An Antimatter Experiment Shows Surprises Near Absolute Zero

For decades, researchers have toyed with antimatter while searching for new laws of physics. These laws would come in the form of forces or other phenomena that would strongly favor matter over antimatter, or vice versa. Yet physicists have found nothing amiss, no conclusive sign that antimatter particles — which are just the oppositely charged twins of familiar particles — obey different rules.

That hasn’t changed. But while pursuing precision antimatter experiments, one team stumbled upon a puzzling finding. When bathed in liquid helium, hybrid atoms made from both matter and antimatter misbehave. Whereas buffeting from the stew would throw the properties of most atoms into disarray, hybrid helium atoms maintain an unlikely uniformity. The discovery was so unexpected that the research team spent years checking their work, redoing the experiment, and arguing about what might be going on. Finally convinced that their result is real, the group detailed their findings today in Nature.

“It’s very exciting,” said Mikhail Lemeshko, an atomic physicist at the Institute of Science and Technology Austria who was not involved with the research. He anticipates that the result will lead to a new way to capture and scrutinize elusive forms of matter. “Their community will find more exciting possibilities to trap exotic things.”

Chill Antiprotons

One way to gauge the properties of atoms and their components is to tickle them with a laser and see what happens, a technique called laser spectroscopy. A laser beam with just the right energy, for instance, can briefly push an electron to a higher energy level. When it returns to its previous energy level, the electron emits light of a particular wavelength. “This is, if you want, the color of the atom,” said Masaki Hori, a physicist at the Max Planck Institute of Quantum Optics who uses spectroscopy to study antimatter.

In an ideal world, experimentalists would see every single hydrogen atom, say, shining with the same sharp hues. An atom’s “spectral lines” reveal natural constants, such as the electron’s charge or how much lighter the electron is than the proton, with extreme precision.

But ours is a flawed world. Atoms careen about, crashing into neighboring atoms in chaotic ways. The constant jostling deforms the atoms, messing with their electrons — and therefore the host atom’s energy levels. Shine a laser at the distorted particles and each atom will respond idiosyncratically. The cohort’s crisp intrinsic colors get lost in rainbowlike smears.

Spectroscopy practitioners like Hori spend their careers fighting this “broadening” of spectral lines. For instance, they might employ thinner gases where atomic collisions will be rarer — and energy levels will stay more pristine.

That’s why a hobby project of Anna Sótér, at the time a graduate student of Hori’s, initially seemed counterintuitive.

In 2013, Sótér was working at the CERN laboratory on an antimatter experiment. The group would assemble hybrid matter-antimatter atoms by firing antiprotons into liquid helium. Antiprotons are the negatively charged twins of protons, so an antiproton could occasionally take an electron’s place orbiting a helium nucleus. The result was a small cohort of “antiprotonic helium” atoms.

The project was designed to see if spectroscopy in a helium bath was possible at all — a proof of concept for future experiments that would use even more exotic hybrid atoms.

But Sótér was curious about how the hybrid atoms would react to different temperatures of helium. She convinced the collaboration to spend precious antimatter repeating the measurements inside increasingly chilly helium baths.

“It was a random idea from my side,” said Sótér, now a professor at the Swiss Federal Institute of Technology Zurich. “People were not convinced it was worth it to waste antiprotons on it.”

Where the spectral lines of most atoms would have gone completely haywire in the increasingly dense fluid, widening perhaps a million times, the Frankenstein atoms did the opposite. As the researchers lowered the helium bath to icier temperatures, the spectral smudge narrowed. And below about 2.2 kelvins, where helium becomes a frictionless “superfluid,” they saw a line nearly as sharp as the tightest they had seen in helium gas. Despite presumably taking a battering from the dense surroundings, the hybrid matter-antimatter atoms were acting in improbable unison.

Unsure what to make of the experiment, Sótér and Hori sat on the result while they mulled over what could have gone wrong.

“We continued to argue for many years,” Hori said. “It was not so easy for me to understand why this was the case.”

A Close Call

In time, the researchers concluded that nothing had gone awry. The tight spectral line showed that the hybrid atoms in superfluid helium aren’t experiencing atomic collisions in the billiard-ball manner that’s typical in a gas. The question was why. After consulting with various theorists, the researchers landed on two possible reasons.

One involves the nature of the liquid surroundings. The atomic spectrum abruptly tightened when the group chilled the helium into a superfluid state, a quantum mechanical phenomenon where individual atoms lose their identity in a way that permits them to flow together without rubbing against one another. Superfluidity takes the edge off atomic collisions in general, so researchers expect foreign atoms to experience only mild broadening or even a limited amount of tightening in some cases. “Superfluid helium,” Lemeshko said, “is the softest known thing you can immerse atoms and molecules into.”

But while superfluid helium may have helped the hybrid atoms become their most isolationist selves, that alone can’t explain just how well-behaved the atoms were. Another key to their conformity, the researchers believe, was their unusual structure, one brought about by their antimatter component.

In a normal atom, a tiny electron can venture far from its host atom, especially when excited by a laser. On such a loose leash, the electron can easily bump into other atoms, disturbing its atom’s intrinsic energy levels (and leading to spectral broadening).

When Sótér and her colleagues swapped zippy electrons for lumbering antiprotons, they drastically changed the atom’s dynamics. The massive antiproton is much more of a homebody, staying close to the nucleus where the outer electron can shelter it. “The electron is like a force field,” Hori said, “like a shield.”

Still, this rough theory only goes so far. The researchers still cannot explain why the spectral broadening reversed as they switched from gas to liquid to superfluid, and they have no way to calculate the degree of tightening. “You need to be predictive, otherwise it’s not a theory,” Hori said. “It’s just hand-waving.”

Super Tools

In the meantime, the discovery has opened up a new realm for spectroscopy.

There are limits to what experimentalists can measure using low-pressure gases, where atoms zoom around. This frantic motion creates more of the distracting broadening, which researchers combat by slowing the atoms down with lasers and electromagnetic fields.

Sticking atoms in a liquid is a simpler way of holding them relatively still, now that researchers know that getting particles wet won’t necessarily wreck their spectral lines. And antiprotons are just one species of exotic particle that can get placed in orbit around a helium nucleus.

Hori’s group has already applied the technique to fabricate and study “pionic” helium, in which an extremely short-lived “pion” particle replaces an electron. The researchers have made the first spectroscopic measurements of pionic helium, which they described in Nature in 2020. Next, Hori hopes to use the method to bring the kaon particle (a rarer relative of the pion) and the antimatter version of a proton-neutron pair to heel. Such experiments may allow the physicists to measure certain fundamental constants with unprecedented precision.

“This is a new capability that didn’t exist before,” Hori said.

Editor’s note: Natalie Wolchover contributed reporting to this article.