FIRE antimatter at matter and you would expect to see some spectacular fireworks. But not always, it seems. Sometimes, antimatter can bounce off matter, casting doubt on the long-held notion that a collision between matter and antimatter always makes for an explosive annihilation.
In 2004, Evandro Lodi Rizzini of Brescia University in Italy and colleagues reported the results of an experiment using a beam of antiprotons created by the Low Energy Antiproton Ring at CERN, near Geneva, Switzerland. They fired the antiprotons into a long aluminium cylinder filled with either hydrogen, deuterium or helium gas. The antiprotons, which had energies ranging from 1 to 10 kiloelectronvolts (keV), were duly annihilated in the gas. The team was able to explain the annihilation of nearly 70 per cent of the antiprotons, which had been slowed down and captured by an atom of gas (Physical Review A, vol 70, p 032501).
But for the remaining 30 per cent of the cases, there was a puzzle: though the antiprotons annihilated inside the cylinder, the delay before annihilation was such that these particles must have travelled a distance greater than the length of the cylinder.
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Now the team has reanalysed the data and shown that in these unexpected cases, the antiprotons actually bounced off the end wall of the cylinder. Only after deflecting off the wall were the antiprotons finally captured by an atom of gas and annihilated.
“When antiprotons were fired into a gas-filled aluminium cylinder, some actually bounced off the end wall before annihilating”
The team’s calculations show that nearly 50 per cent of antiprotons could be deflected off the end wall if they had energies as low as 0.5 keV. The work will be published soon in the journal Physical Review A.
According to Lodi Rizzini, the work has implications for the search for localised regions of antimatter in our galaxy. In theory, matter and antimatter should have been created in equal amounts after the big bang, and the absence of antimatter in the universe remains a mystery. Astronomers have searched for clumps of antimatter by looking for radiation spewing out from the edge of such regions – where antimatter and matter should be busy annihilating each other – but to no avail.
The new results suggest that the border between regions of matter and antimatter may not generate as much energy as previously thought, so astronomers should refine their search, says Lodi Rizzini.
He adds that the phenomenon needs to be studied in more detail using dedicated antiproton experiments operating at low energies. CERN’s Antiproton Decelerator, due to start such experiments in the next year or two, would be ideal for the job.
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