Probing dark force at BES-III/BEPCII

We study an experimental search of a GeV scale vector boson at BES-III/BEPCII. It is responsible for mediating a new U(1)d interaction, as recently exploited in the context of weakly interacting massive particle dark matter. At low energy e+e− colliders this dark state can be efficiently probed. We discuss the direct productions of this light vector U boson and the decay of this state with BES-III data and its foreseen larger data. In particular, we show that Higgs'-strahlung in the dark sector can lead to multilepton signatures, which probe the physics range for kinetic mixing parameter ϵ∼10−4-10−3 over a large portion of the parameter space.