Widening the Axion Window via Kinetic and St\"uckelberg Mixings
  We point out that kinetic and St\"uckelberg mixings that are generically
present in the low energy effective action of axions can significantly widen
the window of axion decay constants. We show that an effective super-Planckian
decay constant can be obtained even when the axion kinetic matrix has only
sub-Planckian entries. Our minimal model involves only two axions, a
St\"uckelberg U(1) and a modest rank instanton generating non-Abelian group.
Below the mass of the St\"uckelberg U(1), there is only a single axion with a
non-perturbatively generated potential. In contrast to previous approaches, the
enhancement of the axion decay constant is not tied to the number of degrees of
freedom introduced. We also discuss how kinetic mixings can lower the decay
constant to the desired axion dark matter window. String theory embeddings of
this scenario and their phenomenological features are briefly discussed.
