The quark-gluon plasma (abbreviation QGP) is a state of matter at extremely high temperatures or baryon densities. Here the confinement of quarks and gluons is removed, which is why these particles show a quasi-free behavior.
It is assumed that the universe passed through this state in the first fractions of a second after the Big Bang. In today's universe, the QGP exists at most in the center of neutron stars, where some theories predict another phase characterized by color superconductivity.
The use of heavy ion accelerators enables the research of the quark-gluon plasma (QGPs) in the laboratory. Corresponding experiments with particle accelerators are carried out at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, at the European Nuclear Research Center CERN in Geneva (Large Hadron Collider, LHC) and at the Relativistic Heavy Ion Collider (RHIC) on Long Island, New York. Of particular interest is the study of the phase transition from confinement to QGP.
At RHIC, gold atomic nuclei are accelerated to 99.9% of the speed of light in the accelerator ring and then shot at each other. Particle detectors are used to study the resulting products. The atomic nuclei decay into tens of thousands of matter particles due to the huge energies and temperatures (several trillion Kelvin). It can be shown that in the first fractions of nanoseconds after the collision, pressure fluctuations inside the collided particles are balanced in a way that suggests a state of matter similar to a liquid: a quark-gluon plasma has been created (for the shape of the QGP, see below).
Another indication of the occurrence of a QGP state analogous to a liquid in thermal equilibrium is a smaller number of jets, cone-shaped particle bursts from the collided atomic nuclei. This is explained by the fact that the particles are slowed down so much by the QGP and thus become less energetic that less energy remains for a jet.
The high energy density at the penetration of two colliding atomic nuclei lets the partons (i.e. the quarks and gluons) move quasi-free. In this phase, the partons interact with each other through inelastic collisions until a state of equilibrium occurs; this is called the quark-gluon plasma. Due to the internal pressure, the plasma expands and cools in the process. If the temperature falls below the critical temperature, the hadronization of the partons begins. The so-called chemical equilibrium is reached when the composition of the particle species no longer changes. If there are no more inelastic interactions between the generated particles, this is called thermal equilibrium.
Current measurements at RHIC and Large Hadron Collider take place in the state of high energies and low particle density (low baryochemical potential). Current results indicate a so-called crossover transition (this is only gradual, "smeared" as it were, in contrast to a sharp "phase transition"). Another indication for the existence of the QGP would be the detection of a first-order or second-order (critical point) phase transition at higher baryochemical potentials. The search for transitions from crossover to sharp phase transition behavior is currently performed at RHIC and LHC, respectively, and in the future at GSI in Darmstadt.
Indirect detection possibilities
The state of deconfinement, i.e., the existence of the QGP, is too short-lived to be readily detected directly. In addition, predictions of direct signatures such as energy density or temperature are highly model dependent. For this reason, indirect signatures must usually be used.
One of them is the enrichment of strange quarks, or of strange-quark-containing particles (for example the φ-meson) in the QGP after hadronization (Berndt Müller, Johann Rafelski 1982). This is because the energy needed to produce an s s pair is present exactly at the temperature above which the resolution of nucleons and hadrons into quarks and gluons, i. e. i.e., the formation of a QGP, is expected. s s pairs are increasingly produced at this temperature in the QGP by the merger of gluons: g + g → s s. In addition, some energy states are occupied by lighter quarks, so that beyond a certain point the production of s s pairs is preferred.
Other signatures include, for example, the suppression of relatively high-energy particles caused by the high energy loss when crossing the QGP, or the breakup or melting of heavy quarkonia such as the J/ψ meson J / ψ J/\psi or the Υ meson Υ (Helmut Satz, Tetsuo Matsui 1986).
A QGP proof requires the measurement of many different signatures and a theoretical model for the QGP that can explain these signatures. Based on numerical simulations and experimental findings, the transition to the quark-gluon plasma is thought to occur at a temperature of about 4-1012 Kelvin and belongs to the universality class of the three-dimensional Ising model. Three-dimensional because of the fact that of the four dimensions of the special relativity theory at high temperatures the variable time is omitted; Ising model (n=1) because as in this model (except for the sign) only one single degree of freedom dominates, for example the strandess or anti-strandess degree of freedom. The specified universality class is also possessed by ordinary fluids.
Since the commissioning of the LHC at CERN in Geneva, an accelerator currently (2016) operating at 6.5 TeV per proton and allowing, among other things, the production of quark-gluon plasmas by collisions of lead nuclei, direct detections have also become possible. This is reported in an article in the Physics Journal. The authors write: "The stopping power of quark-gluon matter is even so large that it can almost completely stop high-energy partons. This can already be seen in event images during data taking."
Bound states of heavy quarks and their antiquarks, e.g., in the bottomonium, are another probe: here one sees with the LHC when comparing 1s, 2s, and 3s states of the Υ concretely the plasma polarization as a change of the potential.
Older evidence (as of August 2005, source RHIC) suggests that the cohesion between quarks and gluons in the quark-gluon plasma is not completely removed, but that there are still strong interactions and mergers. Thus, at least at energies just above the formation energy, the quark-gluon plasma behaves more like a liquid (but not a superfluid) than a gas. This is true for temperatures around ≈160 MeV. Only at even higher energies do the elementary particles gain complete freedom.
Furthermore, since 2008, a discussion is underway about a hypothetical precursor state of the quark-gluon plasma, the so-called glasma state. This corresponds to an amorphous (glass-like) condensate, similar to the way one gets so-called "metallic glasses" (i.e., amorphous metals) in solid-state physics for some metals or metal alloys below the liquid state.