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.
Categories: hep-ex