Cosmological Constraints on Decaying Dark Matter

TL;DR

This work provides a comprehensive cosmological analysis of decaying dark matter by incorporating CMB, Type Ia supernova, Ly$\alpha$ forest, large-scale structure, and weak lensing data within an EFT framework. It derives decay-background and perturbation evolution, including a reionization model with a phenomenological parameter $f$ and a combined optical-depth constraint, and uses MCMC to extract lifetime bounds. The results show strong constraints on DM lifetimes, with $\Gamma^{-1} \gtrsim 100$ Gyr for negligible reionization and $(f\Gamma)^{-1} \gtrsim 5.3\times10^8$ Gyr for significant reionization, translating into stringent bounds on particle-physics couplings (typically $\lesssim 10^{-22}$ for $m_{\rm DM}\sim100$ GeV) across several representative models. The analysis demonstrates how cosmological data constrain beyond-Standard-Model scenarios (e.g., gauge mediation, minimal supergravity, and little Higgs) and highlights the synergy and potential tensions with astrophysical constraints, pointing toward future probes like 21 cm observations to refine our understanding of decaying dark matter.

Abstract

We present a complete analysis of the cosmological constraints on decaying dark matter. Previous analyses have used the cosmic microwave background and Type Ia supernova. We have updated them with the latest data as well as extended the analysis with the inclusion of Lyman-$α$ forest, large scale structure and weak lensing observations. Astrophysical constraints are not considered in the present paper. The bounds on the lifetime of decaying dark matter are dominated by either the late-time integrated Sachs-Wolfe effect for the scenario with weak reionization, or CMB polarization observations when there is significant reionization. For the respective scenarios, the lifetimes for decaying dark matter are $Γ^{-1} \gtrsim 100$ Gyr and $ (f Γ) ^{-1} \gtrsim 5.3 \times 10^8$ Gyr (at 95.4% confidence level), where the phenomenological parameter $f$ is the fraction of the decay energy deposited in baryonic gas. This allows us to constrain particle physics models with dark matter candidates through investigation of dark matter decays into Standard Model particles via effective operators. For decaying dark matter of $\sim 100$ GeV mass, we found that the size of the coupling constant in the effective dimension-4 operators responsible for dark matter decay has to generically be $ \lesssim 10^{-22}$. We have also explored the implications of our analysis for representative models in theories of gauge-mediated supersymmetry breaking, minimal supergravity and little Higgs.

Figures (5)

• Figure 1: Posterior probability density function of the decay rate $\Gamma$. Solid line: using all the datasets. Dashed line: CMB + SN + LSS + Ly$\alpha$. Dotted line: CMB only. The probability density function is normalized as $\int P(\Gamma)d\Gamma =1$.
• Figure 2: Constraints on the early universe CDM density parameter $\Omega_{cdm,e}$ and decay rate $\Gamma$, using all the datasets, is plotted on the left panel. For comparison, the present day CDM density parameter $\Omega_{cdm}$ and decay rate $\Gamma$ is plotted on the right panel. The inner and outer contours correspond to 68.3% and 95.4% confidence levels, respectively.
• Figure 3: CMB power spectrum for different dark matter decay rate, assuming the decayed particles are relativistic and weakly interacting. For the CDM density parameter, we choose $\Omega_{cdm,e}h^2$ to be the same as WMAP-5yr median $\Omega_mh^2$. For the other cosmological parameters we use WMAP-5yr median values. By doing this, we have fixed the CDM to baryon ratio at recombination. In a similar plot in Ichiki et al.Ichiki:2004vi$\Omega_{cdm0}h^2$ is instead fixed. Therefore the height of first peak, which has dependence on CDM to baryon ratio at recombination, will significantly change as one varies the decay rate. In this plot the red line corresponds to a stable dark matter . The blue dotted line corresponds to dark matter with a lifetime 100 Gyr, and the blue dashed line 27 Gyr. The data points are WMAP-5yr $<T T>$ spectrum mean values and errors (including instrumental errors and cosmic variance).
• Figure 4: The marginalized posterior likelihood of the total optical depth and that of the DM decay reionization parameter.
• Figure 5: The marginalized 2D likelihood contours. The inner and outer contours correspond to 68.3% and 95.4% confidence levels, respectively.