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

We consider the effects of neutralino-stau (chi-<(tau)over tilde>) coannihilations on the cosmological relic density of the lightest supersymmetric particle (LSP) <(chi)over tilde> 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)over tilde> is approximately a U(1) gaugino (B) over tilde, and constraints on the cosmological relic density Omega((B) over tilde)h(2) yield an upper bound on mg. We show that in regions of parameter space for which the cosmological bound is nearly saturated, coannihilations of the (B) over tilde with the <(tau)over tilde>, the next lightest sparticle, are important and may reduce significantly the B relic density. Including also (B) over tilde coannihilations with the (e) over tilde and <(mu)over tilde>, we find that the upper limit on m<(mu)over tilde>, is increased from about 200 GeV to about 600 GeV in the constrained MSSM, with a similar pew upper limit expected in the MSSM. (C) 1998 Elsevier Science B.V. All rights reserved.