Constraints on Axion Models from $K^+\toπ^+ a$

We explore a new class of axion models in which some, but not all, of the left-handed quarks have a Peccei-Quinn symmetry. These models are potentially afflicted by flavour changing neutral currents. We derive the bounds on the Peccei-Quinn symmetry-breaking scale from bounds on the $K^+\to π^+ a$ branching ratio, showing that in some cases they are even stronger than the astrophysical ones, but still not strong enough to kill off the models.

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