Observations of the Radiation Amplitude Zero effect and of the Diboson Longitudinal-Longitudinal Interactions in High $p$$_{T}$ Region using W±Z Production with the ATLAS Detector

This thesis focuses on the studies of di-boson polarization states in W±Z production, presenting two novel measurements using pp collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV. The WZ decaying to leptonic final states is considered. The data was collected at the Large Hadron Collider (LHC) with the ATLAS experiment, utilizing an integrated luminosity of 140.1 fb−1 . The first measurement pertains to the Radiation Amplitude Zero effect. By studying the depth of distributions involving the rapidity differences between the W lepton and the Z boson (∆Y3Z) and between the W boson and the Z boson (∆YWZ), an agreement is observed between the measured depth in the background-subtracted data consisting only of the transverse-transverse WZ contribution and the Monte Carlo (MC) prediction. The second measurement involves the determination of the longitudinal-longitudinal WZ polarization fraction. This measurement is performed in regions with predominantly s-channel WZ events, characterized by high $p$$_{T}^{Z}$ values in two fiducial phase spaces: $p$$_{T}^{Z}$ > 100 GeV or $p$$_{T}^{Z}$ > 200 GeV. To extract the polarization fraction, a multivariate analysis based on a Boosted Decision Trees discriminant is employed. A maximum log-likelihood fit is then executed, yielding a longitudinal-longitudinal polarization fraction ($f$00) of 0.25+0.06 −0.06 for the $p$$_{T}^{Z}$ > 200 GeV region, with an observed (expected) significance of 3.4σ (4.6σ). In the p$_{T}^{Z}$ > 100 GeV region, the longitudinal-longitudinal polarization fraction is found to be 0.17+0.021 −0.022, with an observed (expected) significance of 7.9σ (8.8σ). Notably, this marks the first measurement of the longitudinal-longitudinal WZ production in the high-$p$$_{T}^{Z}$ phase space. In addition to the analysis, this thesis covers the contribution of the University of Michigan ATLAS group to the ATLAS New Small Wheel Muon Spectrometer upgrade, particularly the integration and commissioning of the small-strip Thin Gap Chambers electronics. Detailed procedure regarding the integration work, especially the readout chain electronics validations, and detector noise studies is described in the thesis.