Dark matter in the light of LEP

Abstract

LEP constraints suggest that cold matter particle candidates must be heavier than previously favoured. The LEP limit on Z 0→invisible neutrals implies that any Majorana fermions annihilating via the Z 0 to give the critical closure density must weigh more than about 14 GeV, if the present Hubble expansion rate is 50, km s -1 Mpc -1, a limit which can rise to about 19 GeV for Dirac fermions when underground 76Ge experiments are taken into account. It is pointed out that in both the Dirac and Majorana cases the Z 0 couplings must be weaker than those of neutrinos, unless the masses exceed ∼ 1 2 mz . These arguments on mass and interaction strength should be considered in future dark matter searches. We exemplify these generic arguments by identifying regions of parameter space in the minimal supersymmetric extension of the standard model where the lightest supersymmetric particle (LSP) can have the critical density. In some of these regions the LSP is mainly a higgsino and its mass much larger than the Majorana fermion bound mentioned above. In other regions the LSP is mainly a gaugino, in which case m LSP ≳20 GeV.