Title:Â (Non-)Perturbative Dynamics of a Light QCD Axion
Abstract: The QCD axion is a pseudo Nambu-Goldstone boson that dynamically solves the strong CP problem and can can constitute cold dark matter. In the canonical scenario, the QCD axion mass-coupling relation implies that, for a given mass, the interaction strength lies below the reach of most experiments, which target lighter or more strongly coupled axions. However, no fundamental principle requires these canonical properties. If a light QCD axion is discovered, the so-called Z_N model---invoking N exact Standard Model copies coupled by the QCD axion---remains one of the few natural explanations. Yet in a Z_N scenario only a fraction---first assumed to be 1/N---of the possible cosmological evolutions yield a solution to the strong CP problem. Moreover, these evolutions are generically susceptible to non-perturbative dynamics that alter the axion relic abundance. We present the first perturbative statistical treatment of the CP-solution probability, and the first non-perturbative lattice simulations of the Z_N axion, revealing a significant reduction in the probability of solving the strong CP problem relative to the 1/N expectation, and an O(1) suppression of the axion dark matter abundance.