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.
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