Two million Swiss francs granted to search for new physics

What the matter around us is made of and how its smallest building blocks interact – all that is explained by the so-called Standard Model of particle physics. But it has long been known that this model, as conclusive as it is in itself, cannot explain all actual observations. Most stunningly, it cannot explain why, after the Big Bang, matter and antimatter were not left over in equal amounts; these would have completely annihilated each other. In short, the fact that we observe matter in the universe requires new physics beyond the Standard Model.

Muons can help in the search for new physics

Philipp Schmidt-Wellenburg at PSI is among those looking for traces of this new physics. Now he wants to set up a new experiment for this, which will be called muEDM and will make use of elementary particles called muons. Muons are similar to electrons, but around 200 times heavier. Schmidt-Wellenburg will measure one property of muons more precisely than ever before: their electric dipole moment. The European Research Council has selected his project for funding of around two million Swiss francs.

"PSI is the perfect place for this experiment," Schmidt-Wellenburg explains, "because here we have the highest intensity muon source of its kind in the world."

This funding will enable Schmidt-Wellenburg to employ three doctoral candidates and two postdoctoral researchers. Together with them, he will develop and set up a precursor experiment at one of PSI's muon beamlines over the course of the five-year funding period.

Complement to worldwide experiments

"This experiment has never been done anywhere," the physicist says, explaining the challenge. But he has high hopes for success because of a recent measurement of another property of the muon, its magnetic dipole moment, made by a research group at Fermilab near Chicago, Illinois, USA. They confirmed a comparatively large discrepancy between the measurement and the value predicted by the Standard Model.

Besides other experiments around the world – including one at the Large Hadron Collider at CERN in Geneva that revealed a deviation from the Standard Model in the decay of so-called B mesons – an experiment has been running at PSI for many years that has the potential to detect the same kind of new physics: the nEDM experiment, which measures the electric dipole moment of the neutron. Schmidt-Wellenburg himself has worked on this in the past. "The muon experiment that is now planned is another approach that could give us crucial clues about the new physics," Schmidt-Wellenburg says. The fact that his new research project has now been selected by the ERC means that in the future he will be devoting himself less to neutrons and more to muons.

However, the financial support is not paid out by the ERC itself. Because, as of this year, Switzerland is currently not participating in the European funding program, Schmidt-Wellenburg will instead receive the same amount of funding from the Swiss State Secretariat for Education, Research and Innovation.

Muons on a circular path

His actual experiment, Schmidt-Wellenburg explains, will only be about the size of a kitchen table, and thus significantly more compact than the setup of the sister experiment at Fermilab, which is more than 14 metres in diameter. One of the special features Schmidt-Wellenburg will need is a unique combination of a strong magnetic field and a strong electrical field: Since muons are electrically charged particles, magnets can be used to force them onto a circular path. In the muEDM experiment, it should be possible to measure how, over time, the “spin” of the muons lifts out of the plane of the circular particle movement. The spin of a muon is a quantum mechanical property of the particle, which can be thought of as a tiny bar magnet. The spin has an orientation that reacts to certain influences. The combination with the strong electric field would keep the spin pointing along the circular path – but only if the muon has no electric dipole moment. In other words: If a change in the direction of the muon spins is measurable, the researchers can use this to detect and measure the electric dipole moment of the muon.

Schmidt-Wellenburg concludes: "If we can measure that the orientation of the muon spins changes while the muons themselves remain on this circular path, we will be able to draw conclusions about the physical effects that in turn would establish physics beyond the Standard Model – one day enabling us, we hope, to be able to explain why there was far more matter than antimatter after the Big Bang."

Schmidt-Wellenburg is not the only one excited about his new experiment. "There is great international interest in supporting this project at PSI. Researchers from the United Kingdom, Italy, Germany, and the United States, among others, have already announced that they want to participate. I am also looking forward to this collaboration," says the physicist.

Text: Paul Scherrer Institute/Laura Hennemann