CERN Accelerating science

The ALPHA (Antihydrogen Laser PHysics Apparatus) collaboration at CERN is testing Charge-Parity-Time (CPT) symmetry through precise measurements with antihydrogen atoms and in the future will measure antihydrogen's free fall acceleration in Earth's gravitational field. The antihydrogen atoms are created by slowly merging cold plasmas of antiprotons and positrons. The production rate is highly sensitive to the parameters of these plasmas and typically only a few atoms have been available at a time for measurements.

Antihydrogen, composing an antiproton and positron, is the only bound state of two antiparticles yet to be synthesised, making for an enticing system to study the purported symmetry of matter and antimatter. As antihydrogen does not occur naturally in the observable universe, any study of this atom requires it to be synthesised in a lab, which the ALPHA experiment is routinely able to do. However, the absolute numbers are small and efficient detection is crucial for the experiment.

The Einstein Equivalence Principle (EEP) has never been directly examined with an antimatter test body. To address this, ALPHA is planning to measure the Earth’s gravitational field using antihydrogen atoms as test masses. The experiment calls for the careful release of antiatoms from a magnetic trap and requires precise characterization of the magnetic fields that are used.

The bound state of an antiproton and positron, antihydrogen, is an ideal test particle for comparisons between matter and antimatter as hydrogen has been studied extensively through history both experimentally and theoretically. The Antihydrogen Laser Physics Apparatus (ALPHA) collaboration has made significant progress on antihydrogen trapping, cooling, and spectroscopy in recent years.In a new apparatus, ALPHA-g, the collaboration aims to probe the effects of gravity on antimatter.