Search for heavy Majorana neutrinos in 𝑒±𝑒± and 𝑒±𝜇± final states via WW scattering in 𝑝𝑝 collisions at √𝑠 = 13 TeV with the ATLAS detector

A search for heavy Majorana neutrinos in scattering of same-sign 𝑊 boson pairs in proton–proton collisions at √𝑠 = 13 TeV at the LHC is reported. The dataset used corresponds to an integrated luminosity of 140 fb−1, collected with the ATLAS detector during 2015–2018. The search is performed in final states including a same-sign
𝑒𝑒 or 𝑒𝜇 pair and at least two jets with large invariant mass and a large rapidity difference. No significant excess of events with respect to the Standard Model background predictions is observed. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. New constraints are set on the values of the |𝑉 𝑒𝑁 |2 and |𝑉 𝑒𝑁 𝑉 ∗ 𝜇𝑁 | parameters for heavy Majorana neutrino masses between 50 GeV and 20 TeV, where 𝑉𝓁𝑁 is the matrix element describing the mixing of the heavy Majorana neutrino mass eigenstate with the Standard Model neutrino of flavour 𝓁 = 𝑒, 𝜇. The sensitivity to the Weinberg operator is investigated and constraints on the effective 𝑒𝑒 and 𝑒𝜇 Majorana neutrino masses are reported. The statistical combination of the 𝑒𝑒 and 𝑒𝜇 channels with the previously published 𝜇𝜇 channel is performed.