Antiprotons in superfluid
Antiprotonic helium atom suspended in liquid helium in the superfluid state. The antiproton is protected by the electron shell of the helium atom and so avoids immediate annihilation.  | © Image: Christoph Hohmann (LMU München / MCQST)

Antiprotons in superfluid

A new way for sensitive measurements of antimatter

1. April 2022 | by Max-Planck-Institut für Quantenoptik

A team of scientists at CERN led by MPQ physicist Masaki Hori has found that a hybrid antimatter-matter atom behaves in an unexpected way when submerged in superfluid helium. The result may open a new way for antimatter to be used to study the properties of condensed matter, or to search for antimatter in cosmic rays.

When taking a glimpse into the shadowy world of antimatter, researchers have to rely on elaborate technical tricks to keep their samples of antimatter from coming into contact with the normal matter that surrounds us. This isolation is critically important because antimatter and matter immediately destroy each other on contact. An international team of scientists led by the Max Planck Institute of Quantum Optics (MPQ) in Garching has nevertheless combined matter and antimatter into curious hybrid atoms of helium that remain stable for short periods of time. Now the researchers from Italy, Hungary, and Germany have submerged the bizarre atoms into liquid helium and cooled it down to temperatures close to absolute zero - where the helium changes into a so-called superfluid state.

The results of the experiments carried out at the European Organization for Nuclear Research CERN in Geneva surprised the scientists because of the precise and sensitive way that the antimatter-matter hybrid atoms reacted to laser light despite the dense liquid that surrounded the atoms.

"Experiments on antimatter are particularly exciting with regards to the fundamental laws of physics," says Masaki Hori, the team leader. For example, the Standard Model of particle physics - the basis of scientists' current understanding of the structure of the universe and the forces acting within it - requires that particles and their antiparticles differ in the sign of their electric charge. An antiproton - the counterpart of the positively charged proton, a building block of atomic nuclei - carries a negative charge. According to the Standard Model the other properties are identical. "In our past experiments, we have found no evidence that the masses of protons and antiprotons differ in the slightest," notes Hori. "If any such difference could be detected, however small, it would shake the foundations of our current view of the world."

Research leader Masaki Hori at the so called ASACUSA experiment in CERN. Photo: CERN

Electron out, antiproton in

To create the exotic helium atoms containing antiprotons, the researchers used the Antiproton Decelerator at CERN - a globally unique facility that slows down the antimatter particles created in collisions of energetic protons. The slow velocity of the antiprotons makes them ideal for experiments such as those conducted by Hori's team. The researchers mixed the slow antiprotons with liquid helium cooled to a temperature of a few degrees above absolute zero, or minus 273 degrees Celsius, trapping a small part of the antiprotons in atoms of helium. The antiproton replaced one of the two electrons that normally surround a helium atomic nucleus - forming a structure that remained stable long enough to be studied spectroscopically.

"Until now, it was thought that antimatter atoms embedded in liquids could not be investigated by high resolution spectroscopy using laser beams," Hori reports. This is because the intense interactions between the densely packed atoms or molecules of the liquid lead to a strong broadening of the spectral lines. These lines are images of resonances in which the energy absorbed from the laser beam excites the atoms. They are thus a kind of fingerprint that identifies each atom. The exact position of the resonance line on the frequency scale as well as the shape reveal the properties of the atom under investigation - and the forces acting on the antiparticle. But the broadening of the lines obscures this information because it is virtually smeared. Hori and his team have now succeeded for the first time in preventing the "smearing" of the spectral lines in a liquid.