Puzzles of the Strong Interaction
The Standard Model of particle physics knows 4 fundamental forces: gravitation, electromagnetism, the strong force and the weak force. The two latter one does only experience in subatomic physics. The strong force is responsible for the binding of protons and neutrons in nuclei, but more fundamentally it describes the interaction of quarks, which are basic building blocks of matter. Quarks have a peculiar property called color – this has nothing to do with optical properties, but is a new quality – a quantum number. The strong interaction is carried by gluons, which are exchanged between the quarks and carry color themselves. (The name "color" was chosen because it comes in three versions or dimensions, like the three fundamental colors red, green and blue.) This color has never been directly observed, as nature does not allow free quarks, but only combinations of quarks, where the colors combine to a neutral state ("white"). This is a peculiar property of the strong interaction, which confines quarks e.g. inside a proton. In addition, the masses of the constituents of a proton, which can be explained by the recently discovered Higgs particle, are much too small to explain the total mass of a proton. Apparently a large fraction of this mass has to be created by the strong interaction in the binding process. In the theory, this is related to the spontaneous breaking of a fundamental symmetry, the chiral symmetry. Both these interesting phenomena, confinement and chiral symmetry breaking, are not fully understood theoretically.
The Quark-Gluon Plasma
At high temperature this behavior is expected to change. The theory of the strong interaction, quantum chromodynamics (QCD), predicts that confinement vanishes above a critical temperature, which is of the order of 1012 Kelvin, or equivalently in the units used in particles physics, 150-200 MeV. A new state of matter is reached, where quarks and gluons are liberated. This phase transition is illustrated in the picture on the right hand side. The upper panel shows a system consisting of a collection of hadrons (e.g. protons and neutrons), also called a hadron gas. The individual hadrons can be understood as independent “bags” confining quarks and gluons. At higher temperature or density these bags will start to overlap (middle panel), and the boundaries between the hadrons loose their meaning. Ultimately, beyond the critical temperature, all bags together form a large volume, in which quarks and gluons can move freely - deconfinement sets in. The new state, a quark-gluon-plasma, has very different properties compared to a hadronic system. The interaction changes, as mentioned above, leading to deconfinement. In addition, theoretical calculations indicate that the chiral symmetry, which is broken at low temperature, will be restored. This should lead to significant changes in the apparent masses of particles. The study of such systems will tell us about theses fundamental properties and should thus greatly enhance our understanding of the interaction itself.