  Assuming that the lightest supersymmetric particle (LSP) is the lightest
neutralino, we present a detailed exploration of neutralino-stau coannihilation
channels, including analytical expressions and numerical results. We also
include neutralino coannihilations with the selectron and smuon. We evaluate
the implications of coannihilations for the cosmological relic density of the
LSP, which is assumed to be stable, in the constrained minimal supersymmetric
extension of the Standard Model (CMSSM), in which the soft
supersymmetry-breaking parameters are universal at the supergravity GUT scale.
We evaluate the changes due to coannihilations in the region of the MSSM
parameter space that is consistent with the cosmological upper limit on the
relic LSP density. In particular, we find that the upper limit on the mass of
the neutralino is increased from about 200 GeV to about 600 GeV in the CMSSM,
and estimate a qualitatively similar increase for gauginos in the general MSSM.
