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 <(chi)over tilde>, we present a detailed exploration of neutralino-stau (<(chi)over tilde> - <(tau)over tilde>) coannihilation channels, including analytical expressions and numerical results. We also include <(chi)over tilde> coannihilations with the (e) over tilde and <(mu)over tilde>. 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 m(<(chi)over tilde>) 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. (C) 2000 Elsevier Science B.V. All rights reserved.