RUI: A Search for Long-Range Spin-Spin Interactions and Thallium-Fluoride Investigations

Amherst College, Amherst MA

Investigators

Abstract

Elementary particles have an intrinsic property called spin--they act as if they were constantly spinning around like tops. Just as a tops precess in the presence of gravity, the spins of fundamental particles precess in a magnetic field. This precession is the basis of nuclear magnetic resonance which is the underlying physics used in the medical diagnostic known as magnetic resonance imaging (MRI). Recently developed precision optical techniques have allowed the study of interactions with particle spins with unprecedented precision. The researchers will use these precision techniques as tools to investigate the fundamental forces and symmetries of nature. At the most basic level, our present understanding of nature is summarized in the "Standard Model' of particle physics. This model requires four fundamental forces (gravitational, electromagnetic, strong and weak) to describe all of reality as it is presently known. In one experiment, the investigators will look for a new long-range force between particle spins that can't be described by the Standard Model. To optimize their search, they will measure the interaction of their laboratory spins with the sum of all of the electron spins within the Earth. In their other experiment, the researchers hope eventually to see if the fundamental laws of nature might be asymmetric in time. This breaking of "time symmetry" can be studied by looking for the precession of a nuclear spin in an electric field. Here the experimental sensitivity can be increased by using a very cold beam of molecules. Additional time asymmetry (beyond that which has already been observed) is believed to be necessary to explain the existence of our universe. Without time-reversal violation, our universe would have produced equal amounts of matter and anti-matter. Their mutual annihilation would not have allowed for the formation of galaxies, stars, planets and life. Recently, the researchers created the first map of the electron-spin density within the Earth. These "geo-electrons" constitute the largest polarized spin source known. Precision measurement of spin-precession frequencies in laboratories at the surface of the Earth as a function of their applied magnetic-field direction, allows one to look for long-range spin-spin interactions (LRSSI) between the geo-electrons and the laboratory spins. In the first proposed experiment, a refined spin-precession apparatus will be constructed which is both well calibrated and relatively immune to AC light effects. This should allow at least an order of magnitude improvement in the sensitivity of these measurements to LRSSI. If an effect is seen it would suggest the existence of a new force of nature. In current models this force might be associated with an ultra-light vector meson, a "dark" photon, the "unparticle," or torsion gravity. In the second proposed experiment, critical parameters that determine the viability of an electric-dipole moment (edm) experiment in thallium fluoride (TlF) will be investigated using an ultraviolet laser and a cold molecular source. Specifically, the researchers hope to demonstrate the existence of a cycling transition in TlF and to measure the efficiency with which TlF can be ablated from a surface. If the results of these measurements are favorable, TlF will then be proposed as a candidate system for a cold-beam precision measurement of the edm of the thallium nucleus. The discovery of a permanent nuclear edm would imply a violation of time symmetry and could help explain the existence of our matter-dominated universe.