Work package number
Start date
Activity Type
Activity number and acronym
Work package title

Theory of Relativistic Heavy Ion Collisions

Beneficiary number
Organization legal name
(in italics the Research Units)
Short name
Activity leaders
(in bold the spokesperson)
Human effort
Centre national de Recherche Scientifique
18 (18)

CNRS/IN2P3 Laboratoire de physique subatomique et des technologies associées, Nantes



J. Aichelin
18 (18)

Istituto Nazionale di Fisica Nucleare

3 (39)

INFN Laboratori Nazionali del Sud

V. Greco
3 (20)
INFN Sezione di Firenze
F. Becattini
Forschungszentrum Dresden-Rossendorf e. V.
B. Kämpfer
Gesellschaft für Schwerionenforschung mbH
J. Wambach

6 (24)


Johann Wolfgang Goethe – Universität Frankfurt am Main

C. Greiner

4 (91)

Justus Liebig Universität Giessen
W. Cassing
Helsingin yliopisto
K. J. Eskola

6 (26)


Commissariat a l’Energie Atomique


CEA – Istitut de Physique Théorique


Y. Ollitrault

MTA KFKI Reszecske- Es Magfizikai Kutatointezet
T. Biro

7 (32)


Universitetet i Bergen


L. Csernai

3 (16)

Other involved institutions not receiving EC funds

Activity leaders

Estimated human effort involved in the WP

University of Oslo (Norway)

L. Bravina


University of Wroclaw (Poland)

K. Redlich

Univerzity Mateja Bela, Banska Bystrica (Slovakia)
B. Tomasik

Consejo Superior de Investigaciones Científicas (Spain)

C. Manuel

The quest of the properties of dense and hot matter is one of the outstanding challenges of present day nuclear and particle physics. Thousands of physicists at BNL, CERN and GSI design, construct and carry out the experiments which are necessary to determine these properties in ultrarelativistic heavy ion reactions. In these experiments huge numbers of particles are measured simultaneously. Unfortunately the observables reflect the desired information only very indirectly. In order to reveal the QCD-matter properties explicitly a strong support from theory is necessary.

The interpretation of the heavy ion experiments presents a great challenge for theory. During the various stages of the reaction very different physical processes occur which have in the past been addressed by separate physics communities. In the first stage of the interaction the collision energy is high enough so that perturbative Quantum Chromodynamics (QCD) is applicable. The experimental results, however, suggest that many-body effects are far from being negligible, and that they may even dominate this phase. Thus models based on the Color Glass Condensate (CGC) have been advanced and Weibel instabilities have been considered as a possible mechanism, yet the fast thermalization is still far from being understood. The success of hydrodynamical studies indicates that at the end of this initial phase the system is close to a local equilibrium. In order to study how the system expands, hydrodynamical as well as other dynamical studies need input from QCD Lattice calculations or possibly even from AdS-CFT string theories. These two fields have until recently been remote to those who study the transport of matter.

QCD predicts that during the expansion a phase transition (or a cross-over) takes place, in which the quark-gluon gas condensates to a hadron gas. The properties of this transition can presently be studied from first principles only for a static system through Lattice Gauge simulations. In heavy ion experiments, however, this transition takes place in an environment which expands with a sizeable fraction of the speed of light. Hence transport models or hydrodynamics are needed. These models require as input not only the lattice-QCD results but also the properties of hadrons close to the phase transition, which traditionally have been calculated in effective theories.

The unprecedented complexity of the heavy ion reactions calls for an unprecedented collaboration of nuclear and particle theory, of transport approaches as well as of computational physics. It is the purpose of the proposed network to provide the frame of such a collaboration of different branches of theory which are traditionally not used to work together, as well as to foster and to extend the collaboration which have been successfully developed under FP6.


The network will focus on four topics at the borderline of the different subfields of nuclear and particle physics:

1. Initial phase physics (in bold the leading institution) (Annecy, Banska Bystrica, Barcelona, Bergen, Frankfurt, Jyväskylä, Paris).

Understanding the physics in the initial phase of the heavy ion collisions, i.e., the dynamics related to primary production of QCD quanta and to the early thermalization stage. Here the objective is to develop the various proposed phenomena (nuclear effects in primary collisions, many body collisions, CGC, instabilities, perturbatively calculable hard probes of dense matter, applications of new gauge/gravity duality ideas) into quantitative predictions which can be checked against experimental results.

Year 1: Assessment of the different ideas and models. This includes an understanding of the mechanisms of the instabilities, especially if the current is modified by two body collisions of the charge carriers. Assessment of the influence of the 3-body processes (qq→qqg, etc) on cascade type calculations (and the cancellations (Bloch-Nordsieck, Lee-Nauenberg) which render these cross sections finite).

Years 2 and 3 (half): Determination of observables which can be predicted in these conceptually quite different models and which can be subsequently compared with experimental observables. Study of the question whether the fast equilibrium times which are seen experimentally are compatible with the approaches.

2. Heavy flavor physics (in bold the leading institution) (Nantes, Annecy, Banska Bystrica, Barcelona, Budapest, Catania, Geneva, Frankfurt, Giessen, Jyväskylä, Oslo, Paris, Rossendorf).

Heavy flavor particles will be created in processes which can be described by perturbative QCD (pQCD). Therefore their distribution at the moment of creation is known. While interacting with the expanding system this distribution will be changed and therefore the final distribution carries information on the expanding system. Heavy flavor mesons, in particular also the quenching of heavy flavor jets, are probably one of the key observables which allow to study the plasma properties almost directly. This study calls for a collaboration between particle physics, hadron physics, transport theories and lattice gauge calculations to study the properties of heavy flavor hadrons in matter.

Years 1 and 2: Study of the interaction of heavy flavor quarks with other plasma particles using reactions with two and three particles in the exit stage. Evaluation of the transport coefficients and energy loss. Comparison of the results with energy loss calculations for light quarks and from more phenomenological approaches.

Years 3 (half): Embedding this approach into a transport theory which describes the expansion of the system.

3. The chiral/confinement phase transition (in bold the leading institution) (Nantes, Banska Bystrica, Bergen, Budapest, Catania, Geneva, Darmstadt, Firenze, Frankfurt, Giessen, Jyväskylä, Lyon, Oslo, Rossendorf, Wuppertal, Wroclaw).

Here the objective will be to study how this transition takes place in an expanding system with the perspective to identify the observables which carry information on the plasma properties. Lattice gauge calculation start to become sufficiently precise to predict hadron properties in matter close to the transition. These results have to be compared with those of the phenomenological approaches using hadronic interactions and those that are used in the transport theories.

Year 1: Assessment of the different scenarios ( effective quark model, extended NJL model, quark droplet and high mass hadron model, Sigma model, etc) and discussion of the similarities and differences of their physical content. Comparison to lattice results.

Years 2 and 3 (half): Quantifying the different models, calculation of the transport coefficients and the equation of state as well as embedding into transport theories (i.e. ideal and viscous hydrodynamics and particle based approaches).

4. Exploratory studies for the development of a transport code for relativistic heavy ion collisions

Heavy ion reactions yield very complex results: in order to explore the physics, transport codes are necessary, which predict how different assumptions of cross sections, of in medium properties of particles, of the presence of a phase transition, etc. show up in the experimental observables. These transport theories are very complex and there is an urgent need to review critically the present approaches and to combine and develop them to a next generation transport approach. The network will fund the exploratory studies including workshops which prepare proposal for funding.

There is a leading institution for each of the first three research activities which is responsible for ensuring communication among participants and for monitoring and reporting progress. The responsible institution will organize the necessary workshops and exchanges. The communication between the three activities will be organized by the spokesperson. A yearly workshop is foreseen in which the results of the different activities will be presented (if possible by postdocs and PhD students) and discussed with colleagues from outside Europe. This will also be the occasion where the young people of the network meet.

The total number of tenured theorists who work on these projects is 60 and the number of postdocs and PhD students is 81. Therefore the research itself will be essentially financed by the participating institutions through the salaries of the researchers. The EC support will finance the additional personnel which is needed to ensure the functioning of the network, that common research projects between different communities are launched and that information is transferred between the groups.

The traveling connected to this project, the mini-workshops which are the backbone of the collaborations, the exchange of young researchers between laboratories and the annual network workshop: this will ensure the coherence and dissemination of knowledge, give the young researchers opportunities to communicate their results to a larger audience and embed the network into the worldwide efforts.


Initial phase physics: Predictions for the different scenarios and assessment of their compatibility with experimental data, with a particular focus on the observables in CERN (delivery month from start date: 30).

Heavy flavor physics: Computer program which describes the heavy flavor dynamics. Identification of the observables which are sensitive to the properties of strongly interacting elementary particle matter (delivery month from start date: 30).

The chiral/confinement phase transition: Quantitative evaluation of the different hadronization scenarios (delivery month from start date: 30).


Following the investment of huge efforts into ultrarelativistic heavy ion experiments, the experimental and theoretical physics communities are working to interpret the results and to assess the underlying physics. The complexity of the theoretical approaches needed to incorporate the relevant processes is commensurate with the difficulty of the experiments. Many theoretical physicists work in this field, usually focussing on particular aspects and subfields. The community feels an urgent need to encourage the various groups to initiate and pursue common research projects and to learn from each other. During the 6th framework program some projects have emerged where this approach already led to common research projects, between lattice and hadron physicists for example. The three topics which will be the center of the present network have recently seen a fast development and arrived at a stage where only common efforts between the different sub-communities will allow further progress.

The network partners represent an almost exhaustive list of the European theory groups which work in the field of ultrarelativistic heavy ion physics and produce of the order of 100 articles per year. The network will help to bridge the gap between different theoretical physics communities and stimulate collaboration. It will also intensify the collaboration between theory and experiment, which is necessary to exploit the complex experimental data, a prerequisite for any theoretical interpretation.

In view of the number of participants and the rather limited funding which will be available, most of the work will be financed from other sources. TORIC support will allow to organize and finance small workshops where the various communities may interact and develop joint projects. It will furthermore support the exchange of physicists to allow a sharing of the very latest knowledge between the different communities and offer young people of smaller groups the possibility to integrate themselves in larger or more experienced groups. Postdocs are necessary to ensure that this communication does not remain on the level of mutual listings of research results but is transformed into common research work.

The success and impact of theoretical activities are usually measured by the number of publications, of citations and of invitations to presentations at conferences. We will ensure from the beginning that publications which are supported by the network are centrally collected and that conference talks are available on one internet site. To survey this will be the task of the postdocs which are employed by the network. During the yearly mini-workshops the progress will be assessed and further development discussed.