Probing a secluded U(1) at B factories

A secluded U(1)S gauge field, kinetically mixed with standard model hypercharge, provides a "portal" mediating interactions with a hidden sector at the renormalizable level, as recently exploited in the context of weakly interacting massive particle dark matter. The U(1)S symmetry-breaking scale may naturally be suppressed relative to the weak scale, and so this sector is efficiently probed by medium-energy e+e- colliders. We study the collider signatures of the minimal U(1)S model, focusing on the reach of B-factory experiments such as BABAR and BELLE. In particular, we show that Higgs strahlung in the secluded sector can lead to multilepton signatures, which probe the natural range for the kinetic mixing angle κ∼10-2-10-3 over a large portion of the kinematically accessible parameter space.