Cosmology’s widely accepted concordance model attributes only about one-sixth of the present mass density of the cosmos to baryonic matter—protons and neutrons. The dominant nonbaryonic mass is presumed to consist of some still-unknown dark-matter particle species, impervious to electromagnetic and strong nuclear interactions. The seminal role assigned to dark-matter particles in galaxy formation requires that they be stable and significantly heavier than the proton. Hence the name WIMPs—weakly interacting massive particles.
Quite apart from cosmological considerations, particle physicists had predicted just such WIMPs by positing a supersymmetry between bosons and fermions in attempts to expand particle theory’s standard model. Supersymmetric models predict an abundance of new heavy species—a bosonic partner for every fundamental fermion and vice versa. But none has yet been found, and mass predictions are varied and vague.
The favored dark-matter candidate at present is χ0, the lightest of a family of uncharged, spin-½ “neutralinos” predicted...
