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Core composition effects on the QCD axion mass limit from neutron star cooling

Fernando Arias-Aragón1,* and Federico Nola2,3,4,†

  • *Contact author: fernando.arias@urjc.es
  • †Contact author: federico.nola@unicampania.it
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Phys. Rev. D 114, 063015 – Published 8 September, 2026

DOI: https://doi.org/10.1103/3frv-1wm7

Abstract

Neutron stars are very dense media in which axions may be produced. This has been used to set a limit on the QCD axion mass, usually under the assumption that only neutrons, protons, electrons, and muons appear in the star core. Given the extreme conditions reached within neutron stars, it is reasonable to consider that other particles, such as hyperons and Δ resonances, may appear in the neutron star core. In this work, we study how the limit on the mass of QCD axions, namely Kim-Shifman-Vainshtein-Zakharov and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) invisible axions, is altered when different equations of state are used, allowing for heavier particles to appear in the neutron star core. We find that this dependence is in general mild and thus reinforces the reliability of the known limit. Additionally, in the DFSZ scenario, it may drive the limit within the sensitivity window for IAXO. This would allow this experiment to discern the composition of neutron star cores if an axion were to be observed within that window.

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Phys. Rev. D 114, 063015– Published 8 September, 2026
  • Received 19 June 2026
  • Accepted 11 August 2026
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