Revisiting the bound on axion-photon coupling from Globular Clusters

Understand

We derive a strong bound on the axion-photon coupling $g_{a\gamma}$ from the analysis of a sample of 39 Galactic Globular Clusters.

• As recognized long ago, the R parameter, i.e.
• the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores.
• In this regard we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for $g_{a\gamma}$.