Medición de las secciones eficaces inclusivas y diferenciales de la producción asociada de un par de quarks top y un bosón w, y búsqueda de leptoquarks en estados finales con múltiples leptones con el detector atlas

Résumé

This dissertation presents a broad spectrum of physics analyses containing multiple leptons and $b$-tagged jets in their final state using data from proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider (LHC). The full Run 2 dataset is used period, which corresponds to an integrated luminosity of 139 fb$^{-1}$. The dissertation is structured along two main axes. The first axis is centred around the measurement of the inclusive and differential cross section of the associated production of a top-quark pair and a $W$ boson ($t\bar{t}W$). This measurement is greatly motivated due to $t\bar{t}W$-like tensions in data reported by the ATLAS and CMS experiments. The theoretical modelling of this process is quite challenging due to non-negligible higher order corrections, resulting into a great effort to achieve an accurate simulation of this contribution. The measurement is performed both inclusively and differentially, where several observables are unfolded at particle level. The $t\bar{t}W$ prediction is measured to be consistently higher than the SM expectation with an inclusive signal strength of $\mu_{t\bar{t}W}$=1.46$^{+0.14}_{-0.13}$. The second axis focuses on searches for physics beyond the SM (BSM) and, particularly, on searches for leptoquarks. Leptoquarks are one of the most favoured BSM scenarios to describe the intriguing deviations reported in $B$-meson decays by the LHCb experiments and the $B$ factories. This thesis is targeting pair-produced leptoquarks with a mass in the TeV scale each coupling simultaneously into a top quark and a lepton ($e, \mu, \tau$-lepton). This effort is divided in two dedicated searches targeting $t\ell t\ell$ and $t\tau t\tau$ signatures, respectively. No significant excess is found in both searches, and the most restrictive to date upper limits are set on the signal cross section and the branching ratio as a function of the leptoquark mass for several scalar and vector leptoquark models. In the $t\ell t\ell$ search, a scalar leptoquark with a mass about 1.6 TeV is excluded at the 95\% confidence level, assuming it decays exclusively into $te$ or $t\mu$. The most stringent exclusion for a vector leptoquark reaches 2.0 TeV. In the $t\tau t\tau$ search, the corresponding exclusions for the same types of scalar and vector leptoquarks are about 1.4 TeV and 1.8 TeV, respectively. These searches are also combined with other leptoquark searches in ATLAS, significantly improving the sensitivity.