New Dark Matter Candidate in a U(1)$'$ Model

The Minimal Supersymmetric Standard Model (MSSM) suffers from the fine-tuning of the $\mu$ parameter in the superpotential. This is called the ``$\mu$ problem'' in the MSSM. The $U(1)'$-extended MSSM (UMSSM) is a natural extension without the $\mu$-problem or domain wall problem. With a new gauge boson ($Z'$), we revive the possibility for the sneutrino to be the dark matter including the constraints on the relic density from WMAP, bounds on $Z'$ mass from Tevatron and BBN and the constraint on the elastic scattering cross section off nucelon from CDMSII.