​The nucleon, proton or neutron alike, has a radius of about one femtometer (10^-15 m). The matter inside the nucleon is distributed in a way that is far from uniform and static. How is it possible to study its spatial structure and its evolution over time, knowing that these are very closely linked due to the relativistic nature of the system?

One way consists of projecting antiprotons onto a target containing protons. The proton-antiproton pairs annihilate, releasing an amount of energy that allows for the creation of other particles. The physicists are seeking to study the "mix" of the six quark-antiquark pairs that precede the annihilation, over a sequence as short as 10^-24 seconds! This is why they exclusively select the events leading to the creation of an electron-positron pair and measure the probability of its occurrence. The reasons why this value is so interesting to the researchers is that it is sensitive to the electromagnetic fields present within the mixture before annihilation. It also provides access to its "time-like" region.

This experiment called PANDA (Antiproton Annihilation at Darmstadt), currently in preparation at the Helmholtz Centre for Heavy Ion Research (GSI), will use antiprotons produced by the Facility for Antiproton and Ion Research (FAIR). A team from IRFU has performed a detailed simulation predicting the signals expected from this machine, set to achieve unmatched accuracy. The PANDA experiment will make it possible for the first time to access individual quantum properties of the proton, electric as well as magnetic, in the explored region.