Dark Matter Coannihilation with a Lighter Species

We propose a new thermal freeze-out mechanism for ultraheavy dark matter. Dark matter coannihilates with a lighter unstable species that is nearby in mass, leading to an annihilation rate that is exponentially enhanced relative to standard weakly interactive massive particles. This scenario destabilizes any potential dark matter candidate. In order to remain consistent with astrophysical observations, our proposal necessitates very long-lived states, motivating striking phenomenology associated with the late decays of ultraheavy dark matter, potentially as massive as the scale of grand unified theories, $M_{\mathrm{GUT}}\sim {10}^{16}\mathrm{GeV}$.

Figure 1

Representative Feynman diagrams for the freeze-out (left) and decay (right) of dark matter $\chi$. The lighter state in the hidden sector ${\chi }^{\prime }$ is assumed to couple to a pair of standard model particles.

Figure 2

Viable parameter space in the $\epsilon -m_{\chi }$ plane for ${\xi }_{\inf }=1$ and $m_{\chi }/m_{{\chi }^{\prime }}=1.5$. $\chi$ freezes out through coannihilations with a lighter unstable particle in the hidden sector ${\chi }^{\prime }$. Along the black contours, the $\chi$ abundance is equal to the measured dark matter energy density for different values of the effective coupling ${\alpha }_{\chi }$. For sufficiently small values of $\epsilon$, the light blue region is excluded by BBN. For larger values of $\epsilon$, this model is constrained from searches for the late decays of dark matter into neutrinos by high-energy neutrino telescopes (yellow). Similar decays into SM singlets are excluded by measurements of the CMB power spectrum (red).