Cold dark matter from heavy right-handed neutrino mixing

We show that, within the seesaw mechanism, an almost decoupled right-handed (RH) neutrino species $N_{\mathrm{DM}}$ with mass $M_{\mathrm{DM}}\gtrsim 100\mathrm{GeV}$ can play the role of dark matter (DM). The $N_{\mathrm{DM}}$’s can be produced from nonadiabatic conversions of thermalized (source) RH neutrinos with mass $M_S$ lower than $M_{\mathrm{DM}}$. This is possible if a nonrenormalizable operator is added to the minimal type I seesaw Lagrangian. The observed DM abundance can be reproduced for $M_{\mathrm{DM}}{\delta }^{1/4}\sim {10}^{-13}{\Lambda }_{\mathrm{eff}}\xi$, where ${\Lambda }_{\mathrm{eff}}$ is a very high energy new physics scale, $\delta \equiv (M_{\mathrm{DM}}-M_S)/M_{\mathrm{DM}}$, and $\xi \lesssim 1$ is a parameter determined by the RH neutrino couplings.