Physicists Take Their Closest Look Yet at an Antimatter Atom

The laws of physics, as experts currently understand them, dictate the following: Every fundamental particle has an antimatter twin. The electron, quark, and muon, for example, are paired with the positron, antiquark, and antimuon, respectively. Each antiparticle weighs exactly the same as its twin, but exhibits precisely the opposite electric charge. If the twins meet each other, they annihilate, often to produce light.

Since physicists discovered the first antimatter particle in 1932, the substance has become, in some ways, quite mundane. Researchers have found that lightning in thunderstorms generates positrons; when they meet nearby electrons, the two annihilate each other. Bananas, which contain trace amounts of radioactive potassium, emit a positron every 75 minutes. When they come into contact with electrons, the two also promptly annihilate, with no noticeable effect.

Still, experts understand very little about antimatter. Because of the substance’s tendency to disappear in tiny poofs of light, researchers have had difficulty holding on to it long enough to perform experiments. But over the past two decades, physicists at CERN in Switzerland have been developing special magnets, containers, and lasers for steering, storing, and studying antimatter more closely. Now, they can finally trap it long enough to examine antimatter up close, in a bid to learn more about how it twins with matter.

Publishing in Nature today, physicists working on a CERN experiment called ALPHA have measured new properties of antihydrogen, the antimatter twin of the hydrogen atom. In contrast with hydrogen, which consists of a negatively-charged electron orbiting a positive proton nucleus, antihydrogen consists of a positively-charged positron orbiting a negative antiproton nucleus.

For the experiment, ALPHA’s physicists measured part of antihydrogen’s spectrum, the signature light that quantum particles emit. The frequencies, or colors, of this emitted light, reveal information about antihydrogen’s internal structure, such as the trajectory of its positron as it whizzes around the antiproton nucleus. Antihydrogen should emit specific frequencies spanning from infrared and red to violet and ultraviolet, but ALPHA focused on its emissions in the ultraviolet. To measure this part of antihydrogen’s spectrum, they induced the anti-atoms to emit light by beaming a pulsed laser at them. “The idea is to measure the colors of light and compare it to hydrogen,” says physicist Jeffrey Hangst, the spokesperson of ALPHA’s 50-member collaboration.

To make antihydrogen, the ALPHA team used CERN’s particle colliders and other machines, which produce antiprotons and positrons. For this experiment, they mixed about 90,000 antiprotons with 3 million positrons at a time, at half a degree above absolute zero. Such cold temperatures are necessary to slow down antimatter, so that the particles don’t knock into their surroundings and vanish themselves out of existence. These mixtures produced just 30 antihydrogen atoms, which they collected in a long cylinder, roughly the diameter of a paper towel tube, that is held in vacuum. Accumulating the particles over two hours, they managed to collect about 500 anti-atoms. Then, they beamed a pulsing laser at the antihydrogen, which caused the anti-atoms to emit light, whose colors they measured.

They repeated this process with several batches of antihydrogen to measure the frequencies of its ultraviolet emissions to 12digits of accuracy. As a quantum mechanical object, the positron obeys strange rules, in that it is only allowed to move along certain paths with respect to the antiproton nucleus. These prescribed paths are related to the frequencies of light in antihydrogen’s spectrum. By measuring the spectrum precisely, they can consequently describe better the relationship between the positron and antiproton nucleus in antihydrogen.

The ALPHA study of antihydrogen fits into a bigger goal in physics—to find differences between matter particles and their antimatter counterparts. Current physics theory, what physicists call the Standard Model, predicts that the twins should always behave as mirror images of each other. Antihydrogen’s spectrum should match hydrogen’s exactly. The dance between positron and antiproton in antihydrogen should exactly follow that of the electron and proton in hydrogen.

But physicists have long known that the Standard Model is not completely correct. “According to the Standard Model, we shouldn’t even exist,” says physicist Randolf Pohl of the University of Mainz in Germany, who was not involved with the work. If the Big Bang occurred according to the rules laid out by the Standard Model, the universe would have produced about equal amounts of matter and antimatter. “The matter and antimatter would have annihilated a long time ago, and there would not be enough matter left over to form galaxies and stars and planets and humans,” says Pohl. By studying antimatter more closely, physicists like Hangst hope to find clues to why regular matter dominates the universe.

One strategy is to replicate historical hydrogen experiments in antihydrogen, to see if the results are identical. For example, in this latest work, ALPHA adapted a 1947 experiment, first performed on hydrogen atoms by Willis Lamb and Robert Retherford at Columbia University, for antihydrogen. They measured a property in antihydrogen’s spectrum called the Lamb shift, named after Willis Lamb, who discovered it in hydrogen. Lamb’s work led to the realization that, when illuminated by a certain type of laser light, hydrogen emits two very similar but ultimately distinct shades of ultraviolet, which physicists had previously believed to be just one frequency. To explain why hydrogen emits both colors, physicists developed the new theory of quantum electrodynamics, which forms the basis of particle physics theory today. Quantum electrodynamics, for example, revealed to physicists that empty space is never really empty—particles pop in and out of existence, a reality that researchers must acknowledge when analyzing the aftermath of every particle collider experiment. Repeating these experiments with antimatter could yield similar breakthroughs, says Pohl.

ALPHA found that antihydrogen exhibited a Lamb shift identical to hydrogen’s, conforming to the Standard Model’s prediction that the twins should behave identically. So that means the team didn’t find any new leads as to why the universe exists. But they are still excited, because they now have a solid recipe for creating, storing, and manipulating hundreds of antimatter particles for hours.

Hangst and his colleagues have been gradually building up to this experiment for more than 25 years. Antihydrogen does not naturally occur on Earth; physicists first synthesized it in 1995 at CERN. But these particles moved at nearly the speed of light and disappeared in 40 billionths of a second. It would take another seven years before physicists could produce near-motionless antihydrogen that would not immediately knock into regular matter and annihilate. And it wasn’t until 2010 that they could successfully trap and store antihydrogen. Hangst’s team can now perform experiments for up to 24 hours at a time on the antihydrogen. “When we started, there were many skeptics,” says Hangst. “They thought we would never make antihydrogen. And if we made it, we would never trap it. And if we trapped it, we would never have enough to measure.”

Next, Hangst’s team wants to study how antihydrogen falls. “The idea is to trap a bunch of antihydrogen, release it, and see what happens to it,” says Hangst. Standard physics theory actually does not predict how antihydrogen would behave in Earth’s gravity, and some researchers speculate that it might even fall upward. Whatever happens, it’ll be a surprise.

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