Radiative Quarkonium decays into an Invisible ALP

Abstract: In this talk the ALP production from radiative quarkonium decays is presented. To this purpose, the relevant cross-section is computed from a $d=5$ effective Lagrangian containing simultaneous ALP couplings to $b(c)$-quarks and photons. The interplay between resonant and non-resonant contributions is shown to be relevant for experiments operating at $\sqrt{s}=m_{\Upsilon(nS)}$, with $n=1,2,3$, while the non-resonant contribution dominates at the $\Upsilon(4S)$ resonance. These effects imply that the experimental searches performed at with different experimental setups are sensitive to complementary combinations of ALP couplings. To illustrate these results, constraints from existing BaBar, Belle and Belle II data on ALPs decaying into invisible final states are derived. Finally, constrains from the recent BESIII measurements of the $J/\Psi \to \gamma \,a$ decay rate are also included for comparison.