Towards a Measurement of the Electric Dipole Moment of Rn.

Abstract

The observed baryon asymmetry in the universe requires a greater degree of CP violation than is contained in the CP-violating Standard Model processes discovered thus far. Since the permanent electric dipole moment (EDM) of a particle, atom, or molecule is a CP-odd observable, any nonzero measurement of an EDM above the small Standard Model background would indicate a new source of CP violation. The nuclear structure of 223Rn is expected to enhance its sensitivity to CP-violating interactions relative to 199Hg, currently the most sensitive atomic EDM result, motivating the development of the Radon EDM experiment. In anticipation of radon production at TRIUMF, we performed a series of studies to improve the design and predict the expected precision of the Radon EDM experiment. We designed and tested a prototype gas transfer apparatus that collects a sample of noble gas from a beam and transfers it to a measurement cell, achieving a transfer efficiency of about 40%. We recently improved this to greater than 90%. We studied the polarization and relaxation of radon using samples of 209Rn produced at SUNY Stony Brook’s Nuclear Structure Laboratory and polarized it via spin-exchange with optically pumped rubidium. We constructed equations for the anisotropy of the gamma rays emitted by polarized 209Rn as a function of the rubidium polarization, the spin-exchange cross section σSE, the wall-binding temperature T0, the quadrupole relaxation rate Γ∞2 , and the cell temperature. Using rubidium polarization values measured by electron spin resonance, our radon polarization data indicated that a silane-based wall coating improved the relaxation rate in our cells. For T0 = 350 K and σSE = 2.5×10−5˚A 2 , we found Γ∞2 = 0.14±0.02 Hz in uncoated cells and Γ∞2 = 0.042 ± 0.012 Hz in coated cells. We used the coated-cell results to obtain a conservative estimate of the Radon EDM precision, 3 × 10−26e · cm at TRIUMF using gamma-ray anisotropies. Coupled with the enhancement effects, this indicates sensitivity to CP violation similar to that of 199Hg in the first phase of the Radon EDM experiment.