Newly Built Artificial Atom Is Helium With An Antimatter Twist

Artistic impression of the pionic helium atom irradiated by a laser beam. Thorsten Naeser, Dennis Luck, MPQ

Artificial atoms allow scientists to push the laws of physics to the limit, gaining insight into what we are familiar with already and what we are yet to discover. Researchers have now created a new type of artificial atom in which a helium nucleus is orbited not by an electron but by a pion.

Helium is not just for party balloons. It is the second lightest and second most abundant element in the universe. Its nucleus is made of two protons (that have a positive charge) and either one or two neutrons (no charge). Neutrons and protons are both made up of three quarks. This nucleus is orbited by two electrons with a negative charge.

Pions are similar to protons and neutrons but they are composed of one quark and one antiquark. Depending on the type of quark/antiquark mixture, they can have a positive, negative, or neutral charge. You can’t just swap a negative pion for an electron and get a stable atom, but researchers had established in theory that you could do so for a short amount of time. This meta-stable state is long enough to conduct interesting tests – a state researchers have now demonstrated can be obtained.

As reported in Nature, a team lead by Masaki Hori from the Max Planck Institute for Quantum Optics have created pionic helium for the first time. The artificial atom was stable for several nanoseconds, or a few billionths of a second. A short time indeed, but over 1,000 times longer than typical interactions between atomic nuclei and pions.

Achieving this was anything but easy. First, the team had to cool helium down to almost absolute zero. At that temperature, helium is a superfluid, and quantum properties are suddenly macroscopic with weird effects. For example, superfluid helium can climb the walls of its container. To create the pionic helium, researchers shot the helium with pions.

Schematic representation of the experiment. The pion kicks of the electron and the laser give it the extra boost get closer to the nucleus before it is absorbed and break the nucleus apart. Masaki Hori, MPQ

In 98 percent of the events, the team ended up breaking the nuclei, but for the remaining 2 percent, the pions managed to displace one of the electrons and begin orbiting the nucleus. The team then took it a step further by using lasers to give the orbiting pions an extra boost. Thanks to the extra energy, the pions got rid of the second electron as well. This provided an even more intriguing interaction between pions and the nucleus, without the pesky electron in the way.

"This success opens up completely new ways to investigate pions with the methods of quantum optics," Hori explained in a statement.

This technique has exciting possibilities. In a commentary that accompanied the research, Niels Madsen of Swansea University says this approach has the potential to improve the accuracy of the pion mass by up to a factor of 100. And this is not all. Pions can even be used to directly estimate the mass of the muon antineutrino, a particle that could hold the secrets as to why the universe is made of matter and not antimatter.

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