Aad, G.Abbott, B.Abeling, K.Abicht, N. J.Abidi, S. H.Aboulhorma, A.Çetin, Serkant Ali2024-05-192024-05-1920231434-60441434-6052https://doi.org10.1140/epjc/s10052-023-11915-yhttps://hdl.handle.net/20.500.12713/5901A search for Majorana neutrinos in same-sign WW scattering events is presented. The analysis uses root s=13 TeV proton-proton collision data with an integrated luminosity of 140 fb(-1) recorded during 2015-2018 by the ATLAS detector at the Large Hadron Collider. The analysis targets final states including exactly two same-sign muons and at least two hadronic jets well separated in rapidity. The modelling of the main backgrounds, from Standard Model same-sign WW scattering and WZ production, is constrained with data in dedicated signal-depleted control regions. The distribution of the transverse momentum of the second-hardest muon is used to search for signals originating from a heavy Majorana neutrino with a mass between 50 GeV and 20 TeV. No significant excess is observed over the background expectation. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. In addition, the sensitivity to the Weinberg operator is investigated. Upper limits at the 95% confidence level are placed on the squared muon-neutrino-heavy-neutrino mass-mixing matrix element |V-mu N|(2) as a function of the heavy Majorana neutrino's mass m(N), and on the effective mu mu Majorana neutrino mass |m(mu mu)|.eninfo:eu-repo/semantics/openAccess[No Keywords]Article839WOS:0010909832000012-s2.0-85171580591N/A10.1140/epjc/s10052-023-11915-yQ1