A torsion-balance search for ultra low-mass bosonic dark matter

Abstract: We used a stationary torsion balance with a beryllium-aluminum composition dipole to search for ultra low-mass bosonic dark matter coupled to baryon minus lepton number. We set 95% confidence limits on the coupling constant $g_{\rm B-L}$ for bosons with masses between $10^{-18}$ and $10^{-16}$ eV/$c^2$ with the best performance at $m_{\rm DM} = 8\times 10^{-18}$ eV/$c^2$ constraining $g_{B-L}(\hbar c)^{-1/2} < 1 \times 10^{-25}$. This provides a complimentary limit to equivalence-principle experiments that search for ultra low-mass bosons as force-mediating particles.

• The paper introduces a novel torsion-balance experiment to detect ultra low-mass bosonic dark matter coupled to the baryon minus lepton charge (B-L).
• It analyzes 91.4 days of angle measurements over 491,019 mass values, setting a 95% confidence limit on the coupling constant g₍B-L₎ at specific dark matter masses.
• Results provide competitive linear constraints compared to equivalence-principle tests and suggest paths for enhancing sensitivity by an order of magnitude in future experiments.

A Torsion-Balance Search for Ultra Low-Mass Bosonic Dark Matter

This paper presents a detailed search for ultra low-mass bosonic dark matter (ULMB) using a stationary torsion-balance experiment. The focus is on detecting ULMBs that are coupled to the baryon minus lepton number (B-L), a conserved quantity in many unified theories. The study explores dark matter candidates with masses between $10^{-18}$ and $10^{-16}$ eV/$c^2$, where the dark matter behaves as a classical field due to the high number density.

Experimental Approach

The researchers constructed a torsion balance using a pendulum with a beryllium-aluminum composition dipole, exploiting the differing B-L charges to create a charge dipole. The setup aims to detect the differential accelerations induced by the ULMB interactions as a mechanical torque on the pendulum. The apparatus includes a fused-silica fiber torsion oscillator within a vacuum and a mu-metal shield, measured by a high-sensitivity autocollimator.

Data Analysis

The data set comprises 91.4 days of angle measurements, with an extensive mass range search from $0.4 \times 10^{-18}$ to $206.8 \times 10^{-18}$ eV/$c^2$, exploring 491,019 possible masses. The analysis involves fitting signals to multiple days of measurements, discounting data during damping or glitch events. Instrumental basis functions, alongside geocentric vectors for signal extraction, were employed to search for coherent vector boson interactions potentially detectable via torque on the dipole.

Results

The ensuing results place stringent 95% confidence limits on the coupling constant $g_{\rm B-L}$. Notably, at $m_{\rm DM} = 8 \times 10^{-18}$ eV/$c^2$, they constrain $g_{\rm B-L}(\hbar c)^{-1/2} < 1 \times 10^{-25}$. This constraint sets a competitive benchmark against equivalence-principle experiments, which typically observe signals proportional to the squared coupling. Observationally, traditional EP tests require significantly higher sensitivities to match the linear constraints offered here.

Implications and Future Directions

This research introduces an experiment-sensitive approach toward detecting ULMB, broadening the theoretical understanding of dark matter and its interaction characteristics. The findings have implications for models predicting forces mediated by ultra low-mass bosons and suggest the potential for significant advancements in the field of dark matter detection. Future developments may involve optimizing sensitivity through enhancements in pendulum design, autocollimator readings, and seismic isolation, potentially improving constraints on ULMB candidates by an order of magnitude.

The study demonstrates a methodological advance in searching for ULMB dark matter candidates, enriching the theoretical tapestry and prompting further experimental innovations in this continuously evolving field.