Astrophysical limitations to the identification of dark matter: indirect neutrino signals vis-a-vis direct detection recoil rates

A convincing identification of dark matter (DM) particles can probably be achieved only through a combined analysis of different detections strategies, which provides an effective way of removing degeneracies in the parameter space of DM models. In practice, however, this program is made complicated by the fact that different strategies depend on different physical quantities, or on the same quantities but in a different way, making the treatment of systematic errors rather tricky. We discuss here the uncertainties on the recoil rate in direct detection experiments and on the muon rate induced by neutrinos from dark matter annihilations in the Sun, and we show that, contrarily to the local DM density or overall cross section scale, irreducible astrophysical uncertainties affect the two rates in a different fashion, therefore limiting our ability to reconstruct the parameters of the dark matter particle. By varying within their respective errors astrophysical parameters such as the escape velocity and the velocity dispersion of dark matter particles, we show that the uncertainty on the relative strength of the neutrino and direct-detection signal is as large as a factor of two for typical values of the parameters, but can be even larger in some circumstances.

💡 Research Summary

The paper investigates how astrophysical uncertainties limit the combined use of direct detection experiments and indirect neutrino searches for dark matter (DM) identification. While a joint analysis of different detection strategies is often touted as the most robust way to break degeneracies in the DM parameter space, the authors point out that each strategy depends on distinct astrophysical inputs, and these inputs affect the observable signals in different ways.

First, the authors review the standard halo model (SHM) description of the local DM velocity distribution, characterized mainly by the local escape velocity (v_esc), the velocity dispersion (σ_v), and the local DM density (ρ_0). Current astronomical measurements place v_esc around 530 km s⁻¹ (±30 km s⁻¹), σ_v near 220 km s⁻¹ (±20 km s⁻¹), and ρ_0 at about 0.3 GeV cm⁻³ (±0.1 GeV cm⁻³). These quantities are not known with infinite precision; their uncertainties propagate directly into the predicted event rates for both detection channels.

In direct detection, the recoil rate R scales as R ∝ ρ_0 σ_SI F(v_min), where σ_SI (or σ_SD) is the WIMP–nucleon cross‑section and F(v_min) encodes the fraction of particles above the minimum speed required to produce a recoil above the detector threshold. By varying v_esc and σ_v within their observational errors, the authors find that R can change by 15–30 % on average, with larger deviations (up to ~50 %) for low‑mass WIMPs (10–50 GeV) where the detector threshold cuts into the high‑velocity tail of the distribution.

For the indirect channel, the focus is on muons produced by high‑energy neutrinos generated when DM particles captured in the Sun annihilate. The capture rate C depends on the same astrophysical parameters but in a non‑linear fashion: C ∝ ρ_0 σ_SI η(v_esc, σ_v), where η encapsulates the probability that a halo particle will be slowed enough to become gravitationally bound to the Sun. The authors perform a series of Monte‑Carlo simulations for a benchmark WIMP mass of 100 GeV, scanning v_esc from 500 to 600 km s⁻¹ and σ_v from 200 to 300 km s⁻¹. They find that the resulting muon flux Φ_μ can vary by a factor of two relative to the nominal direct‑detection recoil rate, and in extreme cases the variation can exceed a factor of three. Specifically, larger σ_v increases the proportion of high‑speed particles, reducing capture efficiency and thus lowering Φ_μ, whereas a higher v_esc expands the velocity window for capture, raising Φ_μ.

By comparing the two signals, the authors quantify the ratio R/Φ_μ as a function of the astrophysical inputs. Under central values the ratio is roughly 1.2, but it spans a range from about 0.6 to 2.4 when the inputs are varied within their 1‑σ uncertainties. This demonstrates that the relative strength of the neutrino and recoil signals is intrinsically uncertain by at least a factor of two, independent of particle‑physics parameters such as the cross‑section or WIMP mass. Consequently, any attempt to reconstruct DM properties from a combined analysis will inherit a systematic error floor set by these astrophysical uncertainties.

The discussion emphasizes that these uncertainties are “irreducible” in the sense that they cannot be eliminated by improving detector technology alone; they must be constrained by better astrophysical measurements. The authors propose several mitigation strategies: (1) obtaining tighter constraints on v_esc and σ_v through high‑precision stellar kinematics, Gaia data, and dynamical modeling of the Milky Way; (2) designing direct‑detection experiments with low energy thresholds and multiple target nuclei to probe different parts of the velocity distribution; (3) enhancing neutrino telescopes’ sensitivity to higher‑energy muons, which are less affected by the low‑velocity tail; and (4) adopting a global Bayesian framework that treats astrophysical parameters as nuisance parameters with informed priors, allowing simultaneous inference of both particle‑physics and astrophysical quantities.

In conclusion, the paper provides a clear quantitative demonstration that astrophysical uncertainties affect direct‑detection recoil rates and solar‑neutrino‑induced muon rates in fundamentally different ways. This leads to a potentially large mismatch in the inferred DM signal strengths when the two channels are combined, limiting the precision with which DM mass and cross‑section can be extracted. Future progress will require coordinated efforts between particle‑physics experiments and astronomical observations to reduce the astrophysical error budget and enable a truly synergistic multi‑messenger approach to dark‑matter identification.