Heavy Thermal Relics from Zombie Collisions

We propose a new thermal freezeout mechanism which results in dark matter masses exceeding the unitarity bound by many orders of magnitude, without violating perturbative unitarity or modifying the standard cosmology. The process determining the relic abundance is $\chi \zeta^\dagger \to \zeta \zeta$, where $\chi$ is the dark matter candidate. For $m_\zeta < m_\chi < 3 m_\zeta$, $\chi$ is cosmologically long-lived and scatters against the exponentially more abundant $\zeta$. Therefore, such a process allows for exponentially heavier dark matter for the same interaction strength as a particle undergoing ordinary $2 \to 2$ freezeout; or equivalently, exponentially weaker interactions for the same mass. We demonstrate this mechanism in a leptophilic dark matter model, which allows for dark matter masses up to $10^{9}$ GeV.