

What is Quark Matter ? Quarks can only exist in certain combinations of two or three, forming a hadron (pion, proton, kaon,...) A single quark cannot be alone in the vacuum (it would then have infinite energy) it is therefore not a "particle" in the usual sense, like the hadrons. However, the quarks experience one of the fundamental forces (the strong interaction = gluon exchange between quarks). This gluon exchange represents the elementary form of the strong force, whereas the hadronic interaction is an extremely complicated secondary phenomenon: an effective force that we call nuclear force. Ordinary matter, from atomic nuclei to neutron stars, owes its existence, stability and structure to this effective force. The quarks are at the source of this interaction, but they stay confined, in doublets or triplets, to the interior of the hadrons. A more fundamental state of matter, as structured by the strong interaction would be obtained if one could "melt" the hadron bubbles in ordinary nuclear matter (composed of protons and neutrons), such as to deconfine the quarks from the hadron volume to the extended volume of an entire atomic nucleus. Such a state, if it exists, would be called Quark Matter.