Search for heavy top quark partners with charge 5/3 in the single-lepton final state in proton-proton collisions at √s = 13 TeV

A search for the pair production of a vector-like quark, X5/3, a heavy fermionic partner of the top quark with an electric charge of 5/3, is presented. The search is performed in the final state with one lepton, an electron or a muon. Proton-proton collision data collected by the CMS experiment at the LHC from 2016 to 2018, at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb⁻¹, are used. Two decay modes are considered. In the conventional decay mode, the X5/3 is assumed to decay exclusively into a top quark and a W boson. In an alternative, exotic scenario, it is assumed to decay exclusively into a top quark and a charged Higgs boson, assumed to subsequently decay exclusively into top and bottom quarks. No excess of data events over the standard model (SM) expectation is observed. Expected and observed upper limits on the pair production cross section of the X5/3 are set at 95% confidence level as functions of the X5/3 mass. The observed exclusion ranges of the X5/3 mass extend up to 1630 GeV in the decay to SM particles, and up to 1740 GeV in the exotic decay mode for a charged Higgs boson mass of 800 GeV. These are the most stringent limits on the X5/3 decaying to a top quark and a W boson, and the first limits on the X5/3 decay into a top quark and a charged Higgs boson.