The PANDA (Antiproton Annihilation at Darmstadt) Experiment
Understanding the creation of mass - the PANDA experiment at FAIR
We believe that we have identified the fundamental building blocks of nature. Some of them, the quarks, exist only as compounds. Particles that are made up of two or three quarks are called hadrons of which the proton is perhaps the best-known example. Naively the mass of a hadron would be expected to nearly reveal the sum of the mass of its building blocks. However, it turns out that this assumption is far from reality. In the case of protons the mass of the building blocks adds up to only a few percent of the proton mass. The rest of the mass must be generated in a different way. The only explanation is that the interaction between the quarks generates this mass. The planned PANDA experiment at FAIR will study precisely how mass is generated by strong interaction that acts between the quarks.
The PANDA experiment will be done in a unique way by using "antimatter". Why antimatter? When an antimatter particle hits its matter counterpart, Einstein’s famous equivalence between matter and energy kicks in: Antimatter and matter annihilate each other, producing a fireball of energy. This energy is released partly in form of other fundamental particles, called "gluons", which are the particles that bind quarks together through the strong interaction. Therefore a systematic study of the role of gluons in hadrons might reveal the secrets of mass generation due to the strong interaction.
If our understanding of the strong interaction is true, gluons can also form particles. Particles made only of gluons are called "glueballs". Since the annihilation of antimatter with matter produces so many gluons, it is likely that glueballs will form in such a reaction. PANDA is designed to detect these glueballs and measure their masses and other properties. How is this done? Particles like glueballs usually live so briefly that they cannot be measured directly. But the particles into which they decay can be identified and their energies and momenta are measured. This information then allows us to reconstruct the glueball properties.
The PANDA collaboration was formed in 2002. It consists of about 400 physicists from 49 institutions in 16 countries. The PANDA detector itself is a most modern particle physics detector and is located inside the HESR storage ring for antiprotons. It is within this detector that antiprotons circling inside the HESR ring hit protons or other nuclei that cross their path. Since we yet do not know into which particles a glueball decays, the PANDA detector must be able to measure all types of particles. This makes the detector rather complicated equipment. And because these particles are flying in all directions the detector has to surround the target as completely as possible. An additional challenge is the fact that glueballs are a rare species and thus many billions of events must be recorded before a glueball signal can be proven. In the case of PANDA, the detector has to handle 10 million annihilations every second.