Search for a spin-zero particle in Higgs boson decays or top-associated production using the ATLAS detector

This thesis contains two searches for light pseudoscalar particles using 140 $fb^{−1}$ proton-proton (pp) collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider during Run 2. Both searches focus on multi-b final states, using novel techniques for b-quark identification. Only events with two leptons are considered, in order to reduce contributions from difficult-to-model hadronic background processes. The first search explores light pseudoscalar production in Higgs boson decays, H → aa, with an a-boson mass between 12 and 60 GeV. Each pseudoscalar decays to two b-quarks, a → bb, which translates into a final state with four b-quarks. The Higgs boson is produced in association with a Z boson that decays to either electrons or muons. No significant excess of events above the expected Standard Model background is observed. Upper limits at 95% confidence level are set for the branching ratio of the Higgs boson decaying to two light pseudoscalars, BR(H → aa → 4b), between 3% and 25%. The second search targets light pseudoscalar production in association with top quarks, tta. This production mechanism allows to explore the pseudoscalar mass range from 12 to 100 GeV. The tt pair is required to decay leptonically to electrons, muons or both. The light pseudoscalar is studied via its decay to b-quarks. As no significant excess is observed, upper limits for the production cross section of a light pseudoscalar in association with top quarks, σ(pp → tta) × BR(a → bb), are presented. They range between 0.1 and 0.9 $pb^{−1}$.