Scientists at CERN discover “colour” of antimatter

Thomas McMullan Read more April 10, 2018

In a world first, researchers at CERN have measured the spectral structure of an antimatter atom, revealing the mysterious antiparticle’s “colour”.

The discovery is part of a wider aim to understand the relationship between matter and antimatter; the slippery substance that is puzzlingly absent in our observable universe. To understand antimatter would be to throw light on the origins of matter, life, everything. It’s no easy task when you consider that antimatter is incredibly difficult to produce.

Individual antimatter particles are slightly easier to produce, as the Alpha (no relation) team did in its latest study. Because hydrogen is one of the best-understood atoms in the universe, the scientists focused on producing and containing antihydrogen atoms. They were the first to do so in 2010, and have now measured antihydrogen’s optical properties using spectrographic lasers.

To create the antihydrogen, the team first used an antiproton decelerator to lower the energy of the particles using a magnetic field, and therefore capture them from a collision of protons. These antiprotons were then paired with positrons – the positively charged, antimatter counterpoint to electrons – to make antihydrogen atoms. Don’t try this at home, unless you live in CERN.

The scientists then measured the spectrum of antihydrogen as it moved from its lowest to highest energy state, which is described in a paper published in Nature at an 1S-2S transition. The result is a spectral line, which illustrates how much light gets absorbed at different frequencies. Instead of creating a rainbow, the spectral line is primarily useful because it can be compared to that of hydrogen.

“What we’re looking for is if hydrogen in matter and antihydrogen in antimatter behave in the same way,” says Jeffrey Hangst, a group leader at Alpha. “This is our most precise test to date for that question, and so far they look the same.”

According to the results from this experiment, antihydrogen’s transition from 1S to 2S has the same spectrographic reading as hydrogen. The instruments at CERN were able to glean this result to 12 decimal places, whereas hydrogen’s transition can be measured to 15 decimal places. There’s always scope, therefore, that differences may lie outside of the current levels of precision, but it so far looks like both have identical readings.

If there were to be some divergence, that would be a huge discovery, potentially paving the way to a better understanding of the nature of matter. The result is nevertheless significant, and an important milestone in measuring the structure of antimatter.

How about other antimatter atoms, you ask. The next step up in that direction would be antihelium, which has been artificially produced, but not without great difficulty. To create enough of it to be tested with spectroscopy is currently well out of reach of CERN’s powers, meaning that – at least for now – our hopes for understanding the nature of antimatter lie with antihydrogen.

Image credit: CERN