Extragalactic and galactic gamma-rays and neutrinos from annihilating dark matter

We describe cosmic gamma-ray and neutrino signals of dark matter annihilation, explaining how the complementarity of these signals provides additional information that, if observable, can enlighten the particle nature of dark matter. This is discussed in the context of exploiting the separate galactic and extragalactic components of the signal, using the spherical halo model distribution of dark matter. We motivate the discussion with supersymmetric extensions of the standard model of particle physics. We consider the minimal supersymmetric standard model (MSSM) where both neutrinos and gamma-rays are produced from annihilations. We also consider a gauged B-L, baryon number minus lepton number, extension of the MSSM, where annihilation can be purely to heavy right-handed neutrinos. We compare the galactic and extragalactic components of these signals, and conclude that it is not yet clear which may dominate when looking out of the galactic plane. To answer this question, we must have an understanding of the contribution of halo substructure to the annihilation signals. We find that different theories with indistinguishable gamma-ray signals can be distinguished in the neutrino signal. Gamma-ray annihilation signals are difficult to observe from the galactic center, due to abundant astrophysical sources; but annihilation neutrinos from there would not be so hidden, if they can be observed over the atmospheric neutrinos produced by cosmic rays.

💡 Research Summary

This paper presents a comprehensive theoretical study of the high‑energy gamma‑ray and neutrino signals that would be produced today by the annihilation of dark matter particles. The authors separate the expected signal into two components: a Galactic contribution arising from annihilations within the Milky Way halo, and an extragalactic contribution from annihilations throughout the cosmological large‑scale structure. Both components are calculated using a spherical halo model: the halo mass function is taken from the Sheth‑Tormen prescription, each halo is assigned a Navarro‑Frenk‑White (NFW) density profile, and a concentration‑mass relation based on recent N‑body simulations is employed. A minimum halo mass of 10⁻⁶ M⊙ is assumed, and the calculations integrate over redshift, including attenuation of gamma rays by the extragalactic background light (EBL).

Two particle‑physics frameworks are examined in detail. The first is the Minimal Supersymmetric Standard Model (MSSM) with the lightest neutralino (χ̃⁰₁) as the dark‑matter candidate. The authors discuss the four classic regions of the MSSM parameter space—bulk, focus‑point (or hyperbolic), co‑annihilation, and funnel—highlighting the dominant annihilation channels in each (bb̄, τ⁺τ⁻, W⁺W⁻, ZZ, or resonant A‑mediated bb̄/τ⁺τ⁻). These channels imprint distinct spectral shapes on both the gamma‑ray and neutrino fluxes. For example, focus‑point models with a sizable Higgsino component produce hard W‑boson final states, leading to a high‑energy gamma‑ray bump, whereas bulk models are softer and dominated by b‑quark fragmentation.

The second framework is a gauged U(1)_{B‑L} extension of the MSSM. In this scenario the right‑handed sneutrino (˜N) is the lightest supersymmetric particle. Annihilation proceeds mainly via ˜N ˜N → Nᶜ Nᶜ, where the heavy right‑handed neutrino Nᶜ (mass ≈ 135 GeV) subsequently decays into a Standard‑Model Higgs and a light neutrino. The Higgs (taken to be ≈120 GeV) decays predominantly to WW* and bb̄, generating secondary photons and neutrinos. Because the primary annihilation products are neutrinos, the resulting neutrino flux is comparatively large, while the gamma‑ray component is sub‑dominant and arises only from secondary Higgs decays.

The paper then evaluates the observable spectra. For gamma rays, the extragalactic component is integrated over redshift, accounting for cosmological expansion and EBL absorption, while the Galactic component is obtained by line‑of‑sight integration through the Milky Way halo. The authors find that the Galactic center region is overwhelmed by astrophysical gamma‑ray sources (pulsars, supernova remnants, etc.), making a dark‑matter gamma‑ray line or continuum difficult to extract. In contrast, high‑energy neutrinos suffer far less background contamination; the atmospheric neutrino flux is the main obstacle, but its spectrum is well understood. Instruments such as IceCube, with good angular resolution at TeV–PeV energies, could potentially isolate a dark‑matter‑induced excess, especially if the signal is concentrated toward the Galactic center or dwarf spheroidal galaxies.

A critical source of uncertainty is the contribution from halo substructure. Small subhalos and sub‑subhalos boost the annihilation rate because the signal scales with the density squared. Depending on the assumed subhalo mass function and concentration, the total flux can be enhanced by factors of a few up to an order of magnitude. The authors stress that current N‑body simulations cannot resolve the lowest‑mass halos, so the true boost factor remains uncertain. Similarly, the inner density profile of the Milky Way (cored vs. cusped) strongly influences the Galactic component.

The authors conclude that gamma‑ray and neutrino observations are highly complementary. Gamma‑ray spectra provide detailed energy information but are often masked by astrophysical backgrounds, whereas neutrinos offer a cleaner probe of the annihilation core, especially for models like the B‑L extension where neutrinos dominate the final state. Detecting both signals would allow simultaneous constraints on the dark‑matter particle mass, annihilation cross‑section, dominant channels, and on the astrophysical distribution of dark matter (halo profile, substructure). The paper emphasizes that future improvements in detector sensitivity (e.g., CTA for gamma rays, IceCube‑Gen2 for neutrinos) together with refined theoretical modeling of halo substructure will be essential to turn these indirect searches into decisive tests of supersymmetric dark‑matter scenarios.