Level Crossing between QCD Axion and Axion-Like Particle
  We study a level crossing between the QCD axion and an axion-like particle,
focusing on the recently found phenomenon, the axion roulette, where the
axion-like particle runs along the potential, passing through many crests and
troughs, until it gets trapped in one of the potential minima. We perform
detailed numerical calculations to determine the parameter space where the
axion roulette takes place, and as a result domain walls are likely formed. The
domain wall network without cosmic strings is practically stable, and it is
nothing but a cosmological disaster. In a certain case, one can make domain
walls unstable and decay quickly by introducing an energy bias without spoiling
the Peccei-Quinn solution to the strong CP problem.
