The thesis of Micah P. Ledbetter, entitled Progress Toward a Search for a Permanent Electric Dipole Moment in Liquid ¹²⁹Xe, has been placed on deposit.

Any member of the University wishing to read the thesis may do so. Any objections should be submitted to me in writing. The principal advisor for this work was Prof. Michael Romalis

ABSTRACT

Here we report progress toward the measurement of a permanent electric dipole moment (EDM) in hyperpolarized liquid ¹²⁹Xe which violates invariance under both parity and time reversal. The standard model (SM) predicts atomic EDMs well beyond current experimental limits while many natural extensions to the SM predict EDMs within the expected sensitivity of current experiments. Hence the search for a non-zero EDM is viewed as an ideal test for new physics.

Liquid ¹²⁹Xe is an attractive medium in which to perform such a search because it has a high number density and the nuclear spin precession can be efficiently detected using SQUID (superconducting quantum interference device) magnetometers, yielding very high signal-to-noise ratios. For experimentally realizable parameters it should be possible to achieve a sensitivity of \sim 10^-32 e-cm for one day of integration, several orders of magnitude beyond current experimental limits on EDMs.

In preparation for performing a search for an EDM in liquid xenon, we have conducted a thorough experimental and theoretical investigation of the spin dynamics of hyperpolarized liquid ¹²⁹Xe. In a highly polarized liquid magnetic dipolar interactions can occur on time scales considerably shorter than diffusion and strongly influence spin precession. We find that there are two dramatically different regimes depending on the tip angle that the magnetization forms with the holding magnetic field. For small tip angles the system is insensitive to perturbations, a result known as spectral narrowing, yielding considerably extended free induction decays. For large tip angles the system develops a dynamical instability so that spin precession due to a small magnetic field gradient is amplified exponentially relative to the non-interacting case. In principle, this amplification can be quite large, leading to enhanced sensitivity of spin precession measurements when noise in the detection system is much greater than spin-projection noise.

Experimentally, we have achieved amplification of spin precession due to a small applied field gradient by a factor of 9.5 relative to the non-interacting case in the large tip angle regime. Numerical simulations indicate that high order magnetic field gradients are limiting the amplification and considerable improvement is expected with further optimization of high order gradients. In the small tip angle regime we have realized an extension of the free induction decay by up to a factor of 100 compared to what it would be in the absence of dipolar interactions. We discuss how these two different regimes can be used in a search for an EDM in liquid xenon and analyze the expected sources of systematic effects.

Daniel Marlow
Chair, Dept. of Physics