A New Limit on Lorentz- and CPT-Violating Neutron Spin Interactions Using a Potassium-Helium Comagnetometer

Gravity and quantum mechanics are expected to unify at the Planck scale described by an exceedingly large energy of 1019 GeV. This regime is far from the reach of the highest energy colliders, but tests of fundamental symmetries provide an avenue to explore physics at this scale from the low energy world. Proposed theories of quantum gravity suggest possible spontaneous breaking of Lorentz and CPT symmetry that have so far been unobserved. This thesis presents a test of a Lorentz- and CPT-violating background field constant across the solar system coupling to the neutron spin. A comagnetometer with polarized atomic vapors K and 3He suppresses magnetic fields but remains sensitive to non-magnetic spin couplings. In a tabletop setup, the comagnetometer measures the equatorial components to be b&d15;nX = (0.1 +/- 1.6) x 10-33 GeV and b&d15;nY = (2.5 +/- 1.6) x 10-33 GeV, improving the previous neutron coupling limit by a factor of 30.;This work utilizes the CPT-II apparatus, a second generation comagnetometer installation which features a compact design and evacuated bell jar enclosing all optics for improved long term stability. A rotating platform provides frequent reversals of the apparatus orientation and represents a significant improvement in a Lorentz violation search. The comagnetometer is also a sensitive gyroscope, so reorientation with respect to Earth's rotation contributes significantly to the signal. Reversals along both the north-south and east-west directions provide a separation of systematic effects from maximal and minimal gyroscope backgrounds. Systematic background effects associated with reversals of the apparatus are also identified and removed. Several novel features of comagnetometer behavior are also explored including the response to an ac magnetic field and magnetic field gradients.;Furthermore, a compact, nuclear spin source containing 9 x 10 21 hyper-polarized 3He spins is designed and constructed. The spin source is used in conjunction with the first generation apparatus CPT-I for a test of proposed long-range spin-dependant forces on the scale of 50 cm. The spin source supports accurate real-time monitoring of the 3He polarization, an active magnetic field cancelation system, and efficient spin reversals with losses below 2.5 x 10-6 per flip. This experiment leads to a new limit on neutron coupling to light pseudoscalar and vector particles, including torsion, along with constraints on possible couplings in recently proposed models involving unparticles and spontaneous breaking of Lorentz symmetry. This measurement improves the previous limit by a factor of 500 and reaches an energy resolution of 10-34 GeV, the highest energy resolution of any atomic experiment.