Currently, the universe seems to be composed entirely of matter – the only antimatter around is created at places like CERN. Yet theories predict that exactly equal amounts of matter and antimatter would have been created in the Big Bang. So where did all the antimatter go?

This new research, undertaken by the ALPHA experiment at CERN's Antiproton Decelerator (AD) in Geneva, is the first time that the electric charge of an anti-atom has been measured to high precision. Measuring the electric charge of antihydrogen atoms is a way to study any subtle differences between matter and antimatter which could account for the lack of antimatter in the universe.

In a paper published in the journal Nature Communications, the ALPHA experiment reports a measurement of the electric charge of antihydrogen atoms, finding it to be compatible with zero to eight decimal places. This is the first time that the charge of an anti-atom has been measured to high precision and confirms expectations that the charges of its constituents, the positron and antiproton, are equal and opposite.

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Professor Mike Charlton, who leads the UK effort in ALPHA from Swansea University said: "This is the very first study which has made a precise determination of a property of antihydrogen. This advance was only possible using ALPHA's trapping technique, and we are optimistic that further developments of our programme will yield many such insights in the future. We look forward to the restart of the AD program in August, so that we can continue to study antihydrogen with ever increasing accuracy."

Antiparticles should be identical to matter particles except for the sign of their electric charge. So while the hydrogen atom is made up of a proton with charge +1 and an electron with charge -1, the antihydrogen atom consists of a charge -1 antiproton and a charge +1 positron. However, that matter and antimatter are not exact opposites - nature seems to have a one-part in 10 billion preference for matter over antimatter. However as it is not known why, it is important to measure the properties of antimatter to great precision: the principal goal of CERN's AD experiments.

ALPHA achieves this by using a complex system of particle traps that allow antihydrogen atoms to be produced and stored for long enough periods to make detailed studies. Understanding the matter-antimatter asymmetry is one of the greatest challenges in physics today. Any detectable difference between matter and antimatter could help solve the mystery and open a window to new physics.

To measure the charge of antihydrogen, the ALPHA experiment studied the trajectories of antihydrogen atoms released from the trap in the presence of an electric field. If the antihydrogen atoms had an electric charge, the field would deflect them, whereas neutral atoms would be undeflected. The result, based on 386 recorded events, gives a value of the antihydrogen electric charge as (-1.3±1.1±0.4) × 10-8, the plus or minus numbers representing statistical and systematic uncertainties on the measurement.

With the restart of CERN's accelerator chain getting underway, the laboratory's antimatter research programme is set to resume.

UK funding of the ALPHA project has come largely from the EPSRC over a period of more than 15 years, and supports teams from Physics Departments in the Universities of Swansea, Liverpool and Manchester.