Abstract
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.
Highlights
Sufficiently cold to avoid conflict with structure formation bounds [1], which otherwise exclude [2] a completely secluded 3 → 2 freezeout [3]
We demonstrate that the strongly interacting massive particle (SIMP) mechanism remains efficient in setting the dark matter relic density, while cosmic microwave background (CMB) constraints on unstable relics can be robustly avoided
The SIMP mechanism finds its most attractive realizations in the context of confining gauge theories with chiral symmetry breaking [4], where the pseudo Nambu-Goldstone bosons play the role of dark matter, and 3 → 2 annihilations are mediated by the Wess-Zumino-Witten (WZW) action [5, 6]
Summary
We take a phenomenological approach and discuss the effective field theory (EFT) of hidden QCD, remaining agnostic about specific ultraviolet (UV) completions. After diagonalization of the gauge kinetic and the mass terms, the physical dark photon. We do not specify the origin of the A mass, which could arise from the Stuckelberg mechanism or from the coupling to a dark Higgs field. The second and third lines of L display pieces of the WZW action that arise due to the presence of the gauge fields. Assuming the underlying microscopic theory to be vector-like (so that the dark baryon number B is anomaly-free), we have the freedom to gauge a linear combination of Q and B. We will require that the semi-annihilations ππ → πA be sub-leading to 3 → 2 processes during DM freezeout (see section 5.5), which imposes a lower bound mA 2mπ on the mass of the dark photon [10]
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