Revisiting the Bound on Axion-Photon Coupling from Globular Clusters

We derive a strong bound on the axion-photon coupling ${g}_{a\ensuremath{\gamma}}$ 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 ... View More