Multi-Step Cascade Annihilations of Dark Matter and the Galactic Center Excess

TL;DR

This paper investigates how multi-step dark-sector cascades in dark matter annihilation can explain the Galactic Center gamma-ray excess, broadening spectral signatures and expanding viable parameter space.

Contribution

It provides a model-independent analysis of multi-step dark-sector cascades and their impact on gamma-ray spectra, highlighting conditions that fit the observed excess.

Findings

Multi-step cascades broaden photon spectra, allowing for heavier dark matter.

Hierarchical cascades favor dark matter masses between 20-150 GeV.

Degenerate cascades can accommodate higher dark matter masses.

Abstract

If dark matter is embedded in a non-trivial dark sector, it may annihilate and decay to lighter dark-sector states which subsequently decay to the Standard Model. Such scenarios - with annihilation followed by cascading dark-sector decays - can explain the apparent excess GeV gamma-rays identified in the central Milky Way, while evading bounds from dark matter direct detection experiments. Each 'step' in the cascade will modify the observable signatures of dark matter annihilation and decay, shifting the resulting photons and other final state particles to lower energies and broadening their spectra. We explore, in a model-independent way, the effect of multi-step dark-sector cascades on the preferred regions of parameter space to explain the GeV excess. We find that the broadening effects of multi-step cascades can admit final states dominated by particles that would usually produce too sharply peaked photon spectra; in general, if the cascades are hierarchical (each particle decays to substantially lighter particles), the preferred mass range for the dark matter is in all cases 20-150 GeV. Decay chains that have nearly-degenerate steps, where the products are close to half the mass of the progenitor, can admit much higher DM masses. We map out the region of mass/cross-section parameter space where cascades (degenerate, hierarchical or a combination) can fit the signal, for a range of final states. In the current work, we study multi-step cascades in the context of explaining the GeV excess, but many aspects of our results are general and can be extended to other applications.