Producing the GeV Galactic Center Excess via Cosmic Ray-Dark Matter Scattering

This paper proposes a novel mechanism where cosmic-ray protons scattering off dark matter in the Milky Way halo generate the observed GeV Galactic Center gamma-ray excess, offering a viable alternative to conventional dark matter annihilation or pulsar models.

The Gist

Imagine the center of our galaxy, the Milky Way, as a bustling cosmic city. For years, astronomers have noticed a strange, bright "glow" of high-energy light (gamma rays) coming from this central hub. It's like seeing a streetlamp that is far too bright for its surroundings.

For a long time, scientists thought this glow was caused by two main things:

1. Dark Matter Annihilation: Two invisible "ghost" particles crashing into each other and vanishing in a flash of light.
2. Millisecond Pulsars: Tiny, super-fast spinning neutron stars acting like cosmic lighthouses.

However, these explanations have some holes. For instance, if dark matter were annihilating everywhere, we should see similar bright glows in the small "satellite" galaxies orbiting ours, but we don't.

The New Idea: The Cosmic Pinball Machine

In this paper, the authors propose a completely different way to explain the glow. Instead of dark matter particles crashing into each other, they suggest that dark matter particles are getting hit by cosmic rays.

Think of it like a game of cosmic pinball:

• The Cosmic Rays: These are high-speed protons (particles from space) zooming through the galaxy like pinballs shot from a machine.
• The Dark Matter: These are the invisible bumpers or targets sitting in the center of the galaxy.
• The Collision: When a fast cosmic ray proton smashes into a dark matter particle, it doesn't just bounce off. Instead, it transfers energy, causing the dark matter to "excite" or change state.

The paper explores two specific ways this "pinball game" creates the light we see:

1. The "Bouncy Ball" Scenario (Inelastic Model)
Imagine a light, stable dark matter particle (let's call it a "soft ball"). A fast cosmic ray hits it and knocks it into a heavier, excited state (a "hard ball"). This heavy ball is unstable and immediately breaks apart, but instead of just falling apart, it splits into the original soft ball and two flashes of light (photons).

• Why it works: The heavy ball only forms where the "soft balls" (dark matter) are packed tightly together. Since dark matter is most dense at the Galactic Center, the light flashes happen mostly there, explaining why we don't see the same glow in the sparse satellite galaxies.

2. The "Direct Hit" Scenario (Elastic Model)
In this version, the cosmic ray hits the dark matter, and the collision directly sprays out a high-energy photon, like a spark flying off a grinding wheel. This happens through a specific interaction involving invisible force-carrier particles (mediators) that act like the gears in the machine.

The Results: A Perfect Match
The authors ran the numbers for both scenarios. They found that if the dark matter particles are relatively light (less than 1 GeV, which is very light for a dark matter particle) and the collisions happen just right, the pattern of light produced matches the "glow" observed by the Fermi telescope almost perfectly.

• The Fit: Their new "pinball" models fit the data just as well as the old "annihilation" or "pulsar" models.
• The Advantage: This new mechanism naturally explains why the glow is concentrated in the center (where dark matter is dense) and why it's missing from the outskirts (where dark matter is sparse).

Checking the Rules
Before celebrating, the authors checked if their idea breaks any known laws of physics. They looked at:

• Direct Detection Experiments: Could we have already seen these particles in underground labs? They found that their proposed particles are light enough and interact weakly enough that current detectors wouldn't have spotted them yet.
• Other Observations: They checked constraints from particle accelerators and supernova cooling. Their models pass these tests, meaning they are physically possible.

The Bottom Line
This paper offers a fresh perspective: The mysterious glow at the center of our galaxy might not be dark matter killing itself, but rather dark matter getting a "cosmic high-five" from fast-moving protons. It's a new way to look at the same old mystery, one that fits the data just as well as the leading theories but solves some of their lingering problems.

Technical Summary

Technical Summary: Producing the GeV Galactic Center Excess via Cosmic Ray-Dark Matter Scattering

Problem Statement
The Galactic Center Excess (GCE) refers to an unexplained surplus of GeV-energy gamma rays observed by the Fermi Large Area Telescope (Fermi-LAT) emanating from the Milky Way's center. While conventional explanations attribute this signal to the annihilation of Weakly Interacting Massive Particles (WIMPs) or a population of unresolved millisecond pulsars (MSPs), these models face challenges. Specifically, WIMP annihilation models often predict gamma-ray emission from dwarf spheroidal galaxies (dSphs) that is not observed, while cosmic-ray outburst models typically predict emission along the galactic disk rather than being concentrated at the center. Furthermore, existing literature on cosmic-ray (CR) scattering with dark matter (DM) has suggested that such processes yield insufficient photon flux to account for the GCE, particularly when considering WIMP-scale masses.

Methodology
The authors propose a novel mechanism where high-energy gamma rays are generated via the scattering of hadronic cosmic rays (specifically protons) off sub-GeV dark matter particles in the Milky Way halo. Unlike annihilation, which scales with the square of the dark matter density ($\rho_{DM}^2$), this scattering process scales linearly with $\rho_{DM}$, naturally concentrating the signal where dark matter density is highest (the Galactic Center) while suppressing emission in dSphs and the galactic disk.

The study analyzes two specific theoretical frameworks:

1. Inelastic Dark Matter (iDM): A model featuring two dark matter species, a lighter stable state ($\chi_1$) and a heavier state ($\chi_2$) with a mass splitting $\Delta$. A cosmic-ray proton up-scatters $\chi_1$ to $\chi_2$ ($\chi_1 + p \to \chi_2 + p$). The heavier state subsequently decays via a three-body process ($\chi_2 \to \chi_1 + 2\gamma$) mediated by a scalar particle $\phi$, producing two photons.
2. Elastic Dark Matter (eDM): A single-component fermion dark matter model ($\chi$) interacting with protons via a light scalar ($\phi$) and a light vector boson ($Z'$). This model facilitates a 2-to-3 scattering process ($\chi + p \to \chi + \gamma + p$) where a high-energy photon is produced directly in the final state via $Z'$ and $\phi$ exchange.

The authors compute the differential photon flux by integrating the cosmic-ray proton flux (modeled as a cylinder with a 10 kpc radius and 4 kpc half-height) over the dark matter density profile (generalized Navarro-Frenk-White profile). They calculate the differential cross-sections for the scattering processes and evaluate the resulting energy spectra. To validate the models, they compare their predicted spectra against the GCE data from Cholis et al. [21], as well as benchmark spectra from WIMP annihilation ($DM \to b\bar{b}$) and MSPs. They also assess constraints from direct detection experiments (XENON1T, PandaX-4T, LZ), beam-dump experiments (NA62, BaBar), and astrophysical limits (supernova cooling).

Key Contributions

• Novel Mechanism: The paper introduces a sub-GeV dark matter scattering mechanism that generates GeV-scale photons, addressing the insufficiency of previous CR-DM scattering proposals which relied on heavier WIMP masses.
• Two Viable Models: The authors present concrete benchmark points for both inelastic and elastic dark matter scenarios that successfully reproduce the observed GCE spectral features.
• Spatial Consistency: The model naturally explains the absence of a comparable excess in dwarf galaxies due to the lower dark matter density and reduced cosmic-ray flux in those regions, a feature that distinguishes it from annihilation-based models.
• Constraint Satisfaction: The proposed benchmark points are shown to be consistent with current limits from direct detection experiments, laboratory searches for light mediators, and astrophysical constraints.

Results
The authors identify specific benchmark points (BPs) for both the iDM and eDM models (detailed in Table I) that yield gamma-ray energy spectra peaking between 1 and 5 GeV.

• Spectral Fit: As shown in Figure 2 and Table II, the predicted spectra from the CR-DM scattering models provide a goodness-of-fit ($\chi^2$) to the GCE data that is comparable to the best-fitting models of dark matter annihilation and millisecond pulsars.
• Parameter Space: The results demonstrate that a good fit is not restricted to a single fine-tuned parameter set but persists across a nontrivial region of the parameter space.
• Constraints: The models satisfy constraints from low-threshold direct detection experiments (predicting $\mathcal{O}(1)$ signal events for a 1 ton-year exposure, which falls below observed limits) and laboratory searches for light mediators. The specific choice of mediator masses and couplings ensures compliance with limits from NA64, BaBar, and DELPHI, as well as self-interaction limits from the Bullet Cluster.

Significance and Claims
The paper claims to open a "new avenue for interpreting gamma-ray observations of the Galactic Center." By demonstrating that cosmic-ray scattering off sub-GeV dark matter can produce a gamma-ray spectrum consistent with the GCE, the authors offer an alternative to the standard annihilation and pulsar paradigms. The authors modestly note that they focus exclusively on fitting the energy spectrum and do not perform a morphological analysis due to the strong model dependence of background modeling in that domain. They conclude that while their benchmark models satisfy current constraints, future multi-messenger observations (e.g., via neutrino detectors like DUNE or Hyper-K) and next-generation gamma-ray telescopes (such as the Advanced Particle-astrophysics Telescope) could provide further insights or probe additional signals from these dark matter scenarios. The work suggests that the GCE could be a signature of dark matter interactions rather than annihilation, provided the dark matter mass is in the sub-GeV range.