Probing Hidden Sectors with St\"uckelberg U(1) Gauge Fields
  We propose a framework in which visible matter interacts with matter from a
hidden sector through mass mixings of St\"uckelberg U(1) gauge fields. In
contrast to other Z' mediation scenarios, our setup has the added appealing
features that (i) the choice of Z's can be significantly broadened without
necessarily introducing unwanted exotic matter and (ii) there can be sizable
tree-level interactions between the visible and hidden sectors. String theory
embeddings of this scenario and their phenomenological features are briefly
discussed.
