WIMP Dark Matter and Neutrino Mass from Peccei-Quinn Symmetry

The Peccei-Quinn anomalous global U(1)PQ symmetry is important for solving the strong CP problem with a cosmologically relevant axion. We add to this the simple (but hitherto unexplored) observation that it also has a residual Z2 symmetry which may be responsible for a second component of dark matter, i.e. an absolutely stable weakly interacting singlet scalar. This new insight provides a theoretical justification for this well-studied simplest of all possible dark-matter models. It also connects with the well-studied notion of generating radiative neutrino mass through dark matter. Two such specific realizations are proposed. In our general scenario, dark-matter detection is guaranteed at existing direct-detection experiments or axion searches. Observable signals at the Large Hadron Collider are discussed.