Antiatoms, All Out of Energy and Ready for Work

by Adrian Cho 

Out with a bang. In this artist's rendition, an antihydrogen atom rattles around the ALPHA trap before escaping to create a pair of pions.

Credit: CERN/ALPHA collaboration

Just 6 months ago, physicists reported that they had trapped atoms made of antimatter for a fraction of a second. Now, the same team has held on to individual atoms of antihydrogen, each of which consists of an antiproton bound to a positron, for up to 15 minutes. That's long enough for an atom to lose all of its internal energy and settle into its least-energetic "ground state," a prerequisite for probing its inner workings. The result takes physicists a key step closer to their decades-old goal of precisely comparing hydrogen to antihydrogen in hopes of finding a flaw in a key symmetry between matter and antimatter.

"I'm delighted that if we just turn on a trap, a couple of antihydrogen atoms will stick around long enough to reach the ground state," says Gerald Gabrielse, a physicist at Harvard University who invented the general trapping scheme for antihydrogen and leads a competing experiment known as ATRAP.

As in their previous work, researchers with the ALPHA experiment used a cylindrical array of electrodes to capture in electric fields a puff of antiprotons and a puff of positrons, the antimatter partners of electrons. Working at the European particle physics laboratory, CERN, near Geneva, Switzerland, the team used an additional electric field to slosh the cloud of 15,000 antiprotons through the 1 million positrons, allowing the particles to form antihydrogen atoms. When that happens, the positively charged positron and the negatively charged antiproton cancel each other's charges. Uncharged atoms cannot be bound by an electric field, so the physicists used a magnetic field to grasp the few atoms that formed.

To prove they had trapped an antihydrogen atom, the team first applied an electric field to sweep out any remaining antiprotons and positrons and then, after a delay, turned off the magnetic trap. Eventually, a liberated antimatter atom would then drift into the electrodes, annihilating on contact with ordinary matter to produce a telltale spray of particles called pions. In November, ALPHA researchers reported that they had trapped antihydrogen atoms for 0.172 seconds. This time, they waited longer to turn off the magnetic trap, and in seven of 16 attempts, they held an atom for 1000 seconds. They even succeeded in one of three attempts to hold an atom for 2000 seconds, they report online today in Nature Physics.

That's enough time for an antihydrogen atom to lose its internal energy and reach its ground state, says Jeffrey Hangst, a physicist at Aarhus University in Denmark and leader of the ALPHA team. An atom can possess only certain amounts of energy, so its internal states form a ladderlike arrangement of increasing energy. Each antihydrogen atom forms high on the ladder and works its way down by radiating photons. The ALPHA team didn't prove that its long-held atoms made it to the ground state, but calculations showed they must have. "I would bet my house that they're in the ground state," Gabrielse says.

"This is clearly a very, very important experimental step forward," says Ryugo Hayano, a physicist at the University of Tokyo and leader of the competing Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) experiment at CERN. Researchers would like to measure the arrangement of internal states in antihydrogen and compare it with that in hydrogen, which is known to a precision of one part in 10¹⁴. Any difference would violate a symmetry between matter and antimatter known as charge parity time reversal (CPT) symmetry, which requires, for example, a particle and its antiparticle to have the same mass and lifetime. And if CPT symmetry does not quite hold, then neither can a symmetry of space and time called Lorentz invariance that is the basis for Einstein's theory of special relativity. But to make such measurements, scientists first need to make antihydrogen in its ground state.

It may be years before researchers can measure antiatoms precisely enough to make a stringent test of CPT symmetry, however. Moreover, in spite of its early lead, it's not clear that the ALPHA team will be first to succeed. The ASACUSA team also aims to make such measurements using a different approach with a free-floating beam of antihydrogen atoms. And Gabrielse and the ATRAP team, also working at CERN, are taking a slightly different tack from the ALPHA team by trying to trap many more antihydrogen atoms to make precision measurements easier. Bona fide experiments on antihydrogen are poised to begin. But the race for the prize results will be a marathon, not a sprint.