Aad, G.Aakvaag, E.Abbott, B.Abdelhameed, S.Abeling, K.Abicht, N. J.Zwalinski, L.2025-11-102025-11-1020250031-90071079-7114https://doi.org/10.1103/bpln-ccqlDal Santo, Daniele/0000-0001-7176-7979; Corchia, Federico Anea Guillaume/0000-0002-1788-3204; Ventura González, Salvador/0000-0001-5246-0779; Matousek, Jan/0000-0002-2174-5517; Ventura González, Salvador/0000-0001-5246-0779; Clark, Allan Geoffrey/0000-0001-8341-5911This Letter reports the first evidence of electroweak production of same-sign W boson pairs where at least one of the W bosons is longitudinally polarized and the most stringent constraint to date for the production of two longitudinally polarized same-sign W bosons. The dataset used corresponds to an integrated luminosity of 140 fb(-1) 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 W bosons with at least one, or two longitudinally polarized W bosons. The observed (expected) significance of the production with at least one longitudinally polarized W 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 W bosons.eninfo:eu-repo/semantics/openAccessArticle2-s2.0-10501741432910.1103/bpln-ccql