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The Compressed Baryonic Matter Experiment

The goal of the research program on nucleus-nucleus collisions at the Facility for Antiproton and Ion Research (FAIR) is the investigation of highly compressed nuclear matter. Matter at very high densities exists in neutron stars and in the core of supernova explosions. In the laboratory, super-dense nuclear matter can be created in the reaction volume of relativistic heavy-ion collisions. The baryon density and the temperature of the fireball reached in such collisions depend on the beam energy. In other words, by varying the beam energy one may, within certain limits, produce different states and phases of strongly interacting matter.

Figure 1: A schematic phase diagram of strongly interacting matter.


Mapping the phase diagram of strongly interacting matter

The phases of strongly interacting matter are shown schematically in figure 1. The "liquid" phase is realized in atomic nuclei at zero temperature and at saturation density (300 million tons/cm3). At low densities, the nucleons (i.e. protons and neutrons) behave like a gas. As the temperature and the density are raised, the nucleons are excited into "baryon resonances" which subsequently decay into pions and nucleons. This mixture of nucleons, baryonic resonances and mesons is called hadronic matter. This hadronic phase is represented by the white area in figure 1. At very high temperatures the hadrons melt and their constituents, the quarks and gluons, form a new phase of matter, the so called quark-gluon plasma. This "deconfinement" phase transition from hadronic matter to quark-gluon matter takes place at a temperature of about 170 MeV (at net baryon density zero) which is 130 thousand times hotter than the interior of the sun. Such conditions did exist in the early universe a few microseconds after the big bang and can be created in heavy ion collisions at ultra-relativistic energies as provided by the Relativistic Heavy Ion Collider (RHIC) in Brookhaven and by the Large Hadron Collider (LHC) at CERN. In highly compressed cold nuclear matter - as it may exist in the interior of neutron stars - the baryons also lose their identity and dissolve into quarks and gluons. The critical density at which this transition occurs, however, is not known. The same is true for the entire high-density area of the phase diagram. At very high densities and low temperatures, beyond the deconfinement transition, a new phase is expected: the quarks are correlated and form a color superconductor. At the "critical point" the deconfinement/chiral phase transition is predicted to change its character. Heavy-ion collisions at FAIR energies permit the exploration of the "terra incognita" of the QCD phase diagram in the region of high baryon densities. This research program is complementary to the investigations performed at RHIC and LHC.

Figure2: Sketch of the expansion phase of a U+U collision at 23 GeV/nucleon beam energy at different time steps: initial stage where the two Lorentz-contracted nuclei overlap (left), high density phase (middle), and final stage ("freeze-out") when all hadrons have been formed (right). Projectile and target nucleons are illustrated in red, exited baryons in blue, mesons in yellow. Different particles are created in different stages of the collisions or escape from the interaction region at different times. Almost 1000 charged particles are created in such a collision, most of them are pions.


Exploring fundamental properties of QCD

By exploring the phase diagram, one probes the strong interaction andits underlying theory, Quantum Chromo Dynamics (QCD). In particular, fundamental properties of QCD such as confinement and the broken chiral symmetry, which is related to the origin of hadron masses, can beexplored in heavy-ion collisions. A quantitative understanding of thesetwo phenomena is still lacking and hence poses a challenge for future research. An experimental approach to these problems is to search formodifications of hadron properties in a dense and hot nuclear mediumand for deconfined matter consisting of quarks and gluons.


Physics goals and key observables

In the region of highest baryon densities and moderate temperatures the QCD phase diagram is only little explored. Baryon densities of up to about 3 times that of nuclei can be produced and have been investigated in heavy-ion collisions at the present SIS18 accelerator of GSI. The highest net baryon densities are expected for nuclear collisions in the beam energy range between 10 and 40 GeV/u. The energy range up to 15 GeV/u was pioneered at the AGS in Brookhaven. In a second generation experiment the energy range from 10 to 40 GeV/u should be scanned searching for:

  • in-medium modifications of hadrons in dense matter;
  • indications of the deconfinement phase transition at high baryon densities;
  • the critical point providing direct evidence for a phase boundary;
  • exotic states of matter such as condensates of strange particles.

The approach of the CBM experiment towards these goals is to measure simultaneously observables which are sensitive to high density effects and phase transitions (see figure 2 for an illustration).
In particular, the research program is focused on the investigation of:

  • short-lived light vector mesons (e.g. the ρ-meson) which decay into electron-positron pairs. These penetrating probes carry undistorted information from the dense fireball;
  • strange particles, in particular baryons (anti-baryons) containing more than one strange (anti-strange) quark, so called multistrange hyperons (Λ, Ξ, Ω);
  • mesons containing charm or anti-charm quarks (D, J/Ψ);
  • collective flow of all observed particles. event-by-event fluctuations.

In the CBM experiment, particle multiplicities and phase-space distributions, the collision centrality and the reaction plane will be determined. For example, the study of collective flow of charmonium and multi-strange hyperons will shed light on the production and propagation of these rare probes in dense baryonic matter.
The simultaneous measurement of various particles permits the study of cross correlations. This synergy effect opens a new perspective for the experimental investigation of nuclear matter under extreme conditions.

CBM 3D models

CBM Wiki pages
the internal webspace of the CBM experiment (internal)



Discussions related to the CBM Experiment (internal)

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