Neutralino-stau coannihilation and the cosmological upper limit on the mass of the lightest supersymmetric particle

We consider the effects of neutralino-stau (χ−τ̃) coannihilations on the cosmological relic density of the lightest supersymmetric particle (LSP) χ̃ in the minimal supersymmetric extension of the Standard Model (MSSM), particularly in the constrained MSSM in which universal supergravity inputs at the GUT scale are assumed. For much of the parameter space in these models, χ̃ is approximately a U(1) gaugino B̃, and constraints on the cosmological relic density ΩB̃h2 yield an upper bound on mB̃. We show that in regions of parameter space for which the cosmological bound is nearly saturated, coannihilations of the B̃ with the τ̃, the next lightest sparticle, are important and may reduce significantly the B̃ relic density. Including also B̃ coannihilations with the ẽ and μ̃, we find that the upper limit on mχ̃ is increased from about 200 GeV to about 600 GeV in the constrained MSSM, with a similar new upper limit expected in the MSSM.