(Electric Dipole Moment of the Electron in Diatomic Molecules)

Principal Investigator:

Timo Fleig,


Malaya K. Nayak (Mumbai), Trond Saue (Toulouse), Mikhail G. Kozlov (St. Petersburg)


Malika Denis

The Search for Physics Beyond the Standard Model

Theory Project

The EDM of the electron cannot be measured directly. In all modern experiments an energy shift or a transition energy in an atom or molecule due to an EDM is measured, from which the particle's EDM can be interpreted by means of a many-body calculation. The determination of this factor which relates the atomic physics to the particle physics scale is one of the major objectives of our work. Thus, the knowledge of the internal electric field interacting with the electron in the many-particle system is required. We have recently implemented the respective P,T-odd interaction Hamiltonian into our Kramers-Restricted Configuration Interaction method. This method allows for the computation of P,T-odd matrix elements as expectation values over correlated four-component relativistic CI wavefunctions.

The new method has been and is currently being applied to various diatomic molecules which have been identified as promising candidates in the search for the electron EDM. Among them are HfF+, ThO, ThF+, and WC. The currently most constraining upper bound to the electron EDM has been obtained from experimental and theoretical studies on ThO (ACME collaboration and own work). The measure for the P,T-odd interaction is the effective electric field at the position of an unpaired electron in the molecule (for an electron pair in a closed shell the energy shifts cancel out). The studies on heavy-element molecules are challenging and typically require an accurate description of molecular electronically excited states, including the effects of special relativity. Our approaches take all atomic electrons into account explicitly, and limit the number of explicit electrons for correlation calculations to valence and outer core shells.

If the experimental upper bound to the molecular EDM is interpreted in terms of the electron EDM by dividing this upper bound by the calculated effective electric field, the electron EDM upper bound results. It currently (January 2019) has the value |de| < 1.3 x 10-29 e cm. The figure to the left shows the ensuing constraint on theories going beyond the SM (click on the image).

Strictly speaking, only theories to the right of the eEDM bar are still valid possibilities as extensions to the SM. However, accidental cancellations of for instance eEDM and ENSPS effects may lead to a too low upper bound. Moreover, NP theories are susceptible to varying predictions of EDMs based on freedom in their parameter spaces.

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