Charge Asymmetric Cosmic Ray Signals From Dark Matter Decay

The PAMELA and Fermi measurements of the cosmic-ray electron and positron spectra have generated much interest over the past two years, because they are consistent with a significant component of the electron and positron fluxes between 20 GeV and 1 TeV being produced through dark matter annihilation or decay. However, since the measurements are also consistent with astrophysical interpretations, the message is unclear. In this paper, we point out that dark matter can have a more distinct signal in cosmic rays, that of a charge asymmetry. Such charge asymmetry can result if the dark matter’s abundance is due to a relic asymmetry, allowing its decay to generate an asymmetry in positrons and electrons. This is analogous to the baryon asymmetry, where decaying neutrons produce electrons and not positrons. We explore benchmark scenarios where the dark matter decays into a leptophilic charged Higgs boson or electroweak gauge bosons. These models have observable signals in gamma rays and neutrinos, which can be tested by Fermi and IceCube. The most powerful test will be at AMS-02, given its ability to distinguish electron and positron charge above 100 GeV. Specifically, an asymmetry favoring positrons typically predicts a larger positron ratio and a harder (softer) high energy spectrum for positrons (electrons) than charge symmetric sources. We end with a brief discussion on how such scenarios differ from the leading astrophysical explanations.

💡 Research Summary

The paper addresses the intriguing excesses observed by PAMELA and Fermi in the cosmic‑ray electron and positron spectra between roughly 20 GeV and 1 TeV. While such excesses can be accommodated by both dark‑matter (DM) scenarios (annihilation or decay) and conventional astrophysical sources (pulsars, supernova remnants, etc.), the authors argue that a distinctive signature—charge asymmetry—could decisively differentiate a DM origin.

The central idea is that if the present‑day abundance of DM is set by a relic particle–antiparticle asymmetry (an “asymmetric dark matter” scenario), then the surviving DM particles (denoted χ) are not their own antiparticles. Consequently, χ can decay preferentially into positively charged leptons (ℓ⁺) together with a neutral particle (ν̄ or a gauge/Higgs boson), while the conjugate decay producing ℓ⁻ is absent. This mirrors the familiar baryon asymmetry where neutron β‑decay yields electrons but no positrons. The authors construct six benchmark models that realize this mechanism.

In the simplest class, χ is a fermion that decays to a charged boson X⁻ (either a Standard‑Model W⁻ or a leptophilic charged Higgs H⁻) plus a lepton ℓ⁺ (ℓ = μ, τ). The neutral partner X⁰ can be a Z⁰ or Higgs h⁰, decaying to ν̄. Three representative decay patterns are considered: (i) χ → W⁻ℓ⁺ only; (ii) χ → W⁻ℓ⁺, Z⁰ν̄, h⁰ν̄ with branching ratios 2 : 1 : 1 (analogous to a right‑handed neutrino); and (iii) χ → H⁻ℓ⁺ with H⁻ → τ⁻ν̄, a leptophilic Higgs that predominantly yields taus. The authors note that the first two patterns are in tension with PAMELA antiproton limits, motivating the third, “safer” scenario.

Model parameters are chosen to reproduce the observed high‑energy softening of the combined electron‑plus‑positron spectrum: χ masses of 3.5 TeV (μ‑channel) and 7 TeV (τ‑channel), and a charged Higgs mass of 150 GeV. Using standard cosmic‑ray propagation tools, they compute the local DM contribution to the positron‑to‑electron flux ratio Φₑ⁺/Φₑ⁻. All asymmetric benchmarks display a rising ratio with energy, in contrast to charge‑symmetric decays (χ → ℓ⁺ℓ⁻) which predict a roughly constant or slowly varying ratio. The μ‑channel models, especially the H⁻ ℓ⁺ decay, yield the strongest asymmetry, while τ‑channel models give a more modest effect.

Fitting to PAMELA positron‑fraction data and Fermi/HESS electron‑plus‑positron spectra, the authors find that charge asymmetry resolves a normalization tension: symmetric decays require a shorter lifetime to match PAMELA, which then overshoots the Fermi flux; asymmetric decays can simultaneously accommodate both with a lifetime ≈ 10²⁶ s. The positron fraction in the best‑fit asymmetric models can approach, though not exceed, 0.5 at the highest energies; a value above 0.5 would constitute a “smoking‑gun” signal.

The paper also examines complementary observables. Gamma‑ray constraints from the isotropic Fermi‑LAT background are satisfied, particularly for the H⁻ μ⁺ model, where the extragalactic component dominates and the Galactic center contribution remains subdominant. Neutrino fluxes from χ decays are within IceCube’s current limits but could become detectable with future exposure, especially for τ‑channel decays that produce harder ν̄ spectra.

A key experimental test is the charge‑discriminating capability of AMS‑02, which can measure the positron fraction up to several hundred GeV with high precision. The authors argue that AMS‑02 should be able to confirm (or refute) the predicted rise in Φₑ⁺/Φₑ⁻, the harder positron spectrum (∝ E⁻²·² versus ∝ E⁻³·¹ for electrons), and the softer electron spectrum characteristic of asymmetric decays.

Finally, the authors contrast their DM‑induced charge asymmetry with astrophysical sources. Pulsars are expected to emit electrons and positrons symmetrically, so any observed charge imbalance would favor a DM origin. Moreover, the distinct spectral slopes (electron α ≈ 3.1–3.2 for asymmetric models versus α ≈ 3.0 for symmetric ones) provide an additional discriminant. The paper concludes that charge‑asymmetric DM decay offers a concrete, testable signature that could break the degeneracy between dark‑matter and astrophysical explanations of the cosmic‑ray lepton excesses, and that forthcoming data from AMS‑02, Fermi‑LAT, and IceCube will be decisive.