Search for a new light particle produced in association with a top quark pair with the ATLAS Detector

This thesis presents the first dedicated search for a new light pseudoscalar particle decaying into a pair of muons, and produced in association with a top quark pair. Pseudoscalars are well motivated phenomenologically in various theories beyond the Standard Model of particle physics, such as 2HDM+$a$ and NMSSM, and can provide insights into new physics. The search targets specific decays of the signal process, where one of the top quarks decays into an electron or muon, leading to two distinct regions with three leptons: an electron and two muons ($e\mu\mu$) and three muons final states ($\mu\mu\mu$), within a dimuon invariant mass window of $12 - 77~$GeV. The search uses data recorded by the ATLAS detector at CERN, from 2015 to 2018, at centre of mass energy of $√s = 13$ TeV, corresponding to an integrated luminosity of $139~\text{fb}^{−1}$. Considering statistical and systematic uncertainties, a $95\%$ confidence level upper limit is set on the cross-section times the branching ratio of the pseudoscalar particle decaying into muons and signal significance of the decay process is presented.