Light scalar top quarks and supersymmetric dark matter

A stable neutralino χ~⁰₁, 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 t~₁. We show that for natural values of the neutralino mass, <~300 GeV, the χ~⁰₁-t~₁ mass difference has to exceed ~10 to 30 GeV if χ~⁰₁ is to contribute significantly to the dark matter. Scenarios with smaller mass splitting, where t~₁ is quite difficult to detect at collider experiments, are thus cosmologically disfavored. On the other hand, for small t~₁-χ~⁰₁ mass splitting, we show that coannihilation allows superparticle masses well beyond the reach of the CERN LHC, m_χ~⁰1~5 TeV, without ``overclosing'' the Universe.