Nuclear Physics Group

Radon Electric Dipole Moment

Figure 6 : At left a particle with intrinsic spin angular momentum in the upward direction (suggested by the circumferential arrow of "rotation") is shown with both a magnetic dipole moment (mu) and an electric dipole moment (d). The relationship between the common magnetic dipole moment mu and the intrinsic spin remains unchanged under both the parity (P) and time-reversal (T) operations. The relationship between an EDM and the intrinsic spin, however, changes sign under both P and T reversal. A particle with a non-zero EDM thus transforms into a distinguishable particle under the P and T operations, and is said to violate these symmetries. While parity violation (due to the weak nuclear interaction) is well known in the current laws of physics, time-reversal violation is not and a non-zero particle EDM has not yet been discovered.

As illustrated in Fig. 6, the orientation of an intrinsic electric dipole moment (EDM) of a particle, relative to its intrinsic angular momentum or spin, changes sign under both the parity (P) and time-reversal (T) operations. A non-zero permanent EDM for an elementary particle or atom can thus only arise from the polarization of the system by parity and time-reversal violating fundamental interactions. No such particle EDM has ever been measured, despite 6 decades of searches with ever increasing experimental sensitivity.

The global effort to perform more and more precise searches for particle EDM's is motivated by our desire to understand the fundamental symmetries of the laws of physics and the most basic origins of matter in our universe. All of the experimental evidence available to date supports the theoretically motivated expectation that all of the physical laws remain unchanged under the combined operation of CPT-reversal, where CP-reversal exchanges particles with their anti-particle partners. Assuming the validity of this CPT symmetry, the observation of a time-reversal (T) violating electric dipole moment would imply the existence of new fundamental CP-violating fundamental interactions in nature.

Additional sources of CP-violation are required to account for the observed imbalance between matter and anti-matter in our universe, as the CP-violation within the currently known Standard Model of particle physics is far too weak to do so. Many of the proposed models of physics beyond the Standard Model, such as supersymmetry (SUSY), theories with multiple Higgs bosons, and left-right symmetric models, naturally include such additional sources of CP-violation that could account for the cosmic matter/antimatter asymmetry. These CP-violating interactions also generically lead to predictions for particle electric dipole moments at, or tantalizingly close to, the current level of experimental sensitivity. EDM measurements thus provide demanding tests of physics beyond the Standard Model, and important constraints on the models of new physics required to explain the matter/antimatter asymmetry of the universe.

In collaboration with researchers from the University of Michigan, Simon Fraser University, and TRIUMF, we are currently exploring a program to search for electric dipole moments in odd-mass isotopes of the radioactive noble gas radon (Rn). A particular deformation of the nucleus known as octupole deformation, is predicted to enhance the sensitivity of these isotopes to an underlying CP-odd interaction by 2-3 orders of magnitude. Large quantities of the neutron-rich Rn isotopes of interest, 221,223Rn, will become available from new actinide production targets at the ISAC facility, making these Rn isotopes a potentially attractive systems for a high-precision EDM search program.

Our group at the University of Guelph leads the nuclear structure measurements for these odd-A Rn isotopes with the GRIFFIN Spectrometer at ISAC-I that will establish their octupole deformation properties and candidacy for a high-precision EDM search program at ISAC.