Torsion pendulum searches for macroscopic spin-interactions as a window on new physics

The interactions of intrinsic particle spin offer a powerful window into important aspects of new physics, such as hidden high energy symmetries and CP-violation. This dissertation describes a new spin 20-pole torsion pendulum made with alternating high and low spin-density magnets. We operated the pendulum 4.1 mm from a spin source and 2.0 mm from a mass source, reaching the thermal noise level of the torsion fiber in all of our experiments. This allowed us to probe electron spin dipole-dipole and monopole-dipole interactions to greater sensitivities and over shorter distances than any previously test. We found no evidence of new interactions. Our monopole-dipole experiments set new constraints for exchange boson masses of ~1- 500 mueV. Our dipole-dipole experiments set new constraints for all interaction lengths, down to 10 -15 of electromagnetic strength at infinite range.