Naturally light invisible axion in models with large local discrete symmetries
We show that by introducing appropriate local $Z_N(N\geq13)$ symmetries in electroweak models it is possible to implement an automatic Peccei-Quinn symmetry keeping at the same time the axion protected against gravitational effects. Although we consider here only an extension of the standard model and a particular 3-3-1 model, the strategy can be used in any kind of electroweak model. An interesting feature of this 3-3-1 model is that if: {\it i)} we add right-handed neutrinos, {\it ii)} the conservation of the total lepton number, and {\it iii)} a $Z_2$ symmetry, the $Z_{13}$ and the chiral Peccei-Quinn $U(1)_{\rm PQ}$ are both accidental symmetries in the sense that they are not imposed on the Lagrangian but they are just the consequence of the particle content of the model, its gauge invariance, renormalizability and Lorentz invariance. In addition, this model has no domain wall problem.