GBAR starts with a straight line: on 1 March the experiment installed its first component – a linear accelerator.

GBAR (Gravitational Behaviour of Antihydrogen at Rest) is an antimatter experiment that will measure the freefall of antihydrogen atoms in the Earth’s gravitational field. The effect of gravity on antimatter is an open question of physics. While theories exist as to whether antimatter will behave like matter or not, so far only a proof of principle experiment has been performed by the ALPHA collaboration.

Located in the Antiproton Decelerator (AD) hall, GBAR is the first of five experiments that will be connected to the new ELENA (Extra Low ENergy Antiproton) deceleration ring. It will use antiprotons supplied by ELENA and positrons created by the newly installed linac to produce antihydrogen ions (antihydrogen atoms with one additional positron).

In sharp contrast to the LHC’s chain of big accelerators and fast particles, the world of antimatter is small and its particles are as slow as they come. The GBAR linac is only 1.2 metres long. It produces electrons and accelerates them to 10 MeV, towards a tungsten target. In the collision, positrons for the antihydrogen ions are created and are later trapped by a magnetic field.

Before they are turned into antihydrogen ions, the antiprotons go through several stages of energy reduction. Starting with a 5.3 MeV antiproton beam in the AD, ELENA reduces the energy by a factor of 50 to just 100 KeV. In April 2016, GBAR will be equipped with its own decelerator, which will bring down the energy of the antiprotons to just 1 KeV.

“With the positrons from the linac, we will create a cloud of electron-positron pairs, called positroniums. When the antiprotons from ELENA pass through the positronium target, they will catch positrons and turn into antihydrogen ions,” explains Patrice Pérez, GBAR’s spokesperson. Indeed, positrons and electrons can very briefly bind together into an exotic atom before annihilating.

While antihydrogen ions are much harder to produce than antihydrogen atoms, their positive charge makes them significantly easier to manipulate. With the help of lasers, their velocity will be reduced to half a metre per second. This will allow them to be navigated to a fixed point. Then, trapped by an electric field, one of their positrons will be removed with a laser, which will make them neutral again. The only force acting on them at this point will be gravity and they will be free to make a 20-centimetre fall, during which researchers will observe their behaviour.

The technology at GBAR has never been used before, which makes it a pioneering experiment. According to the schedule, by September 2018 the installation of all parts will be completed and recording of the first data can begin.

The results might turn out to be very exciting. As Pérez explains: “Einstein’s Equivalence Principle states that the trajectory of a particle is independent of its composition and internal structure when it is only submitted to gravitational forces. If we find out that gravity has a different effect on antimatter, this would mean that he was wrong and that we know very little about the universe.”

Five other experiments are installed at the Antiproton Decelerator, two of which – AEGIS and ALPHA – are also studying the effect of gravity on antimatter.