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Some Properties of Multi-Component Axion Dark Matter

Hai-Jun Li

TL;DR

This paper proposes a multi-component dark matter scenario where cold DM consists of the QCD axion plus many ultra-light ALPs, enabled by axion mixing in a string axiverse. It develops the formalism for post-mixing energy densities under adiabatic level-crossing, exploring two-, three-, and general multi-component cases and showing how dominance shifts with the QCD axion regime and ALP-decay-constant hierarchies. The authors find that in the light QCD-axion case the lightest ALP often dominates after mixing, while in the heavy case, the QCD axion and non-lightest ALPs can both dominate depending on ALP decay constants; under certain conditions the QCD axion can still dominate the DM budget. They also present a concrete type IIB string axiverse framework with $\, ext{O}(100)\,$ axions, generating a spectrum of ultra-light ALPs and a QCD axion with hierarchical masses and decay constants that realize both regimes. The setup broadens the conventional single-component DM paradigm and offers a structured path to realize observable multi-component DM signals within high-dimensional axion theories.

Abstract

We introduce a mechanism for multi-component dark matter (DM) that originates from axion mixing and present some of its defining properties. In this context, multi-component DM implies that the cold DM is composed of the QCD axion and many ultra-light axion-like particles (ALPs). This framework can be realized in the type IIB string axiverse with hierarchical axion masses and decay constants. Our investigation reveals that in the light QCD axion scenario, the energy density of the lightest ALP often dominates after mixing. On the other hand, in the heavy QCD axion scenario, both the QCD axion and non-lightest ALPs may dominate, depending on the ALP decay constants. Under certain conditions, the QCD axion can dominate the DM budget. Finally, we briefly discuss a theoretical framework featuring $\sim\mathcal{O}(100)$ axions, with hierarchical axion masses and decay constants.

Some Properties of Multi-Component Axion Dark Matter

TL;DR

This paper proposes a multi-component dark matter scenario where cold DM consists of the QCD axion plus many ultra-light ALPs, enabled by axion mixing in a string axiverse. It develops the formalism for post-mixing energy densities under adiabatic level-crossing, exploring two-, three-, and general multi-component cases and showing how dominance shifts with the QCD axion regime and ALP-decay-constant hierarchies. The authors find that in the light QCD-axion case the lightest ALP often dominates after mixing, while in the heavy case, the QCD axion and non-lightest ALPs can both dominate depending on ALP decay constants; under certain conditions the QCD axion can still dominate the DM budget. They also present a concrete type IIB string axiverse framework with axions, generating a spectrum of ultra-light ALPs and a QCD axion with hierarchical masses and decay constants that realize both regimes. The setup broadens the conventional single-component DM paradigm and offers a structured path to realize observable multi-component DM signals within high-dimensional axion theories.

Abstract

We introduce a mechanism for multi-component dark matter (DM) that originates from axion mixing and present some of its defining properties. In this context, multi-component DM implies that the cold DM is composed of the QCD axion and many ultra-light axion-like particles (ALPs). This framework can be realized in the type IIB string axiverse with hierarchical axion masses and decay constants. Our investigation reveals that in the light QCD axion scenario, the energy density of the lightest ALP often dominates after mixing. On the other hand, in the heavy QCD axion scenario, both the QCD axion and non-lightest ALPs may dominate, depending on the ALP decay constants. Under certain conditions, the QCD axion can dominate the DM budget. Finally, we briefly discuss a theoretical framework featuring axions, with hierarchical axion masses and decay constants.
Paper Structure (9 sections, 33 equations, 3 figures)

This paper contains 9 sections, 33 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Single-component axion dark matter
  3. Multi-component axion dark matter
  4. Two-component
  5. Three-component
  6. Multi-component
  7. A framework with hierarchical axion masses and decay constants
  8. Conclusion
  9. More details about the three-component axion DM scenario

Figures (3)

  • Figure 1: Schematic illustration regarding the transfer of axion energy density in the three-component axion DM scenario. Notice that the evolutionary processes in the light and heavy QCD axion scenarios are similar, with the main difference lying in the relationship between the axion decay constants. The red, blue, and purple solid lines represent the mass eigenvalues $m_{e_i}$, and the corresponding arrows indicate the directions of axion energy density transfer. The evolution direction of the cosmic temperature is from right to left. This plot can also be extended to the multi-component scenario.
  • Figure 2: The ratios of the energy densities $\rho_{A_i,0}/\rho_{a_0,0}$ in the light QCD axion scenario. Here, we set $f_{a_0}=10^{12}\, {\rm GeV}$, and $f_{A_i}$ consist of 10 random numbers within the range of $10^{9.5}-10^{10.5}\, {\rm GeV}$. Notice that we have neglected the contribution of axion masses in this context.
  • Figure 3: Same as figure \ref{['fig_ratio_1']} but for the heavy QCD axion scenario. Here, we set $f_{a_0}=10^{12}\, {\rm GeV}$, and $f_{A_i}$ consist of 10 random numbers within the range of $10^{13.5}-10^{14.5}\, {\rm GeV}$.