Search for Millicharged Particles Using Optically Levitated Microspheres

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We report results from a search for stable particles with charge > $10^{-5}$ e in bulk matter using levitated dielectric microspheres in high vacuum.

• No evidence for such particles was found in a total sample of 1.4 ng, providing an upper limit on the abundance per nucleon of 2.5 x $10^{-14}$ at the 95% confidence level for the material tested.
• These results provide the first direct search for single particles with charge < 0.1 e bound in macroscopic quantities of matter and demonstrate the ability to perform sensitive force measurements using optically levitated microspheres in vacuum.