Search for heavy Majorana neutrinos in same-sign dilepton channels in proton-proton collisions at $\sqrt{s}=$ 13 TeV

A search is performed for a heavy Majorana neutrino (N), produced by leptonic decay of a W boson propagator and decaying into a W boson and a lepton, with the CMS detector at the LHC. The signature used in this search consists of two same-sign leptons, in any flavor combination of electrons and muons, and at least one jet. The data were collected during 2016 in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb${}^{-1}$. The results are found to be consistent with the expected standard model backgrounds. Upper limits are set in the mass range between 20 and 1600 GeV in the context of a Type-I seesaw mechanism, on |$V_{\mathrm{e}\mathrm{N}}$|${}^2$, |$V_{\mu \mathrm{N}}$|${}^2$, and |$V_{\mathrm{e}\mathrm{N}}$$V_{\mu \mathrm{N}}^{\ast }$|${}^2$ / (|$V_{\mathrm{e}\mathrm{N}}$|${}^2$ + |$V_{\mu \mathrm{N}}$|${}^2$), where $V_{\mathrm{\ell }\mathrm{N}}$ is the matrix element describing the mixing of N with the standard model neutrino of flavor $\mathrm{\ell }=$ e, $\mu$. For N masses between 20 and 1600 GeV, the upper limits on |$V_{\mathrm{\ell }\mathrm{N}}$|${}^2$ range between 2.3 $\times$ 10${}^{-5}$ and unity. These are the most restrictive direct limits for heavy Majorana neutrino masses above 430 GeV.