Searches for new neutral and charged scalars with multiple top and bottom quarks in Run 2 pp collisions at 13 TeV with the ATLAS detector

This thesis presents two searches for new scalars using the 139 fb^-1 proton-proton collision data at a center-of-mass energy of 13 TeV collected by the ATLAS detector at the CERN Large Hadron collider during Run 2. Both searches look for multi-jet final states with one electron or muon and events are categorised according to the multiplicity of jets and how likely these are to have originated from the hadronisation of a bottom quark. Parameterised feed-forward neural networks are used to discriminate between signal and background and included in maximum-likelihood fits to the data for the various mass hypotheses. The first search is dedicated to charged Higgs bosons, predicted by various theories Beyond the Standard Model and motivated by the inadequacy of the Standard Model to explain some experimental phenomena. The work focuses on heavy charged Higgs bosons, heavier than the top quark, decaying to a pair of top and bottom quarks. They are assumed to be produced in association with a top and a bottom quarks. The search is performed in the mass range between 200 and 2000 GeV. No significant excess of events above the expected Standard Model background is observed, hence upper limits are set for the cross-section of the charged Higgs boson production times the branching fraction of its decay. Results are interpreted in the context of hMSSM, various Mh125 scenarios and 2HDM+a. The second search targets flavour-changing neutral-currents decays of top quarks into a new scalar decaying into a pair of bottom quarks. This novel study probes for a scalar on a broad mass range between 20 and 160 GeV and branching ratios below 0.1%. In the case of the Higgs boson, branching ratios are predicted within the Standard Model to be heavily suppressed. Several Beyond the Standard Model theoretical models predict new particles and enhanced branching ratios. In particular, simple extensions involve the Froggatt-Nielsen mechanism, which introduces a scalar field with flavour charge, the so-called flavon, featuring flavour violating interactions. As no significant excess is observed, upper limits for both possible FCNC decays are extracted. In addition, limits are set for the process involving the Standard Model Higgs.