Glimmers from the Axiverse

Understand

We study axion-photon couplings in compactifications of type IIB string theory.

• We find that these couplings are systematically suppressed compared to the inverse axion periodicity, as a result of two effects.
• First, couplings to the QED theta angle are suppressed for axion mass eigenstates that are light compared to the mass scale set by stringy instantons on the cycle supporting QED.
• Second, in compactifications with many axions the intersection matrix is sparse, making kinetic mixing weak.