Single top production in association with a WZ boson pair at the CMS experiment

This thesis presents a comprehensive study of the single top quark production in association with a WZ boson pair (tWZ), culminating in the first observation of this process. Measurements of rare processes are crucial to improve our understanding of the Standard Model (SM) and improve the sensitivity to loop-induced new physics. These represent a crucial part of the Large Hadron Collider (LHC) physics program. In this context, the tWZ process provides a unique probe of the top quark-electroweak sector of the SM and offers sensitivity to possible effects of new physics, which can be systematically studied within the Standard Model Effective Field Theory (SMEFT) framework. This thesis addresses the challenges associated with the computation of SMEFT predictions and uses them to measure constraints on the Wilson coefficients of relevant SMEFT operators, also providing their projections for the High-Luminosity LHC. A measurement of the inclusive tWZ production cross section is performed using proton-proton collisions recorded by the CMS experiment at centre-of-mass energies of 13 and 13.6 TeV, corresponding to an integrated luminosity of 200 fb−1. The analysis tackles several experimental challenges, for example an efficient identification of the tWZ process with machine learning, and a precise prediction of the background from misidentified leptons. The inclusive tWZ cross section is measured independently at 13 and 13.6 TeV, and a simultaneous fit yields an observed statistical significance exceeding 5 standard deviations, thus establishing the first observation of the tWZ process.