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 $4b$, $4\tau$, and $2b2\tau$ 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 $b\bar{b}$, $c\bar{c}$, light quarks, gluons, and effective $4b$ and $2b2\tau$. Parts of the ${\tau }^{+}{\tau }^{-}$ and effective $4\tau$ 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 $r=m_{\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{r}}\mathrm{/}{m}_{\mathrm{D}\mathrm{M}}\lesssim 0.4$. Among the four-body benchmarks, $4\tau$ 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.