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Untangling the question of antimatter mass

Boffins spin up positronium

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A group of researchers at University of California Riverside hopes to chip away at one of physics’ ‘question of questions’ – why the blazes we’re here at all. Their hope is to make electron/positron pairs live long enough to measure the positron’s mass and find out if it’s different to the electron.

It’s a puzzle that resists solution: somehow, just after the Big Bang, the Universe we inhabit ended up with a theoretically-unexpected characteristic: a little more matter than antimatter. This violation of symmetry (baryogenesis – at least, I think that’s how it’s spelled and what it means) gives rise to the fundamental particles of ordinary matter.

If matter and antimatter can be demonstrated to have somehow different properties, it would be a step in the right direction: it would at least give us a pointer as to what might give rise to the matter/antimatter difference.

The Riverside researchers are taking the first tentative steps to look at one such possibility – that matter and antimatter have different mass.

To provide a useful test of the mass of an antimatter particle, you need one that exists for long enough to respond to measurement – always difficult, since matter and antimatter annihilate on contact.

The aim of the Riverside project is to create positronium (an electron and its antimatter counterpart, a positron, in a bound state) to survive for as much as ten milliseconds – ten thousand times longer than it usually exists, according to Allen Mills, an assistant project scientist working under David Cassidy in the university’s Department of Physics and Astronomy.

Their approach, described here in R&D, is to have the positronium electron/positron pair exist in an atom in a Rydberg state. In this highly excited state, the “outermost” electrons orbit at maximum distance from the nucleus (observing that we’re using “orbit” as a simple model of the atom, commenters please don’t bother reminding me that the model isn’t complete).

The distance separating the bound electron and positron gives them a longer-than-usual survival time.

"Using lasers we excited positronium to what is called a Rydberg state, which renders the atom very weakly bound, with the electron and positron being far away from each other," said Cassidy. "This stops them from destroying each other for a while, which means you can do experiments with them."

So far, though, the team has only achieved a 10-to-100 times longer lifetime for the positronium.

"But that's not enough for what we're trying to do," Cassidy said. "In the near future we will use a technique that imparts a high angular momentum to Rydberg atoms. This makes it more difficult for the atoms to decay, and they might live for up to 10 milliseconds — an increase by a factor of 10,000 — and offer themselves up for closer study."

With longer-lifetime atoms to work with, Cassidy says, the laboratory will be able to create beams of the excited atoms – and the deflection of those beams due to gravity might tell them whether positronium’s mass is different to that of normal matter.

While other experiments have created long-lived antimatter (CERN has created anti-hydrogen with a thousand-second lifetime), what's notable about the Riverside experiment is that it seeks to create a long-lived antimatter that can be put in a beam.

A difference – which, by the way, Cassidy says he doesn’t expect to see – would go some way, at least, to explaining the asymmetry that gives the universe more matter than antimatter. ®

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