Particle Dark Matter Physics: An Update

This write–up gives a rather elementary introduction into particle physics aspects of the cosmological Dark Matter puzzle. A fairly comprehensive list of possible candidates is given; in each case the production mechanism and possible ways to detect them (if any) are described. I then describe detection of the in my view most promising candidates, weakly interacting massive particles or WIMPs, in slightly more detail. The main emphasis will be on recent developments. Invited talk at the Fifth Workshop on Particle Physics Phenomenology, Pune, India, January 1998. 1) Introduction: The Need for Exotic Dark Matter Dark Matter (DM) is, by definition, stuff that does not emit detectable amounts of electromagnetic radiation. At present its existence can therefore only be inferred [1] from the gravitational pull it exerts on other, visible, celestial bodies. The best evidence of this kind comes from the study of galactic rotation curves. Here one measures the velocity with which globular stellar clusters, gas clouds, or dwarf galaxies orbit around (other) galaxies, including our own Milky Way. If the mass of these galaxies was concentrated in their visible parts, the orbital velocity at large radii R should decrease like 1/ √ R. Instead, in nearly all cases on finds that it remains approximately constant out to the largest radius where it can be measured. This implies that the total mass M(R) felt by an object at radius R must increase linearly with R. Studies of this type imply that 90% or more of the mass of (large) galaxies is dark; this is a lower bound, since it is not known where the growth M(R) ∝ R cuts off (as it must, since the total mass of a galaxy is obviously finite). Cosmologists like to express mass densities averaged over the entire visible Universe in units of the critical density ρc ∼ 10g/cm; the dimensionless ratio is then called Ω, with Ω = 1 corresponding to a flat Universe. Analyses of galactic rotation curves imply