Split SIMPs with Decays
  We discuss a minimal realization of the strongly interacting massive particle
(SIMP) framework. The model includes a dark copy of QCD with three colors and
three light flavors. A massive dark photon, kinetically mixed with the Standard
Model hypercharge, maintains kinetic equilibrium between the dark and visible
sectors. One of the dark mesons is necessarily unstable but long-lived, with
potential impact on CMB observables. We show that an approximate "isospin"
symmetry acting on the down-type quarks is an essential ingredient of the
model. This symmetry stabilizes the dark matter and allows to split
sufficiently the masses of the other states to suppress strongly their relic
abundances. We discuss for the first time the SIMP cosmology with sizable mass
splittings between all meson multiplets. We demonstrate that the SIMP mechanism
remains efficient in setting the dark matter relic density, while CMB
constraints on unstable relics can be robustly avoided. We also consider the
phenomenological consequences of isospin breaking, including dark matter decay.
Cosmological, astrophysical, and terrestrial probes are combined into a global
picture of the parameter space. In addition, we outline an ultraviolet
completion in the context of neutral naturalness, where confinement at the GeV
scale is generic. We emphasize the general applicability of several novel
features of the SIMP mechanism that we discuss here.
