Light interacts with antimatter

Breakthrough in research into the enigmatic antimatter

Why is there something and not rather nothing? Or to put it another way: Why does matter dominate our universe, when matter and antimatter should have been extinguished shortly after the Big Bang? Researchers have long been concerned with the mysterious matter-antimatter imbalance. The Standard Model of particle physics does not provide an explanation for the violation of this so-called CP symmetry. With a breakthrough at the European Nuclear Research Center Cern near Geneva, it is now possible to measure antimatter with unprecedented precision.

As scientists from the alpha experiment based at Cern announce in the current issue of the specialist journal "Nature", they have for the first time succeeded in slowing down anti-hydrogen with laser light and thus cooling it down to temperatures close to absolute zero. It was "by far the most difficult experiment we have ever carried out," says Jeffrey Hangst, spokesman for the Alpha experiment, STANDARD. A decade ago, laser cooling of antimatter sounded like science fiction.

Laser cooling

The laser cooling method has been known for four decades and is part of the standard repertoire of many physics laboratories. In 1997 Steven Chu, Claude Cohen-Tannoudji and William D. Phillips were awarded the Nobel Prize in Physics for their development. Laser cooling uses laser beams to slow down the natural movement of atoms. "Photons have no rest mass, but they do have momentum," says Hangst. "At the atomic level, this momentum can be very significant."

If the direction and frequency of the laser are correct, the light particles slow the atoms down. "Imagine an atom moving in the direction of the light source and absorbing a photon. A small shock is experienced and this affects the speed," says Hangst. This allows atoms to be cooled down to just above absolute zero.

"Five or six miracles at the same time"

If one wants to cool antimatter with laser light, however, several difficulties arise. The first challenge arises from generating antimatter at all. "It's very difficult to produce the right kind of light to interact with hydrogen," says Hangst. This is an enormous challenge even for hydrogen, all the more so for anti-hydrogen.

"So many things have to work at once to do this experiment," says Hangst. "You have to string together five or six miracles and make them all work at once - then the experiment is a success."

Where did all the antimatter go?

The first successful laser cooling of antimatter is a "game changer" for Hangst in its research. The production of anti-hydrogen near absolute temperature zero allows much more precise measurements of its internal structure and its behavior under the influence of gravity. Comparing such measurements with those of normal hydrogen also reveals differences between matter and antimatter. "Any possibility of testing the symmetry between matter and antimatter more precisely is of course extremely interesting," says Hangst.

But why is it so important to measure the puzzling symmetry violation of matter and antimatter over and over again with even greater precision? "I would like to turn this question around," says Hangst. "How could one resist not doing it? There is obviously something here in the nature of the universe that we do not understand: why matter dominates in our universe, why is it Antimatter gone? " For him it is "a really great motivation that it might be possible to take a closer look at these questions with such experiments". The project is also very challenging technically and experimentally. "I can't think of anything more difficult to spend my time with." (Tanja Traxler, April 2nd, 2021)