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

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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 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.

• 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 $69 \pm 7$ events, corresponding to an observed signal significance of 6.5 standard deviations.
• The measured fiducial signal cross section is $\sigma^{\mathrm {fid.}}=2.89^{+0.51}_{-0.48} \mathrm{(stat.)} ^{+0.29}_{-0.28} \mathrm{(syst.)}$ fb.