Scientists Have Measured the 'Color' of Antimatter for the First Time

It’s a finding three decades in the making.

Kate Lunau

Kate Lunau

ALPHA's Jeff Hangst at CERN's Antimatter Factory. Image: CERN

When the universe was formed in the Big Bang, there were equal parts matter and antimatter—but today, our universe is almost entirely made up of matter. When matter and antimatter encounter each other, they annihilate into pure energy. So why did matter survive those violent moments after the Big Bang, whereas antimatter is now extremely rare?

Some small difference between the two has to account for it, and could help explain how our matter-filled universe came to be. A new study, published Wednesday in Nature, offers the most precise direct measurement of antimatter yet—showing its spectral structure “in unprecedented colour,” according to CERN in Geneva, where the experiment is based. So far, antimatter looks like the regular matter we know, but more precise experiments allow for more accurate comparisons, and eventually will shed light on the universe’s preference for one over the other.

The ALPHA team at CERN works with antihydrogen, the antimatter twin of hydrogen. (The same group was the first to trap cold antihydrogen atoms, in 2010.) Hydrogen has been extremely well-studied, making it a useful jumping-off point for this kind of work.

To obtain antihydrogen, ALPHA scientists used the Antiproton Decelerator to make antiprotons, which they bound up with positrons. Antihydrogen is tricky to work with, since it’s at risk of annihilating with matter. So the next step involves securing antihydrogen inside a magnetic trap. Then scientists can beam a laser at it to perform their measurements.

In 2016, the ALPHA team did a similar experiment using the same technique to take a spectroscopic measurement of antihydrogen. What’s different now is the level of precision—a factor of 100 times improved, according to CERN.

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This time, scientists could measure the spectral shape, or spread in colours, of antihydrogen from its lowest-energy state to its first excited state (what’s called the 1S to 2S transition). Turns out it looked a lot like hydrogen.

“This particular measurement in hydrogen is the most precisely measured quantity in physics,” ALPHA collaborator Scott Menary, professor of physics at York University in Toronto, told me over the phone. “It’s measured to 15 decimal places.” Being able to compare this measurement to the equivalent in antihydrogen, he continued, would be the “holy grail” of the field. “In the recent paper [on antihydrogen], we got it to 12 decimal places.”

So far, Menary told me, antihydrogen and hydrogen look the same, “unfortunately,” he added, as any small difference could have sparked new ideas and experiments. Even so, the pace is moving quickly now and even more precise results are likely within reach. The finding announced this week, according to ALPHA spokesperson Jeffrey Hangst, represents a "paradigm shift" that's been three decades in the making.

ALPHA: A new era of precision for antimatter research. Video: CERN/YouTube

"Matter and antimatter are so fundamental to the laws of physics," Makoto Fujiwara of the ATLAS experiment’s Canadian contingent told me in 2016, after the earlier results were announced. "If we find any significant difference, we'll really have to rewrite the history of the universe."

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