First observation of the electroweak production of a W$^+$W$^-$ pair plus two jets in the leptonic channel in $\sqrt{s}$ = 13 TeV pp collisions

The first observation of the electroweak production of a W$^+$W$^-$ pair in association with two jets, with both W bosons leptonically decaying, is reported. The data sample corresponds to an integrated luminosity of 138 fb$^{-1}$ of proton-proton collisions at centre-of-mass energy of 13TeV collected by the CMS detector. Events are selected by requiring two leptons (electrons or muons) and two jets with large pseudorapidity separation and high invariant mass. Events are categorised based on the flavour of final state leptons. A signal is observed (expected) with a significance of 5.6 (5.2) standard deviations and the measured fiducial cross section is 10.2 $\pm$ 2.0 fb. The results are in agreement with the Standard Model predictions of 9.1 $\pm$ 0.6 fb.