Cold dark matter from heavy right-handed neutrino mixing

We show that, within the seesaw mechanism, an almost decoupled right-handed (RH) neutrino species N-DM with mass M-DM greater than or similar to 100 GeV can play the role of dark matter (DM). The N-DM's can be produced from nonadiabatic conversions of thermalized (source) RH neutrinos with mass M-S lower than M-DM. This is possible if a nonrenormalizable operator is added to the minimal type I seesaw Lagrangian. The observed DM abundance can be reproduced for M-DM delta(1/4) similar to 10(-13) Lambda(eff)xi, where Lambda(eff) is a very high energy new physics scale, delta equivalent to (M-DM - M-S)/M-DM, and xi less than or similar to 1 is a parameter determined by the RH neutrino couplings.