Measurement of the electric dipole moment of the 129Xe

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Florian Kuchler

TU München - Excellence Cluster Universe
Boltzmannstr. 2/EXC
85748 Garching

Tel: +49-89-35831-7156
Fax: +49-89-3299-4002
In a complementary effort we are trying to measure an EDM with the isotope 129Xe based on a novel technique. Our approach will show significantly different systematic effects and (in principle) has a potential sensitivity close to predictions by the Standard Model. The development of a novel method at the same helps to develop technologies for the neutron EDM project. The method should overcome intrinsic systematic limitations of most current experiments. By performing this small scale project locally, we can use many synergies in physics and technological aspects with the larger nEDM project. We will use liquefied laser-polarized xenon micro-droplets, condensed on a micro-structure. The nuclear spin is investigated using superconducting coils inside a magnetically well controlled environment. The goal for the next one-two years is to conceptually prove the method. This technically challenging exploration can be a milestone in the field, potentially lowering experimental bounds by three orders of magnitude already in the prototype. Due to its microscopic nature, this type of experiment also enables us to test short distance effects or exotic (Axion-like) couplings with much higher sensitivity compared to a measurement deploying a neutron EDM apparatus.

At the Excellence Cluster "Universe" we are setting up a new experiment to measure the electric dipole moment (EDM) of the diamagnetic atom 129-Xenon. Our novel approach is based on an array of sub-millimeter droplets of hyper-polarized liquid Xenon placed in a low magnetic field (on the order of µT). Resulting low Larmor-precession frequencies (on the order of Hz) and small sample size enable us to apply rotating electric fields perpendicular to the magnetic field direction rather than parallel. By surrounding the Xenon droplets with eight individual electrodes the electric field can be controlled to co-rotate with the Xenon spins. This electric field can be kept rotating at a constant phase to the Larmor-precession of the Xenon atoms, which leads to a decoupling of the Larmor-precession and a precession due to an interaction of the electric field with an EDM of the Xenon atoms. With this technique we expect a huge increase in sensitivity potentially testing EDM predictions of Standard Model extentions, eg. Supersymmetry. For details see further information or contact us.

Our current goal will be demonstration of a low-field NMR experiment on hyper-polarized liquid Xenon. While the Xenon sample is kept at 160K, we're utilizing highly sensitive SQUID current sensors cooled to 4.2K to measure the precession signal of the liquid Xenon sample.

The Xenon EDM lab

In 2008 we started setting up the experimental equipment for our Xenon EDM experiment. Our lab is located at the Maier-Leibnitz Laboratory (MLL) in Garching, where we moved to at the end of 2010.

A gas system supplies the appropriate gas mixture for polarizing Xenon. Xenon, Nitrogen and Helium are filled into a glass cell, which contains a small amount of solid Rubidium (Isotope 87). The Rubidium is necessary since 129-Xenon cannot be polarized directly but only via spin-exchange with polarized Rubidium vapour. This polarization method is called spin-exchange optical pumping (SEOP).

To control the flow of polarized gas we built non-magnetic, remote-controlled high vacuum glass-valves, which are directly attached to the aforementioned glass cell.

Our current goal will be demonstration of a low-field NMR experiment on hyper-polarized liquid Xenon. While the Xenon sample is kept at 160K, we're utilizing highly sensitive SQUID current sensors cooled to 4.2K to measure the precession signal of the liquid Xenon sample.

Typically, we use partial pressures of 200-500 mbar Xenon, 100 mbar Nitrogen and 3500 mbar Helium, where Nitrogen is needed to quench unwanted transitions during optical pumping and the high amount of Helium accounts for pressure broardening of the absorption line to increase the amount of absorbed laser-light. Recently we've rebuilt the gas system introducing mass-flow controllers to supply a continous flow of the gas-mixture. This allows for production of larger amounts of liquid hyper-polarized Xenon. The gas has to be transferred several meters to the Xenon EDM experiment via the SEOP setup, where Xenon gas is spin-polarized.

Xenon Polarizer

Xenon EDM Cryostat

The cryostat for the Xenon EDM measurement is placed inside a 3,5m x 3,5m x 5m styrofoam house for temperature stabilisation of the cylindrical magnetic shields. The insulation vacuum system containing the Xenon sample and a specialized SQUID system is located inside the 5-layer Mu-metall shield (shielding factor ~50000) to create an almost zero magnetic-field environment.
Hyper-polarized Xenon gas has to be transfered into the cryostat through the Mu-metall shields after it is spin-polarized in the SEOP setup. To perform the experiment the hyperpolarized Xenon has to be liquified and kept at a temperature of 161-165 Kelvin. The liquid Xenon is then enclosed by a micro-fabricated structure forming sub-millimeter droplets of spin-polarized 129-Xenon. In very close vicinity below these droplets superconducting pick-up coils are employed to observe induction signals of precessing Xenon atoms. The pick-up coils made of Niobium wire on a Sapphire rod are kept at 4.2 Kelvin to maintain the superconducting state. Highly sensitive SQUID current sensors cooled to 4.2 Kelvin are attached to each of the pick-up coils enabling measurement of magnetic signals down a few fT (10e-15 Tesla!).