Search for composite dark matter with optically levitated sensors (2007.12067v2)

Abstract: Results are reported from a search for a class of composite dark matter models with feeble, long-range interactions with normal matter. We search for impulses arising from passing dark matter particles by monitoring the mechanical motion of an optically levitated nanogram mass over the course of several days. Assuming such particles constitute the dominant component of dark matter, this search places upper limits on their interaction with neutrons of $\alpha_n \leq 1.2 \times 10^{-7}$ at 95\% confidence for dark matter masses between 1--10 TeV and mediator masses $m_\phi \leq 0.1$ eV. Due to the large enhancement of the cross-section for dark matter to coherently scatter from a nanogram mass ($\sim 10^{29}$ times that for a single neutron) and the ability to detect momentum transfers as small as $\sim$200 MeV/c, these results provide sensitivity to certain classes of composite dark matter models that substantially exceeds existing searches, including those employing kg-scale or ton-scale targets. Extensions of these techniques can enable directionally-sensitive searches for a broad class of previously inaccessible heavy dark matter candidates.

Search for Composite Dark Matter with Optically Levitated Sensors

The search for dark matter (DM) is a critical endeavor in physics, with various terrestrial experiments attempting to detect weak interactions with visible matter. The paper "Search for composite dark matter with optically levitated sensors" explores a novel method using optically levitated microspheres to probe long-range dark matter interactions. This approach marks an innovative use of macroscopic force sensors to detect potential DM-nugget interactions described by classical long-range forces mediated by a light force carrier, $\phi$.

Methodology

The paper employs nanogram-scale optomechanical sensors, specifically SiO$_2$ spheres optically levitated in a vacuum, to identify impulses that may result from passing DM particles. This experiment places upper limits on dark matter interactions using a theoretical composite model in which the dark matter consists of large "nuggets" that interact via a Yukawa potential. The premise is to detect tiny momentum transfers resulting from DM interactions through a sensor’s center-of-mass motion.

The experiment represents a significant technical achievement. By using feedback cooling, the spheres' motion is cooled to around 200 µK, providing the necessary sensitivity to measure nanometer displacements due to potential dark matter interactions. The paper reports successful calibration with known forces acting on the sensors and confirms an ability to resolve momentum transfers as small as 0.15 GeV.

The signals are sought by monitoring deviations in the mechanical motion of these suspended sensors, distinguishing between potential DM signals and noise. Moreover, the paper explores mediator masses ranging to 0.1 eV, with high sensitivity between dark matter masses of 1-10 TeV, taking advantage of the large coherent scattering enhancement factor provided by such techniques.

Results and Implications

The first reported limits on DM-neutron interactions place the interaction strength at $\alpha_n \leq 1.2 \times 10^{-7}$ for the considered range of masses and mediator parameters. Notably, the sensitivity of micromechanical sensors used far exceeds those of existing large-scale detectors, including those with kilogram- or ton-scale targets. The results suggest substantial sensitivity for detecting certain classes of composite DM models, significantly impacting search strategies by exploring a previously inaccessible parameter space.

Broad Implications

These outcomes have critical implications for future collider and non-collider dark matter search techniques. By demonstrating the potential of macroscopic scale sensors to exceed traditional quantum scale targets, the paper underscores a new method for dark matter detection that could complement existing techniques reliant on nuclear recoils and electromagnetic interactions.

Theoretical Implications

On a theoretical front, this work supports developing models of composite dark matter structure, enhancing our understanding of how such models could be probed experimentally. The results inform the development of more constrained models correlating dark matter interactions with known physical principles, like symmetry and conservation laws.

Future Developments

Given the effectiveness of this approach, future studies could expand upon directional sensitivity using multiple optically trapped particles, enhancing determination of the DM signal’s directionality. Further reduction in detection threshold and improvement in background rejection can significantly increase detection prominence of signal events, potentially leading to groundbreaking discoveries in the search for cosmic dark matter structures.

In conclusion, by leveraging advanced optomechanical systems to expand the parameter space accessible to dark matter searches, this research opens a new frontier for exploring the fundamental constituents of the universe.