Back to the phase space: thermal axion dark radiation via couplings to standard model fermions

Abstract

We investigate the cosmological consequences of axion interactions with standard model fermions accurately and precisely. Our analysis is entirely based on a phase space framework that allows us to keep track of the axion distribution in momentum space across the entire expansion history. First, we consider flavor-diagonal couplings to charged leptons and quantify the expected amount of dark radiation as a function of the coupling strength. Leptophilic axions are immune from complications due to strong interactions and our predictions do not suffer from theoretical uncertainties. We then focus on flavor-diagonal interactions with the three heavier quarks whose masses are all above the scale where strong interactions become non-perturbative. The top quark case is rather safe because its mass is orders of magnitude above the confinement scale, and the consequent predictions are solid. The bottom and charm masses are in more dangerous territory because they are very close to the QCD crossover. We present a comprehensive discussion of theoretical uncertainties due to both the choice of the scale where we stop the Boltzmann evolution and the running of QCD parameters. Finally, we compute the predicted amount of dark radiation expressed as an effective number of additional neutrino species. We compare our predictions with the ones obtained via standard approximate procedures, and we find that adopting a rigorous phase space framework alters the prediction by an amount larger than the sensitivity of future CMB observatories.

16 pages, 9 figures, version published in PRD

References in corpus (21)

• The Atacama Cosmology Telescope: DR4 Maps and Cosmological Parameters
• Axion and neutrino bounds improved with new calibrations of the tip of the red-giant branch using geometric distance determinations
• Probing Physics Beyond the Standard Model: Limits from BBN and the CMB Independently and Combined
• Thermal axion production in the primordial quark-gluon plasma
• Constraints on $\Lambda$CDM Extensions from the SPT-3G 2018 $EE$ and $TE$ Power Spectra
• Constraints on the coupling with photons of heavy axion-like-particles from Globular Clusters
• The Irreducible Axion Background
• Cosmological Bound on the QCD Axion Mass, Redux
• Cosmological constraints on multi-interacting dark matter
• Thermal axion production at low temperatures: a smooth treatment of the QCD phase transition
• Thermal QCD Axions across Thresholds
• Baryonic effects for weak lensing. Part II. Combination with X-ray data and extended cosmologies
• Generalized teleparallel cosmology and initial singularity crossing
• Novel cosmological bounds on thermally-produced axion-like particles
• Production of Thermal Axions across the ElectroWeak Phase Transition
• The DFSZ axion in the CMB
• Exploring galaxies-gravitational waves cross-correlations as an astrophysical probe
• Flavor Violating Axions in the Early Universe
• Cosmic Imprints of XENON1T Axions
• Bound on Quantum Fluctuations in Gravitational Waves from LIGO-Virgo
• Irreducible cosmological backgrounds of a real scalar with a broken symmetry

Cited by in corpus (10)

• Free-Streaming Neutrinos and Their Phase Shift in Current and Future CMB Power Spectra
• Precise predictions for the QCD axion contribution to dark radiation with full phase-space evolution
• Dark Matter Freeze-In and Small-Scale Observables: Novel Mass Bounds and Viable Particle Candidates
• Using $\Delta N_{\mathrm{eff}}$ to constrain preferred axion model dark matter
• Axion Portal to Scalar Dark Matter: Unveiling Stabilizing Symmetry Footprints
• Improved cosmological constraints on axion-lepton interactions
• Axions at the meV Crossroads: Theory, Cosmology, Astrophysics, and Experiments
• Ultralight Dark Matter from the Edge of Field Space
• Seasons of Dark Matter Freeze-In Shaped by the Weather of the Early Universe
• Beyond thermal approximations: Precise cosmological bounds on Axion-Like Particles