Light scalar top quarks and supersymmetric dark matter

A stable neutralino <(chi)over tilde>(0)(1), 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) over tilde(1). We show that for natural values of the neutralino mass, less than or similar to 300 GeV, the <(chi)over tilde>(0)(1)-(t) over tilde(1) mass difference has to exceed similar to 10 to 30 GeV if <(chi)over tilde>(0)(1): is to contribute significantly to the dark matter. Scenarios with smaller mass splitting, where (t) over tilde(1) is quite difficult to detect at collider experiments, are thus cosmologically disfavored. On the other hand, for small (t) over tilde(1)-<(chi)over tilde>(0)(1) mass splitting, we show that coannihilation allows superparticle masses well beyond the reach of the CERN LHC, m(<(chi)over tilde>1)(0)similar to 5 TeV, without overclosing the Universe.