Direct detection of fourth generation Majorana neutrino dark matter
  Heavy stable fourth generation Majorana neutrinos contribute to a small fraction of the relic density of dark matter (DM) in the Universe. Due to its relatively strong coupling to the standard model particles, it can be probed by the current direct and indirect DM detection experiments even it is a subdominant component of the halo DM. We show that the current Xenon100 data constrain the mass of the stable Majorana neutrino to be greater than the mass of the top quark. The effective spin-independent cross section for the neutrino elastic scattering off nucleon is predicted to be $\sim 1.5\times 10^{-44} cm^2$, which is insensitive to the neutrino mass and mixing and can be reached by the direct DM detection experiments in the near future. In the same mass region the predicted effective spin-dependent cross section for the heavy neutrino scattering off proton is in the range of $2\times 10^{-40} cm^2\sim 2\times 10^{-39} cm^2$, which is within the reach of the ongoing DM indirect search experiments. We demonstrate such properties of the heavy neutrino DM in a fourth generation model with the stability of the fourth Majorana neutrino protected by an anomaly-free U(1) gauge symmetry. 
Yu-Feng Zhou
hep-ph