[Paper Review] Alternative dark matter candidates: Axions

Andreas Ringwald|arXiv (Cornell University)|Jan 1, 2016

Dark Matter and Cosmic Phenomena19 references8 citations

TL;DR

This paper reviews axions as a leading candidate for cold dark matter, detailing their cosmological production via the misalignment mechanism and the role of the Peccei-Quinn symmetry in solving the strong CP problem. It establishes that axion dark matter is viable across a mass range of 50 µeV to 15 meV, depending on initial field values and topological defect contributions, and surveys key experimental efforts targeting this mass window through resonant conversion and spin-coupling techniques.

ABSTRACT

The axion is arguably one of the best motivated candidates for dark matter. For a decay constant greater than about 10^9 GeV, axions are dominantly produced non-thermally in the early universe and hence are 'cold', their velocity dispersion being small enough to fit to large scale structure. Moreover, such a large decay constant ensures the stability at cosmological time scales and its behaviour as a collisionless fluid at cosmological length scales. Here, we review the state of the art of axion dark matter predictions and of experimental efforts to search for axion dark matter in laboratory experiments.

Motivation & Objective

• To review the theoretical motivation for axions as a solution to the strong CP problem and their emergence as a leading cold dark matter candidate.
• To analyze the non-thermal production of axions in the early universe via the misalignment mechanism, particularly in pre- and post-inflationary symmetry breaking scenarios.
• To determine the viable axion mass range consistent with the observed dark matter density, incorporating contributions from axion strings and domain walls.
• To summarize current and proposed laboratory experiments searching for axion dark matter across a broad mass spectrum.
• To assess the interplay between theoretical predictions and experimental sensitivity, particularly regarding model-dependent couplings and detection strategies.

Proposed method

• Uses the Peccei-Quinn extended Standard Model (PQSM) to describe axions as pseudo-Nambu-Goldstone bosons arising from spontaneous U(1)PQ symmetry breaking.
• Applies lattice QCD calculations of the topological susceptibility χ(T) to determine the temperature-dependent axion potential and mass, with χ(0) = [75.6(1.8)(0.9) MeV]⁴ as a key input.
• Models axion production via coherent oscillations of the axion field after PQ symmetry breaking, with initial field value θ₀ determining the dark matter yield.
• Evaluates contributions from topological defects (axion strings and domain walls) using lattice simulations of cosmic string and wall networks.
• Derives the axion mass range from the requirement that vacuum realignment and defect decay do not overclose the universe, using ΩA,toth² ≈ 1.6+1.0−0.7 × 10⁻² × (fA / 10¹⁰ GeV)¹.¹⁶⁵.
• Reviews detection techniques including resonant microwave cavity experiments (e.g., ADMX, CULTASK), magnetic resonance (CASPEr), and spin precession (QUAX), all exploiting axion coupling to photons or spin.

Experimental results

Research questions

• RQ1 What is the viable mass range for axion dark matter in post-inflationary PQ symmetry breaking scenarios, given constraints from the observed dark matter density?
• RQ2 How do contributions from axion strings and domain walls affect the total axion dark matter abundance, and what are the implications for the axion mass?
• RQ3 What is the role of the initial misalignment angle θ₀ in determining the axion dark matter density in pre-inflationary versus post-inflationary scenarios?
• RQ4 How do lattice QCD results on the topological susceptibility χ(T) inform the axion mass and potential in the critical temperature range near T ≈ 150 MeV?
• RQ5 Which laboratory experiments are most sensitive to axion dark matter across different mass ranges, and what are their underlying detection mechanisms?

Key findings

• The axion mass is predicted to be mA = 57.0(7) × (10¹¹ GeV / fA) µeV, based on lattice QCD results for the topological susceptibility χ(0) = [75.6(1.8)(0.9) MeV]⁴.
• In the post-inflationary PQ symmetry breaking scenario, the axion mass range relevant for dark matter spans from 50 µeV to 200 µeV, with the lower bound assuming 50% contribution from topological defects.
• For N = 1, the axion dark matter fraction from topological defects is predicted as ΩA,toth² ≈ 1.6+1.0−0.7 × 10⁻² × (fA / 10¹⁰ GeV)¹.¹⁶⁵, leading to a mass range of 50 µeV ≲ mA ≲ 200 µeV.
• If the PQ symmetry is explicitly broken via Planck-suppressed operators, the axion mass range for dark matter can extend up to 15 meV, particularly for N > 1.
• The axion mass range of 50 µeV to 15 meV is consistent with both the observed dark matter abundance and recent astrophysical hints of excess energy loss in stars.
• Experiments such as ADMX, CULTASK, MADMAX, and ABRACADABRA are sensitive across the 10⁻⁶ eV to 10⁻⁴ eV range, while CASPEr and QUAX target lower and higher masses, respectively, via distinct coupling mechanisms.