2961612 20260518221520.0 oai:cds.cern.ch:2961612 cerncds:FULLTEXT cerncds:cms-pas CMS-PAS-B2G-24-023 Search for heavy neutral leptons in top quark decays with same-sign dilepton final states in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$ CMS Collaboration CMS Collaboration CERN LHC CMS 2026 Geneva CERN CERN CMS Monte-Carlo The first search for a Majorana$\text{-}$type heavy neutral lepton (N) produced in top quark decays with the CMS detector is reported. In type$\text{-}$I seesaw models, where the N mixes with the first two generations of neutrinos, these heavy leptons lead to novel top quark decay modes that violate lepton number conservation. We therefore search for top quark pair production in proton$\text{-}$proton collisions, followed by a top quark decay to a bottom quark and a W boson, decaying further into a charged lepton (electron or muon), and N. Here, the Majorana N decays to a W boson and a charged lepton with the same sign as the charged lepton produced with N. The experimental signature of this process is characterized by the presence of same$\text{-}$sign and same$\text{-}$flavor lepton pairs and jets. We use proton$\text{-}$proton collision data corresponding to an integrated luminosity of 138 $\text{fb}^{-1}$ at a center$\text{-}$of$\text{-}$mass energy of 13 TeV, collected during 2016--2018 with the CMS detector at the LHC. The search is performed for N masses between 20 and 100 GeV. Depending on the mass of N, the observed (expected) upper limits at the 95\% CL on the branching ratio of the top quark decay to N, charged lepton, and bottom quark vary between $1.1\times10^{-5}$ ($6.8\times10^{-6}$) and $4.7\times10^{-5}$ ($3.0 \times 10^{-5}$) in the dielectron final state and between $3.8\times10^{-6}$ ($3.4 \times 10^{-6}$) and $2.4\times10^{-5}$ ($1.3 \times 10^{-5}$) in the dimuon final state. http://cms.cern.ch/iCMS/analysisadmin/cadi?ancode=B2G-24-023 Additional information for the analysis (restricted access) 2843919 2209587 http://cds.cern.ch/record/2961612/files/B2G-24-023-pas.pdf Fulltext in PDF 2843888 16489 http://cds.cern.ch/record/2961612/files/Figure_013-a.pdf Figure_013-a Figure 2843889 16545 http://cds.cern.ch/record/2961612/files/Figure_013-b.pdf Figure_013-b Figure 2843890 42383 http://cds.cern.ch/record/2961612/files/Figure_012-b.pdf Figure_012-b Figure 2843891 29142 http://cds.cern.ch/record/2961612/files/Figure_010-e.pdf Figure_010-e Figure 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2843929 5855 http://cds.cern.ch/record/2961612/files/Figure_001-c.png?subformat=icon icon Figure_001-c Figure 2843930 7042 http://cds.cern.ch/record/2961612/files/Figure_001-b.png?subformat=icon icon Figure_001-b Figure 2843931 6587 http://cds.cern.ch/record/2961612/files/Figure_001-a.png?subformat=icon icon Figure_001-a Figure NOTE CMS-PHYSICS-ANALYSIS-SUMMARIES cms-pag-conveners-b2g@cern.ch SzGeCERN Particle Physics - Experiment