Even SIMP miracles are possible

Abstract

Strongly interacting massive particles $π$ have been advocated as prominent dark matter candidates when they regulate their relic abundance through odd-numbered $3 π\to2π$ annihilation. We show that successful freeze-out may also be achieved through even-numbered interactions $X X \to ππ$ once bound states $X$ among the particles of the low-energy spectrum exist. In addition, $X$-formation hosts the potential of also catalyzing odd-numbered $3 π\to2π$ annihilation processes, turning them into effective two-body processes $πX \to ππ$. Bound states are often a natural consequence of strongly interacting theories. We calculate the dark matter freeze-out and comment on the cosmic viability and possible extensions. Candidate theories can encompass confining sectors without a mass gap, glueball dark matter, or $ϕ^3$ and $ϕ^4$ theories with strong Yukawa or self-interactions.