Search for a light charged Higgs boson in <em>t</em> → <em>H</em>^±<em>b</em> decays, with <em>H</em>^± → <em>cs</em>, in <em>pp</em> collisions at √s = 13 TeV with the ATLAS detector

A search for a light charged Higgs boson produced in decays of the top quark, <em>t</em> → <em>H</em>^±<em></em>b with <em>H</em>^± → <em>cs</em>, is presented. This is a direct search for new physics beyond the Standard Model of particle physics. A charged Higgs boson appears in most theories with an extended scalar sector and the <em>cs</em> channel is predicted to be among the dominant decay modes. Extensions to the scalar sector are motivated by the solutions they provide to open questions in particle physics. For example, extensions can yield a solution to the strong CP problem, facilitate baryogenesis and provide a dark-matter candidate. <br> The search analyses 140 fb^-1 of <em>pp</em> collisions data at √s = 13 TeV collected with the ATLAS detector between 2015 and 2018. The focus is set on top-quark pair events, <em>tt̄</em> → <em>WbH</em>^±<em></em>b with <em>W</em> → <em>ℓν</em> (<em>ℓ</em> = <em>e</em>, <em>μ</em>) and <em>H</em>^± → <em>cs</em>, resulting in a lepton-plus-jets final state, characterised by an isolated lepton and at least four jets. Signal events in data are identified by filtering the data for semileptonic <em>tt̄</em> events and reconstructing the <em>tt̄</em> event topology. The dominant <em>tt̄</em> background is suppressed with the help of boosted decision trees that are trained with features related to the kinematics of the top-quark decay products, <em>b</em>- and <em>c</em>-quark tagging information of jets, and the spin of the hadronically decaying boson. <br> No significant signal excess is observed in the BDT score distributions for any of the twelve signal mass hypotheses between 60 GeV and 168 GeV. In that mass range, observed (expected) upper limits at 95% CL on the branching ratio B(<em>t</em> → <em>H</em>^±<em>b</em>), assuming B(<em>t</em> → <em>Wb</em>) + B(<em>t</em> → <em>H</em>^±( → <em>cs</em>)<em>b</em>) = 100%, are set between 0.066% (0.077%) and 3.6% (2.3%). These are the first direct limits on B(<em>t</em> → <em>H</em>^±<em></em>b) in the <em>cs</em> channel in the charged Higgs boson mass range from 60 to 80 GeV and 160 to 168 GeV. <br> The limits from this analysis supersede limits from previous searches in the mass range from 120 to 160 GeV. <br> The improved limits can guide theoreticians to develop and refine models with a charged Higgs boson. Finally, this thesis discusses methods to further improve the search for a charged Higgs boson in the future.