Multivariate Analysis of Polarized Vector Boson Scattering of a W and Z Boson with the ATLAS Detector

The scattering of two longitudinally polarized gauge bosons can be used to test details of the mechanism of electroweak symmetry breaking and to search for physics beyond the Standard Model of particle physics. This process can be studied in the electroweak production of a WZ pair in association with two jets with the final state lνlljj. In this thesis this process was investigated for a center of mass energy $\sqrt{s} = 13$ TeV and an integrated luminosity of 139 fb$^{-1}$. A validation of the simulation of this processes with Madgraph that includes the polarization information of the bosons was performed. A multivariate analysis using Boosted Decision Trees was performed using events with longitudinally polarized W and Z boson (WLZL), longitudinally polarized W boson (WLZX), longitudinally polarized Z boson (WXZL) or at least one longitudinally polarized boson (min1L) as signal and the polarizations that are not part of the signal as well as other non-polarization backgrounds as background. Maximum Likelihood fits to Asimov data were performed using the BDT outputs. Different fit configurations were used including a stat-only fit and fits that include systematic uncertainties of the non-polarization backgrounds. For the WLZL signal all fits lead to significances of less than 1 standard deviation (σ). All fits to the mixed signal configurations WLZX and WXZL result in significances of less than 3 σ. For the min1L signal a significance of about 3 σ could be obtained by the fit if the background polarization was assumed to be constrained by 25% systematic uncertainties. Fits using higher integrated luminosities indicate that evidence of the WLZL process might be seen at the High Luminosity LHC and evidence of WLZX and WXZL in Run 3 if the background polarizations can be sufficiently constrained.