Neutralino Relic Density in a Supersymmetric U(1)' Model

We study properties of the lightest neutralino (\chi) and calculate its cosmological relic density in a supersymmetric U(1)' model with a secluded U(1)' breaking sector (the S-model). The lightest neutralino mass is smaller than in the minimal supersymmetric standard model; for instance, m_\chi < 100 GeV in the limit that the U(1)' gaugino mass is large compared to the electroweak scale. We find that the Z-\chi-\chi coupling can be enhanced due to the singlino components in the extended neutralino sector. Neutralino annihilation through the Z-resonance then reproduces the measured cold dark matter density over broad regions of the model parameter space.