Light scalar top quarks and supersymmetric dark matter

A stable neutralino ̃ \~χ₁⁰, assumed to be the lightest supersymmetric particle, is a favored particle physics candidate for cosmological dark matter. We study coannihilation of the lightest neutralino with the lighter scalar top quark t₁. We show that for natural values of the neutralino mass, 300 GeV, the ̃ \~χ₁⁰-t₁ mass difference has to exceed ~10 to 30 GeV if ̃ \~χ₁⁰ is to contribute significantly to the dark matter. Scenarios with smaller mass splitting, where t₁ is quite difficult to detect at collider experiments, are thus cosmologically disfavored. On the other hand, for small t₁-̃ \~χ₁⁰ mass splitting, we show that coannihilation allows superparticle masses well beyond the reach of the CERN LHC, m_ ̃ \~χ₁⁰~5 TeV, without ``overclosing'' the Universe.