Quark-Gluon Plasma

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The quark-gluon plasma (abbreviation QGP) is a state of matter at extremely high temperatures or baryon densities. Here, the confinement (English confinement ) of the curd and Gluons removed, which is why these particles exhibit a quasi-free behavior.

The quark-gluon plasma in nature

It is believed that the universe went through this state in the first fractions of a second after the Big Bang . In today's universe, the QGP most still exists in the center of neutron stars , some theories there another phase predict that by color superconductivity (Engl. Color superconductivity should be characterized).

Manufacturing on earth

The ALICE experiment at CERN to research the quark-gluon plasma

The use of heavy ion accelerators enables research into quark-gluon plasma (QGPs) in the laboratory. Corresponding experiments with particle accelerators are being carried out at the GSI Helmholtz Center 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. The investigation of the phase transition from confinement to QGP is of particular interest .

At the RHIC, gold atomic nuclei in the accelerator ring are brought to 99.9% of the speed of light and then shot together. The resulting products are examined with particle detectors . The atomic nuclei disintegrate into tens of thousands of matter particles due to the enormous energies and temperatures (several trillion Kelvin ). It can be shown that in the first fractions of a nanosecond after the collision, pressure fluctuations in the interior of the collided particles are balanced out in a way that suggests a state of matter similar to a liquid: a quark-gluon plasma has arisen (to the form of the QGP see below).

Another indication for the occurrence of a QGP state analogous to a liquid in thermal equilibrium is a lower number of jets , i.e. 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 lower in energy that less energy remains for a jet.


The high energy density when two colliding atomic nuclei penetrate allows the partons (i.e. the quarks and gluons) to move almost freely. In this phase the partons interact with one another through inelastic collisions until a state of equilibrium occurs; this is called quark-gluon plasma. Due to the internal pressure, the plasma expands and cools down 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 types no longer changes. If there are no more inelastic interactions between the generated particles, one speaks of thermal equilibrium .

Current measurements at the RHIC and Large Hadron Collider take place in a state of high energies and low particle density (low baryochemical potential ). Current results indicate a so-called crossover transition (in contrast to a sharp “ phase transition ”, this is only gradual, so to speak “smeared”). A further indication for the existence of the QGP would be the proof of a phase transition of first order or second order ( critical point ) at higher baryochemical potentials. The search for transitions from crossover to sharp phase transition behavior is currently being carried out at RHIC or LHC and in the future at GSI in Darmstadt.

Evidence options

Indirect evidence

The state of deconfinement , i.e. the existence of the QGP, is too short-lived to be able to be proven directly. In addition, the predictions of direct signatures such as energy density or temperature are strongly model-dependent. For this reason, indirect signatures must usually be used.

One is the accumulation of Strange quark , or of strangeness -containing particles (for example of the φ-meson ) in QGP after a hadronization ( Berndt Müller , Johann Rafelski 1982). Because the energy required to generate a pair is available at the exact temperature from which the dissolution of nucleons and hadrons into quarks and gluons, i.e. H. the formation of a QGP is expected. Pairs are at this temperature in QGP increasingly produced by the fusion of gluons: . In addition, some energy states are occupied by lighter quarks, so that from a certain point onwards the generation of pairs is preferred.

Further signatures are, for example, the suppression of relatively high-energy particles, which is caused by the high energy loss when crossing the QGP, or the breaking or melting of heavy quarkonia such as the J / ψ meson or the Υ meson ( Helmut Satz , Tetsuo Matsui 1986) .

A QGP attestation 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, it is assumed that the transition to the quark-gluon plasma takes place at a temperature of about 4 · 10 12  Kelvin and belongs to the universality class of the three-dimensional Ising model . Three-dimensional because of the four dimensions of the special theory of relativity at high temperatures the variable time is omitted; Ising model (n = 1) because, as in this model (except for the sign), only a single degree of freedom dominates, for example the strangeness or anti-strangeness degree of freedom. Ordinary liquids also have the stated universality class.

Direct evidence

Since the commissioning of the LHC at CERN in Geneva, an accelerator that currently (2016) works at 6.5 TeV per proton and, among other things, allows the generation of quark-gluon plasmas through the collision of lead nuclei, direct evidence has also become possible. This is reported in an article in the Physik-Journal . The authors write: “The braking power of quark-gluon matter is so great that it can almost completely stop high-energy partons . This can already be seen in the event images during the data acquisition. "

Another probe are bound states of heavy quarks and their antiquarks, e.g. B. in the Bottomonium : Here you can see the plasma polarization as a change in potential when comparing the 1s, 2s and 3s states of the LHC .

to form

Older findings (as of August 2005, source RHIC ) suggest that the cohesion between quarks and gluons in the quark-gluon plasma is not completely broken, but that there are still strong interactions and associations. The quark-gluon plasma behaves more like a liquid (but not like a superfluid) than like a gas, at least at energies just above the formation energy. This applies to temperatures around ≈160 MeV. Only at higher energies do the elementary particles gain complete freedom.

Since 2008, a discussion has been going on 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 what one gets in solid-state physics with some metals or metal alloys below the liquid state so-called “ metallic glasses ” (ie amorphous metals).


  • Spectrum of Science 09/05: Time Travel to the Beginning of Space (pp. 14–15) ( full text )

Web links

Individual evidence

  1. Enhanced production of multi-strange hadrons in high-multiplicity proton – proton collisions . In: Springer Nature (Ed.): Nature Physics . 13, No. 6, 2017, ISSN  1745-2473 , pp. 535-539. doi : 10.1038 / nphys4111 .
  2. ^ RHIC Scientists Serve Up "Perfect" Liquid. Brookhaven National Lab, April 18, 2005, accessed November 23, 2018 .
  3. ^ A b Johann Rafelski, Berndt Müller: Strangeness Production in the Quark-Gluon Plasma . In: American Physical Society (APS) (Ed.): Physical Review Letters . 48, No. 16, April 19, 1982, ISSN  0031-9007 , pp. 1066-1069. doi : 10.1103 / physrevlett.48.1066 .
  4. ^ A b Johann Rafelski, Berndt Müller: Erratum: Strangeness Production in the Quark-Gluon Plasma . In: American Physical Society (APS) (Ed.): Physical Review Letters . 56, No. 21, May 26, 1986, ISSN  0031-9007 , pp. 2334-2334. doi : 10.1103 / physrevlett.56.2334 .
  5. Frithjof Karsch and Helmut Satz: Quantum matter and supercomputers (University of Biefeld, Faculty of Physics).
  6. Christoph Blume, Klaus Rabbertz, Stefan Tapprogge: The strong side of the LHC. In: Physik Journal 11 (2012), Issue 4, 45–49 ( online , example see Fig. 6)
  7. Raju Venugopalan: From Glasma to quark-gluon plasma in heavy-ion collisions , J. Phys. G35 104003, online version .