Antimatter atoms trapped for first time

Posted by Emma Woollacott

Scientists at CERN have succeeded for the first time in capturing and storing antimatter - albeit for only a fraction of a second. The team successfully created 38 hydrogen 'anti-atoms', and hung onto them long enough to study.

Antimatter – or, rather, the lack of it – remains one of the biggest mysteries of science. At the Big Bang, matter and antimatter should have been produced in equal amounts; but the antimatter seems to have since disappeared. One way of working out what might have happened to it is to examine whether antihydrogen behaves in the same way as hydrogen.

CERN first made antihydrogen a full eight years ago, but the antiatoms were destroyed within a few millionths of a second, as soon as they came into contact with the matter of the walls of the experiment.

"Trapping antihydrogen proved to be much more difficult than creating antihydrogen," says Joel Fajans, a professor of physics at UC Berkeley. "ALPHA routinely makes thousands of antihydrogen atoms in a single second, but most are too 'hot' to be held in the trap. We have to be lucky to catch one."

In order to avoid the immediate annihilation of their antimatter, the CERN team used strong, complex magnetic fields to trap it for about a fifth of a second.

The main component of the Minimum Magnetic Field Trap is an octupole - eight-magnetic-pole - magnet whose fields keep antiatoms away from the walls of the trap.

The antiatoms were created and stored during 335 experimental trials, each lasting one second. To form antihydrogen during these sessions, antiprotons were mixed with positrons inside the trap. As soon as the trap's magnet was 'quenched', any trapped anti-atoms were released, and their subsequent annihilation was recorded by silicon detectors.

"Proof that we trapped antihydrogen rests on establishing that our signal is not due to a background," says Fajans.

While many more than 38 antihydrogen atoms are likely to have been captured during the 335 trials, the researchers were careful to confirm that each candidate event was in fact an anti-atom annihilation and was not the passage of a cosmic ray or, more difficult to rule out, the annihilation of a bare antiproton.
               
To discriminate between real events and background, the ALPHA team used computer simulations based on theoretical calculations to show how background events would be distributed in the detector versus how real antihydrogen annihilations would appear.

The next stage will be to carry out experiments on antimatter - crude experiments, but the very first of their kind, says CERN.

"These are significant steps in antimatter research, and an important part of the very broad research programme at CERN," says CERN director general Rolf Heuer.