Constraints on the coupling with photons of heavy axion-like-particles from Globular Clusters

Understand

We update the globular cluster bound on massive ($m_a$ up to a few 100 keV) axion-like particles (ALP) interacting with photons.

• The production of such particles in the stellar core is dominated by the Primakoff $\gamma + Ze\to Ze +a$ and by the photon coalescence process $\gamma+\gamma\to a$.
• The latter, which is predominant at high masses, was not included in previous estimations.
• Furthermore, we account for the possibility that axions decay inside the stellar core, a non-negligible effect at the masses and couplings we are considering here.