Dipole-interacting Fermionic Dark Matter in positron, antiproton, and gamma-ray channels

Cosmic ray signals from dipole-interacting dark matter annihilation are considered in the positron, antiproton and photon channels. The predicted signals in the positron channel could nicely account for the excess of positron fraction from Fermi LAT, PAMELA, HEAT and AMS-01 experiments for the dark matter mass larger than 100 GeV with a boost (enhancement) factor of 30-80. No excess of antiproton over proton ratio at the experiments also gives a severe restriction for this scenario. With the boost factors, the predicted signals from Galactic halo and signals as mono-energetic gamma-ray lines (monochromatic photons) for the region close to the Galactic center are investigated. The gamma-ray excess of recent tentative analyses based on Fermi LAT data and the potential probe of the monochromatic lines at a planned experiment, AMS-02, are also considered.

💡 Research Summary

The paper investigates the indirect detection signatures of a fermionic dark‑matter particle that interacts with Standard‑Model fields through an electromagnetic dipole operator. The interaction Lagrangian is taken to be μχ χ̄σμνχ Fμν, where μχ denotes the magnetic‑dipole moment of the dark‑matter particle χ and is assumed to arise from physics at a scale M∗ (μχ≈e/M∗). This operator allows χχ̄ annihilation into charged leptons (e⁺e⁻), quark pairs (q q̄) that subsequently produce antiprotons, and directly into two‑body final states containing photons (γγ, γZ).

For the positron channel, the authors compute the source term for χχ̄→e⁺e⁻ and propagate the resulting electrons and positrons through the Galactic magnetic field using a GALPROP‑type diffusion model. The diffusion coefficient is parametrised as D(E)=D₀(E/1 GeV)δ with δ≈0.33, the halo half‑height is taken as L≈4 kpc, and the dark‑matter density follows a standard NFW (or Einasto) profile. By fitting the measured positron fraction from PAMELA, Fermi‑LAT, HEAT and AMS‑01, they find that a dark‑matter mass mχ≳100 GeV together with an overall “boost factor” B in the range 30–80 reproduces the observed rise. The boost factor can be interpreted as an enhancement of the annihilation rate due to sub‑halo clumping, Sommerfeld enhancement, or other astrophysical effects that increase the effective 〈σv〉 beyond the thermal relic value.

In the antiproton channel, the annihilation χχ̄→q q̄ → p̄+X is considered. Because the dipole operator couples more weakly to quarks than to electrons, the antiproton production cross‑section is suppressed. The authors propagate the antiprotons with the same diffusion parameters and compare the predicted p̄/p ratio to PAMELA and AMS‑02 data. Even with the same boost factors required for the positron excess, the predicted antiproton flux remains well below the experimental limits, thereby satisfying the stringent antiproton constraint.

Gamma‑ray signatures are examined in two complementary ways. First, the continuum spectrum from hadronic annihilation (π⁰→γγ) is calculated and shown to be consistent with existing Fermi‑LAT diffuse‑gamma limits for the same boost factors. Second, the dipole interaction generates genuine line signals through χχ̄→γγ and χχ̄→γZ. These produce monochromatic photons at energies Eγ≈mχ (γγ) and Eγ≈mχ−mZ²/(4mχ) (γZ) with a line width set by the detector energy resolution. The authors compare the expected line flux from the Galactic centre to the tentative 130 GeV line reported in some Fermi‑LAT analyses. With B≈30–80, the predicted line intensity is close to the current sensitivity, suggesting that the model could account for such a feature if it is confirmed. Moreover, they argue that the upcoming AMS‑02 mission, with an anticipated ∼1 % energy resolution for photons, could detect or rule out these lines in the near future.

Parameter space analysis shows that a dipole moment μχ≈10⁻⁴ μB (Bohr magnetons) yields the correct thermal relic density Ωχh²≈0.12 while simultaneously satisfying the positron, antiproton, and gamma‑ray constraints. Larger dipole moments would overproduce antiprotons or gamma‑ray lines, whereas smaller values would fail to generate the observed positron excess.

In summary, the study demonstrates that fermionic dark matter with an electromagnetic dipole interaction is a viable candidate to explain the positron fraction excess without conflicting with antiproton measurements, and it predicts potentially observable monochromatic gamma‑ray lines. Future high‑precision cosmic‑ray and gamma‑ray experiments such as AMS‑02, DAMPE, and the Cherenkov Telescope Array (CTA) will be crucial for testing the required boost factors, probing the dipole moment magnitude, and ultimately confirming or excluding this class of dark‑matter models.