Observation of Electroweak Production of a Same-Sign $W$ Boson Pair in Association with Two Jets in $pp$ Collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS Detector

This Letter presents the observation and measurement of electroweak production of a same-sign $W$ boson pair in association with two jets using $36.1\text{}\text{}{\mathrm{fb}}^{-1}$ of proton-proton collision data recorded at a center-of-mass energy of $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ by the ATLAS detector at the Large Hadron Collider. The analysis is performed in the detector fiducial phase-space region, defined by the presence of two same-sign leptons, electron or muon, and at least two jets with a large invariant mass and rapidity difference. A total of 122 candidate events are observed for a background expectation of $69±7$ events, corresponding to an observed signal significance of 6.5 standard deviations. The measured fiducial signal cross section is ${\sigma }^{\text{fid}}=\phantom{\rule{0ex}{0ex}}{2.89}_{-0.48}^{+0.51}\left(\text{stat}{\right)}_{-0.28}^{+0.29}\left(\text{syst}\right)\text{}\text{}\mathrm{fb}$.