The project aims at table-top experimental investigation of fundamental symmetries of nature along three thrusts: (1) measuring atomic parity violation (APV) in ytterbium, (2) searching for spatio-temporal variation of the fine-structure "constant" alpha and APV in dysprosium, and (3) conducting discrete symmetry tests in barium. The three apparatus are well-positioned to perform APV measurements across a chain of isotopes (Ytterbium, Dysprosium, Barium), yielding information about how the neutron radius varies, with ramifications to supernovas, neutron stars, and the creation of heavy elements. These experiments provide a unique window into weak interactions within atomic nuclei.
Our present understanding of the laws of Nature is largely based on the concepts of symmetry and invariance. For example, we generally assume that fundamental laws of physics do not change with time. Many symmetries of nature, however, are broken, for example, the symmetry between the actual world and its mirror image (the so-called left-right asymmetry a.k.a. parity violation). While experiments with high-energy colliders and astronomical observations provide powerful laboratories for the study of fundamental symmetries, small-scale table-top experiments such as the ones in this project can be competitive with other techniques in certain areas. Moreover, they are also capable of providing unique scientific information that is difficult to obtain by other means. Such small-scale experiments are ideal for training the new generation of hands-on scientists.
The project was devoted to the study of fundamental symmetries of Nature using modern spectroscopic techniques with atoms with relatively complex spectra, specifically, the rare earth atoms and xenon. One atom of our experimental focus has been dysprosium (Dy), where we performed a sensitive search for a possible temporal variation of the fundamental fine-structure constant (alpha). We performed the spectroscopy of radio-frequency transitions between nearly degenerate, opposite-parity excited states in dysprosium. Theoretical calculations predict that these states are very sensitive to variation of the fine-structure constant owing to large relativistic corrections of opposite sign for the opposite-parity levels. The near degeneracy reduces the relative precision necessary to place constraints on variation of alpha, competitive with results obtained from the best atomic clocks in the world. Additionally, the existence of several abundant isotopes of Dy allows isotopic comparisons that suppress common-mode systematic errors. We measure a rate of alpha variation consistent with zero. The same data were used to constrain a dimensionless parameter characterizing a possible coupling of alpha to a changing gravitational potential. Using similar techniques, we also performed a joint test of local Lorentz invariance and the Einstein equivalence principle for electrons, using long-term measurements of the transition frequency between the two nearly degenerate states of dysprosium. The experiment constrained Lorentz violation for electrons, matching or improving the best laboratory and astrophysical limits by up to a factor of 10, and improved bounds on gravitational redshift anomalies for electrons by two orders of magnitude. It is crucial for precision measurements of this kind to be able to control possible systematic effects. As part of this project, we measured the differential polarizability between the nearly degenerate, opposite parity states in dysprosium and the differential blackbody radiation induced Stark shift of these states. We concluded that ac Stark-effect related systematics would not limit a search for variation of the fine-structure constant, using dysprosium, down to the level of a few parts in 1017, for two measurements of the transition frequency one year apart. In a separate sub-project, we considered a possible parity-violation effect in atoms due to an effect not considered previously. We proposed methods for extracting limits on the strength of P-odd interactions of pseudoscalar and pseudovector cosmic fields with electrons, protons and neutrons. Candidates for such fields are dark matter (including axions) and dark energy, as well as several more exotic sources described by standard-model extensions. Calculations (done by our Australian collaborators) of parity nonconserving amplitudes and atomic electric dipole moments induced by these fields were performed for H, Li, Na, K, Rb, Cs, Ba+, Tl, Dy, Fr, and Ra+. From these calculations and existing measurements in Dy, Cs and Tl, we constrained the interaction strengths of the parity-violating static pseudovector cosmic field with an electron, and with a proton. Finally, with our collaborators from Greece, we identified a pair of near-degenerate states of opposite parity in atomic xenon (Xe), for which parity- and time-odd effects are expected to be enhanced by the small energy separation. The Stark effect of several Xe states was measured, and new atomic states in Xe were identified, which have not been observed before. The project has highlighted the utility of complex atoms for fundamental-physics research, complementing the work done at large accelerator facilities.
