Foreground dependence of the 20 GeV Galactic halo gamma-ray excess and its proposed connection to the LUX-ZEPLIN event

Abstract

We assess the dark matter interpretation of the 20 GeV gamma-ray halo excess reported by Totani and its proposed connection to the LUX-ZEPLIN (LZ) high-energy nuclear-recoil event, which independently favors dark matter masses above about 200 GeV. Using fifteen years of Fermi-LAT data, we reproduce the excess under the original assumptions and test it against 281 Galactic diffuse emission configurations, three descriptions of the Fermi bubbles, four of Loop I, and uniform and X-ray-brightness-weighted eROSITA bubbles templates. A component correlated with a Navarro-Frenk-White (NFW) annihilation template remains with finer spatial binning and the instrumental response included. Outside the Fermi bubbles footprint, it is detected at nominal significances of about 4-16$σ$ over 5.6-130 GeV, falling to 1-7$σ$ when the enlarged boundaries of a recent substructured bubbles model are also excluded. With the substructured model, which improves the fit substantially, the component remains at 7-18$σ$ in four of six representative foreground configurations. However, its amplitude and spectrum depend strongly on the foreground model, and no tested foreground yields a single NFW normalization across the sky, including with additional spatial freedom in the eROSITA bubbles. Simulations show that foreground mismodeling can produce spurious NFW-correlated emission of the observed amplitude, and that residual-derived bubbles templates can bias the recovered spectrum. The 420 GeV $W^+W^-$ spectrum motivated by the LZ connection describes the data for some foreground models but is disfavored by others. These results disfavor interpreting the full excess as annihilation in a smooth NFW halo and weaken the case for a common origin with the LZ event, but they do not exclude a dark matter contribution to the persistent halo-like residual. [abridged]