An observation is reported of the electroweak production of a W+W− pair in association with two jets, with both W bosons decaying leptonically. The data sample corresponds to an integrated luminosity of 138 fb−1 of proton-proton collisions at √s = 13 TeV, collected by the CMS detector at the CERN LHC. 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. Events are categorized based on the flavor
of the final-state leptons. A 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 ± 2.0 fb and
this value is consistent with the standard model prediction of 9.1 ± 0.6 fb.