Compressed supersymmetry and natural neutralino dark matter from top squark-mediated annihilation to top quarks

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Abstract
The parameters of the minimal supersymmetric standard model appear to require uncomfortably precise adjustment in order to reconcile the electroweak symmetry breaking scale with the lower mass limits on a neutral Higgs scalar boson. This problem can be significantly ameliorated in models with a running gluino mass parameter that is smaller than the wino mass near the scale of unification of gauge couplings. A compressed superpartner mass spectrum results; compared to models with unified gaugino masses, the ratios of the squark and gluino masses to the lightest superpartner mass are reduced. I argue that in this scenario the annihilation of binolike neutralino pairs to top-antitop quark pairs through top-squark exchange can most naturally play the crucial role in ensuring that the thermal relic dark matter density is not too large, with only a small role played by coannihilations. The lightest superpartner mass must then exceed the top-quark mass, and the lighter top squark cannot decay to a top quark. These conditions have important implications for collider searches.
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