Gamma Lines without a Continuum: Thermal Models for the Fermi-LAT 130 GeV Gamma Line

TL;DR

This paper explores models explaining a 130 GeV gamma-ray line detected by Fermi-LAT, proposing resonant and cascade annihilation mechanisms to reconcile the observed line with dark matter properties.

Contribution

It introduces two novel mechanisms—resonant and cascade annihilation—to account for the gamma-ray line without conflicting with existing dark matter constraints.

Findings

Resonant annihilation enhances cross-section during freezeout.

Cascade annihilation involves p-wave and s-wave processes.

Models predict new particles near the dark matter mass, with TeV-scale charged states.

Abstract

Recent claims of a line in the Fermi-LAT photon spectrum at 130 GeV are suggestive of dark matter annihilation in the galactic center and other dark matter-dominated regions. If the Fermi feature is indeed due to dark matter annihilation, the best-fit line cross-section, together with the lack of any corresponding excess in continuum photons, poses an interesting puzzle for models of thermal dark matter: the line cross-section is too large to be generated radiatively from open Standard Model annihilation modes, and too small to provide efficient dark matter annihilation in the early universe. We discuss two mechanisms to solve this puzzle and illustrate each with a simple reference model in which the dominant dark matter annihilation channel is photonic final states. The first mechanism we employ is resonant annihilation, which enhances the annihilation cross-section during freezeout and allows for a sufficiently large present-day annihilation cross section. Second, we consider cascade annihilation, with a hierarchy between p-wave and s-wave processes. Both mechanisms require mass near-degeneracies and predict states with masses closely related to the dark matter mass; resonant freezeout in addition requires new charged particles at the TeV scale.