High-Frequency Search for New Sub-Millimeter Range Forces

Indiana University, Bloomington IN

Investigators

Abstract

This award supports an experimental search for spin-dependent forces below 1 millimeter. Present experimental limits allow for undiscovered forces in nature several million times stronger than gravity acting over distances resolvable by the unaided eye. Theoretical models developed over the past few decades that attempt to explain why interactions involving the strong nuclear force always conserve certain spacetime symmetries make specific predictions of spin-dependent forces in the sub-millimeter range. The proposed experiment thus represents an excellent opportunity for discoveries in this range. 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. Spin-polarized materials with high spin density but low intrinsic magnetism, including ferrimagnets that exhibit orbital compensation of the magnetism associated with the aligned electron spins, will be investigated as test masses. With modest aligned-spin densities but good control of magnetic backgrounds, the projected sensitivity of the experiment is several orders of magnitude greater than the current best limits at ranges below 1 millimeter. Spin is a fundamental property of elementary particles and a crucial aspect of quantum physics. The discovery of a 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 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 the "dark matter" hypothesized to pervade all of space in order to explain the apparent mass distributions of galaxies. The mysterious "dark energy" postulated to explain the observed expansion of the universe also seems to point to a length scale on the order of tens of microns as a special range at which previously undetected phenomena might appear. During its execution, this fundamental project will also provide all participants (including several 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. Success of the proposed experiment will support the IUCSS goal to establish a long-term program of coordinated experimental and theoretical investigation of new physics at sub-millimeter length scales.