$U(1{)}_{\chi }$, Seesaw Dark Matter, and Higgs Decay

It has recently been pointed out that the underlying symmetry of dark matter may well be $U(1{)}_{\chi }$ (coming from $SO(10)\to SU(5)\times U(1{)}_{\chi }$) with the dark parity of any given particle determined by $(-1{)}^{Q_{\chi }+2j}$, where $Q_{\chi }$ is its $U(1{)}_{\chi }$ charge and $j$ its spin angular momentum. Armed with this new insight, previous simple models of dark matter are reinterpreted, and a novel idea is proposed that light seesaw dark matter exists in analogy to light neutrinos and is produced by the rare decay of the standard-model Higgs boson.