Search for heavy top quark partners with charge 5e/3 in the single lepton final state

A search is presented for a very heavy fermionic partner of the top quark with charge 5e/3 ($X_{5/3}$), which is expected to decay into a top quark and a W boson. Events consistent with pair production of $X_{5/3}$, where one of the four W bosons from the decays of $X_{5/3}$ and top quarks decays leptonically, and all the other W bosons decay hadronically, are analyzed. The results from this semi-leptonic final state are then combined with same-sign dileptons signature to enhance the sensitivity. No excess of data above SM expectations is observed and 95$\%$ CL upper limits on the $X_{5/3}$ production cross section are set using the data corresponding to an integrated luminosity of 2.2 $fb^{-1}$ collected by the CMS detector at center-of-mass energy of 13 TeV.