New Limits on Light Dark Matter-Nucleon Scattering

TL;DR

This work addresses the challenge of constraining sub-GeV dark matter that interacts hadronically with the Standard Model. By building a low-energy effective theory and employing heavy baryon and chiral perturbation theory, the authors compute the DM–nucleon cross section and DM–photon scattering rates, deriving strong bounds from Big Bang Nucleosynthesis and from rare Kaon decays, as well as an irreducible freeze-in abundance. They show that DM in equilibrium with photons during BBN is excluded for m_χ ≲ 0.5 MeV, and that meson decay bounds can surpass BBN in the quark-coupled case; an irreducible freeze-in abundance often imposes even tighter limits, sometimes dominating over structure-formation and direct-detection constraints. The results imply that future direct-detection efforts targeting MeV-scale DM must reach cross sections as low as ~10^−35 cm^2, guiding experimental strategies and UV-model development for hadronically interacting DM.

Abstract

We derive new bounds on hadronically-interacting, sub-GeV mass dark matter. First, we show that one-loop interactions with photons can be sufficient to maintain equilibrium between the dark matter and Standard Model sectors at MeV temperatures, resulting in constraints from Big Bang Nucleosynthesis. Using chiral perturbation theory, we find that this leads to an upper bound on the dark-matter--nucleon scattering cross-section that is orders of magnitude stronger than existing astrophysical constraints. Furthermore, we show that even if these interactions remain out of equilibrium, there is an irreducible freeze-in abundance of dark matter that can easily overclose the universe. We also compute new bounds from rare Kaon decays that can provide even stronger constraints. Our results have significant implications for future direct detection experiments aiming to search for MeV-scale dark matter.

Figures (4)

• Figure 1: Feynman diagrams contributing to $\langle \gamma\gamma | \sum_q m_q \bar{q} q | 0 \rangle$ at leading order. There is an additional diagram obtained by interchanging the photons in the first diagram.
• Figure 2: Constraints on the dark-matter--nucleon cross-section. The red shaded regions are excluded by BBN, requiring that the dark matter is out of equilibrium with the SM at $T=10$ MeV. The orange region is excluded by $K^+\to\pi^+\bar{\chi}\chi$. The grey shaded regions are excluded by dark matter overabundance, with the dashed grey lines denoting where the low-temperature freeze-in process $\gamma\gamma\to\bar{\chi}\chi$ saturates the observed relic abundance, assuming instantaneous reheating with $T_\text{RH}=10\,$MeV. Along the grey dot-dashed lines the observed relic abundance is obtained via freeze-out. The blue and green regions are excluded by existing bounds from structure formation Rogers:2021bylBuen-Abad:2021mvc and direct detection experiments SENSEI:2023zdfPandaX:2023xgl, respectively.
• Figure 3: Feynman diagrams contributing to $\langle \gamma\gamma | \alpha_s GG | 0 \rangle$ at leading order, where the loops contain $\pi^\pm$ or $K^\pm$. There are two additional diagrams obtained by interchanging the photons in the diagrams of the first column.
• Figure 4: Feynman diagrams contributing to $K^+ \to \pi^+ \bar{\chi} \chi$ at leading order in the low-energy effective theory.