Antimatter experiment shows surprising results near absolute zero

(ORDO NEWS) — For decades, researchers have been playing with antimatter in search of new laws of physics.

These laws could manifest as forces or other phenomena that strongly favor matter over antimatter, or vice versa. However, physicists have found nothing unusual, no convincing evidence that antimatter particles – which are oppositely charged twins of familiar particles – obey different rules.

It hasn’t changed. But while conducting precision experiments with antimatter, one team stumbled upon a mysterious find. When bathing in liquid helium, hybrid atoms made of matter and antimatter behave incorrectly.

While the rampage of the stew would throw the properties of most atoms into disarray, helium hybrid atoms retain a surprising uniformity.

The discovery was so unexpected that the research team spent years testing their work, repeating the experiment, and arguing about what might be going on. Finally convinced of the reality of their result, the group detailed their findings today in the journal Nature.

“It’s very interesting,” said Mikhail Lemeshko, an atomic physicist at the Austrian Institute of Science and Technology, who was not involved in the study. He expects the result to lead to a new way of capturing and scrutinizing the elusive forms of matter. “Their community will find more interesting opportunities to capture exotic things.”

Cooled antiprotons

One way to evaluate the properties of atoms and their components is to tickle them with a laser and see what happens, a technique called laser spectroscopy. For example, a laser beam with the right energy can bring an electron to a higher energy level for a short time.

When it returns to its previous energy level, the electron emits light of a certain wavelength. “It’s the color of the atom, if you like,” says Masaki Hori, a physicist at the Max Planck Institute for Quantum Optics, who uses spectroscopy to study antimatter.

In an ideal world, experimenters would see that each hydrogen atom, say, shines with the same sharp hues. The spectral lines” of an atom show natural constants, such as the charge of an electron or how much an electron is lighter than a proton, with extreme precision.

But our world is not perfect. Atoms rush about, chaotically crashing into neighboring atoms. The constant jostling deforms the atoms, changing their electrons and hence the energy levels of the host atom. Shine a laser on distorted particles, and each atom will react in its own way. The clear natural colors of the cohort are lost in iridescent strokes.

Spectroscopy specialists like Hori spend their careers fighting this “broadening” of spectral lines. For example, they can use thinner gases, where atomic collisions are less frequent – and the energy levels remain cleaner.

This is why the project of Anna Soter, who was Hori’s graduate student at the time, initially seemed counterintuitive.

In 2013, Soter worked at the CERN laboratory on an experiment to study antimatter. The group assembled matter-antimatter hybrid atoms by firing antiprotons into liquid helium.

Antiprotons are the negatively charged twins of protons, so an antiproton can occasionally take the place of an electron in the orbit of a helium nucleus. As a result, a small cohort of “antiprotonic helium” atoms was obtained.

The project was designed to test whether helium bath spectroscopy is possible at all – a proof of concept for future experiments that will use even more exotic hybrid atoms.

But Sauter was interested in how the hybrid atoms would react to different helium temperatures. She convinced the collaboration to spend precious antimatter to repeat measurements in increasingly cool helium baths.

“It was a random idea on my part,” says Sauter, now a professor at the Swiss Federal Institute of Technology in Zurich. “People weren’t sure it was worth spending antiprotons on this.”

Whereas the spectral lines of most atoms might go completely awry in an increasingly dense liquid, expanding perhaps millions of times, Frankenstein’s atoms did the exact opposite. As the researchers lowered the helium bath to icier temperatures, the spectral spot narrowed.

And below 2.2 kelvins, where helium becomes a “superfluid” without friction, they saw a line nearly as sharp as the narrowest they had seen in gaseous helium. Even though the hybrid matter/antimatter atoms are supposedly hit by the dense environment, they act in incredible unison.

Unsure what to do with this experiment, Soter and Hori were left alone with the result, pondering what could have gone wrong.

“We kept arguing for years,” Hori says. “It was not so easy for me to understand why this happened.”

Over time, the researchers came to the conclusion that nothing terrible happened. The narrow spectral line showed that hybrid atoms in superfluid helium do not experience atomic collisions in a billiard ball, as they usually do in a gas. The question was why. After consulting with various theorists, the researchers settled on two possible causes.

One of them is related to the nature of the liquid medium. The atomic spectrum narrowed dramatically when the group cooled the helium to a state of superfluidity, a quantum mechanical phenomenon in which individual atoms lose their individuality, allowing them to flow together without rubbing against each other.

Superfluidity takes the edge off atomic collisions in general, so the researchers expect foreign atoms to experience only slight broadening or even limited contraction in some cases. “Superfluid helium,” said Lemeshko, “is the softest thing known, in which atoms and molecules can be immersed.”

But while superfluid helium may have helped hybrid atoms become the most isolated, that alone cannot explain how well the atoms behaved. Another key to their correspondence, according to the researchers, was their unusual structure, due to the antimatter component.

In an ordinary atom, a tiny electron can travel far from the host atom, especially when excited by a laser. On such a free leash, an electron can easily collide with other atoms, disrupting the internal energy levels of its atom (which leads to a broadening of the spectrum).

When Sauter and her colleagues swapped nimble electrons for bulky antiprotons, they radically changed the dynamics of the atom. The massive antiproton has become much more of a homebody, staying close to the nucleus where it can be sheltered by an outer electron. “An electron is like a force field,” says Hori, “like a shield.”

However, this crude theory has gone so far. The researchers still cannot explain why the spectral broadening changed as the transition from gas to liquid and superfluidity progressed, and they have no way to calculate the degree of narrowing. “You have to be predictable, otherwise it’s not a theory,” Hori said. “It’s just arm waving.”

Meanwhile, this discovery opened up a new field for spectroscopy.

There are limits to what experimenters can measure using low-pressure gases in which atoms move. This frantic movement creates more of a distracting broadening, which the researchers combat by slowing down the atoms with lasers and electromagnetic fields.

Cramming atoms into a liquid is an easier way to keep them relatively still, now that researchers know that getting particles wet won’t necessarily destroy their spectral lines. And antiprotons are just one of the exotic particles that can be put into orbit around a helium nucleus.

Hori’s group has already used this technique to make and study “pion” helium, in which an extremely short-lived “pion” particle replaces an electron. The researchers made the first spectroscopic measurements of pion helium, which they described in the journal Nature in 2020.

Next, Hori hopes to use this method to study the kaon particle (a rarer relative of the pion) and an antimatter version of the proton-neutron pair. Such experiments could allow physicists to measure certain fundamental constants with unprecedented precision.

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


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