Irrational Axions as a Solution of The Strong CP Problem in an Eternal Universe

Abstract

We exhibit a novel solution of the strong CP problem, which does not involve any massless particles. The low energy effective Lagrangian of our model involves a discrete spacetime independent axion field which can be thought of as a parameter labeling a dense set of $θ$ vacua. In the full theory this parameter is seen to be dynamical, and the model seeks the state of lowest energy, which has $θ_{eff} = 0$. The processes which mediate transitions between $θ$ vacua involve heavy degrees of freedom and are very slow. Consequently, we do not know whether our model can solve the strong CP problem in a universe which has been cool for only a finite time. We present several speculations about the cosmological evolution of our model.