An Antimatter Experiment Shows Surprises Near Absolute Zero

 

In antiprotonic helium, one of the helium atom’s two electrons has been replaced by an antiproton.

Researchers were surprised by the results of an experiment using hybrid matter-antimatter atoms.

Researchers have been tinkering with antimatter for decades in quest of new physics principles. These principles might take the shape of forces or other phenomena that strongly favour matter over antimatter, or the other way around. Physicists, on the other hand, have found no evidence that antimatter particles, which are simply the oppositely charged twins of known particles, follow different rules.

That isn't going to change. However, when doing precision antimatter research, one team made an unexpected discovery. Hybrid atoms made up of matter and antimatter misbehave when submerged in liquid helium. The properties of ordinary atoms would be thrown into disorder if they were buffeted by the stew, but hybrid helium atoms preserve an unusual regularity. The discovery was so surprising that the researchers spent years double-checking their work, redoing the experiment, and debating what was going on. The team published their findings in Nature after they were finally sure that their outcome was legitimate.

"It's really intriguing," said Mikhail Lemeshko, an atomic physicist at the Austrian Institute of Science and Technology who wasn't involved in the study. He expects the outcome will lead to a new means of capturing and scrutinising obscure types of matter. "Their society will discover new and interesting ways to capture strange creatures."

Chill Antiprotons

Laser spectroscopy is a technique for determining the properties of atoms and their constituents by tickling them with a laser and watching what happens. For example, a laser beam with the correct energy can briefly push an electron to a higher energy level. The electron emits light of a certain wavelength when it returns to its former energy level. "This is the atom's colour," Masaki Hori, a physicist at the Max Planck Institute of Quantum Optics who studies antimatter with spectroscopy, said.

Experimenters in an ideal world would see every single hydrogen atom, for example, sparkling with the same bright hues. The "spectral lines" of an atom show natural constants with extraordinary precision, such as the electron's charge or how much lighter the electron is than the proton.

However, we live in a flawed world. Atoms careen around, slamming into one another in a disorderly manner. The frequent jostling deforms the atoms, tampering with their electrons and, as a result, the energy levels of the host atom. If you shine a laser at the deformed particles, each atom will react differently. The crisp intrinsic hues of the group are lost in rainbowlike blurs.

Spectroscopists like Hori devote their lives to combating this "broadening" of spectral lines. Thinner gases, for example, would result in fewer atomic collisions, allowing energy levels to remain more stable.

That's why Anna Sótér's hobby project, when she was a graduate student of Hori's, looked illogical at first.

Sótér was working on an antimatter experiment at the CERN laboratory in 2013. Antiprotons would be fired into liquid helium to create hybrid matter-antimatter atoms. Because antiprotons are protons' negatively charged twins, an antiproton could occasionally take the place of an electron orbiting a helium nucleus. As a result, a tiny group of "antiprotonic helium" atoms emerged.

The goal of the effort was to see if spectroscopy in a helium solution could be done at all, as a proof of concept for future studies with even more unusual hybrid atoms.

Sótér, on the other hand, was curious about how the hybrid atoms might behave to varied helium temperatures. She persuaded the collaboration to expend valuable time repeating the measurements in helium baths that were becoming progressively cold.

Sótér, who is currently a professor at the Swiss Federal Institute of Technology Zurich, commented, "It was a wild thought from my side." "People weren't sure that wasting antiprotons on it was worth it."

In the more thick fluid, normal atoms' spectral lines would have gone utterly berserk, enlarging possibly a million times, but the Frankenstein atoms performed the exact opposite. The spectral smear narrowed when the researchers dropped the helium bath temperature to lower levels. They noticed a line about as sharp as the tightest they have seen in helium gas below around 2.2 kelvins, where helium becomes a frictionless "superfluid." The hybrid matter-antimatter atoms were functioning in improbable synchronisation despite probably taking a beating from their dense environment.

Sótér and Hori lingered on the results, unsure what to make of the experiment, while they pondered what went wrong.

Hori recalled, "We continued to dispute for many years." "It was difficult for me to comprehend why this was thus."

Super Tools

Meanwhile, the discovery has expanded the scope of spectroscopy.

Experimenters can only quantify so much with low-pressure vapours in which atoms rush around. This frenzied motion causes additional distracting broadening, which researchers prevent by using lasers and electromagnetic fields to slow down the atoms.

Sticking atoms in a liquid is an easier approach to keep them reasonably stationary given that scientists know that getting particles wet doesn't always mean their spectral lines are ruined. Antiprotons are just one type of unusual particle that can be found orbiting a helium nucleus.

Hori's team has already used the technology to make and examine "pionic" helium, which is made by replacing an electron with an extremely short-lived "pion" particle. The team published their findings in Nature in 2020, describing the first spectroscopic measurements of pionic helium. Hori aims to utilise the technology to bring the kaon particle (a rarer relative of the pion) and the antimatter form of a proton-neutron couple to heel in the future. Physicists may be able to measure key fundamental constants with remarkable precision as a result of such experiments.

Hori explained, "This is a new capacity that didn't exist previously."

Reference:

• Sótér, A., Aghai-Khozani, H., Barna, D. et al. High-resolution laser resonances of antiprotonic helium in superfluid 4He. Nature (2022). https://doi.org/10.1038/s41586-022-04440-7