Prospects for axion detection in natural SUSY with mixed axion-higgsino
  dark matter: back to invisible?
  Under the expectation that nature is natural, we extend the Standard Model to
include SUSY to stabilize the electroweak sector and PQ symmetry to stabilize
the QCD sector. Then natural SUSY arises from a Kim-Nilles solution to the SUSY
mu problem which allows for a little hierarchy where mu ~ f_a^2/M_P ~ 100-300
GeV while the SUSY particle mass scale m(SUSY)~ 1-10 TeV >> mu. Dark matter
then consists of two particles: a higgsino-like WIMP and a SUSY DFSZ axion. The
range of allowed axion mass values m(axion) depends on the mixed axion-higgsino
relic density. The range of m(axion) is actually restricted in this case by
limits on WIMPs from direct and indirect detection experiments. We plot the
expected axion detection rate at microwave cavity experiments. The
axion-photon-photon coupling is severely diminished by charged higgsino
contributions to the anomalous coupling. In this case, the axion may retreat,
at least temporarily, back into the regime of near invisibility. From our
results, we urge new ideas for techniques which probe both deeper and more
broadly into axion coupling versus axion mass parameter space.
