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Download File A search for supersymmetric phenomena in final states with high jet multiplicity at the ATLAS detector Matthew N.K. Smith Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2017 c 2017 Matthew N.K. Smith All rights reserved A search for supersymmetric phenomena in final states with high jet multiplicity at the ATLAS detector Matthew N.K. Smith ABSTRACT Proton-proton collisions at the Large Hadron Collider provide insight into fundamen- tal dynamics at unprecedented energy scales. After the discovery of the Higgs boson by the ATLAS and CMS experiments completed the Standard Model picture of par- ticle physics in 2012, the focus turned to investigation of new phenomena beyond the Standard Model. Variations on Supersymmetry, which has strong theoretical underpinnings and a wide potential particle phenomenology, garnered attention in particular. Preliminary results, however, yielded no new particle discoveries and set limits on the possible physical properties of supersymmetric models. This thesis de- scribes a search for supersymmetric particles that could not have been detected by earlier efforts. The study probes collisions with a center of mass energy of 13 TeV detected by ATLAS from 2015 to 2016 that result in events with a large number of jets. This search is sensitive to decays of heavy particles via cascades, which result in many hadronic jets and some missing energy. Constraints on the properties of reclus- tered large-radius jets are used to improve the sensitivity. The main Standard Model backgrounds are removed using a template method that extrapolates background be- havior from final states with fewer jets. No excess is observed over prediction, so limits are set on supersymmetric particle masses in the context of two different theo- retical models. Gluino masses below 1500 and 1600 GeV, respectively, are excluded, a significant extension of the limits set by previous analyses. Contents Contentsi List of Figures iii List of Tables ix 1 Introduction1 2 The Standard Model4 2.1 The Particle Composition of the Standard Model6 2.2 The Dynamics of the Standard Model9 2.3 Shortcomings 12 3 Jets 15 3.1 Connecting Collisions to Jets 16 3.2 Jet Algorithms 21 3.3 Conclusions 28 4 Supersymmetry 29 4.1 Motivations 29 4.2 The Particle Composition of SUSY 33 4.3 SUSY Breaking 37 4.4 SUSY and the LHC 39 4.5 Conclusions 42 i 5 The LHC and ATLAS 43 5.1 The Large Hadron Collider 43 5.2 The ATLAS Detector 50 5.3 Triggers 63 5.4 Data Quality 64 5.5 Simulation 65 5.6 Conclusion 66 6 Searching for SUSY in Multijet Events 68 6.1 Motivation 69 6.2 Signal models 72 6.3 Object definitions and Event Cleaning 80 6.4 Event Selection 85 6.5 Standard Model Backgrounds 90 6.6 Systematic Uncertainty 99 6.7 Statistical Methods and Results 108 6.8 Summary 119 7 Conclusion 120 Bibliography 122 Appendix A: The Particle Spectra of the pMSSM 132 Appendix B: Control Region Variables 135 Appendix C: Signal Region Reach 147 ii List of Figures 2.1 A recent summary of Standard Model cross sections as measured by AT- LAS. Theory and experiment agree closely across all channels.5 2.2 An example of a second order Feynman diagram, for an interaction be- tween a photon and a charged fermion (e.g., an electron). As long as momentum is conserved at each vertex, the intermediate momenta of k and k0 are not constrained, and are free to diverge. 13 3.1 Measurements of the strong coupling αS as a function of energy scale Q [35] 19 3.2 A set of example PDFs, the MSTW from 2008, next-to-leading order (NLO). This plot shows the probability that a specific particle constituent of the proton will interact as a function of the momentum fraction x and the overall momentum scale Q2, to a one-sigma confidence level [36]. 19 3.3 A recent example of a di-jet event measured by ATLAS 21 3.4 A recent example of a twelve-jet event measured by ATLAS 22 2 4.1 One-loop quantum corrections to the Higgs mass parameter mH , for (a) a fermion f and (b) a scalar particle S [44]. 31 4.2 The dependence of the SM couplings αi on energy scale [45]. 33 4.3 The dependence of the MSSM couplings αi on energy scale [45]. Now the three couplings meet at around the µ = 1016 GeV scale. 37 4.4 Example summary limits on parameter space for (a) the Gtt simplified model of gluino production and (b) stop pair production from the ATLAS SUSY group [64]. 41 5.1 An aerial view of Geneva and surroundings with the LHC ring superim- posed. Courtesy of the ATLAS experiment [65]. 44 iii 5.2 Photo of the LHC tunnel and beam pipe. Courtesy of the ATLAS exper- iment [65]. 45 5.3 The injection system of the LHC and the placement of the four main LHC experiments around the ring [70]. 46 5.4 Cumulative luminosity versus day delivered to ATLAS during stable beams and for high energy p-p collisions [75]. 48 5.5 The peak interactions per crossing, averaged over all colliding bunch pairs (a), and the peak luminosity per fill (b), in 2016 [75]. 49 5.6 Mean number of interactions per crossing in 2015 and 2016, weighted by luminosity. [75]. 50 5.7 A slice of the ATLAS detector demonstrating how outgoing particles from the collision interact with the various materials of the detector layers [65]. 51 5.8 A schematic of the ATLAS detector, with people for scale [78]. 53 5.9 The structure of the ATLAS inner detector showing its component sensors. Figure (b) is missing the IBL, which was installed between Run-1 and Run-2 [79]. 55 5.10 The composition and layout of the calorimeters in the ATLAS detector [84]. 58 5.11 The `accordion' structure of the LAr electromagnetic barrel. Courtesy of the ATLAS experiment [65]. 60 5.12 Schematic showing the structure and optical readout system of a wedge in the tile calorimeter [87]. 60 5.13 Diagram of the ATLAS muon system, in the context of the whole detector [89]. 62 5.14 The cumulative luminosity versus time delivered to ATLAS (green), recorded by ATLAS (yellow), and deemed good quality data (blue) in (a) 2015 and (b) 2016 (ongoing) [75]. 65 6.1 Cumulative mass reach of ATLAS SUSY searches [64]. 71 iv 6.2 Feynman diagram of the 2-step decay process. 73 6.3 (a), the Run-1 limits for the 2-step simplified model, including the most sensitive analyses in this region [112] and (b), the first Run-2 limits from the 2015 multijet search [63]. 74 6.4 The distribution of the generated points in the 2-step grid with the ex- pected and observed limits from the 2015 multijet analysis [63]. 75 6.5 Particle spectra for a sample of potential SUSY models which are excluded by the multijet analysis and not excluded by any other analysis. Circled particles are the gluino, Higgsinos, and the bino LSP. Further examples are available in Appendix7. 76 6.6 The truth-level kinematic distributions a selection of pMSSM signal sam- ples in comparison to a similar mass Gtt point. The dashed lines show Run-1 signal selections for the multijet analysis (top) and 2-6 jet analysis (centre and bottom). 78 6.7 The limits from 8 TeV in the pMSSM plane. The various searches are complimentary. 79 6.8 Expected exclusion contours in the 2-step (left) and pMSSM (right) planes Σ for signal region selections with MJ > 340; 420; 500; 580 GeV. 88 6.9 The number of jets for the tt¯ and W +jets control regions for 7 (top), 8 Σ (center) and 9 (bottom), with MJ > 340 GeV. 93 Σ ¯ 6.10 The sum of large-R jet mass MJ for the tt and W +jets control regions Σ for 7 (top), 8 (center) and 9 (bottom), with MJ > 340 GeV. 94 p miss ¯ 6.11 The distribution of ET = HT for the tt and W +jets control regions for Σ 7 (top), 8 (center) and 9 (bottom), with MJ > 340 GeV. 95 p miss 6.12 The distribution of ET = HT at increasing jet multiplicities, prior to Σ any cut on MJ . The ratios are far from steady, indicating that a template directly from the six-jet region would be ill-advised. 98 v Σ ¯ 6.13 The correlation between MJ and HT for data (left) and tt MC simulation (right). 98 Σ 6.14 Six-jet template shapes for various cuts on MJ . Templates with a tighter Σ MJ cut overlap. 99 6.15 The six-jet template region (left) and the seven-jet template validation re- Σ Σ gion (right) for various cuts on MJ . Application of a cut on MJ improves the template prediction of the multijet background. 100 6.16 Results of the template method in the six signal regions (without the p miss ET = HT cut) for greater than 8, 9 and 10 jets. Shown is the distribution p miss of ET = HT with leptonic backgrounds normalized to their post-fit values.101 6.17 The degree of closure (after performing the background fit) observed in each closure region. The prediction is given by the sum of the multijet template prediction and the leptonic background. The solid lines are the predicted numbers of events and the points are the observed numbers.
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