Constraining Gravitational Dark Matter with LHAASO and Fermi-LAT

We use diffuse Galactic high energy gamma ray data from LHAASO and Fermi-LAT to constrain gravitationally produced decaying dark matter (DM). Focusing on four benchmark candidates: a dark photon, a heavy right-handed neutrino (RHN), a pseudo-Nambu-Goldstone boson (pNGB), and a non-minimally coupled scalar we derive bounds on the DM mass and its couplings to the visible sector. For dark photons, RHNs, and pNGBs, the combined data constrain the relevant interaction strength to $\lesssim\mathcal{O}(10^{-30})$ for DM masses $\gtrsim\mathcal{O}$(TeV), while the non-minimally coupled scalar is limited to $\lesssim\mathcal{O}(10^{-10})$. Moreover, photon-dark photon oscillations yield strong constraints for massive dark photon beyond 10 GeV, closing a region of parameter space previously left unconstrained.

💡 Research Summary

This paper presents a comprehensive analysis constraining models of gravitationally produced decaying dark matter (DM) using the latest high-energy gamma-ray data from the LHAASO observatory and the Fermi-LAT satellite. The core premise is that gravity, due to its universal coupling to all forms of energy, serves as the most natural portal between the visible Standard Model sector and dark matter. Even though gravitational interactions are Planck-suppressed, DM can be efficiently generated in the early universe via the “gravitational freeze-in” mechanism during reheating. If such DM is not absolutely stable, its subsequent decays into Standard Model particles, particularly photons, could contribute to the observed diffuse Galactic gamma-ray flux.

The study focuses on four well-motivated DM candidates, treated as benchmark models:

1. Dark Photon: A hidden U(1) gauge boson that mixes kinetically with the standard photon via a parameter ε.
2. Heavy Right-Handed Neutrino (RHN): A Majorana fermion that can generate light neutrino masses via the seesaw mechanism and decays via Yukawa couplings (Y_ν).
3. Pseudo-Nambu-Goldstone Boson (pNGB): A pseudo-scalar particle, described by an effective field theory, that decays into pairs of electroweak gauge bosons (γγ, Zγ, ZZ, WW) with a strength parameterized by coefficients C_ii/f_φ.
4. Non-minimally Coupled Scalar: A real scalar singlet coupled linearly to the Ricci scalar (ξ S R), enabling both its gravitational production and its Planck-suppressed decay into all possible Standard Model particle pairs.

For each model, the authors calculate the DM production yield via minimal gravitational interactions (mediated by the massless graviton), ensuring it matches the observed relic density. This process imposes an upper bound on the reheating temperature T_rh, typically around 10^14-10^15 GeV for TeV-scale DM. They then compute the DM decay lifetime and the subsequent differential gamma-ray flux expected from DM decays in the Milky Way halo, assuming a Navarro-Frenk-White density profile.

The calculated fluxes are compared against the measured diffuse gamma-ray spectra from the inner Galactic plane as observed by LHAASO (TeV-PeV range) and Fermi-LAT (GeV-TeV range). The key results, summarized in Figure 1 of the paper, are stringent upper limits on the coupling parameters as a function of DM mass:

• For dark photons, RHNs, and pNGBs with masses ≳ 1 TeV, the combined data constrain the relevant interaction strengths (ε, Y_ν, C_ii/f_φ) to be extraordinarily feeble, typically ≲ O(10^{-30}). This region of parameter space is precisely where gravitational production yields the correct relic abundance, making these astrophysical constraints highly relevant for such scenarios.
• For the non-minimally coupled scalar, the bounds are comparatively weaker (ξ ≲ O(10^{-10}) for PeV-scale DM) because its decay rate is itself Planck-suppressed.

A significant additional result concerns the “photon-dark photon oscillation” phenomenon. The paper derives new constraints by considering the conversion of a gravitationally produced dark photon flux into observable SM photons during propagation through the Galaxy. As shown in Figure 2, this analysis provides powerful limits on the kinetic mixing parameter ε for dark photon masses above ~10 GeV, reaching ε ≲ 10^{-3} to 10^{-5}. This closes a gap in the parameter space that was previously unprobed by laboratory experiments such as beam dumps, fixed-target setups, and colliders.

In conclusion, this work demonstrates the powerful synergy between ultra-high-energy astrophysical observatories like LHAASO and Fermi-LAT in probing particle physics models of dark matter that interact only through gravitational or extremely feeble couplings. It shows that diffuse gamma-ray observations can constrain not only the particle nature of DM (its mass and couplings) but also provide a window into early universe cosmology by probing the reheating temperature. The findings underscore the role of multi-messenger astrophysics in exploring physics far beyond the reach of terrestrial experiments.