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Torsion Balance Experiments Enable Direct Detection of Sub-eV Dark Matter

Shigeki Matsumoto, Jie Sheng, Chuan-Yang Xing, Lin Zhu

Abstract

Light dark matter with sub-eV masses has a high number density in our galaxy, and its scattering cross section with macroscopic objects can be significantly enhanced by coherence effects. Repeated scattering with a target object can induce a measurable acceleration. Torsion balance experiments with geometric asymmetry are, in principle, capable of detecting such signals. Our analysis shows that existing torsion balances designed to test the Equivalence Principle already place the most stringent constraints on DM-nucleon scattering in the $(10^{-2}, 1)\,$eV mass range.

Torsion Balance Experiments Enable Direct Detection of Sub-eV Dark Matter

Abstract

Light dark matter with sub-eV masses has a high number density in our galaxy, and its scattering cross section with macroscopic objects can be significantly enhanced by coherence effects. Repeated scattering with a target object can induce a measurable acceleration. Torsion balance experiments with geometric asymmetry are, in principle, capable of detecting such signals. Our analysis shows that existing torsion balances designed to test the Equivalence Principle already place the most stringent constraints on DM-nucleon scattering in the eV mass range.

Paper Structure

This paper contains 6 sections, 19 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Dark Matter-Induced Acceleration
  3. EP-Testing Torsion Balance Exps
  4. Constraints
  5. Discussion and Conclusions
  6. Appendix A -- Screening Effects

Figures (2)

  • Figure 1: Form factors of the test bodies used in Braginskii et al. Braginskii:1971tn (left) and Eöt-Wash (2008 & 2012) Wagner:2012ui (right). In the former, the spherical radii are $R_\mathrm{Al}=0.35\,\mathrm{cm}$ and $R_\mathrm{Pt}=0.18\,\mathrm{cm}$; in the latter, the effective outer radius is $R=0.95\,\mathrm{cm}$.
  • Figure 2: Constraints on the DM--nucleon cross section from existing EP tests (colored solid lines). The dashed black curve shows the projected sensitivity of an asymmetric rotating torsion balance Luo:2024ocg. The gray shaded regions are excluded by supernova cooling and DM-induced background forces. The dotted curve indicates possible screening by a thick iron wall.