Searches for the lack of antimatter in the universe remain in the annoying state of uncertainty

Researchers paste foils on the side of an ultra-sensitive detector designed to capture light.

The first particles in the universe formed after a hot and dense lump exploded. Physicists believe that in the extreme conditions of the Big Bang, light turned into matter: electrons, protons and neutrons, which later became a part of us.

But physicists are not sure about how exactly this transformation took place. In the 90s, physicists showed that they can transform light into matter, pushing two beams of radiation of extremely high energy. They also found that light at the same time creates an equal amount of antimatter. The very first particles of matter were to meet with their relatives from the field of antimatter and annihilate. The explosion is no more matter.

But, obviously, there is matter. For some reason, after the Big Bang, matter has formed more than antimatter, and physicists do not know for what. “This is one of the biggest mysteries of the universe,” says physicist Don Lincoln from Fermilab .
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Over the past 50 years in the laboratories and in the equations they have been hunting for processes that produce more matter than antimatter. One of the candidates: a predicted radioactive process in which two neutrons in an atom turn into two protons. Theorists believe that in this process, known as neutrinoless double beta decay , there are two electrons and no antimatter. Two new pieces of matter appear in the universe, and detectors must be able to detect them. If this process occurred quite a few times after the Big Bang, it can explain where this extra matter came from.

But here's the catch: no one has ever seen two neutrons turn into two protons. From previous experiments and calculations, it is clear that this process is most likely to occur in certain atoms, for example, in germanium and xenon atoms. When in the germanium atom two neutrons become protons, the atom turns into a new element, selenium. In a recent paper published in Nature, researchers use data from their ultra-sensitive detector to calculate that it would take more than 10 ²⁵ years for half of a germanium crystal to turn into selenium through such decay. This is a quadrillion times the age of the universe. “This is actually a very rare event,” says physicist Peter Grabmayr, one of the participants in the Germanium Detector Array experiment (GERDA) and one of the authors of the work.



Grabmeyr not afraid of such chances. To confirm that such a process takes place, it is not necessary to convert half of the crystal into selenium. It is necessary to detect the decay of only a few atoms. If any atom from their 36-kilogram germanium crystal turns into selenium, it will be able to detect the energy of the two electrons that have appeared, which will look like light when it collides with a detector. To prevent other radiation sources, like cosmic rays, from affecting the detector, a germanium crystal was placed in a tank with liquid argon, at a depth of 1,400 meters below the mountain in the center of Italy.

The possibility remains that they will never discover this process, as Lincoln says. “But this is only an opinion,” he says. - I would not support him. I would not be surprised if such an experiment refuted my intuition. ”

In the meantime, physicists are exploring other processes that can explain the fact that the Universe consists of matter. In particular, they want to find all the differences between antimatter and matter, since any discrepancy may explain why their fates in the early Universe turned out to be different. Last December, the Alpha experiment at CERN measured the properties of anti-hydrogen, but found no unexpected differences from hydrogen. In January, the Beauty experiment at the Large Hadron Collider found that when a particle called a lambda-b baryon decays, its decay products scatter not at such angles as its antimatter counterpart.

In the next ten years, Fermilab plans to build a 1,300-km underground particle accelerator from Illinois to South Dakota — the Deep Underground Neutrino Experiment (DUNE) [deep underground experiment with neutrinos]. The goal of the experiment is to launch neutrino and antineutrino rays over long distances, says Lincoln. If neutrinos behave differently from antineutrinos, this may help reveal another reason why there is more matter in the Universe than antimatter.

These searches will be beneficial, even if they don’t find anything as a result, says Grabmeyr. Their goal is to understand the rules by which the universe works. If the process for which Grabmeyr is ill does not exist, this fact can be used to exclude many of the hypotheses now proposed.

The Grabmeyer group plans to monitor germanium and signs of radioactive decay for another two years. In the end, they want to use up to a ton of germanium in their detector. More germanium - more likely to see decay. “At some stage we will find it,” says Grabmeyr. But for now they are just waiting.