Evidence for the production of a Higgs boson in association with two top quarks with the ATLAS detector A search in the H → b¯b channel and in combination with √ other Higgs boson decays at s = 13 TeV John Andrew Raine School of Physics and Astronomy CERN-THESIS-2018-044 26/03/2018 A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering 2018 “It is not our part to master all the tides of the world, but to do what is in us for the succour of those years wherein we are set, uprooting the evil in the fields that we know, so that those who live after may have clean earth to till. What weather they shall have is not ours to rule.” - Gandalf the White 2 Contents 1 Introduction 13 2 The Standard Model 15 2.1 Overview of the Standard Model . 15 2.2 Theoretical Motivation for Measuring ttH¯ ............... 29 2.3 Monte Carlo Simulation . 33 3 Statistical Methods 37 3.1 Statistical Analysis . 37 3.2 Boosted Decision Trees . 44 4 LHC and the ATLAS Experiment 49 4.1 The Large Hadron Collider . 49 4.2 The ATLAS Detector . 52 5 Object Reconstruction 65 6 Modelling and Event Selection 77 6.1 Signal and Background Modelling . 77 6.2 Event Preselection . 88 7 Search for ttH¯ H → b¯b at 13 TeV 93 7.1 Analysis Strategy . 94 7.2 Systematic Uncertainties . 126 8 Results 137 8.1 Search for ttH¯ H → b¯b ......................... 137 8.2 Combination with other Searches . 151 9 Colour Connection of b-quarks 157 9.1 Phenomenological Motivation . 158 9.2 Colour Flow in ttH¯ H → b¯b ...................... 161 10 Conclusion 169 References 173 Total Word Count: 44600 3 4 Abstract In this thesis, the search for the production of the Higgs boson in association with two top quarks is presented. The main focus of this work is on the analysis optimised for the decay of the Higgs boson to a b-quark pair. The analysis is performed using 36.1 fb−1 of pp collision data at a centre of mass energy √ s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during 2015 and 2016. The signal strength of ttH¯ in relation to the Standard Model prediction for a Higgs boson with a mass of 125 GeV is measured to be +0.64 µttH¯ = 0.87−0.61, with signal strengths greater than 2.0 excluded at the 95% confidence level. The combination of this analysis with searches targeting additional Higgs boson decay modes is subsequently presented. The measured signal strength in relation to the Standard Model prediction is µttH¯ = 1.2±0.3. This corresponds to an observed (expected) significance for ttH¯ of 4.2σ (3.8σ), constituting evidence for the ttH¯ production mode. Finally, a study into the ability to observe and model the colour connection of b-quarks in ttH¯ H → b¯b and tt¯ + jets events is presented. The jet pull angle observable is used to investigate the effect of colour connection on jet substructure. Such an observable is found to be sensitive to the underlying colour structure in events, showing differences between b-quarks which decay from a colour singlet in comparison to a colour octet. However, the effect is found to be small and a larger dataset is required to measure the effect in ttH¯ events. 5 6 Declaration and Copyright No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. i The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the Copyright) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. 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Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property University IP Policy,1 in any relevant Thesis restriction declara- tions deposited in the University Library, The University Library’s regula- tions2 and in The University’s policy on Presentation of Theses. 1http://documents.manchester.ac.uk/display.aspx?DocID=24420 2http://www.library.manchester.ac.uk/about/regulations/ 7 8 Acknowledgements First of all, I want to say a big thank you to everyone I have had the pleasure to meet and work with over the past three and a half years. Without the support and encouragement from so many people, as well as the countless experiences I’ve been fortunate to enjoy, I can’t imagine I’d be in the same position as I am now. To my family I owe the biggest thanks. You’ve always been there for me, driving me to pursue my goals in life. It’s not always been the easiest journey but knowing you’re there has helped me get through the bumps in the road. To all my friends, you’ve made the past few years far more than just working towards a PhD. Be it with beers on a balcony, barbecuing in the rain, skiing in incredible alpine vistas, the countless coffee breaks, or enjoying the swiss diet of wine and cheese, it wouldn’t have been the same without you. And to Marie, I can’t thank you enough for helping me relax and stay focused. Writing this thesis would have been a lot harder without you. To Yvonne, thank you for all the guidance and feedback you’ve provided. You’ve encouraged me to consider new ideas and I’ve learnt a lot under your supervision. And of course, thank you for encouraging the group to bring in chocolates and cake! And to everyone in Manchester, as well as those based out at CERN, the atmosphere you created was an absolute joy to work in. I would also like to thank everyone I worked with on the ttH¯ H → b¯b analysis, with additional thanks to Lisa and Georges. It has been great fun working on such a complex analysis with such a fantastic group of people. 9 10 Preface In 2014 the author joined the High Energy Particle Physics group at the Uni- versity of Manchester, becoming a member of the ATLAS Collaboration. In the time since then the author has contributed to the operation of the ATLAS experiment and worked on the search for ttH¯ in the H → b¯b decay channel during Run 2 of the LHC. The author worked on the analysis from its start to conclusion. This analysis has since been published in PRD [1] and has been combined with three other searches for ttH¯ using the same dataset collected by the ATLAS experiment [2]. The notable contributions from the author in this thesis are as follows. An online monitoring algorithm for the efficiency of reconstructing tracks used in the ATLAS trigger system was developed by the author and is described in Section 4.2.5. In the search for ttH¯ H → b¯b , the author focused on the dilepton channel, working on and developing the whole analysis strategy. In ad- dition, he performed the full chain of the analysis from preparing the datasets to running the statistical analysis. Several background modelling studies in the dilepton channel were performed by the author. These mostly concerned the Z+jets background using a data-driven method to derive the correction fac- tors for Z+Heavy Flavour jets. But in addition to this, the author investigated the modelling of the tt¯ + HF background and its impact on the sensitivity and stability of the statistical analysis. With regards to the analysis strat- egy, the author performed and optimised the event categorisation presented in Chapter 7. The reconstruction BDT in the dilepton channel, described in the same chapter, was also developed by the author. On top of this, he opti- mised the choice of variables and trained the classification BDTs used as the final discriminants in all dilepton signal regions. The statistical analysis in the dilepton channel was performed by the author, before entering into combina- tion with the semileptonic channel for the final ttH¯ H → b¯b measurement. It subsequently entered into the final ttH¯ combination, the results of which are presented in Chapter 8. For the ttH¯ combination, orthogonality checks 11 Contents Contents were carried out to ensure that there was no overlap between channels in the H → b¯b and multilepton analyses. Finally, the author implemented the pull angle in ttH¯ H → b¯b , an observable used previously in tt¯ analyses to look at the modelling of colour structure in collisions, and studied whether such an effect is observable with the current dataset and whether it could be useful in future searches.