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CERN boffins zap antimatter with ultraviolet lasers in the hope of revealing the secret symmetry of the universe

If you can't measure it, you can't research it

A team of European researchers have succeeded in slowing down antimatter in a study that could lead to more accurate measures of this strangely elusive substance and help confirm the fundamental symmetry of nature.

Antimatter has certain properties – such as electric charge – which are inverted from those of normal matter. In this anti-universe, the anti-electron (aka positron) has a +1 electrical charge, and the antiproton has a −1 electric charge. However, anti-particles do have the same mass as their matter counterparts.

Antimatter is also tricky to work with. If an anti-particle comes into contact with its counterpart, "they annihilate one another, leaving behind pure energy."

As well as providing endless plot devices for science-fiction writers – and powering the Starship Enterprise – antimatter promises to help scientists examine some questions about the fundamental nature of the universe if they could only get the stuff to slow down.

A research team led by Jeffrey Hangst, a physics professor at Denmark's Aarhus University, has demonstrated they can do just that in a study published in Nature last week.

Using an ultraviolet laser beam, the team zapped anti-hydrogen – the antimatter counterpart of hydrogen (the simplest atom) consisting of a positron orbiting an antiproton – at a precise frequency designed to allow the anti-H atoms to absorb the electromagnetic energy.

Accounting for the Doppler effect from the perspective of the moving particles, this tuned signal exactly matched the photon energy needed to be absorbed by the atoms to promote them from their ground state to an excited state.

"The atoms then spontaneously returned to the ground state by emitting another photon in a random direction," explained Masaki Hori, a research group leader in antimatter spectroscopy at the Max Planck Institute, in an accompanying article.

This photo-emission slowed down the anti-atom by an order of magnitude, from 300kmh down to below 50kmh. "This reduction in speed corresponds to a cooling of the atoms," Hori said.

Laser cooling has been a standard feature of research in atomic physics for more than a decade and is used in quantum degenerate gases, quantum information, atomic clocks, and tests of fundamental physics, the paper authors said. However, this is the first time it has been successfully applied to antimatter.

It might be an important avenue for research because closely probing the properties of antimatter in experiments would be a good way to test out some basic assumptions about the universe. For example, so-called charge, parity, and time-reversal (CPT) symmetry says that "if all of the matter in the Universe were simultaneously replaced with antimatter and transformed into its mirror image, and the flow of time-reversed, then the resulting hypothetical universe would be indistinguishable from our own at the microscopic level," Hori explained.

These ideas were developed by Julian Schwinger, Wolfgang Pauli, and others in the 1950s. But antimatter was always too short-lived to test back then, disappearing as it does in a puff of quantum-mechanical energy as soon as it interacts with matter.

In the 1990s, researchers succeeded in bottling antimatter for longer periods in magnetic fields, but the problem of these particles moving too fast to observe in detail remained.

Hangst and the team's work on using ultraviolet laser to slow down antimatter is "a major step towards this goal," Hori added. ®

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