What is Known About the Sensitivity of Pa-229 to Time-Reversal Violating Interactions?

Electric dipole moments (EDMs) are signatures of time-reversal (T), parity (P), & charge-parity (CP) violation. CP violation beyond the Standard Model is generally believed to be required to explain the observed prevalence of matter over antimatter in the universe. Protactinium-229 (t = 1.5 d, I = 5/2) is mostly sensitive to T- and P-violating interactions originating within the nucleus. The best limits on these types of exotic interactions are derived from the atomic EDM limit for Mercury-199. Because of its unusual pear-shaped nuclear structure (octupole deformation), Pa-229 is expected to have a physics sensitivity that is orders of magnitude higher than Hg-199. The exact sensitivity enhancement factor is currently unknown but may be as high as six orders of magnitude higher than Hg-199. We will review what is currently known about the nuclear structure of Pa-229 in relation to time-reversal violation and prospects for future experiments to clarify this issue.