Olof Lundberg Searches for Dark Matter and Large Extra Dimensions in Monojet Final States with the ATLAS Experiment Department of Physics Stockholm University 2016 Doctoral Dissertation 2016 Fysikum Stockholm University Roslagstullsbacken 21 106 91 Stockholm c Olof Lundberg, Stockholm University 2016 ISBN 978-91-7649-429-5 Cover illustration: c Klara Arvidsson Printed in Sweden by Holmbergs, Malmö 2016 Distributed by Publit Abstract This thesis presents searches for evidence for Weakly Interacting Massive Particles (WIMPs) and Extra Dimensions in proton-proton collisions recorded by the ATLAS experiment at the CERN Large Hadron Collider (LHC). The WIMP is one of the main candidates to constitute the particle content of Dark Matter. Extra Dimensions are introduced in several theories in order to explain the apparent weakness of gravity when compared to the other interactions in Nature. Theories with WIMPs as well as Extra Dimensions can manifest them- selves at the LHC, with experimental signatures characterized by an energetic hadronic jet associated with large missing momentum. These signatures are known as monojet signatures, and are investigated in this thesis. −1 The firstp analysis is performed using L = 20.3 fb of proton-proton colli- sions at s = 8 TeV recorded in the ATLAS Run 1. The secondp analysis is per- formed using L = 3.2 fb−1 of proton-proton collisions at s = 13 TeV recorded in the ATLAS Run 2. No significant excess over the expected background is found in either of the analyses. New exclusion limits are set at 95% confidence level on Dark Matter particle production. New limits are also set on graviton production in the so-called ADD scenario with Extra Dimensions. Contents Acknowledgements1 Preface5 Overview................................5 About This Thesis............................6 Author’s Contribution..........................8 I Theory and Previous Research 11 1 The Standard Model of Particle Physics 13 1.1 Introduction............................ 13 1.2 Particles and Interactions - an Overview............. 14 1.3 Symmetries in Gauge Theories.................. 16 1.4 Quantum Chromodynamics.................... 19 1.5 Perturbative Method and Feynman Diagrams.......... 21 1.6 Shortcomings of the Standard Model............... 24 2 Beyond the Standard Model 29 2.1 Introduction............................ 29 2.2 Extra Spatial Dimensions..................... 29 2.3 Dark Matter............................ 37 2.4 On the Validity of Effective Theories............... 52 II Experimental Facilities 53 3 Experiment 55 3.1 The Large Hadron Collider.................... 55 3.2 The ATLAS Detector....................... 57 4 Electronic Noise in the Tile Calorimeter 69 4.1 Hadron Showers and Calorimetry................. 69 4.2 The ATLAS Tile Calorimeter................... 70 4.3 Signal Reconstruction in the Tile Calorimeter.......... 73 4.4 Electronic Noise: Definition and Usage............. 75 4.5 Digital Noise........................... 76 4.6 Cell Noise............................. 77 4.7 Noise for ATLAS Monte Carlo Simulations........... 78 4.8 TileCal Low Voltage Power Supplies............... 79 4.9 Electronic Noise in 2011..................... 80 4.10 Electronic Noise with New LVPS................. 81 4.11 Conclusions and Further Developments............. 83 5 Particle Identification with ATLAS 87 5.1 Primary Vertices.......................... 87 5.2 Electrons............................. 88 5.3 Muons............................... 92 5.4 Jets................................ 94 5.5 Photons.............................. 97 5.6 Overlapping Object Removal................... 97 5.7 Missing Transverse Momentum................. 98 5.8 Invariant and Transverse Mass.................. 99 5.9 Systematic Uncertainties..................... 100 6 Physics Simulations in ATLAS 103 6.1 Introduction............................ 103 6.2 Describing the Proton at High Momentum Transfer....... 104 6.3 Event Generation......................... 104 6.4 From Generated Events to Reconstructed Objects........ 108 III The Monojet Analysis 109 7 New Physics in the Monojet Final State 111 7.1 Introduction............................ 111 7.2 Data Collection.......................... 114 7.3 Signal Region Definitions..................... 114 7.4 Backgrounds........................... 116 8 Monte Carlo Simulations 119 8.1 Background Samples....................... 119 8.2 ADD Signal Samples....................... 122 8.3 WIMP Dark Matter Samples................... 132 8.4 Modeling Systematic Uncertainties................ 134 9 Data Driven Background Estimations 143 9.1 W Plus Jets and Z Plus Jets Backgrounds............. 143 9.2 Multi-Jet Background in the Signal Regions........... 168 9.3 Non-Collision Background.................... 168 9.4 Run 1 Results........................... 169 9.5 Run 2 Results........................... 172 IV Results and Conclusions 175 10 Interpretations and Limits 177 10.1 Introduction............................ 177 10.2 Comparison of Observed Data to Expectation.......... 178 10.3 Statistical Hypothesis Tests.................... 183 10.4 Model Independent Limits.................... 185 10.5 ADD Graviton Production.................... 186 10.6 WIMP Pair Production...................... 195 11 Conclusions 205 Sammanfattning på svenska 209 Bibliography 212 1 Acknowledgements Particle Physics, like life, is a collective effort. Much like quarks, particle physi- cists must interact strongly, and none of the nice-looking figures or results pre- sented in this thesis had been even remotely possible without the help and sup- port of others. For this support I am forever grateful. I want to thank a few key people in these five years of research. First and foremost, my warmest thanks to my supervisor Christophe Clé- ment. For your constant support and your patience through all my questions concerning everything from read-out electronics to the timescales of the early Universe. Most of all, thanks for your enthusiasm. You made me believe that we were actually going to find Dark Matter, and even though in the end we didn’t, you made the road to better experimental constraints much more fun to walk. I also want to thank my second supervisor Barbro Åsman for all the en- couragement. A huge thanks also to my co-worker for most of the analysis time Valerio Rossetti, for all the help, for all the analysis code that filled my folders with Italian swear words, and for making the long hours feel shorter. To Gabriele Bertoli, thanks for being the computer-wiz I most definitely am not. To Katarina Bendtz a very special thanks, not only for all the help with these final months of theory and statistics workshops, but also for being the best con- fidante and partner-in-crime imaginable. To Paweł Klimek and Maja Tylmad, a large thanks for the spinning-chair help sessions. I’m afraid I took more help than I gave, but I hope I’m forgiven. To Christian Ohm, thanks for Genftown hospitality, for beers in the fridge and for coffee by the lake on slow mornings. To all colleagues in the SU Particle Physics group and in the KTH ATLAS group, thanks for creating such a warm and supportive environment. In par- 2 ticular I would like to thank the entire present and previous Ph.D. staff in the SU and KTH ATLAS groups - Christian, Mattias, Karl, Maja, Paweł, Marcel, Kattis, Martin, Ablet, Simon, Olga, Maryon, Gabriele, Wayne, Anna, Nabila, Vladimir, Jelena, Emma, Edvin and Giulia - for lengthy discussions and inspi- ration. Thanks to all others at SU who have helped or guided me in one way or other. To my previous supervisors Christoph Bargholtz and Per-Erik Tegnér for introducing me to experimental physics. To friends around Fysikum then and now - Klas, Kate, Istvan, Rickard Jonas, others - for making life here easier. To Chad Finley, Joakim Edsjö, Bo Holmberg, Klas Hultqvist and Per-Erik Tegnér a big thanks for taking your time to go through my work and probe my knowledge at different defense events. You made both me and my texts better. Thanks to all the Tile community for being warm and supportive through my first nervous ATLAS steps. This goes especially to Claudio Santoni, Carlos Solans and Luca Fiorini for supervision, to Alexander Solodkov for all the help at all sorts of crazy hours, and to Brian Martin for showing me the ropes. To all the monojet people, thanks for the productive collaboration. Special thanks to the conveners: Philippe Calfayan, Jalal Abdallah, Arely Cortes Gon- zalez and David Šalek, who have all been encouraging and helpful from the moment I joined the group and all the way through the second analysis. Large thanks also to Valerio Ippolito, Cora Fischer and Ruth Pöttgen who at different stages of the analyses took time from their own work to help me understand var- ious analysis concepts. Thanks to Andy Buckley for helping me straighten out the graviton event generation. To my family and friends in the macroscopic world - parents, siblings, Lolo Lions, political allies - thanks for everything. You might perhaps not understand everything in this thesis (I’m still not sure I do), but I hope you do understand that none of it had been possible without you. A special thanks to Klara Arvids- son for drawing beautiful elephants at short notice. Finally to Karin I don’t know what words could possibly be enough to tell you how important you have been for the realization of this thesis. Thank you five thousand times around the Moon, to Jupiter and back. Olle, May 2016 3 List of Acronyms ADC Analog to Digital Converter ADD Arkani-Hamed, Dimopolous, Dvali