On-Shell Mediators and Top-Charm Dark Matter Models for the Fermi-LAT Galactic Center Excess

Abstract: An excess in gamma-rays from the galactic center observed by the Fermi Large Area Telescope has been proposed as a possible signal of dark matter annihilation. Recently, the Fermi collaboration showed that systematic errors broaden the range of spectral shapes for this excess. We demonstrate fits to this range for (1) flavor-violating annihilations to top-charm pairs and (2) annihilations to on-shell bosonic mediators which decay to Standard Model quarks in a boosted frame. Annihilation of 40 - 100 GeV DM to pairs of spin-1 mediators provide a good fit to the Fermi-LAT spectrum with a normalization consistent with a thermal relic. Top-charm modes and annihilation to three pseudoscalar mediators can fit the spectral shape but typically require non-thermal annihilation cross sections.