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 is 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. Dominant background contributions originate from the nonprompt lepton background, t¯tZ/γ∗ , t¯tH, WZ, and ZZ. The nonprompt lepton background is estimated using a data-driven "fake-rate" technique, which is validated in a dilepton validation region. In the dilepton channel, the misidentification of lepton charge contributes as an additional background, "charge misID", and is estimated from data, while the other background processes are modelled using simulations. In events with two leptons, the t¯tW signal is differentiated from backgrounds by a multiclass Neural-Network (NN). This network is trained with the simulated t¯tW, t¯tZ, t¯tH, t¯tγ, and nonprompt lepton events from t¯t simulation. The NN algorithm leverages kinematic variables associated with lepton and jets. Trilepton events are classified based on jet and b-tagged jet counts, in addition to the sum of lepton charges. Besides two signal regions, two control regions defined with three or four leptons are employed to constrain the WZ, ZZ, and t¯tZ/γ∗ backgrounds. The inclusive cross section is measured as 868 ± 40 (stat) ± 51 (syst) fb, representing the most precise measurement of t¯tW 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 t¯tW+ and t¯tW− 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).