The 20 GeV Galactic Halo Excess: Pixel-Level Confirmation and Consistency with Sub-TeV WIMP Annihilation

Abstract

A recent analysis of 15 years of Fermi-LAT data reported a spherically symmetric, halo-like component of the Galactic diffuse emission that peaks near 20GeV. We independently reproduce this cell-aggregated analysis, then extend it to a pixel-level likelihood on the native $0.125^\circ$ maps, adding energy-dependent point-spread-function forward folding and masking bright sources. Both methods replicate the 20GeV halo spectrum, with the pixel-level normalisation ${\sim}20\%$ above the cellwise fit across NFW emissivity scalings $ρ^p$, $p \in 1,2,2.5$. This 20GeV halo is a high-latitude feature, distinct from the inner-Galaxy excess, and consistent with sub-TeV dark matter (WIMP) annihilation. It is centrally concentrated, strongly disfavouring extragalactic emission. Fitting prompt $s$-wave annihilation spectra, best-fit masses are $m_χ\simeq 0.55$TeV ($W^+W^-$) and $0.72$TeV ($b\bar{b}$) with $\langleσv\rangle \simeq 1\times10^{-24}~\mathrm{cm^3\,s^{-1}}$, in $\sim\!4$-$5\times$ tension with dwarf spheroidal galaxy limits. However, accounting for foreground modelling and $J$-factor systematic uncertainties widens the tension window to $R\simeq1.6$-$9.3$, leaving the $s$-wave interpretation viable. To close the tension, we consider alternative particle dark matter models. $p$-wave annihilation misses relic abundance constraints by $\sim\!7$ orders of magnitude. A decay interpretation evades dwarf limits but is disfavoured by the isotropic gamma-ray background. The only viable velocity structure consistent with dwarf limits, present-day halo rates, and relic density is low-velocity-enhanced annihilation (resonant Sommerfeld or Breit-Wigner). This supplies the required $\approx\!45\times$ boost from a thermal relic. Fully resolving the dwarf tension requires a fine-tuned resonance peaking at the halo velocity and falling for colder systems.