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.