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<p>Axions coupled to thermal non-Abelian gauge fields may have cosmological significance. As the heat bath defines a frame, its influence depends separately on energy and momentum. A light-like momentum (<i>k</i> ≈ <i>ω</i>) is relevant for the axion contribution to the effective number of light neutrinos, ∆<i>N</i><sub>eff</sub>, whereas a vanishing momentum (<i>k</i> = 0) plays a role for warm natural inflation or ultralight dark matter, and has been employed in lattice estimates (both classical and quantum-statistical) of the strong sphaleron rate. Focussing on soft energies (<i>α</i><sub>s</sub><i>T</i> ≪ <i>ω</i> ≪ <i>πT</i>), we carry out an HTL computation to show how the domains <i>k</i> = 0 and <i>k</i> ≈ <i>ω</i> interpolate to each other. We then compare with lattice data at <i>k</i> = 0, and connect our analysis to NLO computations at <i>k</i> ≈ <i>ω</i> ≥ <i>πT</i>. Assembling the current best input, we re-investigate light QCD axion decoupling dynamics at <i>T</i> ≥ 200 MeV, showing that efficient interactions in the ultrasoft domain increase ∆<i>N</i><sub>eff</sub> from ∼ 0.03 to ∼ 0.04 at <i>f</i><sub><i>a</i></sub> = 4 × 10<sup>8</sup> GeV.</p>

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Energy and momentum dependence of the soft-axion interaction rate

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月5日

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  • Killian Bouzoud,
  • Jacopo Ghiglieri,
  • Mikko Laine,
  • G. S. S. Sakoda

摘要

Axions coupled to thermal non-Abelian gauge fields may have cosmological significance. As the heat bath defines a frame, its influence depends separately on energy and momentum. A light-like momentum (k ≈ ω) is relevant for the axion contribution to the effective number of light neutrinos, ∆Neff, whereas a vanishing momentum (k = 0) plays a role for warm natural inflation or ultralight dark matter, and has been employed in lattice estimates (both classical and quantum-statistical) of the strong sphaleron rate. Focussing on soft energies (αsT ≪ ω ≪ πT), we carry out an HTL computation to show how the domains k = 0 and k ≈ ω interpolate to each other. We then compare with lattice data at k = 0, and connect our analysis to NLO computations at k ≈ ω ≥ πT. Assembling the current best input, we re-investigate light QCD axion decoupling dynamics at T ≥ 200 MeV, showing that efficient interactions in the ultrasoft domain increase ∆Neff from ∼ 0.03 to ∼ 0.04 at fa = 4 × 108 GeV.