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.
Why do we search for Quark Matter ?
Because this would test the Standard Model of strong interaction (Quantum Chromodynamics = QCD) under circumstances that cannot be obtained in energetic collisions of small objects like hadrons and leptons.
These studies have confirmed the Standard Model at an ever decreasing distance scale, and an ever increasing energy scale, of the interacting quarks, gluons and leptons, to the extreme of uncovering their behaviour as point-like objects carrying a set of quantum numbers.
Other aspects of the Quantum Chromodynamics theory, like the "horror vacui" behaviour of individual quarks (that they have to form colour-neutral bound states in order to survive in empty space) are still ad-hoc assumptions.
They are guesses, derived from the properties of the known hadrons, imbedded into the QCD theory.
The Lattice Gauge Theory prediction of the existence of Quark Matter (an extended structure, at a distance and energy scale intermediate between the extremes of point-like quark scattering and extended hadron and nuclear ground state structure) suggests a test of QCD in this "ad Hoc" sector of the Standard Model.
Furthermore, the initial cosmological expansion (Big Bang) proceeding from a state of extreme energy density down to our familiar world of cold, dilute stable particles, matter, planets, stars, galaxies etc., is supposed to have gone through a Quark Matter state, at intermediate density. Pinning down its thermodynamic properties (if it can be proven to exist) and decay products, upon expansion, we would contribute key data to astrophysics studies.
How do we search for Quark Matter ?
How can we re-create conditions of high energy density as they existed in the early Big Bang ? In fixed target experiments (like NA49 at CERN) we bombard heavy nuclei that are accelerated to near light velocity onto nuclei in a thin metal foil (target).
A head-on collision of a Lead (208 Pb) projectile with a Lead target nucleus, at the SPS beam energy of 160 GeV per nucleon in the Pb projectile, may compress and heat the nuclear matter contained in the two nuclei. (A)
It may thus reach the required energy density (20-fold higher than that of the initial nuclei) in a short-lived "fireball" volume. (B)
After about 8^10-23 sec this state expands, cools down and emits hadrons (pions, kaons, lambdas, phi.....) into our detector system. (C)
To this end we employ about 180000 electronic channels (each consisting of a preamplifier, shaper, analog register and storage, and an ADC) These are multiplexed together into light fibers that take the information from the detectors into the counting house where it is formated into a 12 Megabyte-size format for each event, and written to tape.
This way we record the particle trajectories, and the ionisation strength of each particle. In addition, we measure the flight time of the particles in two Time of Flight (TOF) walls placed behind the TPC's. This allows us to identify and determine momentum of all charged particles produced in Pb+Pb head-on collision. We can also identify neutral "strange particles" (lambdas, antilambdas, K0, phi) by their secondary decay into charged particles.
Any electrons produced in heavy ion collisions escape unscathed. Since many particles produced in such collisions decay into electron pairs, these can be used to study how the collision evolves. Above a certain temperature, quarks and the particles made up from them, lose their mass. Calculations suggest that this should happen at the about the same temperature as deconfinement. Consequently, in deconfined matter some of the particles which decay into electron pairs are expected to be lighter, making them easier to produce, so there will be more of them around, and more electron pairs should come out. When CERN smashed sulphur and lead ions into a gold target, this is exactly what the experiments saw.
Another hint that a QGP might briefly have formed has come from studying the number of particles produced which contain strange quarks, which, like charm quarks, are heavier than those in ordinary matter. The number of strange quarks inside a QGP is expected to be large because even though strange quarks are heavy and normally hard to produce, the QGP system is expected to be more stable the more different kinds of particles it contains. As the QGP expands and cools, the free quarks and gluons group together again into composite particles. Counting the number of these particles containing strange quarks can give information about the evolution of the collision and about whether QGP could have been formed.
Experiments do indeed see the number of strange quarks rise as collisions get hotter, but are not yet at a point where this can be considered conclusive evidence for the formation of QGP.