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We study supersymmetric scenarios where the dark matter is the gaugino of anunbroken hidden U(1) which interacts with the visible world only via a smallkinetic mixing with the hypercharge. Strong constraints on the parameter spacecan be derived from avoiding overclosure of the Universe and from requiringsuccessful Big Bang Nucleosynthesis and structure formation. We find that fortypical values of the mixing parameter, scenarios with neutralino NLSP areexcluded, while scenarios with slepton NLSP are allowed when the mixingparameter lies in the range chi~O(10^(-13) - 10^(-10)). We also show that ifthe gravitino is the LSP and the hidden U(1) gaugino the NLSP, the bounds onthe reheating temperature from long lived charged MSSM relics can beconsiderably relaxed and we comment on the signatures of these scenarios atfuture colliders. Finally, we discuss the case of an anomalously small mixing,chi<<10^(-16), where the neutralino becomes a decaying dark matter candidate,and derive constraints from gamma ray experiments.

Hidden gauginos of an unbroken U(1): Cosmological constraints and phenomenological prospects