Search for Majorana neutrinos in same-sign WW scattering events from pp collisions at √s = 13 TeV

A search for Majorana neutrinos in same-sign WW scattering events is presented. The analysis uses TeV proton–proton collision data with an integrated luminosity of 140 fb 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 as a function of the heavy Majorana neutrino’s mass, and on the effective Majorana neutrino mass.