Supersymmetric dark matter—how light can the LSP be?

Using a very minimal set of theoretical assumptions we derive a lower limit on the LSP mass in the MSSM. We only require that the LSP be the lightest neutralino, that it be responsible for the observed relic density and that the MSSM spectrum respect the LEP2 limits. We explicitly do not require any further knowledge about the MSSM spectrum or the mechanism of supersymmetry breaking. Under these assumptions we determine a firm lower limit on the neutralino LSP mass of 18 GeV. We estimate the effect of improved limits on the cold dark matter relic density as well as the effects of improved LEP2-type limits from a first stage of TESLA on the allowed range of neutralino LSP masses.