Antimatter atom trapped and measured with a laser for first time

By Leah Crane

Hydrogen’s antimatter counterpart has shown its true colours, and they are just what physicists ordered.

Antihydrogen atoms are made of a positron (a positively charged version of the electron) orbiting a negatively charged antiproton. According to the standard model of particle physics, these anti-atoms should absorb and emit light at the same wavelengths as hydrogen. Now antihydrogen’s spectrum has been measured at last, and it confirms the prediction.

Antimatter is notoriously difficult to work with, because the moment it touches normal matter both annihilate in an explosion of smaller particles and radiation. To scrutinise antimatter, physicists have to keep it as cold as possible and trap it using powerful magnetic fields.

Jeffrey Hangst and his colleagues in the ALPHA collaboration, based at CERN near Geneva, Switzerland, managed to trap 14 antihydrogen atoms at once – a dramatic improvement over the one or two atoms of previous experiments. To peek at the spectrum, they shone an intense laser on to the antihydrogen atoms, so that the beam’s energy could be absorbed and re-emitted.

The team found that the lowest-energy photons in the antihydrogen spectrum had the same wavelength as for hydrogen.

Because no one has persuaded an atom of antimatter to interact with a laser before, the measurement is still vastly less precise than our best observations for hydrogen.

Improving the accuracy might flag up any possible discrepancies in energy levels between the two, which would undermine the standard model of physics.

“This is really a milestone that they have achieved,” says Michael Doser of CERN’s AEgIS collaboration, one of ALPHA’s competitors in the race to probe antimatter.

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