  A search is performed for heavy Majorana neutrinos (N) using an event
signature defined by two same-sign charged leptons of the same flavour and two
jets. The data correspond to an integrated luminosity of 4.98 inverse
femtobarns of pp collisions at a centre-of-mass energy of 7 TeV collected with
the CMS detector at the Large Hadron Collider. No excess of events is observed
beyond the expected standard model background and therefore upper limits are
set on the square of the mixing parameter, abs(V[ell N]) squared, for ell = e,
mu, as a function of heavy Majorana-neutrino mass. These are the first direct
upper limits on the heavy Majorana-neutrino mixing for m[N] > 90 GeV.
