Scalar dark matter candidates

Abstract

We investigate the possibility that dark matter could be made of scalar candidates and focus, in particular, on the unusual mass range between a few MeVs and a few GeVs. After showing why the Lee–Weinberg limit (which usually forbids a dark matter mass below a few GeVs) does not necessarily apply in the case of scalar particles, we discuss how light candidates ( m dm< O(GeV)) can satisfy both the gamma ray and relic density constraints. We find two possibilities. Either dark matter is coupled to heavy fermions (but if m dm≲100 MeV, an asymmetry between the dark matter particle and antiparticle number densities is likely to be required), or dark matter is coupled to a new light gauge boson U. The (collisional) damping of light candidates is, in some circumstances, large enough to be mentioned, but in most cases too small to generate a non-linear matter power spectrum at the present epoch that differs significantly from the cold dark matter spectrum. On the other hand, heavier scalar dark matter particles (i.e., with m dm≳ O(GeV)) turn out to be much less constrained. We finally discuss a theoretical framework for scalar candidates, inspired from theories with N=2 extended supersymmetry and/or extra space dimensions, in which the dark matter stability results from a new discrete (or continuous) symmetry.