Dark radiation and small-scale structure problems with decaying particles

TL;DR

The paper tackles tensions in ΛCDM at small scales, namely an apparent excess in $N_{ m eff}$ (dark radiation) and the overabundance of small halos. It proposes a unified mechanism where late decays of a heavy particle produce both DR and a warm DM component, parameterized by $\\Delta N_{\rm eff}$ and a free-streaming length $\\lambda_{FS}$. As a concrete realization, it analyzes a supersymmetric axion model in which saxion decays yield axions (DR) and axinos (WDM). It identifies a viable region, e.g. $m_s \sim 100$ MeV, $F_a \sim 5\times10^{12}$ GeV, $T_R \sim 5\times10^5$ GeV, yielding $\\Delta N_{\rm eff} \sim 0.5$–$1.5$ and $\\lambda_{FS} \sim 0.2$–$1.3$ Mpc, compatible with BBN, CMB, and Lyman-\\alpha constraints. This work provides a natural, testable mechanism to address both dark radiation and small-scale structure tensions within a coherent SUSY-axion framework.

Abstract

Although the standard ΛCDM model describes the cosmic microwave background radiation and the large scale structure of the Universe with great success, it has some tensions with observations in the effective number of neutrino species (dark radiation) and the number of small scale structures (overabundance problem). Here we propose a scenario which can relax these tensions by producing both dark matter and dark radiation by late decays of heavy particle. Thanks to the generation mechanism, dark matters are rather warm so that the small-scale structure problem is resolved. This scenario can be naturally realized in supersymmetric axion model, in which axions produced by saxion decays provide dark radiation, while axinos from saxion decays form warm dark matter. We identify a parameter region of supersymmetric axion model satisfying all known cosmological constraints.

Figures (2)

• Figure 1: Blue dashed lines show the contour plot of $c_mf_m=0.1,0.01,0.001$ which gives $\Delta N_{\rm eff} (\tau_X)=1$ when dark matters are produced from the decay of $X$ (equality in Eq. (\ref{['condition1']})). Red solid lines are the contour plot of $\lambda_{\rm FS}=0.2, 1.3 \, \,{\rm Mpc}$ in the plane of $m/M_X$ and $\tau_X$. In the region between the red lines and $f_m \sim 0.001$, both the dark radiation and small scale structure problems can be explained with the decay of particles.
• Figure 2: Contour plots of $\Delta N_{\rm eff}$ and $\lambda _{\rm FS}$ in the ($m_{\rm s}$,$F_a$) plane with other cosmological constraints. Here we used $T_R=5\times 10^5\ {\rm GeV}$, $m/m_{\rm s}=0.25$. Blue lines denote $\Delta N_{\rm eff}=0.5,\, 1.5$. Red lines show $\lambda_{\rm FS}=0.2, \, 1.3\,{\rm Mpc}$. Black lines correspond to thermal production of the axino $\Omega_{\tilde{a}}^{\rm TP}h^2=0.001,\, 0.01$. Brown lines denote the lifetime of the saxion $10^4\sec,\, 10^6\sec$. The horizontal magenta (dotted) lines represent $\Omega_{\tilde{a}}^{\rm NTP}h^2=0.1$ for $\lambda=0.1, 0.2$ respectively. The green line shows the BBN (solid) and CMB (dashed) constraint and the lower region is allowed.