Dark Matter in Axion Landscape
  If there are a plethora of axions in nature, they may have a complicated
potential and create an axion landscape. We study a possibility that one of the
axions is so light that it is cosmologically stable, explaining the observed
dark matter density. In particular we focus on a case in which two (or more)
shift-symmetry breaking terms conspire to make the axion sufficiently light at
the potential minimum. In this case the axion has a flat-bottomed potential. In
contrast to the case in which a single cosine term dominates the potential, the
axion abundance as well as its isocurvature perturbations are significantly
suppressed. This allows an axion with a rather large mass to serve as dark
matter without fine-tuning of the initial misalignment, and further makes
higher-scale inflation to be consistent with the scenario.
