Study of $W^+W^-$ Vector Boson Scattering with the ATLAS detector.

This thesis presents an analysis of the electroweak production of pairs of opposite-sign $W$ bosons in association with two or three jets using data from proton-proton collisions collected by the ATLAS experiment at a center-of-mass energy of $\sqrt{s}=13\,\mathrm{TeV}$, corresponding to an integrated luminosity of $139\,\mathrm{fb}^{-1}$. The analysis is done in the $W^+W^- \rightarrow e^\pm\mu^\mp$ channel, using neural networks to suppress the top and QCD-$WW$ background. A likelihood fit is performed to estimate the signal strength and the corresponding uncertainty. The cross-section was measured in a fiducial region and was found to be $2.62\pm0.50 \text{ fb}$ ($2.20\pm0.47 \text{ fb}$ expected) in agreement with the Standard Model prediction. The observed (expected) significance is $7.06\sigma$ ($6.21\sigma$). This is the first-ever observation and measurement of this process in the ATLAS detector.