Abstract
We update the globular cluster bound on massive (ma 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 γ+Ze→Ze+a and by the photon coalescence process γ+γ→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. Consequently, our result modifies considerably the previous constraint, especially for ma≳50keV. The combined constraints from Globular Cluster stars, SN 1987A, and beam-dump experiments leave a small triangularly shaped region open in the parameter space around ma∼0.5−1MeV and gaγ∼10−5GeV−1. This is informally known as the ALP “cosmological triangle” since it can be excluded only using standard cosmological arguments. As we shall mention, however, there are viable cosmological models that are compatible with axion-like particles with parameters in such region. We also discuss possibilities to explore the cosmological triangle experimentally in upcoming accelerator experiments.
Highlights
Axion-like-particles (ALPs) with masses ma in the keV-MeV range emerge in different extension of the Standard Model, as Pseudo-Goldstone bosons of some broken global symmetry
axion-like particles (ALP) with masses below the MeV scale can have a wide range of implications for cosmology and astrophysics, affecting for example Big Bang Nucleosynthesis (BBN), the Cosmic Microwave Background (CMB) [9,10,11] and the evolution of stars
In this work we have extended the Globular Clusters (GC) bound on the ALP-photon coupling to masses ma >∼ 10 keV, in the region of the parameter space where the Boltzmann suppression of the axion emission rate can no longer be neglected
Summary
Axion-like-particles (ALPs) with masses ma in the keV-MeV range emerge in different extension of the Standard Model, as Pseudo-Goldstone bosons of some broken global symmetry. The number of stars found in the different evolutionary phases depends linearly on the time spent by a star in each of them For this reason, stellar counts provide a powerful tool to investigate the efficiency of the energy sources and sinks in stellar interiors, those that affect the stellar lifetime τ in a given stage of the stellar evolution. Stellar counts provide a powerful tool to investigate the efficiency of the energy sources and sinks in stellar interiors, those that affect the stellar lifetime τ in a given stage of the stellar evolution In this context, the GC R parameter, defined as the number ratio of horizontal branch to red giants branch stars, i.e.:. In Appendix A we compute the photon-axion transition rate from Primakoff conversion and in B we calculate the ALP production rate from Primakoff conversion and photon coalescence
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