Particle Physics

Antimatter charge

Print edition : July 25, 2014

A view of the ALPHA experimental set-up. Photo: ALPHA/CERN

RESEARCHERS of the ALPHA experiment at CERN in Geneva have for the first time measured the electric charge of an anti-atom to high precision. Measuring the electric charge of antihydrogen atoms is a way to study any subtle difference 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 scientists’ expectation that the charges of its constituents, the positron and the antiproton, are equal and opposite.

Though the result is not surprising, it is a fundamental test that matter and antimatter have equal and opposite electric charges. ALPHA is an international collaboration whose aim is stable trapping of antihydrogen atoms, the antimatter counterpart of the simplest atom, hydrogen. By precise comparisons of hydrogen and antihydrogen, the experiment hopes to study fundamental symmetries between matter and antimatter. The ALPHA experiment, which uses the Antiproton Decelerator programme at CERN, is set to restart in August, and according to researchers, antihydrogen properties will continue to be studied 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. Scientists know, 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, they do not know why, so it is important to measure the properties of antimatter to great precision. ALPHA achieves this by using a complex system of particle traps that allows antihydrogen atoms to be produced and stored for long enough periods to make detailed studies. 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, the plus or minus numbers representing statistical and systematic uncertainties on the measurement.