Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Can QCD Axions Survive the Cosmological Constant Problem?

Carsten van de Bruck, C. P. Burgess, Adam Smith

Abstract

Mechanisms that dynamically relax the vacuum energy offer a concrete way to approach the cosmological constant problem, but because relaxation is not confined to the vacuum energy alone it can have consequences for the rest of low-energy physics. We explore this issue using the recently proposed 'yoga' relaxation models as an explicit framework and show how relaxation differentially suppresses 'slow' physics relative to a characteristic timescale set by the mass of the relaxon. It therefore need not alter e.g. Higgs & collider physics but can dramatically change how light scalar fields participate in cosmology. We revisit the QCD axion in this setting and show that the suppression of the axion's vacuum potential reshapes its behaviour on cosmological timescales while leaving fast, high-energy processes unaffected. The result is to alter the axion mass-coupling relation away from the standard QCD band, driving it into a regime already ruled out by observational constraints. In particular, suppression of the vacuum axion potential allows the QCD matter-induced potential to dominate even for matter densities relevant to cosmology and everyday matter, potentially driving the axion away from the CP-conserving minimum for QCD-motivated parameters. We conclude that conventional QCD axions are unlikely to remain viable in their standard form within vacuum-energy relaxation frameworks.

Can QCD Axions Survive the Cosmological Constant Problem?

Abstract

Mechanisms that dynamically relax the vacuum energy offer a concrete way to approach the cosmological constant problem, but because relaxation is not confined to the vacuum energy alone it can have consequences for the rest of low-energy physics. We explore this issue using the recently proposed 'yoga' relaxation models as an explicit framework and show how relaxation differentially suppresses 'slow' physics relative to a characteristic timescale set by the mass of the relaxon. It therefore need not alter e.g. Higgs & collider physics but can dramatically change how light scalar fields participate in cosmology. We revisit the QCD axion in this setting and show that the suppression of the axion's vacuum potential reshapes its behaviour on cosmological timescales while leaving fast, high-energy processes unaffected. The result is to alter the axion mass-coupling relation away from the standard QCD band, driving it into a regime already ruled out by observational constraints. In particular, suppression of the vacuum axion potential allows the QCD matter-induced potential to dominate even for matter densities relevant to cosmology and everyday matter, potentially driving the axion away from the CP-conserving minimum for QCD-motivated parameters. We conclude that conventional QCD axions are unlikely to remain viable in their standard form within vacuum-energy relaxation frameworks.
Paper Structure (21 sections, 43 equations, 4 figures)

This paper contains 21 sections, 43 equations, 4 figures.

Table of Contents

  1. Introduction
  2. A relaxed attitude towards cosmology
  3. Natural relaxation overview
  4. The UV perspective
  5. The IR perspective
  6. Relevance to the PQ axion
  7. Vanilla QCD axion
  8. Relaxation of the potential
  9. Benchmark axion embeddings
  10. Brane axion
  11. Bulk axion
  12. Relaxation, Fast and Slow
  13. Constraints from matter-dependent potentials
  14. Non-derivative axion-matter couplings
  15. Universal bare mass shift
  16. ...and 6 more sections

Figures (4)

  • Figure 1: Parameter space constraints for the QCD axion taken from AxionLimits, with the canonical QCD band defined by the relations (\ref{['QCDmavsfa']}) and (\ref{['VanillaAxCoupling']}) shown in yellow.
  • Figure 2: Surface plot of the axion--dilaton potential (\ref{['axio-dil pot0']}) in the $(\chi = \sqrt{\frac{3}{2}}\ln(\tau)$, ${\mathfrak a}$) plane for $\varepsilon = 10^{-3}$, and the same values for the constants $v_i$ as were used in Burgess:2021obw. A trough is clearly visible stabilising the dilaton direction, while the axion direction exhibits periodic maxima and minima.
  • Figure 3: An overlay of the constraints from Fig. \ref{['fig:fa vs mass']} with the modified QCD axion superimposed for both the bulk and brane axion variants of the yoga relaxation mechanism shown in blue and black respectively.
  • Figure 4: The region of the $1/F_a$-$m_a$ plane for which the QCD matter potential dominates the vacuum potential (within the Earth, Sun, white dwarves and neutron stars), for the vacuum-axion suppression model described in Hook:2017psm.