⚛️ phenomenology

Constraining Gravitational Dark Matter with LHAASO and Fermi-LAT

This paper utilizes high-energy gamma-ray data from LHAASO and Fermi-LAT to constrain the mass and interaction strengths of four specific gravitationally produced decaying dark matter candidates, establishing stringent limits on their couplings to the visible sector and closing previously unconstrained parameter space for massive dark photons.

Original authors: Basabendu Barman, Arindam Das, Prantik Sarmah, Rakesh Kumar SivaKumar

Published 2026-07-07

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Basabendu Barman, Arindam Das, Prantik Sarmah, Rakesh Kumar SivaKumar

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is a giant, dark ocean. We know there's a massive amount of invisible "dark matter" floating in it, making up about a quarter of everything that exists. For a long time, scientists have wondered: How did this dark matter get here, and is it completely stable, or does it slowly break down?

This paper is like a team of cosmic detectives using two powerful "telescopes" (LHAASO and Fermi-LAT) to look for clues. They are hunting for a very specific type of dark matter: Dark matter that was born purely from gravity and is now slowly decaying (breaking apart) into regular light (gamma rays) that we can see.

Here is the story of their investigation, broken down into simple concepts:

1. The "Ghost" Connection

Gravity is the weakest force in the universe. It's like a whisper compared to the shouting of other forces (like magnetism or electricity). Usually, we think gravity is too weak to create particles. However, the authors suggest that in the very hot, early universe (right after the Big Bang), gravity acted like a universal translator. It could "speak" to everything, including invisible dark matter.

They propose that dark matter particles were "frozen in" during this hot phase, created solely by the gravitational pull of the universe itself, without needing any other special machinery.

2. The Four Suspects (The "Four Horsemen")

The researchers didn't just look for "dark matter" in general; they focused on four specific suspects, or "benchmark candidates," that might be hiding in the shadows:

The Dark Photon: Imagine a regular photon (a particle of light) that has a "dark twin." This twin doesn't interact with normal light but can occasionally swap places with it.
The Heavy Right-Handed Neutrino: Think of neutrinos as ghostly particles that barely touch anything. This suspect is a super-heavy, invisible cousin of those ghosts.
The Pseudo-Nambu-Goldstone Boson (pNGB): A fancy name for a particle that acts like a ripple in a field, similar to how a wave moves across a pond.
The Non-Minimally Coupled Scalar: A simple, invisible ball of energy that has a special, slightly stronger handshake with gravity than the others.

3. The Investigation: Looking for the "Afterglow"

If these dark matter particles are unstable, they eventually decay. When they do, they might turn into gamma rays (super-high-energy light).

The scientists used data from two telescopes to scan the Milky Way galaxy:

Fermi-LAT: Looks at lower-energy gamma rays (like the glow of a campfire).
LHAASO: Looks at incredibly high-energy gamma rays (like the flash of a lightning bolt).

They looked at the "diffuse" glow of the galaxy—the background light that isn't coming from a specific star or black hole, but from everywhere. They asked: "Is there too much gamma-ray light here? If so, could it be coming from our four suspects decaying?"

4. The Findings: Tightening the Noose

The team didn't find a smoking gun (they didn't discover the dark matter yet), but they did something very important: They drew a fence around where the suspects cannot be.

The "Feeble" Limit: For the Dark Photon, Heavy Neutrino, and pNGB, the data showed that if they exist, their connection to our visible world must be incredibly weak. The authors describe this strength as being around $10^{-30}$ . To visualize this: If the force of gravity were the size of a mountain, this interaction would be smaller than a single atom.
The Scalar Exception: The "Scalar" suspect is allowed to have a slightly stronger connection (around $10^{-10}$ ), but still very weak.
The "Oscillation" Trap: For the Dark Photon, they found a new way to catch it. If a Dark Photon tries to turn into a regular photon, the data shows this can't happen often for particles heavier than 10 GeV. This closes a gap in the map where scientists previously thought these particles could hide.

5. Why This Matters

The paper concludes that by combining data from both telescopes, they have ruled out huge sections of the "map" where these types of dark matter could exist.

The "Reheating" Clue: They also figured out that for these particles to exist in the right amounts today, the early universe couldn't have been too hot. It's like saying, "If you bake a cake at this temperature, it would have burned up; therefore, the oven must have been cooler."
Conservative Approach: The authors were careful. They didn't subtract the "noise" of cosmic rays (background static) to make their limits look stricter. Instead, they set conservative bounds, meaning their results are very safe and reliable, even if they aren't the absolute tightest possible limits.

Summary

In short, this paper uses the "glow" of our galaxy to test four specific ideas about how dark matter might be made of gravity. They found that if these particles exist, they are extremely shy, interacting with our world so weakly that they are almost impossible to detect, except for the faintest hints of gamma rays. They have successfully closed off several hiding spots, narrowing the search for the true nature of dark matter.

Technical Summary: Constraining Gravitational Dark Matter with LHAASO and Fermi-LAT

Problem Statement
Dark matter (DM) accounts for approximately 24% of the Universe's matter-energy content, yet its particle nature remains unknown. Gravity, being the only force that couples indiscriminately to all forms of energy, serves as a natural mediator between the dark sector and the Standard Model (SM). While gravitational interactions are Planck-suppressed, they can efficiently produce DM during the high-temperature reheating epoch via the "UV freeze-in" mechanism. If such gravitationally produced DM lacks an exact stabilizing symmetry, it may decay into SM final states, including photons. This decay would contribute to the diffuse Galactic gamma-ray flux. The challenge lies in constraining these extremely feeble interactions and the resulting DM lifetimes, particularly for heavy DM candidates (masses $\gtrsim$ TeV) that have remained largely unexplored by previous theoretical frameworks.

Methodology
The authors utilize diffuse Galactic high-energy gamma-ray data from two complementary observatories:

LHAASO: Provides measurements in the TeV to PeV energy range via its SKA and WCDA arrays.
Fermi-LAT: Maps the corresponding GeV to TeV emission.

The analysis focuses on the inner ( $15^\circ < l < 125^\circ, |b| < 5^\circ$ ) and outer ( $125^\circ < l < 235^\circ, |b| < 5^\circ$ ) Galactic plane regions. The authors model the DM density using the Navarro–Frenk–White (NFW) profile and calculate the differential gamma-ray flux from DM decay using the HDMSpectra package.

Four specific benchmark candidates for gravitationally produced DM are investigated:

Dark Photon ( $X$ ): Produced via minimal gravity mediation and decaying via kinetic mixing with the SM photon.
Heavy Right-Handed Neutrino (RHN): Produced gravitationally and decaying via Yukawa couplings in a $U(1)_X$ extension framework.
Pseudo-Nambu–Goldstone Boson (pNGB): A gauge-singlet pseudoscalar interacting with SM electroweak gauge bosons.
Non-minimally Coupled Scalar ( $S$ ): A singlet scalar coupled to the Ricci scalar via a term $\xi M_P S R$ .

For each scenario, the authors derive the DM yield from the Boltzmann equation to ensure the correct relic abundance, linking the DM mass to the reheating temperature ( $T_{rh}$ ). They then calculate the decay rates (or oscillation probabilities for dark photons) and compare the predicted gamma-ray fluxes against observational data to derive bounds on the DM mass and interaction strengths.

Key Contributions and Results
The paper presents constraints on the parameter space (mass vs. coupling) for these four DM candidates, visualized in Figure 1. Key findings include:

Extreme Suppression of Couplings: The combined LHAASO and Fermi-LAT data constrain the effective interaction strengths for dark photons, RHNs, and pNGBs to $\lesssim \mathcal{O}(10^{-30})$ for DM masses $\gtrsim \mathcal{O}(\text{TeV})$ . For the non-minimally coupled scalar, the interaction strength is limited to $\lesssim \mathcal{O}(10^{-10})$ in the same mass range.
Reheating Temperature Constraints: The analysis identifies specific ranges of reheating temperatures ( $T_{rh}$ ) required to achieve the correct relic abundance for heavy DM. For instance, achieving the correct abundance for PeV-scale dark photons requires $T_{rh} \lesssim 3.2 \times 10^{14}$ GeV.
Photon–Dark Photon Oscillations: The study derives limits on photon–dark photon oscillations induced by kinetic mixing. For dark photon masses above $\sim 10$ GeV, the data yields a constraint on the mixing parameter $\varepsilon \lesssim 10^{-3}$ , closing a region of parameter space previously left unconstrained by low-energy scattering, collider, and fixed-target experiments (as shown in Figure 2).
Comparison with Model-Independent Bounds: The authors compare their results with model-independent constraints from Ref. [8]. They note that while Ref. [8] presents stronger bounds for specific decay channels (e.g., $b\bar{b}$ ), those results rely on subtracting cosmic-ray-induced backgrounds, introducing systematic uncertainties. The current work adopts a conservative approach by not subtracting these backgrounds, ensuring robust limits on the DM model parameters.
Production vs. Decay Consistency: The authors examine whether the same interactions responsible for DM decay can also generate the observed relic abundance. They find that for dark photons and Majorana DM, the inverse decay processes required to produce the correct abundance typically demand couplings ( $\varepsilon$ or Yukawa couplings) that are incompatible with the lifetime bounds derived from gamma-ray observations. However, they note that misalignment mechanisms could naturally account for the abundance in specific mass ranges without conflicting with decay constraints.

Significance and Claims
The paper claims that the combination of LHAASO and Fermi-LAT data probes regions of parameter space characterized by extremely feeble couplings ( $\lesssim 10^{-30}$ ) and very heavy DM masses, domains that have hitherto remained largely unexplored. The work demonstrates that diffuse Galactic gamma-ray observations provide a powerful tool to constrain gravitationally produced decaying DM.

The authors emphasize that their constraints depend primarily on the DM decay channels, while the gravitational origin of the production offers insights into the maximum temperature of the early Universe (reheating temperature). They explicitly state that their bounds are conservative, as they do not account for the potential reduction in DM flux limits that would arise from including conventional astrophysical sources (such as Supernova Remnants and Pulsars) in the background modeling. The study concludes that these observations open a window into early Universe cosmology and the nature of gravity-mediated dark sector interactions.