Codecaying Dark Matter

We propose a new mechanism for thermal dark matter freeze-out, called codecaying dark matter. Multicomponent dark sectors with degenerate particles and out-of-equilibrium decays can codecay to obtain the observed relic density. The dark matter density is exponentially depleted through the decay of nearly degenerate particles rather than from Boltzmann suppression. The relic abundance is set by the dark matter annihilation cross section, which is predicted to be boosted, and the decay rate of the dark sector particles. The mechanism is viable in a broad range of dark matter parameter space, with a robust prediction of an enhanced indirect detection signal. Finally, we present a simple model that realizes codecaying dark matter.

Figure 1

Codecay dark matter timeline. At $T_d$, the SM and dark sector decouple; at $T_{\mathrm{\Gamma }}$ the decay of $B$’s begins to deplete the dark sector density; at $T_f$ the $AA\leftrightarrow BB$ process freezes out, resulting in a relic abundance for the $A$ particles.

Figure 2

Yields ($Y\equiv n/s$) as a function of the SM temperature without cannibalism for a benchmark point $g_A=g_B=1$, $m=1\mathrm{GeV}$, $\sigma =1\times {10}^{-30}{\mathrm{cm}}^2$, ${\mathrm{\Gamma }}_B=6\times {10}^{-23}\mathrm{GeV}$. The (purple/solid) and (red/dotted) lines show the yield for the $A$ and $B$ particles, respectively. For comparison, the (blue/dashed) line shows the yield assuming the DM was in chemical and thermal equilibrium. For this choice of parameters, $x_{\mathrm{\Gamma }}\simeq 300$, while freeze-out occurs at $x_f\simeq 1500$. The dark temperature at freeze-out is $x_f^{\prime }\simeq 5\times 10^6$.

Figure 3

The viable parameter space for codecaying dark matter assuming no cannibalization (left) and a cannibalizing dark sector (right). The central white region shows the range of validity of the model. The different regions show constraints from $N_{\mathrm{eff}}$ (purple); DM decays out of equilibrium (gray); unitarity constraints (green); indirect detection assuming decays into $e^{+}{e}^{-}$ (red/solid) or $\gamma \gamma$ (blue/dashed) excluding the region below the curve. The gap in the $\gamma \gamma$ limit between 10 and 20 GeV is due to the thresholds used in the two recasts. The light gray dotted lines represent contours of constant $\sigma$ with values indicated on the right.