Search for a permanent electric dipole moment of 129 Xe with a He/Xe clock-comparison experiment
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Abstract
The search for sources that violate the combined symmetry of charge conjugation (C) and parity transformation (P) is one of the main concerns of high-precision experiments in fundamental physics. The evidence of a CP-violation beyond the Standard Model of particle physics would be a profound indication for the origin of matter-antimatter asymmetry in the early universe. Possible candidates for this are permanent electric dipole moments of fundamental particles (EDMs), which would cause a breakdown of both parity transformation and time reversal symmetry (T). Through the CPT-theorem, this is directly corresponding to a CP-violation.
In the last decades, various experiments have been performed in order to lower the limits of EDMs with more and more precise measurements. The methodical approach of most of these experiments is the same: According to Pauli's exclusion principle, a finite EDM of a fundamental particle is always coupled to the spin of the particle. The existence of an EDM leads to an additional - although extremely small - energy splitting in the presence of an electrical field, besides the Zeeman splitting induced by a magnetic field. Measuring the resulting tiny frequency shifts of the Larmor precession is an extreme experimental challenge.
In this thesis our experiment for the search of a permanent electric dipole moment of 129Xe (Xe-EDM) is presented. For these kinds of high-precision experiments the method of comagnetometry is a convenient tool. Our approach is to detect the free spin precession of the two hyperpolarized spin samples 3He and 129Xe in the same measurement volume. We are able to measure frequency shifts of the spin precession of the co-located species with a statistical sensitivity in the order of δω ≈ 6 x 10^-10 rad/s. To achieve this excellent value, numerous experimental challenges had to be mastered. For example, the design and development of an exceptionally homogeneous magnetic environment was an important step in order to obtain long coherence times of several hours and to achieve high signal-to-noise ratios. The construction of the experimental setup, the elaboration of experimental procedures, along with particular checks and optimizations are discussed in detail in this work.
With first measurements and a preliminary evaluation we were able to lower the upper limit of the Xe-EDM from a current value of |d(Xe)| < 7.3 x 10^-27 ecm (95% CL) to |d(Xe)| < 1.0 x 10^-27 ecm (95% CL). The continuous development of the experimental conditions and operating procedures will allow us to further improve the achievable sensitivity.