Searches for Exotic Spin-Dependent Forces using High-Frequency Mechanical Oscillators

Indiana University, Bloomington IN

Investigators

Abstract

This award supports an experimental search for fundamental forces of nature beyond gravity and electromagnetism at sub-millimeter length scales. Forces could depend on several properties of the test objects involved, such as mass (like gravity), or charge (like electromagnetism). The proposed experiments concentrate on forces that depend on spin, which can be thought of as the rotation of an object around its axis. Most subatomic particles (including the protons, neutrons, and electrons of ordinary matter) have non-zero spin, which determines their magnetic properties: magnetic forces depend on spin. The discovery of an additional macroscopic force that depends on spin would have enormous implications for physics from subatomic to cosmological scales. A variety of extensions to the Standard Model (the most successful theory of particle physics to date, but widely believed to be incomplete) predict short-range spin-dependent forces. These forces are mediated by particles that could contribute to "dark matter," the otherwise unknown quantity hypothesized to make up about one quarter of the mass of the universe and to explain the apparent mass distributions of galaxies. During its execution, this fundamental project will also provide all participants (including undergraduates who have contributed to this proposal) with experience in mechanical design, vacuum technology, low-noise electronics, semiconductor and magnetic materials processing, and other practical techniques useful in a wide range of science and engineering fields. The experiment will also be a key focus of the Indiana University Center for Spacetime Symmetries (IUCSS), which was founded by the PI and his colleagues to strengthen and publicize IU's growing expertise in the investigation of the structure of spacetime. The experiment uses 1-kilohertz planar oscillators as test masses with a thin shield between them to suppress backgrounds, a technique that has demonstrated the capability to probe micron-scale distances using relatively large (square-centimeter) masses, and to operate at the limit of instrumental thermal noise at room temperature. The principal challenge will be to overcome the anticipated magnetic backgrounds associated with the use of polarized test masses. To this end we have developed spin-polarized test masses with very low intrinsic magnetism. We will use them in our experiments to conduct initial searches for several velocity-independent spin-coupled forces with unprecedented sensitivity below one millimeter, and to search for several more velocity-dependent interactions that are presently unconstrained by experiment. We will also investigate polarized materials that have the potential to improve our experimental sensitivity by several additional orders of magnitude. At the completion of the award period, it is our expectation that, in the absence of a discovery, we will have set limits on as many as eleven previously unconstrained spin-coupled interactions of electrons in the sub-millimeter range, and up to four more limits with sensitivity at least order of magnitude greater than present constraints.