Light scalar top quarks and supersymmetric dark matter

A stable neutralino ${\tilde{\chi }}_1^0,$ 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, $\lesssim 300$ GeV, the ${\tilde{\chi }}_1^0$-$t_1$ mass difference has to exceed $\sim 10$ to 30 GeV if ${\tilde{\chi }}_1^0$ 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 $t_1$-${\tilde{\chi }}_1^0$ mass splitting, we show that coannihilation allows superparticle masses well beyond the reach of the CERN LHC, $m_{{\tilde{\chi }}_1^0}\sim 5$ TeV, without “overclosing” the Universe.