106
J. Aichelin et al.
9.1 Introduction
The heavy-ion experiments at the Relativistic Heavy-Ion Collider (RHIC) at
Brookhaven and at the Large Hadron Collider (LHC) at CERN have produced ample
evidence that a new state of matter, a plasma of quarks and gluons (QGP), is produced
[1]. Such a new state of matter has been predicted by lattice gauge calculations in
which the thermal properties of strongly interacting matter are calculated by solving the partition function based on the Lagrangian of Quantum Chromo Dynamics
(lQCD) [2, 3]. The analysis of these experiments shows that the QGP, produced at
these energies, is characterized by a vanishing chemical potential and it is exactly
this limit in which lQCD calculations are possible.
At very low beam energies, of the order of 1 AGeV, heavy-ion reactions are well
described by hadronic degrees of freedom, baryons, and mesons.
The challenge is now to understand the transition between low energy, hadron
dominated, reactions and that at high energy where quarks and gluons, the constituents of hadrons, are the right degrees of freedom to describe the experimental
findings. This transition regime is characterized by a finite chemical potential or, in
other words, by a finite baryon density. Phenomenological models predicted that for
finite chemical potentials the transition between hadronic matter and a plasma of
quarks and gluons is a first-order phase transition [4, 5] what increases the interest
to study this region.
In order to study nuclear matter at finite baryon densities presently two accelerators are under construction, the Facility for Antiproton and Ion Research (FAIR) in
Darmstadt and the Nuclotron-based Ion Collider fAcility (NICA) in Dubna. They
will become operational in the next years. Moreover, the presently running BESII (Beam Energy Scan) at RHIC, which includes a fixed target program, will also
provide experimental data in this energy regime.
In order to understand the result of these experiments and to interpret the physical message of the different observables, transport approaches have to be employed
which simulate the heavy-ion reaction on a computer. In these approaches, one can
understand the origin of the different phenomena which are encoded in the experimental results. The PHQMD model [6] is such a transport approach which has been
especially developed to address the seminal challenges of the physics at finite baryon
density. These include the origin of the formation of clusters at midrapidity, observed
at all energies, despite a temperature of the expanding fireball at midrapidity of the
order of 100 MeV, an environment in which clusters, which have a typical binding
energy of 8 MeV/nucleon cannot survive. These include as well the origin of the
production of hyper-cluster, clusters with at least one strange baryon, the creation of
multi-strange baryons and last but not least the identification of observables which
signal an eventual first-order phase transition.
The presently available transport approaches are either nonrelativistic, like the
QMD approach [7–9], or propagate single-particle Greens functions, like PHSD,
are not well suited to address cluster formation. The novel Parton-Hadron-QuantumMolecular Dynamics (PHQMD) approach overcomes these limitations. It is based on
J. Aichelin et al.
9.1 Introduction
The heavy-ion experiments at the Relativistic Heavy-Ion Collider (RHIC) at
Brookhaven and at the Large Hadron Collider (LHC) at CERN have produced ample
evidence that a new state of matter, a plasma of quarks and gluons (QGP), is produced
[1]. Such a new state of matter has been predicted by lattice gauge calculations in
which the thermal properties of strongly interacting matter are calculated by solving the partition function based on the Lagrangian of Quantum Chromo Dynamics
(lQCD) [2, 3]. The analysis of these experiments shows that the QGP, produced at
these energies, is characterized by a vanishing chemical potential and it is exactly
this limit in which lQCD calculations are possible.
At very low beam energies, of the order of 1 AGeV, heavy-ion reactions are well
described by hadronic degrees of freedom, baryons, and mesons.
The challenge is now to understand the transition between low energy, hadron
dominated, reactions and that at high energy where quarks and gluons, the constituents of hadrons, are the right degrees of freedom to describe the experimental
findings. This transition regime is characterized by a finite chemical potential or, in
other words, by a finite baryon density. Phenomenological models predicted that for
finite chemical potentials the transition between hadronic matter and a plasma of
quarks and gluons is a first-order phase transition [4, 5] what increases the interest
to study this region.
In order to study nuclear matter at finite baryon densities presently two accelerators are under construction, the Facility for Antiproton and Ion Research (FAIR) in
Darmstadt and the Nuclotron-based Ion Collider fAcility (NICA) in Dubna. They
will become operational in the next years. Moreover, the presently running BESII (Beam Energy Scan) at RHIC, which includes a fixed target program, will also
provide experimental data in this energy regime.
In order to understand the result of these experiments and to interpret the physical message of the different observables, transport approaches have to be employed
which simulate the heavy-ion reaction on a computer. In these approaches, one can
understand the origin of the different phenomena which are encoded in the experimental results. The PHQMD model [6] is such a transport approach which has been
especially developed to address the seminal challenges of the physics at finite baryon
density. These include the origin of the formation of clusters at midrapidity, observed
at all energies, despite a temperature of the expanding fireball at midrapidity of the
order of 100 MeV, an environment in which clusters, which have a typical binding
energy of 8 MeV/nucleon cannot survive. These include as well the origin of the
production of hyper-cluster, clusters with at least one strange baryon, the creation of
multi-strange baryons and last but not least the identification of observables which
signal an eventual first-order phase transition.
The presently available transport approaches are either nonrelativistic, like the
QMD approach [7–9], or propagate single-particle Greens functions, like PHSD,
are not well suited to address cluster formation. The novel Parton-Hadron-QuantumMolecular Dynamics (PHQMD) approach overcomes these limitations. It is based on