Observation of electroweak production of a same-sign $W$ boson pair in association with two jets in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This note presents the observation and fiducial cross-section measurement of electroweak production of a same-sign $W$ boson pair in association with two jets using 36.1 fb$^{-1}$ of proton-proton collision data recorded at a center-of-mass energy of $\sqrt{s}=13$ TeV by the ATLAS detector at the Large Hadron Collider. Events containing two same-sign leptons, electron or muon, and at least two jets with a large invariant mass and rapidity difference, are selected. A total of 122 candidate events is observed for a background expectation of $69 \pm 10$ events, giving an observed (expected) signal significance of 6.9 (4.6) standard deviations. The fiducial cross section for the electroweak production process is measured to be $2.91_{-0.47}^{+0.51}~\textrm{(stat.)} \pm 0.27~\textrm{(sys.)}$~fb.