Probing CP couplings in ttX production at the Run3 of the LHC.
PT-CERN Call 2021/2
Universidade de Coimbra
Laboratório de Instrumentação e Física Experimental de Partículas
In this PhD thesis proposal, we explore CP discrimination, in the associated production of top-quark pairs with generic scalar/pseudo-scalar and vector/axial-vector boson mediators, at the LHC. In addition to the Higgs boson, which may reveal a non-Standard Model (SM) CP (charge-parity) nature of the couplings to heavy quarks, these searches are also looking for hints of Dark Matter (DM) candidates, in simplified models with spin-0 and spin-1 particles, that act as mediators. Events from the associated production (ttX, X=spin-0, spin-1) of top quarks are searched for in the single lepton and dilepton topologies, in proton-proton collisions at the Run3. Following full kinematic reconstruction, a search for specific angular distributions that are sensitive to the CP nature of the mediator couplings, not only to the magnitude of the mixing angle but also to its sign, is expected to be performed with the aim of discriminating, as well, the dominant backgrounds at the LHC from the signals under study. The mass range covered by the searches, focuses on the very low mass region up to masses O(TeV), using the Standard Model Higgs boson mass case, as a benchmark for the CP observables performance. The use of Machine Learning (ML) algorithms, for event classification, is also expected to be performed since it has shown to be particularly promising in the context of High Energy Physics (HEP). These algorithms will be tested in ttX associated production, with completely new coverage of the CP parameter space. The impact on the signal sensitivity is to be quantified in terms of cross-section measurements or 95\% CL limits (in case no New Physics is discovered), using ATLAS data collected at the Run3. State-of-the-art Monte Carlo simulations like MadGraph5 with specific UFO models will be used. In addition, a global fit using ML algorithms will be performed to constrain the Wilson coefficients within the Standard Model Effective Field Theory (SMEFT).