NSF Award
1806297
Precision measurements of elementary particles' spins can provide new insight into the fundamental laws of nature. Elementary particles have an intrinsic property called spin which makes them act as if they are constantly rotating like mechanical tops. Just as 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 fidelity. This project will use these precision techniques as tools to investigate the fundamental forces and symmetries of nature. At the most basic level, physicists' present understanding of nature is summarized by 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 all of the aligned 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 is increased by using a beam of very cold 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. <br/><br/>In 2013, 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 the 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 LRSSI measurements. 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, the researchers will continue their investigation of critical parameters that will ultimately determine the sensitivity of the thallium fluoride (TlF) electric-dipole moment (edm) experiment that is presently being constructed at Yale by the CeNTREX collaboration. Specifically, the researchers hope to continue to improve their measurements of optical cycling in TlF and to demonstrate that this cycling can be used to exert optical forces on TlF. These optical forces will be used to transversely cool a cryogenic molecular beam of TlF. This transverse cooling should increase the sensitivity of the TlF edm experiment by about an order of magnitude. With this additional sensitivity it is possible that a permanent nuclear edm will be discovered. If this edm is found, it would imply a violation of time symmetry and could help explain the existence of our matter-dominated universe.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
