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Tentative evidence for excess GeV-scale gamma rays from the galactic centerhas been corroborated by several groups, including the Fermi collaboration, onwhose data the observation is based. Dark matter annihilation into standardmodel particles has been shown to give a good fit to the signal for a varietyof final state particles, but generic models are inconsistent with constraintsfrom direct detection. Models where the dark matter annihilates to mediatorsthat subsequently decay are less constrained. We perform global fits of suchmodels to recent data, allowing branching fractions to all possible fermionicfinal states to vary. The best fit models, including constraints from theAMS-02 experiment (and also antiproton ratio), require branching primarily tomuons, with a $\sim 10-20\%$ admixture of $b$ quarks, and no other species.This suggests models in which there are two scalar mediators that mix with theHiggs, and have masses consistent with such a decay pattern. The scalar thatdecays to $\mu^+\mu^-$ must therefore be lighter than $2m_\tau \cong 3.6$ GeV.Such a small mass can cause Sommerfeld enhancement, which is useful to explainwhy the best-fit annihilation cross section is larger than the value needed fora thermal relic density. For light mediator masses $\sim (0.2-2)$ GeV, it canalso naturally lead to elastic DM self-interactions at the right level foraddressing discrepancies in small structure formation as predicted bycollisionless cold dark matter.

Multimediator models for the galactic center gamma ray excess