Abstract: The Standard Model (SM) of particle physics, despite its astounding success, is known to be incomplete. It cannot explain, for example, the dominance of matter over anti-matter in the observable Universe. Attempts to explain this imbalance require an extra source of CP violation, and models incorporating this led to the existence of relatively large electric dipole moments (EDM). A measurement of a large EDM would represent a clear signal of the violation of the CP symmetries. 

The SM predicts an EDM far lower (10−30) than the experimental reach, making necessary the study of systems, such as atoms or molecules, that can amplify the signal. However, in these systems, the atomic electrons screen any direct observation of the dipole moment of the nucleus, but it can still be probed through the so-called Schiff moment. Theoretical models, which depend on the nuclear structure, are then needed to deduce an EDM from any observed Schiff moment. In order to improve the accuracy of such models, increased knowledge of the structure is needed. 

Measurements of 199Hg diamagnetic atoms currently hold the most stringent limit on an EDM. For the Schiff moment calculation, important nuclear structure information includes the strengths of E2 and E3 transitions that connect the ground state of 199Hg to its excited states. Performing measurements of E2 and E3 transition strengths in 199Hg are very challenging. As such, several experiments on 198Hg and 200Hg at the Maier-Leibnitz Laboratorium of the Ludwig-Maximilians Universitat Munchen were conducted. A deuteron beam bombarded the target of the compound of 198Hg32S, producing scattering particles that were separated and detected using the quadruple three-dipole (Q3D) magnetic spectrograph. Very high-statistics data sets were collected from this reaction. The cross-section angular distributions were used to provide information on the spin and parities and ultimately used to determine the excitation matrix elements. Combined with our results for 200Hg, we have important data on the adjacent even-even systems against which nuclear structure models can be compared. 

Bio: Sally Valbuena is a graduate student in experimental nuclear physics, supervised by Dr. Paul Garrett in the Department of Physics at the University of Guelph, Canada. Her research in nuclear structure explores properties of radionuclides through β-decay spectroscopy to reveal new shapes of nuclei, revolutionizing the considerations established in textbooks. Her current work is motivated by fundamental symmetry tests, intending to measure a fundamental constant of particle physics.