Observation of vector boson scattering $W^{+}W^{-}$ pair production in proton-proton collisions at a center-of-mass energy of 13 TeV with the CMS detector.

The observation of vector boson scattering (VBS) $W^{+}W^{-}$ pair production, with the two $W$ bosons decaying leptonically, has been reported by the CMS Collaboration at the CERN LHC. The data sample analyzed corresponds to an integrated luminosity of $138 {fb^{-1}}$ of proton-proton collisions at $\sqrt{s}=13$ TeV collected by the CMS detector. Events are selected by requiring exactly two opposite-sign leptons (electrons or muons) and two jets with large pseudorapidity separation and high dijet invariant mass. Machine learning techniques are employed to deal with the irreducible background from the QCD-induced production of $W$ bosons and the large background from the production of top quarks pair. The VBS $W^{+}W^{-}$ signal is observed with a significance of 5.6 standard deviations (5.2 expected) with respect to the background-only hypothesis. The measured fiducial cross-section is $10.2\pm$ 2.0 ${fb}$, in agreement with the standard model prediction of $9.1\pm$ 0.6 ${fb}$.