First observation of the electroweak production of a leptonically decaying W$^{+}$W$^{-}$ pair in association with two jets 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 decaying leptonically, is reported. The data sample corresponds to 138 fb$^{-1}$ of proton-proton collisions at $13$ TeV. Events are selected by requiring two leptons (electrons or muons) and two jets with large pseudorapidity separation and high invariant mass. Events are categorized based on the flavour of final state leptons. A signal is observed with a significance of 5.6 standard deviations and the measured fiducial cross section is $10.2 \pm 2.0$ fb. The fiducial volume is required to have two oppositely-charged leptons in the $\lvert\eta\rvert < 2.5$ region with $p_T > 25$ GeV and $p_T > 13$ GeV respectively, having an invariant mass greater than $50$ GeV; additionaly, at least two jets in the $\lvert\eta\rvert < 4.7$ region with a cone size $\Delta R = 0.4$ and $p_T > 30$ GeV are selected, having a pseudorapidity gap larger than $2.5$ and an invariant mass greater than $300$ GeV.