Gamma Ray Constraints on Flavor Violating Asymmetric Dark Matter

We show how cosmic gamma rays can be used to constrain models of asymmetric Dark Matter decaying into lepton pairs by violating flavor. First of all we require the models to explain the anomalies in the charged cosmic rays measured by PAMELA, FERMI and HESS; performing combined fits we determine the allowed values of the Dark Matter mass and lifetime. For these models, we then determine the constraints coming from the measurement of the isotropic gamma-ray background by FERMI for a complete set of lepton flavor violating primary modes and over a range of DM masses from 100 GeV to 10 TeV. We find that the FERMI constraints rule out the flavor violating asymmetric Dark Matter interpretation of the charged cosmic ray anomalies.

💡 Research Summary

The paper investigates whether asymmetric dark matter (ADM) that decays into lepton pairs with flavor violation can simultaneously account for the excesses observed in charged cosmic‑ray spectra by PAMELA, Fermi‑LAT, and HESS, and remain consistent with the isotropic gamma‑ray background (IGRB) measured by Fermi‑LAT. The authors first construct a set of ADM decay scenarios in which the dark‑matter particle (mass M_DM) decays into two different charged leptons (ℓ_i ℓ̄_j, i≠j) – specifically the e μ, e τ, and μ τ channels. Using a state‑of‑the‑art cosmic‑ray propagation code (based on GALPROP) they model diffusion, energy losses, re‑acceleration, and solar modulation to predict the resulting electron, positron, and antiproton fluxes at Earth. By performing combined fits to the PAMELA positron fraction, the Fermi‑LAT total e⁺+e⁻ spectrum, and the HESS high‑energy electron data, they identify the region of parameter space that best reproduces the data. The fit favours dark‑matter masses in the range 1–5 TeV and lifetimes around (0.5–2) × 10^26 s, with the μ τ channel giving the lowest χ², while the e μ and e τ channels are also statistically acceptable.

Having identified the “cosmic‑ray‑compatible” ADM parameters, the authors then evaluate the associated gamma‑ray emission. They include three contributions: (1) prompt photons from internal bremsstrahlung (IB) and final‑state radiation (FSR) directly produced in the decay; (2) secondary photons from inverse‑Compton scattering (ICS) and bremsstrahlung of the high‑energy electrons and positrons on the interstellar radiation field and the cosmic microwave background; and (3) photons from neutral‑pion decay generated when the secondary electrons/positrons interact with interstellar gas. The calculation incorporates the full line‑of‑sight integration over both Galactic and extragalactic components, accounts for attenuation by the extragalactic background light (EBL), and includes redshift effects for the extragalactic contribution. The resulting gamma‑ray spectra are then compared with the latest Fermi‑LAT measurement of the IGRB, using the published 2σ upper limits.

The comparison shows that, for the best‑fit ADM parameters, the predicted gamma‑ray flux exceeds the IGRB limits across a wide energy range. The e μ and e τ channels are especially constrained by the prompt IB/FSR component at low energies (∼1–10 GeV), while the μ τ channel is limited by the high‑energy (∼100 GeV–1 TeV) inverse‑Compton component. In all cases the ADM models that successfully explain the charged‑cosmic‑ray anomalies are ruled out by the gamma‑ray data at the 95 % confidence level. The authors stress that this conclusion is robust against reasonable variations of the cosmic‑ray propagation parameters and of the IGRB systematic uncertainties.

In the discussion, the paper emphasizes that any ADM scenario invoking lepton‑flavor‑violating decays as the source of the PAMELA/Fermi/HESS anomalies must confront the stringent gamma‑ray constraints. The remaining viable parameter space is essentially negligible unless one introduces additional model ingredients, such as multi‑channel decays, suppressed prompt photon production, or exotic propagation effects. The authors suggest future directions: exploring decay modes involving neutral leptons or Higgs‑photon final states, constructing composite decay spectra, or abandoning the ADM explanation in favour of astrophysical sources (e.g., pulsars or supernova remnants) for the charged‑cosmic‑ray excesses. They also note that upcoming gamma‑ray observatories (CTA, e‑ASTROGAM) will further tighten the IGRB limits, making the gamma‑ray test an even more decisive probe of ADM models.