Why is there matter in the universe at all? To date there has been no conclusive answer to this question. Our understanding of the Big Bang is based on the assumption that equal amounts of antimatter and matter were created. But matter and antimatter cancel each other out. In a universe where there is a perfect balance between particles and antiparticles, matter and antimatter should have destroyed one another a long time ago. Our existence is proof of the fact that this balance is violated – physicists talk about fundamental symmetries in this context.

Simon Stellmer from the Institute of Atomic and Subatomic Physics at TU Wien now intends to use a sophisticated high-precision experiment to uncover the violation of this symmetry. This experiment will involve mercury atoms and a series of quantum physics technologies. His research is supported by an ERC Starting Grant, one of the most substantial and prestigious research grants in Europe.

Elementary particles can be used in the study of symmetries
The violation of CP symmetry, as it is known, is a highly efficient mechanism which can lead to an imbalance in the number of particles and antiparticles. In this case, CP symmetry determines that the laws of physics would not change if you mirrored the space and swapped the positive and negative charges at the same time. This symmetry does not apply exactly in our universe though – it is very slightly violated.

“The extent of the CP violation predicted by the standard particle physics model cannot explain the observed imbalance between matter and antimatter,” says Simon Stellmer. “It is at least six to eight orders of magnitudes too small.” We therefore need to expand on the theory of the standard model. The first thing to find out here is the actual extent of the CP violation in the universe at present. And the best way to do this is by investigating tiny elementary particles.

“It is clear that the CP violation leads to an asymmetry in the charge distribution of small particles,” explains Simon Stellmer. The electrical charges of elementary particles are not distributed entirely symmetrically but are slightly skewed in one direction – this is known as an electric dipole moment. “This means that elementary particles, such as electrons, are not perfect spheres in reality,” says Simon Stellmer. 

Scientists have tried again and again to measure this, in electrons, neutrons, and in atoms too, but to date no one has succeeded in identifying a violation of the symmetry – particles appear to be perfect spheres, even when you look at them in extreme close-up. “The measurement precision is simply not sufficient,” says Simon Stellmer. “But if we can measure a little more precisely, it should soon at least be possible to refute all those theories that predict a particularly pronounced degree of CP violation, for example certain supersymmetry theories.”

Ultracold mercury atoms
Simon Stellmer now intends to measure this electric dipole moment using mercury atoms: “We need atoms that are heavy but not radioactive and can be cooled using lasers – mercury is the best solution for this.” There have been previous attempts to measure the electric dipole moment of mercury atoms, but Stellmer now intends to fundamentally change and develop these experiments. “We will take the mercury atoms into the quantum laboratory and cool them to just above absolute zero to ensure we have the best possible level of control. Our intention is to achieve a significantly better degree of accuracy than was possible with the methods previously available.

Hamburg, Innsbruck, Vienna
Simon Stellmer knows the numerous technical tricks that are necessary for this like no other: while working on his dissertation at the University of Innsbruck he succeeded in creating the first Bose-Einstein condensate with ultracold strontium. Prior to that, he studied in his home town of Hamburg and, after completing his dissertation at Innsbruck, he transferred to the Institute of Atomic and Subatomic Physics at TU Wien in 2013. He will now use the ERC grant from the European Union, worth two million euros, to set up his own research group and will then spend the next five years exploring the universe's mysterious symmetries.

For more information, please contact:
Dr Simon Stellmer
Institute of Atomic and Subatomic Physics
TU Wien
Stadionallee 2, 1020 Vienna
T: +43 (0)1 58801 141856
simon.stellmer@tuwien.ac.at