The latest results from the Antihydrogen Laser Physics Apparatus (ALPHA) experiment at the CERN particle-physics laboratory in Geneva have confirmed that the electric charge of antihydrogen is indeed neutral. The experiment has improved the measurement precision of the charge of antihydrogen – a bound antiproton and positron – by a factor of 20 compared with previous results. Because the charge of the antiproton is already known to a similar precision as this latest measurement, the result also helps to refine the bound on the charge of the positron.

One of the big unanswered questions in physics is why our universe contains so much more matter than antimatter today, when equal amounts of both were thought to have formed after the Big Bang. As the Standard Model of particle physics can offer no explanation for this missing antimatter, measuring tiny differences between the behaviour of matter and antimatter could shine a light on this cosmic conundrum.

Current theories say that antimatter particles are identical to their matter counterparts, but with an opposite charge. A hydrogen atom is made up of a positively charged proton and negatively charged electron, and has zero net charge. Similarly, an antihydrogen atom is made up of a negatively charged antiproton and a positively charged positron, and should also be neutral. An asymmetry between matter and antimatter must lie somewhere, and one possible difference is that antihydrogen has some very subtle but measurable net charge. However, this has been difficult to test because of the experimental challenges involved with trapping and holding antihydrogen for long enough to make precise measurements of its charge.

Caught in a trap

ALPHA’s main aim is to study the internal structure of the antihydrogen atom and see if any discernible differences set it apart from regular hydrogen. In 2010, ALPHA was the first experiment to trap 38 antihydrogen atoms for about one-fifth of a second. The team then perfected its apparatus and technique to trap a total of 309 antihydrogen atoms for 1000 s in 2011.

With the launch of their new ALPHA-2 magnetic trap last year, and use of lasers for spectroscopy, ALPHA spokesperson Jeffrey Hangst told physicsworld.com that the team will study the antihydrogen atom’s spectrum and pick up any differences between it and hydrogen, if they exist. In its latest experiment, the team looked at the trajectories of antihydrogen atoms released in the presence of the trap’s electric field. If the antihydrogen atoms have an electric charge, the electric field would deflect them and alter their trajectories.

ALPHA-2’s latest measurement has shown that antihydrogen, just like its matter counterpart, has no charge. “That means the electrical charge of antihydrogen – the antimatter analogue of hydrogen – can be ruled out as the answer to the antimatter question,” says ALPHA team-member Scott Menary, at York University in Canada. Indeed, the result showed that both are electrically neutral at a level 20 times more precise than before. Because the charge of an antiproton is also known to a similar precision, the collaboration has also improved the previous best measurement on the positron charge by a factor of 25.

ALPHA researcher Andrea Capra, at the TRIUMF lab in Canada, says that this result is one piece in the antihydrogen puzzle, the others being comparisons of hydrogen and antihydrogen’s spectra and how antihydrogen responds to gravity, both of which are also being probed by the ALPHA team.

The research is published in Nature.

• To find out more about ALPHA and the study of antimatter, listen to our podcast on “The masters of antimatter”.