In a paper published in the August 3, 2017 issue of the journal Nature, physicists with the ALPHA collaboration, a multinational project based at CERN, report the first detailed observation of spectral lines from an antihydrogen atom, the antimatter counterpart of the simplest atom, hydrogen.

Matter and antimatter are believed to be mirror images of one another, and so the spectral lines of antimatter atoms should be precisely the same as those of their normal atom counterparts. Whether or not this is true is unknown.

Until now, physicists have only had glimpses of antimatter spectral lines, and comparisons with normal matter spectral lines have been coarse.

“The existence of antimatter is well established in physics, and it is buried deep in the heart of some of the most successful theories ever developed,” said co-author Professor Mike Charlton, from Swansea University.

“But we have yet to answer a central question of why didn’t matter and antimatter, which it is believed were created in equal amounts when the Big Bang started the Universe, mutually self-annihilate?”

“We also have yet to address why there is any matter left in the Universe at all. This conundrum is one of the central open questions in fundamental science, and one way to search for the answer is to bring the power of precision atomic physics to bear upon antimatter.”

The ALPHA collaboration has made antihydrogen by replacing the proton nucleus of the ordinary atom by an antiproton, while the electron has been substituted by a positron.

“One of the challenges we face is that matter and antimatter annihilate when they come into contact with one another,” said co-author Justine Munich, a PhD candidate at Simon Fraser University.

“We have to keep them apart. We can’t just put our anti-atoms into an ordinary container. They have to be trapped or held inside a special magnetic bottle.”

In 2016, the physicists used UV light to detect the so-called 1S-2S transition between positron energy levels.

Now, they have used microwaves to flip the spin of the positron. This resulted not only in the first precise determination of the antihydrogen hyperfine splitting, but also the first antimatter transition line shape, a plot of the spin flip probability versus the microwave frequency.

“The data reveal clear and distinct signatures of two allowed transitions, from which we obtain a direct, magnetic-field-independent measurement of the hyperfine splitting,” the researchers said.

“From a set of trials involving 194 detected atoms, we determine a splitting of 1,420.4 ± 0.5 MHz, consistent with expectations for atomic hydrogen at the level of four parts in 10,000.”

“By studying the properties of anti-atoms we hope to learn more about the Universe in which we live,” said co-lead author Professor Michael Hayden, also from Simon Fraser University.

“We can make antimatter in the lab, but it doesn’t seem to exist naturally except in miniscule quantities.”

“Why is this? We simply don’t know. But perhaps antihydrogen can give us some clues.”

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M. Ahmadi et al. 2017. Observation of the hyperfine spectrum of antihydrogen. Nature 548: 66-69; doi: 10.1038/nature23446