10 GeV dark matter candidates and cosmic-ray antiprotons

Recent measurements performed with some direct dark matter detection experiments, e.g. CDMS-II and CoGENT (after DAMA/LIBRA), have unveiled a few events compatible with weakly interacting massive particles. The preferred mass range is around 10 GeV, with a quite large spin-independent cross section of $10^{-43}$-$10^{-41},{\rm cm^2}$. In this paper, we recall that a light dark matter particle with dominant couplings to quarks should also generate cosmic-ray antiprotons. Taking advantage of recent works constraining the Galactic dark matter mass profile on the one hand and on cosmic-ray propagation on the other hand, we point out that considering a thermal annihilation cross section for such low mass candidates very likely results in an antiproton flux in tension with the current data, which should be taken into account in subsequent studies.

💡 Research Summary

The paper addresses a tension that arises when light (∼10 GeV) dark‑matter (DM) candidates, motivated by a handful of events reported by direct‑detection experiments such as CDMS‑II, CoGENT and, to a lesser extent, DAMA/LIBRA, are confronted with measurements of the cosmic‑ray antiproton flux. The authors begin by noting that the favored region of parameter space for these candidates involves a relatively large spin‑independent scattering cross‑section on nucleons, $10^{-43}$–$10^{-41},\mathrm{cm^{2}}$, and that the interaction is assumed to be dominated by couplings to quarks. In such a scenario the dominant annihilation channel is into light quark–antiquark pairs, which inevitably produce antiprotons as secondary particles.

To quantify the resulting antiproton signal the authors adopt two recent advances: (i) refined determinations of the Galactic DM density profile from stellar kinematics and rotation‑curve analyses, and (ii) state‑of‑the‑art cosmic‑ray propagation models calibrated on a variety of secondary‑to‑primary ratios (B/C, sub‑Fe/Fe, etc.). For the DM distribution they consider both cuspy profiles (Navarro‑Frenk‑White, Einasto) and cored alternatives, emphasizing that the inner halo density strongly influences the source term for antiprotons. For propagation they employ the GALPROP framework with three benchmark parameter sets (MIN, MED, MAX) that span the range of plausible diffusion coefficients, convective wind speeds, and re‑acceleration strengths.

Assuming a standard thermal relic annihilation cross‑section, $\langle\sigma v\rangle\approx3\times10^{-26},\mathrm{cm^{3},s^{-1}}$, the authors compute the antiproton spectrum expected from a 10 GeV WIMP annihilating into $q\bar q$. The predicted flux is then compared with the high‑precision measurements from the PAMELA satellite and the AMS‑02 experiment. Across the entire energy range of interest (∼1–10 GeV), the model consistently overshoots the observed antiproton flux, often by more than two standard deviations, even when the most conservative propagation parameters (MIN) and the most cored DM profile are adopted. The excess persists because the antiproton production is directly proportional to the product of the squared DM density and the annihilation cross‑section, both of which are fixed by the direct‑detection interpretation.

The authors explore possible ways to alleviate the tension. One avenue is to invoke non‑thermal production mechanisms or asymmetric DM models, where the present‑day annihilation rate is suppressed relative to the thermal value. Another possibility is to postulate an unconventional propagation environment with an exceptionally low diffusion coefficient or a very strong convective wind, which would reduce the antiproton flux reaching the Earth. However, such extreme propagation setups are in conflict with other cosmic‑ray observables and with the broader astrophysical literature.

In conclusion, the paper demonstrates that a 10 GeV WIMP with the large spin‑independent cross‑section suggested by recent direct‑detection hints is highly constrained by existing antiproton data, provided the annihilation proceeds with the canonical thermal cross‑section into quarks. The authors argue that any future phenomenological study of low‑mass DM candidates must incorporate antiproton constraints, and that reconciling the direct‑detection signals with cosmic‑ray observations likely requires either a departure from the thermal relic paradigm or a more exotic particle physics model that suppresses hadronic final states. This work thus highlights the importance of multi‑messenger consistency checks in the ongoing search for dark matter.