Neutralino relic density in a supersymmetric U(1)′ model

We study properties of the lightest neutralino (x(0)) 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 (0)(x) less than or similar to 100 GeV in the limit that the U(1)' gaugino mass is large compared to the electroweak scale. We find that the Z(x)(x)(0)(0) 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. (C) 2004 Elsevier B.V. All rights reserved.