2025/05/27 by Jin-Bao Wang, Wang, Jin-Bao, Zhi-Hui Guo +3
Physics and Astronomy · #Atomic and Subatomic Physics Research #Cosmology and Gravitation Theories #Dark Matter and Cosmic Phenomena #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph)
paper · pdf · doi:10.48550/arxiv.2505.21276
openalex publication_date 2025/05/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The axion thermalization rate is a crucial input to determine the hot dark matter bound of axions, resulting from the scattering processes in the thermal bath of early Universe. We demonstrate that the commonly employed axion thermalization rate by including the aπ↔ ππ channel alone is significantly underestimated for the temperature T above 100 MeV. This is obtained through the systematical calculation of the axion-light flavor meson scattering amplitudes within the framework of the chiral unitarization approach, paying special attention to the a K ↔ πK reaction. Hadron resonances appearing in a K ↔ πK amplitudes significantly enlarge the cross sections, which turn out to be much bigger than that of aπ↔ππ. The axion thermalization rate is then substantially enhanced by the a K ↔ πK channel for T\gtrsim 100 MeV. Especially at T≃ 130 MeV, the contribution from the a K ↔πK reaction to the axion thermalization rate exceeds the aπ↔ππ one. Obviously more stringent constraints on the axion parameters are obtained, when confronting the number of extra relativistic degrees of freedom ΔN\rm eff from Planck'18.