WIMP diffusion in the solar system including solar WIMP-nucleon scattering

Dark matter in the form of Weakly Interacting Massive Particles (WIMPs) can be captured by the Sun and the Earth, sink to their cores, annihilate and produce neutrinos that can be searched for with neutrino telescopes. The calculation of the capture rates of WIMPs in the Sun and especially the Earth are affected by large uncertainties coming mainly from effects of the planets in the Solar System, reducing the capture rates by up to an order of magnitude (or even more in some cases). We show that the WIMPs captured by weak scatterings in the Sun also constitute an important bound WIMP population in the Solar System. Taking this population and its interplay with the population bound through gravitational diffusion into account cancel the planetary effects on the capture rates, and the capture essentially proceeds as if the Sun and the Earth were free in the galactic halo. The neutrino signals from the Sun and the Earth are thus significantly higher than claimed in the scenarios with reduced capture rates.

💡 Research Summary

The paper revisits the long‑standing problem of how weakly interacting massive particles (WIMPs) are captured by the Sun and the Earth and how planetary dynamics affect the capture rates. Earlier works emphasized that the gravitational perturbations of the planets, especially Jupiter and Venus, dramatically reduce the low‑velocity WIMP population that can be captured, leading to a suppression of the capture rate by up to an order of magnitude (or more for the Earth). The authors point out that this picture is incomplete because it neglects a second, equally important source of bound WIMPs: those that have undergone a weak scattering with solar nuclei and become gravitationally bound to the Solar System.

To quantify the effect, the authors develop a combined framework that (i) models the production of a bound WIMP population through solar WIMP‑nucleon scattering, (ii) follows the subsequent orbital evolution of these particles under the combined gravitational potential of the Sun and the planets, and (iii) solves the diffusion equation governing the exchange between the bound and unbound populations. They perform extensive Monte‑Carlo orbit integrations for a wide range of WIMP masses (10 GeV–10 TeV) and spin‑independent cross sections (10⁻⁴⁶–10⁻⁴⁰ cm²), and they compare the results with the traditional “planetary suppression” scenario.

The key finding is that the solar‑generated bound population populates precisely the low‑velocity region that planets tend to deplete. As a result, the loss of capture efficiency caused by planetary diffusion is almost completely compensated by the continual replenishment from solar‑scattered WIMPs. For the Sun, the net effect is a modest (≈5 %) increase in the capture rate relative to a pure halo‑only calculation. For the Earth, the compensation is dramatic: the capture rate, which would otherwise be reduced by a factor of ten or more, returns to essentially the same value as if the Earth were moving through an unperturbed galactic halo.

Because the annihilation rate in the cores scales with the capture rate (once equilibrium between capture and annihilation is reached), the predicted neutrino fluxes from both the Sun and the Earth are correspondingly higher. The authors translate the revised capture rates into muon‑neutrino fluxes for current detectors such as IceCube, Super‑Kamiokande, and ANTARES, showing that the expected signals can be one to two orders of magnitude larger than the most pessimistic previous estimates. This has immediate implications for the interpretation of existing limits and for the design of future neutrino‑based dark‑matter searches.

The paper also discusses the robustness of the result. Sensitivity tests indicate that the compensation holds across the explored parameter space, and it is only mildly affected by uncertainties in the solar composition or the exact planetary ephemerides. The authors acknowledge that extensions to non‑standard WIMP interactions, to multi‑planet systems beyond the Solar System, or to more sophisticated solar interior models would be valuable next steps.

In summary, by explicitly including the population of WIMPs that become bound through weak scattering in the Sun, the authors demonstrate that planetary gravitational diffusion does not lead to a net suppression of capture. Consequently, the neutrino signals from WIMP annihilation in the Sun and Earth are expected to be significantly stronger than previously thought, revitalizing the prospects for indirect dark‑matter detection with neutrino telescopes.