Scalar dark matter candidates

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)less than or similar to100 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)greater than or similar to 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. (C) 2004 Elsevier B.V. All rights reserved.