Measurements of $W^+W^-$ production cross sections in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

In this thesis, measurements of cross sections for the production of pairs of oppositely charged $W$ bosons in association with hadronic jets are presented. Integrated and differential fiducial cross sections are obtained using data corresponding to an integrated luminosity of \SI{139}{fb^{-1}} collected in $\sqrt{s} = \SI{13}{TeV}$ proton-proton collisions with the ATLAS detector. These measurements provide an important test of the Standard Model, in particular of the electroweak theory and perturbative quantum chromodynamics. In addition, a precise understanding of $W$ boson pair production is required in measurements of resonant Higgs boson production, and in searches for physics beyond the Standard Model, where it constitutes a dominant background. Events are selected with exactly one electron and one muon of opposite charge, and different requirements are placed on the number of hadronic jets. For the first time at the LHC, measurements are included where no requirements are placed on the jet activity. Background contributions are estimated using a combination of simulation and data-driven techniques. The dominant background from top quark events is precisely estimated using a data-driven method that significantly reduces experimental and modelling uncertainties. The integrated cross section obtained in a fiducial phase space with at least one jet is found to be $\mathrm{258} \pm \mathrm{4\,(stat.)}\, \pm \mathrm{25\,(syst.)\,fb}$, whilst the result in a phase space with any number of jets is $\mathrm{668} \pm \mathrm{5\,(stat.)}\, \pm \mathrm{34\,(syst.)\,fb}$. Both results are consistent with a range of state-of-the-art theoretical predictions. Cross section measurements in a phase space with a dynamic requirement on the event jet activity are also presented.