Spin-Dependent Macroscopic Force Search Using 1 kHz Mechanical Oscillators

Physics beyond the Standard Model arises in many attempts to unify General Relativity and quantum mechanics. Phenomena include macroscopic forces of nature beyond gravity and electromagnetism. A review describes sixteen possible potentials for such forces in the limit of non-relativistic single boson exchange, fifteen of which depend on the intrinsic spin of the interacting fermions. We describe an experimental search with sensitivity to all sixteen potentials in the sub-millimeter range, using planar resonant test masses with operational frequencies near 1 kHz. The test masses have been augmented with a rare-earth iron garnet, a polarized material which exhibits orbital compensation of the magnetism associated with the electron spins, substantially reducing the magnetic backgrounds. We review the progress of the apparatus developed to make optimal use of the garnet, including cooling and calibration systems, and discuss the experimental sensitivity.