The electron’s electric dipole moment
Researchers within the Nikhef collaboration based in Groningen cool down and manipulate molecules to study the fundamental interactions and symmetries of the Standard Model of particle physics, including the electric dipole moment of electrons (eEDM).
eEDM is an abbreviation for the electron’s electric dipole moment. The electron is a fundamental particle that is well known for its negative charge. According to our best understanding of fundamental particles, as described in the Standard Model of particle physics, the electron also has an immeasurably small electric dipole moment – about 10 orders of magnitude smaller than the current best experimental limit.
However, it is known that the Standard Model is incomplete. And as it turns out, many theoretical extensions of the Standard model, proposed in order to fix its shortcomings, predict the electron to have a dipole moment that is so large that it is quite close to the current best experimental limit. This makes a measurement of the eEDM a uniquely sensitive low-energy probe of physics beyond the Standard Model.
To study the dipole moment of an electron, Nikhef researchers make use of polar molecules. They consist of two different atoms bound together through electromagnetic interaction, sharing an electron. The molecule as a whole is electrically neutral, which means it does not fly away under the influence of an electric field. The electrons inside the polar molecule, however, experience a huge electric field – much larger than could be created in a laboratory. This internal electric field is used to measure if the electron has an electric dipole moment.
To this end, stable laser light is employed. To prevent that the molecules move too much, which affects the accuracy of these measurements, the researchers develop special molecular cooling techniques. The principle of increased measurement precision at lower temperatures actually holds for many precision measurements. For this reason similar techniques are applied to cool and control other molecules and atoms. These cold particles are used to precisely measure the violation of mirror symmetry, and to develop ultraprecise clocks.
This research is performed at the Van Swinderen Institute (VSI) for Particle Physics and Gravity of the University of Groningen, which is a member of the Nikhef collaboration. The aim of VSI is to study the fundamental forces of Nature with implications for our Universe. Within the VSI, both theoretical and experimental research is performed, for example to test fundamental symmetries and forces, at the Large Hadron Collider (LHC) and in the areas of physics beyond the Standard Model, holography, string theory and cosmic inflation. The experiments range from precision measurements at low energies as described here, to those with the highest accessible energies at the LHC. In the eEDM programme, the VSI unites its experimental and theoretical efforts, working together with scientists from the Vrije Universiteit (VU) in Amsterdam.
This research programme is a prime example of fundamental scientific research, aimed at gathering basic knowledge about everything around us. At the heart of this type of research is curiosity about what our Universe is made of and how it came to be. There’s much that we know already, for example that all visible matter is built up from atoms, yet many questions remain unanswered.
Fundamental research is not aimed at realizing applications in the short term. Still, one thing is for sure: no one can predict which ground-breaking applications will eventually emerge from this research. History shows that today’s fundamental knowledge forms the breeding ground for tomorrow’s discoveries.
eEDM’s programme leader is Prof. Dr. Steven Hoekstra