Search for an Electric Dipole Moment of 199Hg

University Of Washington, Seattle WA

Investigators

Abstract

Compelling scientific evidence points to the conclusion that the universe evolved from a "Big Bang" some 13 billion years ago. A major unanswered question associated with the Big Bang hypothesis is why there is more matter in the universe than anti-matter; they should have been created in equal quantities. The likely answer to this question is that there is an unknown force in nature, extremely weak, that favors matter over anti-matter. It is straightforward to show that if this new force exists, it will endow elementary particles and atoms with a property known as an "electric dipole moment", a small separation of electric charge along the spin axis of the particle or atom. A measurement of the electric dipole moment of 199Hg (the spin ½ isotope of mercury atoms) will provide unambiguous evidence for this new force that breaks the symmetry between matter and anti-matter. The project serves the national interest by promoting the progress of science. The study of the particles that constitute the universe and the laws of nature that govern interactions between the particles serve to advance our understanding of the physical world and stimulate the development of technologies that have arisen from that understanding. Previous experiments have not been sensitive enough to detect the electric dipole moment of atoms or particles. The 199Hg project will constitute the most sensitive electric dipole moment search ever conducted. As such, it will provide excellent training for students in state-of-the-art experimental atomic physics techniques and high precision measurement techniques. The project will advance the field of nuclear spin magnetometry, which has broad application to measurements of magnetic fields in general. Society will benefit from the training of a new generation of scientists and from a more complete understanding of the universe we live in. High precision table-top experiments, such as this 199Hg project, provide a complementary path to particle accelerators for probing the underlying symmetries and forces of nature. An electric dipole moment of an atom can only exist if time-reversal (T) symmetry is violated, and T symmetry violation is directly related to the matter/anti-matter asymmetry (through the CPT theorem). One measures an electric dipole moment by observing a change in the atomic Larmor (spin precession) frequency of the 199Hg atoms as an external electric field is applied to vapor cells that contain the atoms. This experiment will use a stack of four vapor cells in a common magnetic field. Oppositely directed electric fields will be applied to the two innermost vapor cells while the outer two cells, with no electric fields, will serve as co-magnetometers. The atoms will be spin polarized using resonant 254 nm laser light and the Larmor frequencies will be read-out by the optical rotation of the polarization of non-resonant laser light that traverses the vapor cells. A previous experiment, limited by gradients in the magnetic field, achieved an upper limit on the electric dipole moment of 199Hg of less than 3 x 10-30 e-cm. this new experiment will improve the magnetic field uniformity to push the experimental sensitivity into the 10-31 e-cm range.