AEGIS: The experiment

Does antimatter fall down or up?

Help the AEgIS experiment at CERN to work out how antimatter is affected by gravity. Just join the dots!

Participate in the discussions about this project, and find out more!


The observable universe is composed almost entirely of matter but this is a huge surprise. In fact, every time one tries to make a particle from energy (E=mc²), that particle's antiparticle also appears. Antiparticles are a little bit like the mirror image of particles: they have the same mass as normal matter particles but the opposite charge. When an antiparticle collides with an ordinary matter particle (of the same type) they both annihilate - producing a burst of other particles, antiparticles and radiation.

Antiparticles should interact gravitationally just like particles of ordinary matter because Einstein's weak equivalence principle states that gravity doesn't depend on composition. But no direct measurement of this behavior has been possible since gravity is so much weaker than any of the other forces, and if antiparticles were to behave differently than particles, then gravity would be much more complicated than our current understanding indicates. To actually be sensitive to the very weak force of gravity requires turning off all other forces, by studying neutral antimatter, such as antihydrogen atoms.

The experiment

The AEgIS experiment at CERN aims to shoot antihydrogen atoms horizontally - whereupon they fly and fall - at a wall made of matter. On hitting the wall the antihydrogen annihilates with a matter nucleus in the wall to produce a burst of mostly pions and some heavier particles. These particles travel through a special gel called a photographic emulsion which makes their tracks visible. Pions leave thin tracks while heavier particles leave much fatter tracks.

Tracing these tracks to their point of origin tells the AEgIS team exactly where the annihilation occurred, which in turn allows them to calculate how far each particle travels. They can then work out - from the distance each antihydrogen atom flew and fell - how antimatter interacts with gravity.

Your mission

AEgIS will start producing and annihilating antihydrogen atoms in 2015. In the meantime the team needs to better understand the process of annihilation by shooting antiprotons at different target materials which will result in different kinds of particle bursts. AEgIS scientists need to fine-tune their understanding of annihilation by mapping the particle tracks and counting the number of thin and fat tracks for many particle bursts and different targets.

Humans are way better at interpreting the particle tracks than machines so the AEgIS team needs your big brains and keen eyes to map the particles’ path through the emulsion. All you have to do is find the track's paths and indicate where you think they cross!

AEgIS scientists also want to be able to classify each track as fat (dark) or thin (light, visible dots). The light tracks are pions, and come directly from the annihilation of antiprotons with protons or neutrons; the dark tracks are fragments of the nucleus that the annihilation manages to break apart. Find out more on fragmentation here

The images here have been uploaded directly from AEgIS and have not yet been analyzed by scientists. In fact, this is the first time CERN has released detector data for public analysis. So you can be the first to analyze this data and make a real contribution to CERN’s research.

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