Contraints on radiative dark-matter decay from the cosmic microwave background
Le Zhang, Xuelei Chen, Marc Kamionkowski, Zongguo Si, Zheng Zheng
TL;DR
This work addresses how radiative dark-matter decay affects recombination and reionization by injecting energy that modifies the ionization history and CMB power spectra. The authors modify recombination and CMB codes (RECFAST, CAMB) and perform a full MCMC analysis with cosmological parameters, jointly constraining the decay parameters $\Gamma_{\chi}$ and $\zeta$. They report a 95% CL bound for long lifetimes: $\zeta\Gamma_{\chi} < 1.7\times 10^{-25}\,s^{-1}$, map 2D exclusions in the $\log_{10}\zeta$–$\log_{10}\Gamma_{\chi}$ plane for shorter lifetimes, and forecast Planck's improved sensitivity. The results tighten previous limits by about a factor of 10 and illustrate modest gains from including large-scale structure data due to degeneracies, with Planck expected to significantly improve the constraints. This methodology offers a broad, model-independent approach to constraining decaying dark matter using CMB and LSS data.
Abstract
If dark matter decays to electromagnetically-interacting particles, it can inject energy into the baryonic gas and thus affect the processes of recombination and reionization. This leaves an imprint on the cosmic microwave background (CMB): the large-scale polarization is enhanced, and the small-scale temperature fluctuation is damped. We use the WMAP three-year data combined with galaxy surveys to constrain radiatively decaying dark matter. Our new limits to the dark-matter decay width are about ten times stronger than previous limits. For dark-matter lifetimes that exceed the age of the Universe, a limit of $ζΓ_χ < 1.7 \times 10^{-25} s^{-1}$ (95% CL) is derived, where $ζ$ is the efficiency of converting decay energy into ionization energy. Limits for lifetimes short compared with the age of the Universe are also derived. We forecast improvements expected from the Planck satellite.