Search for heavy right-handed Majorana neutrinos in the decay of top quarks produced in proton$-$proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for heavy right-handed Majorana neutrinos is performed with the ATLAS detector at the CERN Large Hadron Collider, using the 140 $\mathrm{fb}^{-1}$ of proton$-$proton collision data at $\sqrt{s}$ = 13 TeV collected during Run 2. This search targets $t\bar{t}$ production, in which both top quarks decay into a bottom quark and a $W$ boson, where one of the $W$ bosons decays hadronically and the other decays into an electron or muon and a heavy neutral lepton. The heavy neutral lepton is identified through a decay into an electron or muon and another $W$ boson, resulting in a pair of same-charge same-flavor leptons in the final state. This paper presents the first search for heavy neutral leptons in the mass range of 15$-$75 GeV using $t\bar{t}$ events. No significant excess is observed over the background expectation, and upper limits are placed on the signal cross-sections. Assuming a benchmark scenario of the phenomenological type-I seesaw model, these cross-section limits are then translated into upper limits on the mixing parameters of the heavy Majorana neutrino with Standard Model neutrinos.