard Model Higgs boson in association with a top quark pair in the fully hadronic final state 146 7.1. Introduction ............................... 146 7.2. Analysis strategy and event selection ................. 148 7.2.1. Object reconstruction and general preselection ........ 148 7.2.2. Analysis specific event preselection .............. 150 7.3. Matching performance ......................... 154 7.4. Analysis method ............................ 156 7.5. Signal and background modeling ................... 159 7.5.1. Signal Monte Carlo simulation ................ 160 7.5.2. Top quark pair simulation ................... 161 7.5.3. QCD multijet estimation .................... 161 7.6. Control distributions .......................... 162 7.7. Sideband method ............................ 168 7.8. Multivariate discrimination ...................... 171 7.9. Results .................................. 182 7.10. Conclusion and outlook ........................ 185 8. Bucket algorithm at trigger level 187 8.1. Introduction ............................... 187 8.2. Data and event selection ........................ 188 8.3. Adapting the bucket algorithm for trigger jets ............ 190 8.4. Performance ............................... 194 8.4.1. Comparison of trigger and offline jets ............. 194 8.4.2. Construction and labeling of new trigger chains ....... 197 8.4.3. Trigger efficiency in simulated events ............. 198
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