Spectral distortions in the decaying QCD dark matter scenario
We study the QCD--DM scenario by analyzing the imprint of energy injection from decaying dark-sector particles on the spectral distortions (SDs) of the Cosmic Microwave Background (CMB). We adopt a unified framework capable of describing both relativistic and non-relativistic particles, as well as fast and slow decay regimes. Within this approach, we model exponential, power-law, oscillatory, and two-step decays, computing the resulting $μ$- and $y$-type distortions across the parameter space spanned by the confinement scale $a_c$, decay rate $Γ_χ$, energy-transfer efficiency $Σ_χ$, and velocity $v_{χc}$. We find that power-law, oscillatory, and cascade decays can be effectively mapped onto exponential models with appropriate rescaling. The dominant factors controlling SDs are the decay epoch and lifetime, with $v_{χc}$ becoming relevant only in the ultra-relativistic limit. FIRAS observations impose tight constraints on early energy injection, with $μ$-type distortions placing the strongest bounds on $Σ_χ$. Simultaneous matching of both $μ_{\rm firas}$ and $y_{\rm firas}$ breaks the degeneracy between $Γ_χ$ and $Σ_χ$, localizing preferred decay rates around $Γ_χ\lesssim (3.3-4.4)\times10^{-3}~{\rm yr}^{-1}$ and $Σ_χ\lesssim 8.5\times10^{-4}$ for relativistic particles, while fast decays with $Γ_χ\gtrsim 6.5~{\rm yr}^{-1}$ become observationally negligible. Our results show that CMB spectral distortions are a powerful probe of dark-sector dynamics. Future missions such as PIXIE or PRISM could extend current limits by several orders of magnitude and test previously inaccessible regions of parameter space.