Search for heavy neutral leptons in top quark decays with same-sign, same-flavor dilepton final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for a Majorana-type heavy neutral lepton (HNL) produced in top quark decays is reported. The search is based on proton-proton collision data at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$, collected during 2016$-$2018 with the CMS detector at the CERN LHC. In the type-I seesaw model, a Majorana HNL can mediate lepton-number-violating top quark decays. The search targets top quark pair production events in which one top quark decays into a bottom quark and a W boson, which subsequently decays into a charged lepton (electron or muon) and an HNL. The Majorana HNL then decays into a W boson and a charged lepton with the same sign and flavor as the one produced with the HNL. The experimental signature features a same-sign, same-flavor lepton pair and jets. The HNL masses between 20 and 100 GeV are probed. The data agree with the standard model background prediction. Observed (expected) upper limits at 95% confidence level are set on the branching fraction of the top quark decay into an HNL, a charged lepton, and a bottom quark, varying between [1.1 (0.71)] $\times$10$^{-5}$ and [4.8 (3.0)] $\times$10$^{-5}$ in the dielectron channel, and between [3.7 (3.4)] $\times$10$^{-6}$ and [2.4 (1.3)] $\times$10$^{-5}$ in the dimuon channel.