Measuring the electron electric dipole moment using an array of ultracold molecules

The Standard Model of particle physics is one of the greatest achievements of modern science. It has been fabulously successful in classifying the fundamental particles and explaining how they behave. Nevertheless, it is widely accepted that the Standard Model is incomplete because it fails to explain several important observations.

One prominent example is the excess of matter over antimatter in the universe. The Standard Model predicts almost equal amounts of matter and antimatter, but observations show the universe contains only matter. This contradiction is one of the great unsolved problems in modern physics and a major deficiency of our most fundamental theory.

To build a more complete picture of the universe, physicists are striving to reveal what lies beyond the Standard Model. This is an important objective of much of the research being done at gigantic particle accelerators. There is an alternative, and ingenious, way to explore the same problem - measure the shape of an electron.

The new forces needed to explain the matter/antimatter imbalance also make electrons slightly non-spherical. This distortion - known as the electric dipole moment - changes the energy of an electron in an electric field, and that tiny change is amplified when the electron is bound to a molecule. I propose to build an apparatus that uses an array of molecules cooled to microkelvin temperatures to make an extremely precise measurement of the electron's shape.

With very careful measurements, such table-top experiments enable us to probe energies equal to, or even above, those reached by the particle accelerators. Such marvellous precision has become possible by my recent success in cooling molecules to ultracold temperature. That technique is the foundation of this proposal.

My plan is to decelerate a beam of ytterbium monofluoride molecules to rest using the forces exerted by carefully tuned laser light, trap these molecules and cool them to a few microkelvin. Then, I will load them into an array of traps formed by standing waves of light. The array contains millions of individual traps and isolates the molecules from one another, creating a pristine environment for measuring the electron's electric dipole moment.

This measurement is done by watching how the spin of the molecules precesses in applied magnetic and electric fields, similar to the precession of a gyroscope or spinning top in a gravitational field. An electric dipole moment changes this precession rate, though only by the tiniest amount. That tiny change is detected more easily if the spin precesses for a long time.

Because molecules are confined and isolated in the array, the time can be thousands of times longer than in current experiments. That makes my measurement scheme far more sensitive than any other experiment, allowing me to search for new physics with unprecedented precision.