The Sun Can Strongly Constrain Spin-Dependent Dark Matter Nucleon Scattering Below the Evaporation Limit

TL;DR

This work demonstrates that Solar observations remain a potent probe of spin-dependent DM–nucleon scattering well below the conventional evaporation limit. By self-consistently modeling capture, evaporation, and annihilation in the Sun, and by computing indirect signals from neutrinos and long-lived mediator decays, the authors derive world-leading constraints on the SD cross section for $m_\chi$ in the $2{-}4$ GeV range, surpassing direct-detection limits by up to several orders of magnitude. They show that non-equilibrium effects are crucial below $\sim 3$ GeV and that evaporation can suppress signals, yet gamma-ray channels from mediators and neutrino channels provide robust, complementary probes with current data and future Hyper-K projections. The results underscore the Sun’s unique role in dark matter phenomenology and motivate continued multi-messenger searches targeting low-mass DM in astrophysical environments.

Abstract

The Sun is a promising target for dark matter searches due to its ability to accumulate dark matter particles via scattering and catalyze their self-annihilation. However, at low dark matter masses, dark matter particles can also "evaporate" due to subsequent collisions with the hot thermal plasma of the Sun. While several modeling studies have calculated the competitive dynamics of dark matter evaporation and annihilation, observational studies have typically assumed a fixed 4 GeV "evaporation limit", below which dark matter evaporates before it can annihilate. In this paper, we carefully consider the competitive effects of dark matter evaporation and annihilation on the resulting spin-dependent dark matter nucleon cross-section limits, finding that Solar observations can continue to exceed terrestrial constraints by between 1-5 orders of magnitude for dark matter masses between 2-4 GeV for dark matter annihilation to both neutrinos and long-lived mediators.

Figures (4)

• Figure 1: Spin-dependent cross section constraints (solid) and projections (dashed) for the Solar annihilation of DM particles with masses below 4 GeV to both neutrino and $\gamma$-ray final states. For the $\nu\bar{\nu}$ channel: Super-K constraints and Hyper-K projections are in blue, while the previous Hyper-K projection for Jupiter observations is in purple Robles:2024tdh. For $\gamma$-rays from the decay of long-lived mediators: the Fermi-LAT constraint is in teal, while previous studies of Jupiter are shown in orange Leane:2021tjj. The combined limits from direct detection are in gray PICO:2019vscPICASSO:2012ngjCRESST:2022dtlNEWS-G:2024jms, while the neutrino fog is in cyan OHare:2021utq.
• Figure 2: (Left) Total DM annihilation rates (solid) for spin-dependent cross sections of $10^{-37}$ cm$^{2}$ (orange), $10^{-40}$ cm$^{2}$ (teal), and $10^{-44}$ cm$^{2}$ (light blue). Dashed lines show the equilibrium approximation. (Right) Spin-dependent cross section regions where the true annihilation rate is reduced by no more than 10% (violet), 90% (blue) or 99% (green) from half the DM-capture rate, which corresponds to the assumed annihilation rate under the equilibrium assumption.
• Figure 3: DM annihilation signals from the Sun for a DM mass $m_{\chi}=2.5$ GeV using neutrino observations ( Left), which include a diffuse atmospheric muon neutrino background (purple) Super-Kamiokande:2015qek, with angular cuts corresponding to Super-K's angular resolution (magenta) Nguyen:2025ygc. The neutrino fluxes from DM annihilation to $\nu\bar{\nu}$ (light blue) and $\tau^{+}\tau^{-}$ (lime) are shown for a scattering cross-section of $\sigma^{\rm SD}_{\chi p}=10^{-36}$ cm$^{2}$. ( Right:) Results for $\gamma$-ray observations, with the Fermi-LAT solar disk $\gamma$-ray flux shown in orange. The $\gamma$-ray flux from DM annihilation to long-lived mediators $\phi\phi$ are shown for mediator decays into $\gamma\gamma$ (teal), $e^{+}e^{-}$ (violet) and $\mu^{+}\mu^{-}$ (dark blue) for a scattering cross-section of $\sigma^{\rm SD}_{\chi p}=10^{-43}$ cm$^{2}$. Results for $m_{\phi}=1$ GeV (solid lines) and for $m_{\phi}=300$ MeV (dashed lines) demonstrate the advantages of $\gamma$-ray observations for DM signals below the evaporation limit.
• Figure 4: Similar to Fig. \ref{['fig:constraint']}, but for leptonic final states. Results using neutrinos observations of DM annihilation to the $\tau^{+}\tau^{-}$ channel are in lime. Fermi-LAT constraints for long-lived mediator decay to $e^{+}e^{-}$ are in navy blue, and $\mu^{+}\mu^{-}$ are in dark violet. Previous results for long-lived mediator decay to $e^{+}e^{-}$ outside Jupiter from Galileo Probe P2 observations are in brown Li:2022wixYan:2023kdgSingh:2024nou.