  The strong CP problem of QCD is at heart a problem of naturalness: why is the
F\tilde{F} term highly suppressed in the QCD Lagrangian when it seems necessary
to explain why there are three and not four light pions? The most elegant
solution posits a spontaneously broken Peccei-Quinn (PQ) symmetry which
requires the existence of the axion field a. The axion field settles to the
minimum of its potential thus removing the offensive term but giving rise to
the physical axion whose coherent oscillations can make up the cold dark
matter. Only now are experiments such as ADMX beginning to explore QCD axion
parameter space. Since a bonafide scalar particle-- the Higgs boson-- has been
discovered, one might expect its mass to reside at the axion scale f_a~ 10^{11}
GeV. The Higgs mass is elegantly stabilized by supersymmetry: in this case the
axion is accompanied by its axino and saxion superpartners. Requiring
naturalness also in the electroweak sector implies higgsino-like WIMPs so then
we expect mixed axion-WIMP dark matter. Ultimately we would expect detection of
both an axion and a WIMP while signals for light higgsinos may show up at LHC
and must show up at ILC.
