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 Claire Antel born in Johannesburg Oral examination on February 4th, 2019 Enhancing low mass dark matter mediator resonance searches with improved triggering in the ATLAS detector Referees: Prof. Dr. Monica Dunford Prof. Dr. Stephanie Hansmann-Menzemer Abstract v Zusammenfassung Dunkle Materie ist ein wesentlicher Bestandteil unseres Universums, jedoch ist ihre Eigenschaft bisher den Teilchendetektoren entgangen. Dunkle Ma- terie kann in Protonenkollisionen am Large Hadron Collider durch die Her- stellung und den anschließenden Zerfall eines Mediators erzeugt werden, der das Standardmodell mit dem dunklen Sektor koppelt. Neue Suchstrategien zielen darauf ab, den schwer zugänglichen Phasenraum bei geringen Massen zu erschließen, bei dem die beschränkte Triggerbandbreite die Sensitivität auf- grund der vielen niederenergetischen Interaktionen in Protonkollisionen stark einschränkt. Eine dieser Suchstrategien ist die Trigger-Objekt-Level-Analyse. In dieser wird die Bandbreitenbeschränkung umgangen, indem Objekte auf Triggerebene gespeichert werden, die ausschließlich aus einem Teil der vollen Ereignisinformation rekonstruiert wurden. Die Analyse ist daher eng mit den Fähigkeiten des Triggersystems verknüpft. Die vorliegende Arbeit umfasst verschiedene Verbesserungen der Triggerfunktionen des ATLAS-Detektors, um die Sensitivität gegenüber neuer Physik zu steigern. Ein neuer Algo- rithmus zur Identifikation der Protonenkreuzung für hochgesättigte Impulse wurde entwickelt, der den zum Auslösen erforderlichen Energiebereich des First-Level-Triggers auf die neuen hohen Energien des LHC Run 2 erweit- ert. Der Algorithmus wurde erfolgreich beauftragt und für Run 2 dauer- haft aktiviert. Die Leistung des High-Level-Jet-Triggers wird erweitert, um die Spurrekonstruktion auf Triggerebene als Eingabe eines neuen Trigger- Upgrades zu verwenden. Es wird demonstriert, wie dies die Sensitivität gegenüber niederenergetischer Resonanzen der Dunkle Materie in der Trigger- Objekt-Level-Analyse in zukünftigen Datenerfassungen des LHC verbessern wird. Abstract Dark matter comprises a significant component of our universe, but its par- ticle nature has evaded particle detectors thus far. Dark matter may be produced in proton collisions at the Large Hadron Collider through the pro- duction of a mediator that couples the Standard Model to the dark sector. With no hint of new particles in the most accessible mass range, new search strategies aim to access the challenging phase space that lies at low masses, where current trigger bandwidth limitations have strongly constrained the sensitivity owing to the many low energy interactions that occur within pro- ton collisions. One particular search strategy is the Trigger-object Level Anal- ysis, which circumvents bandwidth limitations by recording only objects re- constructed from partial event information at trigger-level and is therefore intimately linked to the capabilities of the trigger system. The body of work herein encompasses various improvements of the triggering capabilities of the ATLAS detector in order to retain and enhance the sensitivity to new physics. A new bunch-crossing identification algorithm is commissioned for highly sat- urated pulses, extending the triggerable energy range of the first-level trigger to the new high energies of the LHC Run 2. The algorithm is successfully com- missioned and permanently activated for Run 2 data taking. The capability of the high-level jet trigger is expanded in order to utilise trigger-level track reconstruction from the input of a new trigger upgrade. It is demonstrated how this will improve the sensitivity to low mass dark matter resonances in the Trigger-object Level Analysis in future data taking runs of the LHC. Acknowledgements I am grateful for having received the opportunity to conduct science at the world’s most powerful particle collider. For this I owe a great debt of gratitude to my supervisor, Monica Dunford and the people of the Heidelberg research training group who funded my years as a doctorate student. My introduction to ATLAS and the Heidelberg group would not have happened without my previous supervisor, Andrew Hamilton. Thanks, Andrew. I wish to thank Monica further for her guidance that lead me to the exciting work that I became involved in. I appreciated her creative ideas, inspirational stories and especially her words of encouragement during the past couple of months. Not once did I leave her office without feeling more motivated or more self-confident. I would like to give thanks to Prof. Dr. Stephanie Hansmann-Menzemer for agreeing to co-review my thesis. The comforts we enjoy within the ATLAS group at the KIP institute are owed to Prof. Hans-Christian Schultz-Coulon, and for this I would like to thank him and express my respect. I would also like to express my appreciation for Petra for dealing with all our badly filled-out paper work as secretary and still being nice about it. I would like to thank Pavel and Martin for the large amount of time they dedicated to reviewing my thesis, as well as Jan, Silvia, Hanno, Martin Klassen, Falk, Philipp and Dan for acting as reviewers for parts of my thesis. I want to thank the L1Calo group at CERN, especially Bruce, who always cared for my well-being, and crazy Eduard, whose words of encouragement meant a great deal to me. Jan and Silvia, thanks for making me laugh. Martin, thank you again for guiding me as technical supervisor through my L1Calo work. I would like to express my appreciation for the TLA team for creating a stimulating working environment and providing feedback on my TLA work. My friends and family were a great source of emotional support during my time here. Merve and Hanno, I really appreciated your friendship, support and day-dreamings of the future. Hanno, I hope one day you will offer me a ride in your fancy RV. Stanislav, you were an insufferable office mate, but otherwise a great friend when no open office windows in the dead of winter were involved. I cannot express enough how grateful I am for my CERN friends and our Alpine adven- tures. Danny, thanks for being my adventure buddy in snowboarding, mountain biking and countless hiking expeditions. I hope we haven’t shared our last sandwich blog post yet. Sam, no one could want for a more life-loving friend to conquer prominent mountains with. Dan, I look forward to being your climbing partner again. Thank you for being infinitely patient with me this year. Finally, I would like to express my deepest gratitude to my parents and my brother. With- out their unwavering support, my time as PhD student would have been a lot harder. I hope to hang out with you guys more frequently in the future. Contents 1 Introduction 1 2 The Standard Model of Particle Physics 5 2.1 Introduction .................................. 5 2.2 Mathematical framework of the Standard Model .............. 7 2.2.1 Electroweak symmetry breaking and the Higgs field ........ 8 2.2.2 Electroweak interactions ....................... 9 2.2.3 Strong interactions .......................... 10 2.3 Beyond the Standard Model ......................... 13 3 Dark Matter 15 3.1 History and observations ........................... 15 3.2 Experimental searches for dark matter particles .............. 16 3.2.1 Empirical knowledge on dark matter ................ 16 3.2.2 Theoretical and practical constraints ................ 17 3.2.3 Weakly interacting massive particles ................ 18 3.2.4 Detection methods .......................... 19 4 The Large Hadron Collider 27 4.1 Introduction .................................. 27 4.2 The injector chain ............................... 27 4.3 LHC design .................................. 28 4.4 Towards High Luminosity LHC ....................... 30 x CONTENTS 5 The ATLAS experiment 31 5.1 The detector .................................. 31 5.1.1 The inner detector .......................... 33 5.1.2 The calorimeter ............................ 34 5.1.3 The muon spectrometer ....................... 36 5.2 ATLAS Computing .............................. 36 5.2.1 Data reconstruction .......................... 36 5.2.2 Simulation ............................... 36 5.3 Data object definitions ............................ 37 5.3.1 Primary tracks and vertices ..................... 38 5.3.2 Jets .................................. 39 5.3.3 Pile-up jets .............................. 43 6 The ATLAS trigger system 47 6.1 The Level-1 trigger .............................. 47 6.1.1 The Level-1 Muon Trigger ...................... 48 6.1.2 The Level-1 Calorimeter Trigger ................... 49 6.2 The High-Level Trigger ............................ 51 6.2.1 Online tracking ............................ 51 6.2.2 Online jet reconstruction ....................... 52 6.3 Data transfer ................................. 52 6.4 The trigger menu ............................... 53 6.5 Data recording streams ............................ 53 6.6 Trigger rates .................................. 54 7 A new Level-1 calorimeter algorithm for enhanced bunch crossing iden- tification 57 7.1 Trigger tower pulse preprocessing ...................... 57 7.1.1 Analogue to digital conversion .................... 59 7.1.2 Pedestal correction and filtering ................... 59 CONTENTS xi 7.1.3 Bunch crossing identification ..................... 61 7.1.4 ADC to energy conversion