Dark Matter Interpretations of the Galactic Center Gamma-Ray Excess with 18 Years of Fermi-LAT Data and the FL16Y Source Catalog

We investigate dark matter (DM) interpretations of the Galactic Center Excess (GCE) using 17.7 years of Fermi-LAT data and the FL16Y source catalog. We test 80 Galactic diffuse-emission models and include correlated systematic uncertainties from Galactic-plane control regions. We derive combined 95% CL annihilation limits from 30 dwarf spheroidal galaxies (dSphs), including J-factor uncertainties, and compare the GCE-preferred regions with these limits and AMS-02 antiproton bounds. For self-conjugate DM, we study two-body annihilation, effective 4b, 4τ, and 2b2τ channels, and cascades through on-shell scalar mediators. For Model X, the best-fitting diffuse-emission model in our template analysis including a DM component, the dSph limits exclude the 68% CL GCE regions for b b, c c, light quarks, gluons, and effective 4b and 2b2τ. Parts of the τ+τ- and effective 4τ regions remain compatible, although their best-fit points exceed these limits. Heavy two-body final states are less constrained but fit the GCE less well. Mediator masses affect both the preferred regions and the constraints. Part of the 68% CL 4b cascade region remains below the dSph limits for r=m mediator/m DM 0.4. Among the four-body benchmarks, 4τ retains a comparatively large preferred region compatible with dSph and antiproton bounds. With the adopted BIG propagation model, antiproton bounds are generally weaker but constrain part of the 4b cascade region. Finally, we examine a secluded singlet fermionic DM benchmark with a predominantly four-bottom cascade.