Masaki Hori along with The ASACUSA project at CERN reveals that a hybrid matter–antimatter helium atom containing an antiproton, the proton’s antimatter analogue in place of an electron, shows an unexpected response to laser light when immersed in superfluid helium. The findings, which were published today in the journal Nature, could lead to many new areas of inquiry.
Masaki Hori ASACUSA co-spokesperson
“Our study suggests that hybrid matter–antimatter helium atoms could be used beyond particle physics, in particular in condensed-matter physics and perhaps even in astrophysics experiments,” says ASACUSA co-spokesperson Masaki Hori. “We have arguably made the first step in using antiprotons to study condensed matter.”
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. Such hybrid atoms, which have an antiproton and an electron surrounding the helium nucleus1, are created by combining antiprotons produced at CERN’s antimatter factory with helium gas with a low atomic density and held at a low temperature.
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 amongst energy levels to be too broad, or even veiled, to estimate the antiproton’s mass relative to the electron.
Therefore, 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.
“This behavior was unexpected,” says Anna Sótér, the experiment’s principal PhD student who 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 behavior 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 extraordinarily little when laser light is shone on it, unlike that of many regular atoms, and hence has trivial 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 utilized 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 overcome before the method can be used in conjunction with other techniques for looking for these types of antimatter.