Searches for heavy vector-like quarks decaying to high transverse momentum W bosons and top- or bottom-quarks and weak mode identification with the ATLAS detector by Steffen Henkelmann B.Sc., The University of Göttingen, 2011 M.Sc., The University of Göttingen, 2013 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Physics) THE UNIVERSITY OF BRITISH COLUMBIA CERN-THESIS-2018-212 29/06/2018 (Vancouver) August 2018 c Steffen Henkelmann 2018 The following individuals certify that they have read, and recommend to the Faculty of Gradu- ate and Postdoctoral Studies for acceptance, the dissertation entitled: Searches for heavy vector-like quarks decaying to high transverse momentum W bosons and top- or bottom-quarks and weak mode identification with the ATLAS detector submitted by Steffen Henkelmann in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics Examining committee: Alison Lister, Physics & Astronomy Supervisor Colin Gay, Physics & Astronomy Supervisory Committee Member Gary Hinshaw, Physics & Astronomy Supervisory Committee Member David Morrissey, TRIUMF Supervisory Committee Member Janis McKenna, Physics & Astronomy University Examiner Donald Fleming, Chemistry University Examiner ii Abstract The precise understanding of elementary particle properties and theory parameters predicted by the Standard Model of Particle Physics (SM) as well as the revelation of new physics phe- nomena beyond the scope of that successful theory are at the heart of modern fundamental particle physics research. The Large Hadron Collider (LHC) and modern particle detectors pro- vide the key to probing nature at energy scales never achieved in an experimental controlled setup before. The assumption that the SM describes nature only up to a certain energy scale Λ can be relaxed if new particles are present. This helps in particular with the so called "fine- tuning" problem which requires large corrections – in the SM – to the bare mass of the Higgs boson in order to be consistent with the observed mass. A possible solution to this problem is the existence of partner particles of the heaviest known fundamental particle, the top-quark. The new partner particles are expected to be up to ten times heavier. Popular examples of theo- ries predicting heavier top-quark partners are supersymmetric theories and theories that add an additional quark sector to the SM which might be a result of an additional spontaneously bro- ken global symmetry. This dissertation documents two searches for heavy top-quark partners, namely vector-like quarks (VLQs), based on the proton proton (pp) collision data collected in 1 2015 and 2016, corresponding to an integrated luminosity of = 36.1 fb− at a center of mass L energy of 13 TeV. It also elaborates on the work that contributed to a successful data taking campaign related to the alignment of the inner most part of the ATLAS detector with emphasis on the identification and mitigation of track parameter biases. No signs for VLQs were found. The strongest lower mass limits on the pair-produced VLQs decaying into W bosons and top- or bottom-quarks are set to 1.35 TeV at the 95% Confidence In- terval exceeding the one TeV scale for the first time. In addition, the analyses were re-interpreted for other expected VLQ decay signatures. iii Lay Summary The fundamental building blocks of matter and their interactions are described in the Stan- dard Model of Particle Physics (SM). The theory has its limitations and is expected to describe nature only up to a certain energy and is assumed to be embedded in a more fundamental theory. Various theories that go beyond the SM predict the existence of new heavy particles. This dissertation presents searches for heavier partners to the heaviest known fundamental particle, the top-quark. Therefore, data recorded by the ATLAS detector, at the most powerful particle accelerator called the Large Hadron Collider at CERN, is analysed to search for these particles. Contributions to the determination of the relative spatial position of the inner most measurement devices of ATLAS, allowing for a successful data taking campaign, are also pre- sented alongside. No new particles were found, but we are able to provide restrictions to their allowed mass range. iv Preface The presented research in this dissertation is based on the data collected by the ATLAS exper- iment and theory predictions from numerous authors. In order to become part of the ATLAS collaboration, every physicist is expected to fulfill a "qualification" task usually in the beginning of the PhD. The successful completion of this project entitles one to be an official author of the experiment. Every author is listed on each publication or public result that leaves the col- laboration and the authorlist is always purely alphabetical. Final results are published in peer reviewed journals. Preliminary results can also be released for certain conferences in the form of conference notes (CONF notes) or made public in form of public documents (PUB notes) or public plots including short descriptions. The targeted journal as well as the type of pub- lic result is determined on mutual agreement between the collaborators and is evaluated on a case-by-case basis. Individuals do not typically claim all public results by the collaboration but instead will high- light those in which they had major contributions. These are generally split into publications, conference and public notes. Most of the theoretical expectations are based on calculations and the simulation of physics processes that are mainly provided by authors not being part of the collaboration. Simulated theory predictions rely on the preparation and validation which is handled by in- dividuals chosen by each physics group of the collaboration. The shared collaborational effort with varying contributions of individual authors can not be fully disentangled. Individual AT- LAS authors are contributing with a minimum of their qualification task during their PhD to the collaborational efforts and as such have the right to analyse the data. The author contributed to performance work of the ATLAS collaboration particularly in the context of the Tracking Com- bined Performance Group and the Top-quark Physics Group. The former resulted in support of the 2015 and 2016 data taking periods and the latter involved contributions to the generation and simulation of theoretical predictions associated with top-quarks. The qualification task was performed in a sub group of the Tracking group, the Inner Detector (ID) alignment team, and v resulted in the update of the ID alignment performance monitoring package in preparation for Run II (the data taking period between 2015 and 2018). Additionally, the author updated and maintained the ID alignment monitoring exploiting the decay products of well known physics 0 processes such as the Z and W boson, J= and KS to determine the relative spatial position of measurement devices in the ID. This is done in real-time during online data taking. The author elongated his contribution to the Tracking group beyond the qualification task through the extension of a methodology to identify particular ID detector deformations (so called weak modes). This was achieved using dimuon decays of the Z boson employed to identify track parameter biases caused by such deformations. The author contributed to the extension of a dual-use tool released by the Tracking group to either assess systematic uncertainties on tracks or to calibrate tracks in situ. Through the usage of this tool, the mitigation of observed biases was made possible without relying on a reprocessing of the recorded data. The initial implemen- tation was performed in collaboration with Paweł Brückman de Renstrom and continued with Michael Ryan Clark. In addition, the author performed measurements of the impact parame- ter resolution at medium to high transverse momenta employing Z boson decays. Consecutive contributions were performed on a consultative basis. The main contributions in the Top-quark Physics group were based on a one-year mandate overseeing and managing the Monte Carlo (MC) sample production for the group and working towards improvements in the assessment of theory predictions and associated uncertainties. In this role, the author provided support to 300 active physicists in the preparation and validation of samples entering the official ATLAS ∼ Monte Carlo event generation and simulation chain. Frequent liaising with group conveners of the Top-quark Physics Group and Physics Coordinators as well as the ATLAS central Monte Carlo (MC) production team was performed. Samples with top-quarks were tested and vali- dated. A subset of these samples resulted in the updated top-quark sample baseline described in Chapter 10 and summarised in Section 12.1. The dissertation text was written by the author and mostly not directly taken from previ- ously published sources. Exceptions to this are parts of Sections 8.1, 8.2.1 and 12.1.2, Chap- ter 12.2, Chapter 13 and Chapter 14 that have appeared in internal (unpublished and non- public) documentations. Figures not including a reference or credit in the caption and not explicitly mentioned in this preface are produced by the author. Figures with the label ATLAS, ATLAS Simulation or ATLAS Preliminary reflect published and public results, and are officially approved by the ATLAS collaboration. The analyses presented in Chapters 13 and 14 have vi been published. Figures with no label or the label ATLAS Internal or ATLAS Work In Progress are results not approved by the ATLAS collaboration albeit depending on either ATLAS data or simulated samples using the ATLAS sample generation or simulation chain. Information needed to support the general understanding of the results presented is sum- marised from sources indicated by the citations in the text. Parts I and II, Chapters 7 and 9 present material that serves as introduction for the reader.