Dissertation Search for supersymmetry in the single lepton final state in 13 TeV pp collisions with the CMS experiment ausgefuhrt¨ zum Zwecke der Erlangung des akademischen Grades einer Doktorin der technischen Wissenschaften unter der Leitung von Prof. Dr. Jochen Schieck und Dr. Robert Schofbeck¨ CERN-THESIS-2017-300 29/01/2018 am Institut fur¨ Hochenergiephysik (HEPHY) der Osterreichischen¨ Akademie der Wissenschaften (OAW)¨ und am Atominstitut (E141) eingereicht an der Technischen Universitat¨ Wien Fakultat¨ fur¨ Physik von Ece As.ilar Matrikelnummer: 1428489 Wien, am 14. December 2017 This thesis is dedicated to the city of Vienna Acknowledgements It took three years for this thesis to come out. If it is written as required, the necessary appreciation for these three years will not fit into one page. For this reason only a brief version of my gratitude is shown here. First of all, I would like to thank Robert Schofbeck¨ for writing this project and choosing me as his student. At every crucial point of the work, he was there and supporting with very beneficial advice. He always encouraged us to reach our best. I also would like to thank Wolfgang Adam. He was always there to discuss problems and finding solutions together. He was always managing to make us happy even in the very stressful times with his calm and positive at- titude. I am also grateful to Prof. Jochen Schieck, my thesis supervisor and director of HEPHY, for providing an inspiring atmosphere within the institute. Furthermore, I want to thank the other jury members, Rosy Nikolaidou and Hubert Kroha for reading this thesis. Moreover, I want to thank all my colleagues at HEPHY for an efficient and at the same time friendly working atmosphere. I further appreciated support from Daniel Spitzbart, Navid k. Rad, Federico Ambrogy, Matheusz Zarucki, Er- ica Brondolin, Ilse Kratschmer, Johannes Brandstetter, Wolfgang Waltenberger and Suchita Kulkarni when I had a ski accident at the end of my PhD. I would like to thank also my ACDVF colleagues, Claudia Seitz, Henning Kirschenmann, Anadi Canepa, Dirk Krucker,¨ Stephan Lammel, Artur Lobanov, Isabell Melzer-Pellmann, Basil Schneider, Anna Stakia, Paris Sphicas and Markus Stoye for their great team work when publishing our analysis. A thank you should also go to all the CMS groups, the LHC team and all the people working at CERN. I would like to thank also Eveline Ess, Nathalie Fortin, Zorica Jelovic, Sonja Weiss. Moreover, thank you to our beloved IT at HEPHY, Dietrich Liko, for his pa- tience and very talented work. He always answered our needs whenever possi- ble including nights and weekends. I would like to thank Rene´ Brun for ROOT, Guido van Rossum for Python and Bram Moolenaar for Vim. A special thank you goes my mom, my brother, his girlfriend Meltem, and my dear friend Aysegul¨ for always providing the support that I need outside the work life. Final but not least, this thesis was made possible with the FWF project P26771- N20 and Doktoratskolleg Particles and Interactions (DK-PI). Thanks for all your encouragement! Abstract A search for Supersymmetry will be presented to turn one more stone in the quest of what is beyond the Standard Model. The search is performed in events with a single charged lepton, multiple jets, and missing transverse energy. The proton-proton collision data were recorded by the CMS experiment during the 2016 run of the LHC at a center-of-mass energy of 13 TeV. The integrated lu- 1 minosity of the dataset corresponds to 35.9 fb− . Although the search is model independent, a simplified model of gluino pair production with masses in the TeV range is used as an example. In the model, each of the gluinos decays to two light quarks and an intermediate chargino, with the latter decaying to a W boson and a neutralino. The neutralino is considered to be the stable lightest supersymmetric particle which results in substantial missing transverse energy in the final state. Only events with a single charged lepton, which can be an electron or a muon, are considered. This requirement provides a clean event topology as well as suppresses most of the multijet events. No b quark is expected in the final state of the tar- geted signal model. Therefore, in the event selection, a veto on b-tagged jets is included. The search uses a powerful discriminating variable to distinguish between background and signal events, and 28 exclusive signal regions are de- fined in di↵erent kinematic observables to enhance sensitivity to a range of di↵erent mass scenarios. The estimation of the Standard Model background yields in the signal regions is performed using data in the control regions. This methodology is verified using simulated samples and data in validation re- gions. Finally, systematic uncertainties related to the background prediction and simulated samples are determined. No significant deviation from the predicted Standard Model background is ob- served. Therefore, stringent upper limits on the cross section of the considered simplified model are set. As a result, gluino masses below 1.9 TeV are excluded for neutralino masses below 300 GeV with 95% confidence level. Contents Introduction 3 1 Supersymmetry: an extension of the Standard Model 4 1.1 Standard Model ................................... 4 1.1.1 Particle content ............................... 4 1.1.2 Particle interactions ............................. 5 1.1.3 From the quantum field theory window ................. 6 1.1.4 Experimental results ............................ 10 1.1.5 Shortcomings ................................ 11 1.1.5.1 Experimental reasons ...................... 11 1.1.5.2 Theoretical reasons ........................ 11 1.2 Supersymmetry as a solution ............................ 13 1.2.1 Algebra of Supersymmetry ......................... 14 1.2.2 Minimal Supersymmetric Standard Model ................ 15 1.2.2.1 Particle decays .......................... 17 1.2.3 Simplified models .............................. 17 1.2.4 Short history of SUSY searches at colliders ................ 18 2 Experimental setup 22 2.1 The LHC at CERN .................................. 22 2.1.1 The CERN accelerator complex ...................... 22 2.1.2 The LHC ................................... 23 2.2 The CMS detector .................................. 25 2.2.1 Superconducting magnet .......................... 26 2.2.2 Tracker .................................... 26 2.2.3 Electromagnetic calorimeter ........................ 27 2.2.4 Hadronic calorimeter ............................ 27 2.2.5 Muon system ................................ 28 2.2.6 Trigger and data acquisition systems ................... 28 2.2.7 Luminosity measurement ......................... 28 2.3 Event simulation ................................... 29 2.3.1 Event generation .............................. 29 2.3.2 Detector simulation ............................. 29 i 3 Object reconstruction and identification 30 3.1 Particle-Flow algorithm ............................... 30 3.2 Tracks and primary vertices ............................ 32 3.3 Jets .......................................... 33 3.3.1 Identification of b jets ............................ 34 3.4 Leptons ........................................ 35 3.4.1 Muons .................................... 35 3.4.2 Electrons ................................... 35 3.4.3 Isolation ................................... 37 3.5 Missing transverse energy ............................. 39 4 Event Selection: Baseline and search regions 43 4.1 SUSY signature ................................... 43 4.1.1 Key variables ................................ 44 4.1.2 Signal samples ................................ 45 4.2 Background processes ................................ 46 4.2.1 Scale factors ................................. 47 4.3 Baseline selection .................................. 49 4.4 Data samples ..................................... 51 4.4.1 Trigger selection ............................... 52 4.5 Event cleaning filters ................................ 54 4.6 Control plots ..................................... 55 4.7 Regions of interest .................................. 57 4.7.1 Signal and control region .......................... 57 4.7.2 Mainband regions .............................. 57 4.7.3 Aggregate regions .............................. 60 5 Background estimation: The RCS method 66 5.1 QCD background estimation ............................ 67 5.2 Background fraction calculations: b-tag multiplicity fit ............. 69 5.3 RCS method in t¯t + jets events ........................... 69 5.4 RCS method in W+jets events ........................... 73 5.5 Validation of the background estimation ..................... 75 6 Systematic uncertainties 80 6.1 Systematic uncertainties on background estimation ............... 80 6.2 Systematic uncertainties on signal modelling .................. 82 6.3 Common systematic uncertainties for signal and background modelling ... 83 ii 7 Results and interpretation 88 7.1 Results of the background prediction ....................... 88 7.1.1 Result of the validation in sideband regions ............... 88 7.1.2 Result of the background prediction in mainband regions ....... 88 7.1.3 Result of background prediction in aggregated regions ......... 89 7.2 Statistical interpretation .............................. 90 7.2.1 Frequentist limit setting procedure .................... 92 7.3 Interpretation on simplified model T5qqqqWW ................. 96 7.4 Comparison to complementary results ...................... 97 Conclusion 102 A