Ruprecht Karls Universität Heidelberg

Experimental search for the electric dipole moment of 129Xe - The MiXed Experiment
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Theoretical Motivation

CP Violation and Permanent Electric Dipole Moments of Particles

Until 1956 it was assumed that all physical processes are invariant under time reversal T, charge conjugation C and spatial or parity inversion P. These symmetries were considered as fundamental properties and every allowed process would transform into an allowed process by applying one of the T, C, or P transformations. Early tests showed that parity was conserved in gravity and in the electromagnetic and strong interaction. Then, in 1956, the famous Wu experiment used the beta decay of polarized 60Co to show that the weak interaction violates P symmetry to the maximum extent. Only the left-handed components of particles (e.g. left-handed neutrinos) and right-handed components of antiparticles (e.g. right-handed anti-neutrinos) participate in weak interactions. Parity transformation leads to particles that do not exist (e.g. right-handed neutrinos or left-handed anti-neutrinos). The same argument is valid for charge conjugation C (e.g. transforming left-handed neutrinos into left-handed anti-neutrinos). Therefore, the separate C and P symmetries are violated in the weak interaction of the SM. However, the combination of both transformations, the CP transformation, leads to particles that do exist (e. g. transforming a left-handed neutrino into a right-handed anti-neutrino). Thus, in the following years, it was assumed (or hoped) that CP symmetry is still conserved.

In 1964, a violation of CP symmetry was found in the decay of neutral Kaons and later in the B-meson system. This CP violation is small (compared to the maximal violation of parity) and a property of the weak interaction. So far, CP violation has only been observed in the weak interaction. The CP violation is well described in the SM as a complex phase factor delta. This phase factor is one of the four free parameters of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which describes the quark-mixing. delta only causes CP violation in flavor-changing processes, and all CP-violating processes that are known today occur through quark interaction involving that phase.

Now to the connection between CP violation and permanent electric dipole moments: A permanent EDM of a fundamental or composite particle must be aligned parallel to the Spin I, as the spin is the only available vector for an eigenstate of the isolated particle. Otherwise, there would have to be a further quantum number that describes the direction of the EDM (parallel or anti-parallel to the spin), and this would lead to additional states that obviously do not exist (Pauli principle). If one now applies P (inverting all spatial coordinates), the electric dipole moment d changes sign, whereas the spin I stays unchanged. Furthermore, under time reversal T, d stays unchanged, but I changes sign. Assuming CPT symmetry conservation, this is equivalent to CP symmetry violation. As such, an EDM is an excellent candidate to look for new sources of CP violation.

Historically, the non-observation of EDMs of particles and atoms has ruled out more speculative models (beyond the Standard Model) than any other single experimental approach in particle physics. The most precise EDM limit was measured in the diamagnetic atom 199Hg (dHg < 3.1 · 10-29e·cm).

Experimental search for the electric dipole moment of 129Xe - The MiXed Experiment

In a new experimental setup which is currently developed in Mainz, Groningen and Heidelberg we will measure the permanent electric dipole moment (EDM) of the 129Xe atom. Our goal is to improve the present experimental limit (dXe < 3 · 10-27e·cm). The precession of co-located 3He/129Xe nuclear spins can be used as ultra-sensitive probe for non-magnetic spin interactions of type Dw ~ dXe · E, where dXe is the electric dipole moment of 129Xe and E is the electric field produced by a voltage between two electrodes. The principle of measurement is to change the direction of E and detect the resulting changes of Dw.

Compared to spin masers, the detection of free spin precession with spin coherence times T > 1 day does not have the systematic limitations of a feedback loop necessary to sustain coherent spin precession.

Literature and Links

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