Research objective — investigating the structure of nuclear matter

In order to investigate the basic structure of the nucleons and the ways in which they interact with one another, atomic nuclei are forced to collide at around 90 percent of the speed of light. This subjects the nucleons to massive pressure, which in turn compresses them.

"In order to investigate the structure of the proton and neutron shells, we try to make the shells of two particles overlap for a short period of time," explains Dr Wolfgang Koenig. He is part of the research group at the GSI large detector HADES and was involved, as technical coordinator, in setting up the experiment.

"The ideal scenario occurs when lots of protons and neutrons collide head on with one another. Then, for a split second, they form a common system with a higher density." This system endures for less than a sextillionth of a second. However, during that time the density of the system forces the quark-antiquark pairs into a reaction — and it is this reaction that makes it possible to study them. These reactions give rise to short-lived, high-mass special particles, which decay into electron-positron pairs.

HADES is designed to detect these electron-positron pairs, which provide information about the properties of the quark-antiquark pairs. And these properties, in turn, could help explain the origins of the nucleons’ mass.

Tracking electrons in a haze of particles

HADES comprises six identical detector systems arranged as sectors of a circle. Their task is to detect the electron-positron pairs that are formed when a quark-antiquark pair (i.e., a part of the cloud within the proton) collides with further protons. In the process, energy is exchanged between the quark-antiquark pair and the protons. The quark-antiquark pair decays almost immediately, resulting in the formation of, amongst other things, electron-positron pairs.

"We measure the electron-positron pairs, because they do not react with components of the atomic nucleus via the strong interaction," Koenig explains. "They convey the information we need straight to the detectors, without any intermediate steps. The problem is that they form extremely rarely — with a probability of 1 to 10,000."

By detecting particular electron-positron pairs, the researchers are able to investigate the way in which the clouds of two nucleons have overlapped. At the same time, all the other particles produced during the reaction are measured. In addition to the electron-positron pairs, the particles of the greatest interest to scientists in this context are those with the property of "strangeness". This is because such particles yield data that ideally complements the study of the electron-positron pairs. During operation, HADES generates an enormous amount of data. Scientists must therefore be able to properly read and interpret the signals from the detectors.

Step by step towards the truth

The HADES experiment poses two elementary questions: „What does the quark-antiquark cloud look like?” and “Why is it there?”

The objective is to create the theoretical framework for a logical theory of the structure and interaction of nucleons. As Koenig explains, the measurements do not always yield complete answers: "The results always leave room for interpretation and prediction. We have to work towards the truth step by step."