Evidence for Longitudinally Polarized W Bosons in the Electroweak Production of Same-Sign W Boson Pairs in Association with Two Jets in pp Collisions at √s = 13 TeV with the ATLAS Detector

This Letter reports the first evidence of electroweak production of same-sign (Formula presented) boson pairs where at least one of the (Formula presented) bosons is longitudinally polarized and the most stringent constraint to date for the production of two longitudinally polarized same-sign (Formula presented) bosons. The dataset used corresponds to an integrated luminosity of (Formula presented) of proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the ATLAS detector during run 2 of the Large Hadron Collider. The study is performed in final states including two same-sign leptons (electrons or muons), missing transverse momentum, and at least two jets with a large invariant mass and a large rapidity difference. Two independent fits are performed targeting the production of same-sign (Formula presented) bosons with at least one, or two longitudinally polarized (Formula presented) bosons. The observed (expected) significance of the production with at least one longitudinally polarized (Formula presented) boson is 3.3 (4.0) standard deviations. An observed (expected) 95% confidence level upper limit of 0.45 (0.70) fb is reported on the fiducial production cross section of two longitudinally polarized same-sign (Formula presented) bosons.