THECONSTRUCTIONOFASCALAREXTENSIONTOTHE STANDARD MODEL AND THE SEARCH FOR A HEAVY SCALAR AT p s = 13 TEV WITH THE ATLAS DETECTOR stefan von buddenbrock A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. School of Physics Faculty of Science CERN-THESIS-2019-333 //2020 University of the Witwatersrand 20 April 2020, Johannesburg, South Africa supervisors: Bruce Mellado Alan S. Cornell “An important part of good scientific strategy is to distinguish between problem areas that might be ripe for grand synthesis, and problem areas where a more opportunistic approach will be more fruitful. A successful theory of something can be more valuable than an attempted Theory of Everything.” Frank Wilczek (A Beautiful Question, 2015) DECLARATION I, Stefan von Buddenbrock (Student number: 567245), am a student registered for the degree of Doctor of Philosophy (Physics) in the academic year 2019. I hereby declare the following: I am aware that plagiarism (the use of someone else’s work without their • permission and/or without acknowledging the original source) is wrong. I confirm that the work submitted for assessment for the above degree is my • own unaided work except where I have explicitly indicated otherwise. I have followed the required conventions in referencing the thoughts and • ideas of others. I understand that the University of the Witwatersrand may take disciplinary • action against me if there is a belief that this is not my own unaided work or that I have failed to acknowledge the source of the ideas or words in my writing. I have written about and presented many of the ideas and results in this • thesis. A list of my previous works that have done so is shown on page ix. I, along with the other co-authors, have intellectual property of these works. Johannesburg, South Africa Stefan von Buddenbrock, 20 April 2020 ABSTRACT Searches for physics beyond the Standard Model (SM) have always been a focus of the particle physics community, more so since the discovery of a Higgs-like boson (h) in 2012. These are typically done using either the top-down or bottom-up methodology. In this thesis, a model is constructed in order to successfully explain a variety of anomalous results from Run 1 and 2 of the Large Hadron Collider (LHC), using a hybrid methodology that iterates between theory and experiment. Initially in 2015, a new heavy scalar boson H was postulated to explain anomalies in the Run 1 LHC data. The result of this early study implied that a heavy scalar boson with a mass around 270 GeV, produced through gluon fusion, could explain these anomalies with a significance of 3s, with a cross section comparable with that of a heavy Higgs boson as well as a dominant h-associated decay mode. Theoretical developments of the model then hinted towards the existence of an additional scalar boson S, which acted as this associated decay product. The S boson, with a mass of around 150 GeV, was likely to decay to final states comprising of multiple leptons. The culmination of these studies found that the H Sh decay mode was ! dominant and could be searched for in final states with multiple leptons and b- tagged jets at the LHC. In 2017 and 2018, using the input from these initial studies, anomalies in the LHC data were successfully explained by the model, as predicted. It was found that the new physics model improved on the SM description of the data at the level of at least 8s with just a single degree of freedom. Should these discrepancies not be resolved, this can be considered as indirect evidence for new physics processes at the LHC, since the current SM tools are unable to provide a reasonable explanation for the anomalies. The potential of correlating this result with a mild excess at 245 GeV in the search for H decaying to four leptons in the ATLAS detector is considered, and future potential developments of the model are discussed. vii PUBLICATIONS The work presented in this thesis has also been presented and published in a number of peer-reviewed journal articles and conference outputs, as well as some unpublished reports. The ideas and results in these works will therefore be of a similar nature to those presented here. A list of these publications is given below: journal articles: S. Buddenbrock, A. S. Cornell, Y. Fang, A. Fadol Mohammed, M. Kumar, • B. Mellado, and K. G. Tomiwa, “The emergence of multi-lepton anomalies at the LHC and their compatibility with new physics at the EW scale,” JHEP 10 (2019) 157, arXiv:1901.05300 [hep-ph] S. von Buddenbrock, A. S. Cornell, E. D. R. Iarilala, M. Kumar, B. Mellado, • X. Ruan, and E. M. Shrif, “Constraints on a 2HDM with a singlet scalar and implications in the search for heavy bosons at the LHC,” J. Phys. G46 no. 11, (2019), arXiv:1809.06344 [hep-ph] ATLAS Collaboration, M. Aaboud et al., “Search for Higgs boson pair pro- • ( ) ( ) duction in the WW ∗ WW ∗ decay channel using ATLAS data recorded at ps = 13 TeV,” JHEP 05 (2019) 124, arXiv:1811.11028 [hep-ex] ATLAS Collaboration, M. Aaboud et al., “Search for dark matter in associa- • tion with a Higgs boson decaying to two photons at ps = 13 TeV with the ATLAS detector,” Phys. Rev. D96 no. 11,(2017) 112004, arXiv:1706.03948 [hep-ex] S. von Buddenbrock, A. S. Cornell, A. Fadol, M. Kumar, B. Mellado, • and X. Ruan, “Multi-lepton signatures of additional scalar bosons beyond the Standard Model at the LHC,” J. Phys. G45 no. 11,(2018) 115003, arXiv:1711.07874 [hep-ph] S. von Buddenbrock, N. Chakrabarty, A. S. Cornell, D. Kar, M. Kumar, T. Man- • dal, B. Mellado, B. Mukhopadhyaya, R. G. Reed, and X. Ruan, “Phenomeno- logical signatures of additional scalar bosons at the LHC,” Eur. Phys. J. C76 no. 10,(2016) 580, arXiv:1606.01674 [hep-ph] S. von Buddenbrock, N. Chakrabarty, A. S. Cornell, D. Kar, M. Kumar, • T. Mandal, B. Mellado, B. Mukhopadhyaya, and R. G. Reed, “The compat- ibility of LHC Run 1 data with a heavy scalar of mass around 270 GeV,” arXiv:1506.00612 [hep-ph] ix conference proceedings: S. von Buddenbrock and B. Mellado, “Anomalies in the production of multi- • ple leptons at the LHC,” arXiv:1909.04370 [hep-ph] S. von Buddenbrock, “Di-Higgs production and the Madala Hypothesis,” in • Proceedings of the HEPP Workshop 2018, A. Cornell and B. Mellado, eds. https: //indico.cern.ch/event/682259/. Accepted for publication S. von Buddenbrock, A. S. Cornell, M. Kumar, and B. Mellado, “The Madala • hypothesis with Run 1 and 2 data at the LHC,” J. Phys. Conf. Ser. 889 no. 1, (2017) 012020, arXiv:1709.09419 [hep-ph] S. von Buddenbrock, “Exploring LHC Run 1 and 2 data using the Madala • hypothesis,” J. Phys. Conf. Ser. 878 no. 1,(2017) 012030, arXiv:1706.02477 [hep-ph] S. von Buddenbrock, “Production of the Madala boson in association with • top quarks,” in The Proceedings of SAIP2017, the 62nd Annual Conference of the South African Institute of Physics, J. Engelbrecht, ed., pp. 187–192. ISBN: 978-0- 620-82077-6 S. von Buddenbrock et al., “Constraining hypothetical extensions to the Higgs • sector at the LHC,” in The Proceedings of SAIP2016, the 61st Annual Conference of the South African Institute of Physics, S. Peterson and S. Yacoob, eds., pp. 158– 163. ISBN: 978-0-620-77094-1 internal notes: S. H. Abidi et al., “Search for heavy ZZ resonances in the ```` final state • using proton–proton collisions at ps = 13 TeV with the ATLAS detector,” Tech. Rep. ATL-ANA-HIGG-2018-09, CERN, Geneva, Sep, 2018 J. Adelman et al., “Supporting note: Selection and performance for the h • ! gg and h Zg analyses, Spring 2017,” Tech. Rep. ATL-COM-PHYS-2017- ! 357, CERN, Geneva, Apr, 2017 S. von Buddenbrock et al., “Supporting note: Search for dark matter in events • with missing transverse momentum and a Higgs boson decaying to two pho- tons in pp collisions at ps = 13 TeV with the ATLAS detector using data in full years of 2015 and 2016,” Tech. Rep. ATL-COM-PHYS-2016-1735, CERN, Geneva, Nov, 2016 X. Ruan et al., “Search for a Higgs boson with the di-photon 2 decay in asso- • ciation with missing energy in p p collisions at ps = 13 TeV with the ATLAS detector,” Tech. Rep. ATL-COM-PHYS-2016-1732, CERN, Geneva, Nov, 2016 S. von Buddenbrock et al., “Search for resonant pair production in H • ! SS, Sh, hh channels decaying to 4W with a four lepton final state at ps = 13 TeV in the ATLAS detector,” Tech. Rep. ATL-COM-PHYS-2016-1622, CERN, Geneva, Feb, 2017 Y. Fang et al., “Search for Higgs boson pair production in the final state of • ggWW ( lnjj) using 36.1 fb 1 of pp collision data recorded at ps = 13 TeV ∗ ! − x with the ATLAS detector,” Tech. Rep. ATL-COM-PHYS-2016-1406, CERN, Geneva, Sep, 2016 L. Cerda Alberich et al., “Supporting note: Selection and performance for the • h gg and h Zg analyses,” Tech. Rep. ATL-COM-PHYS-2016-862, CERN, ! ! Geneva, Jul, 2016 S. von Buddenbrock et al., “Supporting note: Search for dark matter in events • with missing transverse momentum and a Higgs boson decaying to two pho- tons in pp collisions at ps = 13 TeV with the ATLAS detector,” Tech. Rep. ATL-COM-PHYS-2016-708, CERN, Geneva, Jun, 2016 S. von Buddenbrock et al., “Supporting note: Search for a generic heavy boson • using gg + X in p p collisions atps = 13 TeV with the ATLAS detector,” Tech.