Calculations of Neutralino-Stau Coannihilation Channels and the Cosmologically Relevant Region of MSSM Parameter Space

Assuming that the lightest supersymmetric particle (LSP) is the lightest neutralino χ̃, we present a detailed exploration of neutralino-stau (χ̃− τ̃ ) coannihilation channels, including analytical expressions and numerical results. We also include χ̃ coannihilations with the ẽ and μ̃. 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 mχ̃ 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.