Unexpected Behavior of Hybrid Matter–Antimatter Atoms in Superfluid Helium Surprises Physicists at CERN

 

The result may open doors to several lines of research in particle physics and beyond.

When immersed in superfluid helium, a hybrid matter–antimatter helium atom with an antiproton, the proton's antimatter analogue, in place of an electron shows an unexpected response to laser light, according to the ASACUSA project at CERN. The findings, which were published in the journal Nature on March 16, 2022, could lead to various new areas of research.

"Our findings suggest that hybrid matter–antimatter helium atoms could be exploited in fields other than particle physics, such as condensed matter physics and possibly even astrophysics studies," says Masaki Hori, co-spokesperson for ASACUSA. "We have undoubtedly taken the first step toward studying condensed matter with antiprotons."

ASACUSA Experiment. Credit: CERN

The ASACUSA partnership is well-versed in creating hybrid matter–antimatter helium atoms in order to calculate the mass of the antiproton and compare it to the proton's. These hybrid atoms are created by combining antiprotons manufactured at CERN's antimatter factory with helium gas that has a low atomic density and is held at a low temperature, resulting in an antiproton and an electron around the helium nucleus (rather than two electrons around a helium nucleus).

Low gas densities and temperatures have been crucial in antimatter research, which involves determining the light spectrum of hybrid atoms by measuring their response to laser light. High gas densities and temperatures cause spectral lines formed by antiprotons or electrons transitioning between energy levels to be too broad, or even veiled, to estimate the antiproton's mass relative to the electron.

This is why the ASACUSA researchers were surprised to see a drop in the breadth of the antiproton spectral lines when they employed liquid helium in their latest investigation, which has a far higher density than gaseous helium.

Furthermore, they discovered an abrupt additional narrowing of the spectral lines when they reduced the temperature of the liquid helium to levels below the point at which the liquid becomes a superfluid, i.e. flows without resistance.

Masaki Hori, ASACUSA co-spokesperson. Credit: CERN

"This conduct was surprising," says Anna Sótér, who worked on the experiment as a PhD student and is now an assistant professor at ETHZ. "The optical response of the hybrid helium atom in superfluid helium differs dramatically from that of the same hybrid helium atom in high-density gaseous helium, as well as many regular atoms in liquids or superfluids."

The unusual behaviour seen, according to the researchers, is linked to the radius of the electronic orbital, or the distance at which the electron of the hybrid helium atom is positioned. The radius of the hybrid atom's electronic orbital varies very little when laser light is shone on it, unlike that of many regular atoms, and hence has little effect on the spectral lines, even when the atom is immersed in superfluid helium. However, more research is required to corroborate this notion.

The outcome has a number of repercussions. To begin, researchers may build other hybrid helium atoms in superfluid helium, such as pionic helium atoms, employing various antimatter and exotic particles to examine their response to laser light in detail and determine particle masses. Second, the significant narrowing of the lines in superfluid helium suggests that hybrid helium atoms could be utilised to explore this and other condensed-matter phases. Finally, the narrow spectral lines might theoretically be used to look for low-velocity cosmic antiprotons or antideuterons (a nucleus made up of an antiproton and an antineutron) that collide with the liquid or superfluid helium used to cool space experiments or high-altitude balloons. However, a number of technical hurdles must be solved before the method can be used in conjunction with other techniques for looking for various types of antimatter.

References:

“High-resolution laser resonances of antiprotonic helium in superfluid 4He” by Anna Sótér, Hossein Aghai-Khozani, Dániel Barna, Andreas Dax, Luca Venturelli and Masaki Hori, 16 March 2022, Nature.

DOI: 10.1038/s41586-022-04440-7