Neutralino-Stau Coannihilation and the Cosmological Upper Limit on the Mass of the Lightest Supersymmetric Particle

We consider the effects of neutralino-stau coannihilations on the cosmological relic density of the lightest supersymmetric particle (LSP) $\chi$ 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, $\chi$ is approximately a U(1) gaugino ${\tilde B}$, and constraints on the cosmological relic density $\Omega_{\tilde B} h^2$ yield an upper bound on the bino mass. We show that in regions of parameter space for which the cosmological bound is nearly saturated, coannihilations of the $\tilde B$ with the stau, the next lightest sparticle, are important and may reduce significantly the bino relic density. Including also bino coannihilations with the selectrons and smuons, we find that the upper limit on $m_{\chi}$ is increased from about 200 GeV to about 600 GeV in the constrained MSSM, with a similar new upper limit expected in the MSSM.