Storing antihydrogen

Because antimatter annihilates in a flash of energy when it interacts with regular matter, storing it presents a challenge

In 1928 the physicist Paul Dirac proposed that every particle of matter should have an antimatter counterpart. But shortly after the big bang, most of the antimatter disappeared, leaving behind the tiny portion of matter that constitutes the universe we live in today. What happened to swing the balance away from antimatter is one of the greatest puzzles in physics.

Astronomers search for antimatter in space, but it’s hard to come by on Earth. So in order to study it, physicists have to make it themselves. And because antimatter annihilates in a flash of energy when it interacts with regular matter, storing it presents a challenge.

Creating Antihydrogen

The antimatter counterpart to the simplest atom, hydrogen, is a neutral antihydrogen atom, which consists of a positively charged positron orbiting a negatively charged antiproton.

In 1995, physicists at CERN announced that they had successfully created the first atoms of antihydrogen at the Low Energy Antiproton Ring (LEAR). The researchers allowed antiprotons circulating inside LEAR to collide with atoms of a heavy element. Any antiprotons passing close enough to heavy atomic nuclei could create an electron-positron pair; in a tiny fraction of cases, the antiproton would bind with the positron to make an atom of antihydrogen. The antiparticles were highly energetic; each one travelled at nearly the speed of light over a path of 10 metres and then annihilated with ordinary matter after about forty billionths of a second. While creating the antihydrogen was a major achievement, the atoms were too energetic — too “hot”— and didn’t lend themselves to easy study.

In order to understand antimatter atoms, CERN physicists needed more time to interact with them. So they developed techniques to capture and trap antihydrogen for longer periods. The Antiproton Decelerator established at CERN in the late 1990s began providing slower moving, lower-energy antiprotons for antimatter experiments.

In these experiments, electric and magnetic fields hold the antiprotons separate from positrons in a near-perfect vacuum that keeps them away from regular matter. The antiprotons pass through a dense electron gas, which slows them down further.

When the energy is low enough, physicists at the ALPHA experiment use the electric potential to nudge the antiprotons into a cloud of positrons suspended within the vacuum. The two types of charged antiparticles combine into low-energy antihydrogen atoms. Since antihydrogen atoms don’t have an electric charge, the electric field can no longer hold them in place. So instead, two superconducting magnets generate a strong magnetic field that takes advantage of the antihydrogen’s magnetic properties. If the antihydrogen atoms have a low enough energy, they can stay in this magnetic “bottle” for a long time.

Currently the only way to know whether antimatter was actually trapped is to let it annihilate with regular matter. When the magnets are switched off, the antihydrogen atoms escape their trap and quickly annihilate with the sides of the trap. Silicon detectors pick up the energetic flare to pinpoint the antiatom’s position. Only then can the physicists be sure that they had trapped antihydrogen.

Trapping antimatter at CERN

In June 2011, ALPHA reported that it had succeeded in trapping antimatter atoms for over 16 minutes. On the scale of atomic lifetimes, this was a very long time — long enough to begin to study their properties in detail. By precise comparisons of hydrogen and antihydrogen, several experimental groups hope to study the properties of antihydrogen and see if it has the same spectral lines as hydrogen. One group, AEGIS, will even attempt to measure g, the gravitational acceleration constant, as experienced by antihydrogen atoms.

The longer these experiments can trap antihydrogren, the more accurately they can measure it, and physicist will be closer to demystifying antimatter. A new facility called ELENA enables all experiments working at the Antiproton Decelerator to get lower energy and more abundant antiproton beams, making it even easier to produce antihydrogen in large quantities.