Developments in the Search for Non-newtonian Gravity Below the 25 Micron Length Scale PDF Download

Author: Andrew A. Geraci
Publisher:
ISBN: 9780549060581
Category :
Languages : en
Pages : 150

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Book Description
Several recent theories suggest that new physics related to gravity may appear at short length scales. For example, light moduli from string theory or exotic particles in "large" extra dimensions could mediate macroscopic forces of (super) gravitational strength at length scales below a millimeter. Such new forces can be parameterized as a Yukawa-type correction to the Newtonian potential of strength relative to gravity alpha and range lambda. With this motivation, we have built a cryogenic apparatus utilizing micro-cantilevers capable of measuring atto-Newton forces, which now includes a magnetic force calibration. The cantilever is loaded with a rectangular gold prism fabricated by focused-ion-beam milling that serves as a test mass for the experiment. The driving (source) mass is actuated horizontally beneath the cantilever at a vertical separation of order 25 microns. The force between the masses is deduced from the displacement of the cantilever as measured by a fiber-coupled laser interferometer. We perform the measurement at the cantilever resonant frequency, typically of order 300 Hz, while the mechanical driving motion occurs at a sub-harmonic, typically one-third. This is achieved by implementing a density modulation in the drive mass, consisting of alternating gold and silicon sections. For the new magnetic calibration, Co/Pt multi-layer films are deposited on the test mass. The permanent magnetic moment couples to an induced magnetic field gradient as current flows across the meandering gold sections in the drive mass device. The current is turned off for the Yukawa-force search. A mu-metal shield encloses the cryostat to prevent the Earth's field from magnetizing the drive mass. The amplitude and phase of a magnetic or Yukawa signal will change in a predictable way as we vary the equilibrium position of the drive mass oscillation. We utilize this scanning technique as an additional handle to distinguish a signal from background forces. Our most recent experimental constraints on Yukawa-type deviations from Newtonian gravity are more than three times as stringent as our previously published results, and represent the best bound in the range of 5--15 microns, with a 95-percent confidence exclusion of forces with alpha > 14,000 at lambda of 10 microns.