Light Scalar Top Quarks and Supersymmetric Dark Matter

Abstract

A stable neutralino $\chi_1^0$, assumed to be the lightest supersymmetric particle, is a favored particle physics candidate for the cosmological Dark Matter. We study co-annihilation of the lightest neutralino with the lighter scalar top quark $\tilde{t}_1$. We show that for natural values of the neutralino mass, $\lsim 300$ GeV, the $\chi_1^0 - \tilde{t}_1$ mass difference has to exceed $\sim 10$ to 30 GeV if $\chi_1^0$, is to contribute significantly to the Dark Matter. Scenarios with smaller mass splitting, where $\tilde{t}_1$ is quite difficult to detect at collider experiments, are thus cosmologically disfavored. On the other hand, for small $\tilde{t}_1 - \chi_1^0$ mass splitting, we show that co--annihilation allows very large neutralino masses, $m_{\chi_1^0} \sim 5$ TeV, without ``overclosing'' the Universe.