Family Symmetry, Gravity, and the Strong CP Problem

Abstract

We show how in a class of models Peccei--Quinn symmetry can be realized as an automatic consequence of a gauged $U(1)$ family symmetry. These models provide a solution to the strong CP problem either via a massless $u$--quark or via the DFSZ invisible axion. The local family symmetry protects against potentially large corrections to $\overlineθ $ induced by quantum gravitational effects. In a supersymmetric extension, the `$μ$--problem' is shown to have a natural solution in the context of gravitationally induced operators. We also present a plausible mechanism which can explain the inter--generational mass hierarchy in such a context.