A Comprehensive Study of WIMP Models Explaining the Fermi-LAT Galactic Center Excess

TL;DR

This study systematically evaluates WIMP dark matter models to explain the Galactic Center gamma-ray excess, identifying narrow viable parameter regions and highlighting leptophilic vectors and pseudoscalar portals as promising candidates.

Contribution

It provides a comprehensive classification and analysis of WIMP models, incorporating current experimental bounds to identify the most viable scenarios for the GCE.

Findings

Most models are excluded except narrow resonant regions.

Leptophilic vectors and pseudoscalar portals are the most robust options.

Dirac Higgs and Z portals are essentially excluded.

Abstract

The Galactic Center excess (GCE) of GeV $\gamma$ rays may hint at dark matter (DM), yet its origin remains debated. Motivated by this, we survey weakly interacting massive particle (WIMP) models that can fit the GCE while satisfying relic-density, direct-detection (DD), and indirect-detection (ID) bounds. We group candidates into hadronic (Higgs portals; simplified scalar/vector mediators), leptonic ($U(1)_{L_i-L_j}$), and mixed ($U(1)_{B-L}$, $Z$-portal) classes. Across all cases, present DD and dwarf-spheroidal $\gamma$-ray limits exclude wide regions, leaving mainly narrow resonant funnels with $m_{\rm DM}\!\simeq\! m_{\rm med}/2$ and portal couplings $\ll 1$. In hadronic setups, scalar and vector Higgs portals survive only in a thin strip near $m_h/2\simeq62.5$ GeV with portal couplings $\sim 10^{-4}$, while the Dirac Higgs and $Z$ portals are essentially excluded. The UV-complete vector Higgs portal retains resonant bands whose viable portal strength depends on the mixing angle. Simplified scalars allow small windows for complex-scalar or vector DM; Dirac DM is strongly disfavored, whereas a pseudoscalar with Dirac DM remains viable over a broader parameter range. For a simplified $Z'$ mediator, a pure vector coupling leaves only a marginal region, while pure axial is excluded by DD/ID bounds. In leptonic scenarios, inverse-Compton emission is essential: $L_\mu-L_e$ (and, to a lesser extent, $B\!-\!L$) fits the GCE with near-thermal cross sections, while $L_\mu-L_\tau$ is disfavored. Overall, viable WIMP explanations are constrained to finely tuned resonant regime, with leptophilic vectors and pseudoscalar portals emerging as the most robust options.