SIMPly add a dark photon

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Abstract
Pions of a dark sector gauge group can be strongly interacting massive particle (SIMP) dark matter, produced by the freeze-out of 3 → 2 interactions, with naturally large self-interactions. We study if adding a dark photon to the set-up can do it all: i) maintain thermalization with the visible sector, ii) resonantly enhance the 3 → 2 interactions, thus allowing for a perturbative pion description, and iii) provide a velocity dependent self-interaction that can affect small scale structure formation. For Nf = 3 this minimal setup is marginally excluded, as the required kinetic mixing is too small to maintain thermal equilibrium with the SM. Adding an extra dark quark opens up parameter space, and — perhaps somewhat surprisingly — we find that all bounds can be satisfied for dark pion masses mπ ∼ 250 − 600 MeV. Dropping the small scale structure requirement iii), a viable setup is reproduced for dark charges of αd = 0.01 − 1 and a dark pion mass mπ ≥ 30 MeV. Late time annihilations are non-negligible making the SIMP dark pion a bit WIMPy.

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