Scientists at CERN have achieved another milestone in the investigation of antimatter. For the first time, they observed the particular light emitted by an excited atom of antihydrogen, a finding that expands our knowledge on the subject but also opens the door to new possibilities for studying antimatter.
As reported in Nature, researchers from the ALPHA project – a collaboration of physicists based at CERN working on trapping antihydrogen atoms – observed the Lyman-alpha transition for antihydrogen (hydrogen's antimatter counterpart) for the first time.
Physicists are very curious about antimatter. We know antimatter exists but it is not clear to us why the universe is made almost exclusively of matter. All our experiments so far have shown they both behave in the same way, though there are hints at some differences, which researchers hope to discover that could explain this universal disparity.
The Lyman-alpha transition has been observed in matter before, but to observe it behaving in the same way in antimatter adds to the intrigue of why the universe is the way it is.
In regular matter, when an excited electron goes from a higher energy level (or orbital) known as 2P to its ground state it emits an ultraviolet photon at the specific wavelength of 121.6 nanometers.
In antimatter, the role of the electron is played by the positron, which has the same mass but opposite charge. Although antimatter is extremely rare in the universe, positrons can be found quite easily; sodium-22 is a source, for example, which is what they used here. The ALPHA team then made those positrons interact with antiprotons – which are created by shooting a beam of regular protons at a block of metal – forming antihydrogen, and trapped them in a magnetic trap, which stops them from coming into contact with regular matter and annihilating.
They then shot laser pulses at the substance, which excites the positrons, and they transitioned just like regular matter, emitting a photon. The energy transition from one level to the other is quantized, which means it’s always the same value, so the emitted Lyman-alpha photon will always look the same.
“We are really excited about this result,” Jeffrey Hangst, spokesperson for the ALPHA experiment, said in a statement. “The Lyman-alpha transition is notoriously difficult to probe – even in ‘normal’ hydrogen. But by exploiting our ability to trap and hold large numbers of antihydrogen atoms for several hours, and using a pulsed source of Lyman-alpha laser light, we were able to observe this transition.”
Having antihydrogen emit Lyman-alpha light gives scientists a new method to study antimatter. This emission is key to laser-cooling so it could be possible to make cool and dense samples of antihydrogen. This would allow a completely different study of antimatter where precision spectroscopy and gravitational measurements could be made with high precision.