Experimental Hadron Physics
Our main field of research is experimental hadron physics. Here, we investigate the strong nuclear force and how it affects quarks and gluons, the building blocks of all matter known to us.
Protons and neutrons
All matter known to us consists of atoms. However, atoms are not fundamental building blocks, but consist of an atomic nucleus surrounded by a shell of electrons. The atomic nucleus, in turn, is made up of positively charged protons and electrically neutral neutrons. The exact composition of this atomic nucleus largely determines the properties of the atom. It determines which element it is. A single proton forms the nucleus of a hydrogen atom. Two protons and two neutrons form the nucleus of a helium atom.
Quarks and Gluons
Are protons and neutrons fundamental particles? No, they are not. Both are made up of even smaller building blocks called quarks. We now know that there are six different quarks: up, down, strange, charm, bottom, and top. A proton is made up of two up quarks and one down quark. A neutron consists of two down quarks and one up quark. They are held together by the strong interaction. Analogous to the electric charge of electromagnetism, quarks have a colour charge, often referred to as red, green and blue. The force acting between the quarks is transmitted by gluons. These also have a colour charge, which leads to interesting phenomena, but also means that quantum chromodynamics, the theory describing the strong interaction, is very complicated to calculate at low energies. It is precisely these low energies in which our reality takes place. For this reason, it is very important to advance our understanding of the strong interaction at low energies through experimental measurements.
Hadron Spectroscopy
One approach to investigating the strong interaction is hadron spectroscopy. Similar to atomic spectroscopy, this involves investigating excited states of various hadrons, which are particles that feel the strong force. One possible method here is excitation using high-energy photons. For example, the proton or neutron can be excited into a Delta or N* resonance by means of a photon. These decay back to their initial state after an extremely short time, emitting light hadrons such as pions. The more we can learn about the excited states, the more conclusions we can draw about the internal dynamics of the proton or neutron and thus about the strong interaction.
Experiments
INSIGHT@ELSA
The newly planned INSIGHT experiment at the ELSA particle accelerator in Bonn will focus on investigating baryons with strange quarks. To this end, experiments will be conducted using polarised photon beams on polarised targets.
GlueX@JLab
The GlueX experiment at Jefferson Lab in Virginia, USA, specialises in the search for exotic mesons. To this end, photoproduction reactions of a 9GeV photon beam on a hydrogen target are being investigated.
AMBER@CERN
AMBER at CERN is a broad-based experiment at the SPS M2 beam line. The current focus is on measuring the proton radius and preparing for Phase II, which will be dedicated to kaon spectroscopy.