A study of top quark pairs production in association with a W boson

This dissertation presents a measurement of the inclusive cross section of the production top quark-antiquark pairs in association with a W boson in proton- proton collisions at a centre-of-mass energy of 13 TeV. The analysis is conducted on a data set recorded by the CMS detector at CERN’s LHC, corresponding to an integrated luminosity of 138 $fb^{-1}$. The cross section is measured across the full phase space using events characterised by the presence of either two same-charge leptons (e or μ) or three leptons with a total charge sum of ±1. The contribution from τ leptons are included through their decays to e and μ. These events are also required to contain at least two jets, with a minimum of two qualifying as b-tagged jets. In events with two leptons, a multiclass neural network is used to distinguish between the ttW signal and background processes. Events with three leptons are categorised based on the number of jets and of jets originating from b quark hadronisation, and the lepton charges. The inclusive cross section is measured as 868 ± 40 (stat) ± 51 (syst) fb, representing the most precise measurement of ttW inclusive cross section to date. It is higher than, but consistent with, the Standard Model (SM) predictions, falling within two standard deviations. Additionally, the cross sections of ttW+ and ttW− are also measured. These are reported as 553 ± 30 (stat) ± 30 (syst) fb and 343 ± 26 (stat) ± 25 (syst) fb, respectively. The corresponding ratio of these two cross sections is determined to be 1.61 ± 0.15 (stat)^{+0.07}_{-0.05} (syst).