BSM-PM: Measuring Gravity at the Micron-Scale with Optically Levitated Particles: a Renewal Proposal

Gravity is the least well understood force in Nature. Its weakness when compared to the other three Standard Model forces makes gravity particularly challenging to measure precisely in laboratory experiments. There have been several predictions from theories beyond the Standard Model of particle physics, including string theory and supersymmetry, that the Newtonian gravitational inverse square law will break down at some distance below the millimeter scale. In this project, a novel method to test these theories is being developed, which involves using the radiation pressure from a laser beam to suspend and laser cool the motion of a tiny glass bead that serves as a gravitational test mass. When surrounded by a high-vacuum environment the glass bead experiences very little friction and becomes an ultra-sensitive force detector. By introducing a microfabricated silicon and gold source mass near the bead the resulting displacement of the bead can be observed to measure the gravitational attraction between the bead, i.e. test mass, and the source mass. The method is predicted to improve the search for corrections to the gravitational inverse square law at the micron length scale by more than three orders of magnitude. Two graduate students will be broadly trained in experimental physics and nanofabrication. By participating in this highly interdisciplinary research project, students will be well equipped for scientific careers. The fundamental nature of this project can instill a sense of wonder about the natural world in the general public. The nation will benefit from an improved understanding of the high-energy physics related to gravitational physics at the micron length scale, at a fraction of the cost of particle-collider experiments. In this project, an experiment will continue to be developed that makes use of laser-cooled trapped nanospheres to test for Yukawa-type deviations from Newtonian gravity at the micron length scale. Previous results include achieving calibrated zeptonewton-level force sensitivity with optically trapped nanospheres, developing novel techniques to maneuver nanospheres within few-micron distances from a source mass surface, demonstrating interferometric methods for metrology of the distance between the test and source masses, and characterization of several background vibrational and electromagnetic noise sources. This next phase of the project involves (1) a continued investigation of noise sources and systematic errors in preliminary gravity measurements, with a goal of acquiring 10^6 seconds of integrated data in a dedicated Yukawa-force search at the 1-2 micron length scale, and (2) in-parallel development of novel methods for trapping and cooling the levitated nanoparticles, including sympathetic cooling with cold atoms. These new techniques can advance the understanding of gravity at the micron length scale by over three orders of magnitude and may lead to ground-breaking discoveries. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.