Stellar Recipes for Axion Hunters

Abstract: There are a number of observational hints from astrophysics which point to the existence of stellar energy losses beyond the ones accounted for by neutrino emission. These excessive energy losses may be explained by the existence of a new sub-keV mass pseudoscalar Nambu--Goldstone boson with tiny couplings to photons, electrons, and nucleons. An attractive possibility is to identify this particle with the axion -- the hypothetical pseudo Nambu--Goldstone boson predicted by the Peccei--Quinn solution to the strong CP problem. We explore this possibility in terms of a DFSZ-type axion and of a KSVZ-type axion/majoron, respectively. Both models allow a good global fit to the data, prefering an axion mass around 10 meV. We show that future axion experiments -- the fifth force experiment ARIADNE and the helioscope IAXO -- can attack the preferred mass range from the lower and higher end, respectively. An axion in this mass range can also be the main constituent of dark matter.

• The paper demonstrates that DFSZ and KSVZ axion models align with stellar energy loss anomalies, favoring an axion mass near 10 meV.
• It utilizes observational data from red giants, white dwarfs, horizontal-branch stars, and neutron stars to support the axion hypothesis.
• It outlines that upcoming experiments like IAXO and ARIADNE will decisively test axion parameters, connecting theory with observations.

Expert Review of "Stellar Recipes for Axion Hunters"

The paper "Stellar Recipes for Axion Hunters" presents a comprehensive exploration of the potential role of axions, a type of hypothetical pseudoscalar particle, in explaining certain anomalies observed in stellar energy losses. The axion is an elegant solution to the strong CP problem and has been posited as a candidate for dark matter. This investigation focuses on the excess energy loss observed in various astrophysical systems, such as red giants, horizontal-branch stars, white dwarfs, and neutron stars, which may indicate the presence of energy loss mechanisms beyond standard theories.

Key Findings and Models

1. Axion Mass and Coupling Preferences:
   • The study highlights the consistency of two types of axion models, the DFSZ and the KSVZ, with stellar observations. These models provide a good fit to the data, particularly favoring an axion mass around 10 meV.
   • In DFSZ-type models, axions couple to both electrons and nucleons, whereas KSVZ axions primarily interact with nucleons. The authors note the sensitivity of different axion experiments to the axion-photon (g_{a\gamma}) and axion-electron (g_{ae}) couplings, which allows each model to explore different parameter spaces effectively.
2. Stellar Observational Anomalies:
   • By investigating energy loss phenomena in various stars, the potential presence of axions is supported by the observed faster cooling rates, which can be attributed to axion emission.
   • Pulsating white dwarfs, the white dwarf luminosity function, red giants, and horizontal branches show hints of these excess losses, potentially due to axion interactions.
3. Potential for Future Experiments:
   • The authors affirm that forthcoming experiments like the helioscope IAXO and the long-range force experiment ARIADNE will have the capability to probe the proposed axion parameter ranges, effectively testing these theoretical models.
   • These experiments offer the potential to verify the axion interpretations by attacking mass ranges from different angles (both photon and nucleonic couplings).

Implications for Dark Matter

The dual role of axions as a solution to both the strong CP problem and as a dark matter candidate is particularly notable. The proposed axion models accommodate the possibility that axions not only explain stellar observations but also constitute a significant component of the dark matter in the universe. The authors discuss the implications of the Peccei-Quinn symmetry breaking scenario in the early universe, which affects the relic axion abundance and their role in cosmology.

Future Directions

The paper suggests several avenues for ongoing and future research, aiming to resolve the tension between different anomaly signals and to refine axion mass and coupling measurements. The exploration of different axion production mechanisms in stars and the development of more sensitive experimental techniques remain crucial. The future improvements in experimental setups, such as upgraded versions of IAXO, are anticipated to probe deeper into the parameter space, providing more stringent tests of the axion hypothesis.

In conclusion, "Stellar Recipes for Axion Hunters" presents a well-argued case for the existence of axions based on stellar anomalies, fitting them within established theoretical frameworks, and paving the way for future experimental validation. The work underscores the importance of interdisciplinary research that bridges theoretical physics, observational astrophysics, and experimental advances to solve fundamental cosmological questions.