A model of electroweak-scale right-handed neutrino mass
  If neutrino masses are realized through the see-saw mechanism, can the right-handed neutrinos be produced and detected at present and future colliders? The answer is negative in the most popular see-saw scenarios for the simple reason that they are too heavy in these models. However, a simple extension of the Standard Model (SM) particle content, including mirror fermions, two $SU(2)_L$ triplet and one singlet Higgs fields, leads to a scenario in which the see-saw mechanism is realized with the Majorana mass $M_R$ of the right-handed neutrino being of the order of the electroweak scale or smaller. A custodial SU(2) symmetry arising from the two triplet Higgs fields ensures that $\rho=1$ at tree level even when their vacuum expectation values (VEV) which determine the value of $M_R$, can be as large as the electroweak scale. $M_R$ is found to obey the bound $\frac{M_Z}{2} \leq M_R < 246 GeV $ which makes it accessible experimentally (Tevatron, LHC or ILC) since, in our scenario, $\nu_R$'s can couple directly to the Standard Model (SM) gauge bosons. 
P. Q. Hung