PM: RUI: Searching for Optical Cycling in TlF and Long-Range Spin-Spin Interactions

Precision measurements of the spin interactions of elementary particles can provide new insights into the fundamental laws of nature. Elementary particles have an intrinsic property called spin – they act as if they are constantly spinning around like 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. The researchers will use these 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 cannot 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 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. The pursuit of these experiments will provide an enticing introduction to STEM for many undergraduate students and provide continue experiences for the next generation of STEM researchers.

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 is under construction 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) nuclear electric-dipole moment (nEDM) experiment that is presently being constructed at Argon National Lab 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 transverse cool a cryogenic molecular beam of TlF. If successful, transverse cooling could increase the sensitivity of the TlF nEDM experiment by about an order of magnitude.

With this additional sensitivity it is possible that a permanent nEDM will be found. Such a discovery would imply a violation of time symmetry and could help explain the existence of our matter-dominated universe.

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