Searching for Axionlike Particles in Flavor-Changing Neutral Current Processes.

We propose new searches for axionlike particles (ALPs) produced in flavor-changing neutral current (FCNC) processes. This proposal exploits the often-overlooked coupling of ALPs to ${W}^{\ifmmode\pm\else\textpm\fi{}}$ bosons, leading to FCNC production of ALPs even in the absence of a direct coupling to fermions. Our proposed searches for resonant ALP production in decays such as $B\ensuremath{\rightarrow}{K}^{(*)}a$, $a\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$, and $K\ensuremath{\rightarrow}\ensuremath{\pi}a$, $a\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ could greatly improve upon the current sensitivity to ALP couplings to standard model particles. We also determine analogous constraints and discovery prospects for invisibly decaying ALPs.