Experimental Targets for Photon Couplings of the QCD Axion
  The QCD axion's coupling to photons is often assumed to lie in a narrow band
as a function of the axion mass. We demonstrate that several simple mechanisms,
in addition to the photophilic clockwork axion already in the literature, can
significantly extend the allowed range of couplings. Some mechanisms we present
generalize the KNP alignment scenario, widely studied as a model of inflation,
to the phenomenology of a QCD axion. In particular we present KSVZ-like
realizations of two-axion KNP alignment and of the clockwork mechanism. Such a
"confinement tower" realization of clockwork may prove useful in a variety of
model-building contexts. We also show that kinetic mixing of the QCD axion with
a lighter axion-like particle can dramatically alter the QCD axion's coupling
to photons, differing from the other models we present by allowing
non-quantized couplings. The simple models that we present fully cover the
range of axion-photon couplings that could be probed by experiments. They
motivate growing axion detection efforts over a wide space of masses and
couplings.
