Photon proliferation from multi-body dark matter annihilation

Abstract

Multi-body dark matter annihilation is commonly expected to be suppressed by higher-order couplings and phase-space factors, therefore being ignored thus far. We show that, however, this does not hold for a class of nonthermal dark matter scenarios, where the dark matter particle becomes nonrelativistic at temperatures much higher than its mass. We exemplify such a multi-body process via ultralight pseudoscalar dark matter annihilation to diphotons, which leads to a novel photon proliferation effect in the early Universe. As a phenomenological application, we consider the photon temperature shift after neutrino decoupling, showing that the photon proliferation effect can render bounds on the ultralight dark matter couplings stronger than the existing constraints by several orders of magnitude. Our research can be extended to other interactions and dark matter candidates, highlighting the importance of multi-body processes in the early Universe.

Figures (5)

• Figure 1: $N$-body DM annihilation to diphotons via the $a$-$\gamma$-$\gamma$ vertex, where $a$ denotes a pseudoscalar DM particle.
• Figure 2: The upper bound of the DM-photon coupling $g_{a\gamma\gamma}$ from the photon proliferation effect, which is induced from $N$-body DM annihilation via Fig. \ref{['fig:NDM2']}. Current bounds (shaded regions) and detection limits from future experiments (dashed lines) are also shown for comparison.
• Figure 3: The $3\to2$ annihilation process of DM to diphotons via the quartic self-coupling $\lambda$. The $N\geqslant 5$ channels can be induced by attaching more $\lambda$-vertices to the $a$-lines.
• Figure 4: The upper bound on the DM-electron coupling $g_{aee}$, where the photon proliferation effect arises from the $g_{aee}$-inducced quartic DM coupling. Current bounds (shaded regions) and future detection sensitivities (dashed lines) are shown for comparison.
• Figure 5: $N$-body DM annihilation to diphotons via the effective DM-photon coupling, where $\mu_i, \mu'_0$ are used to denote the Lorentz indices in the DM-photon-photon vertices.