SIMPs through the axion portal
  Dark matter could be a thermal relic comprised of strongly interacting
massive particles (SIMPs), where $3 \rightarrow 2$ interactions set the relic
abundance. Such interactions generically arise in theories of chiral symmetry
breaking via the Wess-Zumino-Witten term. In this work, we show that an
axion-like particle can successfully maintain kinetic equilibrium between the
dark matter and the visible sector, allowing the requisite entropy transfer
that is crucial for SIMPs to be a cold dark matter candidate. Constraints on
this scenario arise from beam dump and collider experiments, from the cosmic
microwave background, and from supernovae. We find a viable parameter space
when the axion-like particle is close in mass to the SIMP dark matter, with
strong-scale masses of order a few hundred MeV. Many planned experiments are
set to probe the parameter space in the near future.
