Revisiting the bound on axion-photon coupling from Globular Clusters (1406.6053v2)

Abstract: 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 $g_{a\gamma}$. We show that the estimated value of $g_{a\gamma}$ substantially depends on the adopted He mass fraction Y, an effect often neglected in previous investigations. Taking as benchmark for our study the most recent determinations of the He abundance in H II regions with O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound $g_{a\gamma}<0.66\times 10^{-10}$ GeV$^{-1}$ at 95$\%$ confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.

• The paper refines previous estimates by incorporating helium abundance variations, yielding an upper limit of gₐγ < 0.66×10⁻¹⁰ GeV⁻¹.
• It employs updated stellar models and observations from 39 globular clusters to analyze the HB/RGB ratio for axion effects.
• The findings support axions as dark matter candidates and inform targeted experimental searches like ALPS and IAXO.

Revisiting the Bound on Axion-Photon Coupling from Globular Clusters

The paper presented in the paper "Revisiting the bound on axion-photon coupling from Globular Clusters" addresses the constraints on the axion-photon coupling constant $g_{a\gamma}$, using observations from Galactic Globular Clusters (GCs). Axions, originally proposed to solve the strong CP problem in quantum chromodynamics, have potential implications for dark matter models as well. Their coupling to photons (axion-photon coupling) becomes a point of paper due to its potential effects on astrophysical processes, notably within stellar environments like globular clusters.

Methodology and Analysis

The authors focus on the $R$ parameter, defined as the ratio of the number of stars in the horizontal branch (HB) to those in the red giant branch (RGB) of old stellar clusters. Axion production within stars via photon conversion could reduce the HB star count by impacting stellar energy loss, thereby affecting $R$.

Previous studies had established an upper limit on $g_{a\gamma}$ without considering the effects of helium mass fraction (Y). This research accounts for the variations in Y, an omission in earlier evaluations, and its significant influence on the determination of axion constraints. By using the latest helium abundance determinations from low-metallicity environments comparable to GCs, the paper revises the possible bounds on $g_{a\gamma}$.

Key Results

The analysis, based on a sample of 39 Galactic GCs and state-of-the-art stellar models, leads to a more stringent upper limit on the axion-photon coupling: $$g_{a\gamma} < 0.66 \times 10^{-10} \ \text{GeV}^{-1}$$ at a 95% confidence level. This is argued to be the strongest bound within a broad mass range compared to previous constraints.

Implications

This updated constraint on $g_{a\gamma}$ has significant implications. The paper provides further credence to the potential role of axions as dark matter candidates by narrowing down the coupling parameter space. The findings also have practical bearings, facilitating more targeted experimental searches such as those planned by the ALPS and IAXO collaborations, which seek to detect axions in laboratory settings independent of astrophysical observations.

Additionally, a deeper understanding of stellar evolution processes can be achieved by factoring in possible axion effects, thereby refining predictions related to the lifecycle of stars particularly in older clusters.

Future Directions

This improved constraint invites more precise helium abundance measurements in GC stars, which remain a substantial source of systematic uncertainty. Couplings in the context of axion-like particles (ALPs) also attract attention as further cosmological and astrophysical phenomena potentially hint at their presence.

Continued cross-disciplinary endeavors combining astrophysical observations and particle physics experimentation will be crucial to elucidate the nature of axions and their role within the broader context of cosmic structure and evolution. As theoretical models advance, so too will the techniques required to detect these elusive particles, marking a pivotal area for future research development in the field.