Observation and measurement of the electroweak W${}^{\pm }$W${}^{\pm }$jj process with the ATLAS detector

Measurements of the electroweak sector of the Standard Model (SM) are a way to probe the mechanism of electroweak symmetry breaking at the LHC, and to detect small deviations from the SM predictions, through which the effect of new physics could manifest itself. In this context the scattering of vector bosons (VBS) is a key process. In particular the production of W bosons pairs is a vital test of the mechanism, since its scattering amplitude would increase at high energies violating unitarity, without cancellations of divergences due to exchange involving Z or Higgs bosons. This talk will present an overview of the measurement of the electroweak production of two W bosons with the same electric charge in the signature of two leptons, missing transverse energy and two jets using data recorded by the ATLAS experiment during proton-proton collisions at $\sqrt{s}$= 13 TeV at the LHC corresponding to an integrated luminosity of 36.1 fb${}^{-1}$. The main analysis techniques will be shown, with a focus on the estimation of the background due to the contributions from opposite-charge di-lepton production processes, where the charge of one electron is wrongly reconstructed. An overview of the optimisation procedure will be presented, together with the fit technique that led to the observation of the W${}^{\pm }$W${}^{\pm }$jj electroweak process and the measurement of its fiducial cross-section.