  A search is performed for heavy Majorana neutrinos (N) decaying into a W
boson and a lepton using the CMS detector at the Large Hadron Collider. A
signature of two jets and either two same sign electrons or a same sign
electron-muon pair is searched for using 19.7 inverse femtobarns of data
collected during 2012 in proton-proton collisions at a centre-of-mass energy of
8 TeV. The data are found to be consistent with the expected standard model
(SM) background and, in the context of a Type-1 seesaw mechanism, upper limits
are set on the cross section times branching fraction for production of heavy
Majorana neutrinos in the mass range between 40 and 500 GeV. The results are
additionally interpreted as limits on the mixing between the heavy Majorana
neutrinos and the SM neutrinos. In the mass range considered, the upper limits
range between 0.00015 - 0.72 for |V[eN]|^2 and 6.6E-5 - 0.47 for |V[eN]
V*[muN]|^2 / ( |V[eN]|^2 + |V[muN]|^2 ), where V[lN] is the mixing element
describing the mixing of the heavy neutrino with the SM neutrino of flavour l.
These limits are the most restrictive direct limits for heavy Majorana neutrino
masses above 200 GeV.
