Majorana neutrinos and same-sign dilepton production at LHC and in rare meson decays

We discuss same-sign dilepton production mediated by Majorana neutrinos in high-energy proton proton collisions pp --> l(+)l'X+ for l, l' =e, rho, tau at the LHC energy s(1/2) = 14 TeV, and in the rare decays of the K, D, D-s , and B mesons of the type M+ --> M'-l(+)l'(+). For the pp reaction, assumi ng one heavy Majorana neutrino of mass m(N), we present discovery limits in the (m(N), \UlNUl'N\) plane where flew are the mixing parameters. Taking i nto account the present limits from low-energy experiments, we show that at LHC one has sensitivity to heavy Majorana neutrinos up to a mass m(N) less than or equal to 2-5TeV in the dilepton channels mu mu, tau tau, and mu ta u, but the dilepton states el will not be detectable due to the already exi sting constraints from neutrinoless double beta decay. We work out a large number of rare meson decays, both for the light and heavy Majorana neutrino scenarios, and argue that the present experimental bounds on the branching ratios are too weak to set reasonable limits on the effective Majorana mas ses.