The Large Hadron Collider at CERN, Europe's particle-physics lab near Geneva, Switzerland, currently lies in pieces, with engineers working on boosting its power. At the same time, in a side hall, an upgrade is taking place to an experiment that may allow physicists to measure the properties of atoms of antimatter.
It is a goal that researchers have been chasing since the first antihydrogen atoms were made at CERN in 1995. An antihydrogen atom consists of an antiproton and a positron, which respectively have the same mass as an ordinary proton and electron, but opposite charge. Beyond that, researchers know very little about antihydrogen. “Do matter and antimatter atoms obey the same laws of physics?” asks Jeffrey Hangst, spokesman for ALPHA, one of the collaborative efforts to make and analyze antihydrogen.
The experiments at CERN might also help to explain why there is more matter than antimatter in the visible Universe. The Big Bang should have created equal amounts of the two that would have annihilated on contact. But somehow, matter gained an advantage. Differences have been observed between the behavior of some matter and antimatter particles, such as kaons and mesons, but these are far too small to explain the Big Bang conundrum.
To create antihydrogen atoms, researchers at CERN first make antiprotons by bombarding atoms with accelerated protons, then slow them down by passing them through metallic foil, cool them with cold electrons and trap them with electromagnetic fields. A similar trap accumulates positrons that are emitted by radioactive materials. When the clouds of charged particles are mixed, they make neutral antimatter atoms. But because these have no overall charge, in early experiments they easily escaped the electromagnetic fields used to trap the charged antimatter particles.
By 2002, two collaborations had been able to make as many as 50,000 atoms of antihydrogen, but the atoms quickly annihilated on the walls of their container. It took until 2010 before researchers at ALPHA showed how to trap the atoms using three magnets with a combined field sufficient to restrain antihydrogen, with its tiny magnetic moment. At that time, the antimatter was held for just 170 milliseconds, and only about one atom was trapped for every eight times the group ran the 20–30 minute experiment, says Hangst. But the team has improved its equipment to trap one atom per experiment, and hold it for about 1,000 seconds.
ALPHA is now trying to probe the properties of the anti-atoms. This year, the team reported watching the tracks of hundreds of antihydrogen atoms after they were released from their magnetic cage, to test whether antimatter falls up or down under gravity. The researchers do not yet have an answer, but the experiment works in principle, says Hangst. And in the upgrade, the team is moving in some lasers, with the idea of testing next year whether antihydrogen absorbs and emits light at the same frequencies as hydrogen.
Other teams at CERN are experimenting with different aspects of antimatter, such as how antihydrogen responds to changing magnetic fields. And researchers elsewhere are looking at even more exotic atoms: Ryugo Hayano, a physicist at the University of Tokyo, leads a team studying mixed matter–antimatter atoms, such as antiprotonic helium, in which a helium nucleus is surrounded by one electron and one negatively charged antiproton, an arrangement that lasts for a few microseconds.
In the end, such experiments may not find differences between matter and antimatter that are big enough to explain why the former has prevailed over the latter. But, says Hangst, “one never knows where the new physics might show up. You just have to keep looking.”