Constraints on Axion Models from $K^{+}\to {\pi }^{+}a$

Abstract

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 $K^{+}\to {\pi }^{+}a$ 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.

Comments: 15pp RevTeX, 1 eps fig, uses graphics style. Expanded discussion on massive scalars and pseudoscalars, typos. To appear in Physical Review D