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 $10.2\pm2.0$~fb, in agreement with the standard model prediction of $9.1\pm0.6$~fb.