New Constraints from PAMELA anti-proton data on Annihilating and Decaying Dark Matter

Recently the PAMELA experiment has released its updated anti-proton flux and anti-proton to proton flux ratio data up to energies of ~200GeV. With no clear excess of cosmic ray anti-protons at high energies, one can extend constraints on the production of anti-protons from dark matter. In this letter, we consider both the cases of dark matter annihilating and decaying into standard model particles that produce significant numbers of anti-protons. We provide two sets of constraints on the annihilation cross-sections/decay lifetimes. In the one set of constraints we ignore any source of anti-protons other than dark matter, which give the highest allowed cross-sections/inverse lifetimes. In the other set we include also anti-protons produced in collisions of cosmic rays with interstellar medium nuclei, getting tighter but more realistic constraints on the annihilation cross-sections/decay lifetimes.

💡 Research Summary

The paper exploits the most recent PAMELA measurements of the cosmic‑ray antiproton flux and the antiproton‑to‑proton ratio, which now extend up to ≈200 GeV and show no significant excess over the expected background at high energies. Using these data, the authors derive new limits on dark‑matter (DM) models that produce antiprotons either through annihilation or decay into Standard‑Model final states with sizable hadronic content.

Two complementary constraint sets are presented. In the “background‑free” scenario the authors assume that all observed antiprotons could be of dark‑matter origin, i.e. they neglect the conventional secondary antiproton component generated by cosmic‑ray interactions with the interstellar medium (ISM). This yields the most permissive upper bounds on the annihilation cross‑section ⟨σv⟩ and the most conservative lower bounds on the decay lifetime τ. In the more realistic “background‑included” scenario the secondary antiproton flux is computed with a state‑of‑the‑art propagation code (GALPROP), incorporating diffusion, convection, re‑acceleration, and solar‑modulation effects. The difference between the PAMELA data and the predicted secondary flux is then attributed to a possible DM contribution, leading to tighter but physically credible limits.

The analysis explores a range of propagation parameters (diffusion coefficient D₀, spectral index δ, halo height L, convective wind V_c, and solar modulation potential φ_F) to assess systematic uncertainties. High‑energy antiprotons (>100 GeV) are relatively insensitive to these parameters, making the derived limits robust in that regime. The authors also test different Galactic DM density profiles (NFW, Einasto) and find that a cuspy profile strengthens the constraints modestly.

For annihilating DM, the paper focuses on hadronic channels such as b ={b}, W⁺W⁻, and τ⁺τ⁻, which produce copious antiprotons through hadronisation and subsequent decay of mesons. In the background‑included case, the resulting 95 % C.L. limits are roughly ⟨σv⟩ ≲ 10⁻²⁴ cm³ s⁻¹ for a DM mass of ~1 TeV, significantly below the canonical thermal relic cross‑section (3 × 10⁻²⁶ cm³ s⁻¹) for many masses, thereby excluding a large class of models that aim to explain the PAMELA positron excess via hadronic annihilation. Leptonic channels (e.g., μ⁺μ⁻) generate negligible antiprotons, so the PAMELA antiproton data do not constrain them.

For decaying DM, the authors obtain lower limits on the lifetime τ that typically lie in the range τ ≳ 10²⁶–10²⁸ s for masses between a few hundred GeV and several TeV, again assuming hadronic final states. For example, a 1 TeV DM particle decaying into b ={b} must have τ ≳ 5 × 10²⁶ s when secondary antiprotons are included, and τ ≳ 2 × 10²⁶ s in the background‑free case. These limits are comparable to, and in some cases stronger than, those derived from gamma‑ray observations of dwarf spheroidal galaxies or from the cosmic microwave background.

The paper concludes that the updated PAMELA antiproton spectrum provides a powerful, independent probe of dark‑matter models with hadronic final states. The two‑tiered approach—presenting both optimistic (background‑free) and realistic (background‑included) limits—offers a clear picture of the systematic uncertainties involved. The authors note that forthcoming high‑precision antiproton measurements from AMS‑02, DAMPE, and CALET, extending to even higher energies, will further tighten these constraints and could potentially close the remaining viable parameter space for many annihilating or decaying dark‑matter scenarios.