Light scalar top quarks and supersymmetric dark matter

Abstract

A stable neutralino ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0},$ 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, $\ensuremath{\lesssim}300$ GeV, the ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}$-${t}_{1}$ mass difference has to exceed $\ensuremath{\sim}10$ to 30 GeV if ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}$ 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 ${t}_{1}$-${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}$ mass splitting, we show that coannihilation allows superparticle masses well beyond the reach of the CERN LHC, ${m}_{{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}}\ensuremath{\sim}5$ TeV, without ``overclosing'' the Universe.