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| Main Authors: | , , , |
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| Format: | Preprint |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2403.05621 |
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Table of Contents:
- Hot axions are produced in the early Universe via their interactions with Standard Model particles, contributing to dark radiation commonly parameterized as $ΔN_{\text{eff}}$. In standard QCD axion benchmark models, this contribution to $ΔN_{\text{eff}}$ is negligible after taking into account astrophysical limits such as the SN1987A bound. We therefore compute the axion contribution to $ΔN_{\text{eff}}$ in so-called astrophobic axion models characterized by strongly suppressed axion couplings to nucleons and electrons, in which astrophysical constraints are relaxed and $ΔN_{\text{eff}}$ may be sizable. We also construct new astrophobic models in which axion couplings to photons and/or muons are suppressed as well, allowing for axion masses as large as few eV. Most astrophobic models are within the reach of CMB-S4, while some allow for $ΔN_{\text{eff}}$ as large as the current upper bound from Planck and thus will be probed by the Simons Observatory. The majority of astrophobic axion models predicting large $ΔN_{\text{eff}}$ is also within the reach of IAXO or even BabyIAXO.