Photowalk 2018 - Ciaran McGrath
Antimatter hall (AD) (Image: CERN)

The Gravitational Behaviour of Antimatter at Rest experiment’s acronym refers to the fact that it measures the freefall acceleration under gravity of antimatter, which is denoted by g (pronounced g-bar). It operates in the Antiproton Decelerator (AD) Hall, using antiprotons slowed down by the ELENA facility.

GBAR first combines the antiprotons with two antielectrons to form antihydrogen ions with a positive charge. Although more difficult to produce than the simpler antiatoms, the antimatter ions can be more easily manipulated. Using laser-cooling techniques, these ions are brought to microkelvin temperatures before they are stripped of the additional antielectron, transforming them into antihydrogen atoms. These antihydrogen atoms are then allowed to fall from a height of 20 centimetres, and their annihilation at the end of the fall is recorded.

By measuring the acceleration of antihydrogen under gravity and comparing it with the acceleration of regular hydrogen, GBAR scientists can look for differences in the behaviour of matter and antimatter. In particular, the scientists are testing the Equivalence Principle put forth by Albert Einstein, which states that the trajectory of a particle is independent of its composition and internal structure when it is only submitted to gravitational forces. Observing a difference in the way hydrogen and antihydrogen fall under gravity would demonstrate that this principle is in fact wrong.

GBAR was approved by the CERN Research Board in May 2012 and received its first beam of antiprotons from ELENA in October 2018.