Abstract
We discuss a new variant of the E 6 inspired supersymmetric standard model (E 6SSM) in which the two inert singlinos are exactly massless and the dark matter candidate has a dominant bino component. A successful relic density is achieved via a novel mechanism in which the bino scatters inelastically into heavier inert Higgsinos during the time of thermal freeze-out. The two massless inert singlinos contribute to the effective number of neutrino species at the time of Big Bang Nucleosynthesis, where the precise contribution depends on the mass of the Z′ which keeps them in equilibrium. For example for m Z′ > 1300 GeV we find N eff ≈ 3.2, where the smallness of the additional contribution is due to entropy dilution. We study a few benchmark points in the constrained E 6SSM with massless inert singlinos to illustrate this new scenario.
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