Heavy Thermal Relics from Zombie Collisions

Understand

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.