Stochastic Axion Dark Matter in Axion Landscape
  We study the stochastic axion dark matter scenario in the axion landscape,
where one of the axions is light and stable and therefore explains dark matter.
If the axion mass at the potential is a typical value of the curvature along
the direction, the potential can be well approximated by a quadratic mass term.
On the other hand, if the axion mass happens to be suppressed in the vicinity
of the minimum, the potential may be approximated by a quartic potential plus a
suppressed quadratic one, for which the initial angle, and thus the axion
abundance, can be significantly suppressed compared to the quadratic case. We
delineate the viable parameter region by taking account of various
observational constraints, and find that a broader range of the inflation scale
is allowed. Also, if the curvature of the potential is suppressed over a
certain range of the potential, the onset of coherent oscillations can be
delayed. Then, the axion dark matter with a small decay constant is possible.
We also discuss the {\pi}nflation mechanism to realize the hilltop initial
condition in the stochastic axion scenario.
