It’s a cool achievement (Image: Neils Madsen/ALPHA)

Editorial: Why antimatter matters

ATOMS made of antimatter have been trapped for the first time, a feat that will allow us to test whether antimatter responds to the fundamental forces in the same way as regular matter.

Antiparticles are the oppositely charged twins of normal particles. Since matter and antimatter annihilate on contact, antimatter experiments have been limited to using charged antiparticles, which can be corralled within electromagnetic traps.


Several teams have made antihydrogen atoms in the past, but no one had managed to trap them for detailed experiments as they have no net charge. Now an experiment called the Antihydrogen Laser Physics Apparatus (ALPHA) at the CERN particle physics laboratory near Geneva, Switzerland, has finally managed to ensnare atoms of antihydrogen.

ALPHA produced anti-atoms by combining antiprotons from CERN’s Antiproton Decelerator ring with positrons emitted by a radioactive isotope of sodium. Where it went one better than previous experiments was in being able to manipulate the anti-atoms magnetically.

Even though anti-atoms are electrically neutral, they do behave like tiny magnets and will respond to a magnetic field. This response is so weak, however, that the anti-atoms have to be moving very slowly if they are to be captured magnetically.

With this in mind, the ALPHA team members tried to create sluggish anti-atoms by bouncing antiprotons at -70 °C off much colder positrons at -230 °C. The antiprotons lost energy in the collisions before some finally combined with the positrons to form antihydrogen. The slowest anti-atoms, at a temperature of just -272.5 °C, then became trapped in a powerful cylinder-shaped magnetic field created by superconducting magnets. The field was then turned off so the antihydrogen could annihilate with normal matter, creating particles that silicon detectors picked up.

After 335 runs of the experiment, mixing around 10 million antiprotons and 700 million positrons, only 38 of the antihydrogen atoms the team made were moving slowly enough to be trapped (Nature, DOI: 10.1038/nature09610). “Our efficiency isn’t very good yet,” says ALPHA spokesman Jeffrey Hangst of Aarhus University in Denmark. “We make a lot more antihydrogen than we can trap.” The team expects their success rate to improve when they start using a new antiproton cooling technique tested earlier this year.

The achievement means researchers can now test whether anti-atoms obey the same physical laws as regular atoms. For example, matter and antimatter should absorb and emit light at the same wavelengths, according to the standard model of particle physics.

“This is an encouraging step towards the goal that I laid out long ago – to confine useful numbers of cold antihydrogen atoms long enough for precise laser spectroscopy,” says Gerald Gabrielse of Harvard University. He heads a rival experiment at CERN called ATRAP.

If the spectrum of antihydrogen does not match that of ordinary hydrogen, it would leave the standard model in disarray. But any discrepancies could shed light on the long-standing mystery of why the universe is dominated by matter when the big bang should have created equal amounts of matter and antimatter, Gabrielse says.