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QCDMAT Report Summary

Project ID: 267258
Funded under: FP7-IDEAS-ERC
Country: France

Final Report Summary - QCDMAT (Strongly Coupled QCD Matter)

By colliding heavy nuclei at high energy, one creates in the laboratory very hot and dense states of matter that have existed in our universe only for a brief instant of time, few tens of millisecond after the big bang. This matter, whose properties are governed by Quantum Chromodynamics, the theory of strong interactions, is generically called a quark-gluon plasma. At the presently accessible energies, this plasma appears to behave as a strongly coupled liquid with a relatively small viscosity.

Hydrodynamic studies allow us to extract from the data fundamental properties of the quark-gluon plasma, such as transport coefficients, or the equation of state. Such studies also allow to relate specific correlations detected in the azimuthal distributions of the produced particles to fluctuations of the energy density in the very early stages of the collisions. The hydrodynamic studies carried out during the project have deepened our understanding of the origin of these initial state fluctuations. They have also provided guidance to experimental efforts, making suggestions for new measurements leading to a better determination of the quark-gluon plasma properties.

Early stages of the collisions involve dense systems of gluons. The evolution of these systems with the collision energy are controlled by non linear equations that have been improved during the project to the next-to-leading order accuracy. How such systems of gluons thermalize to reach the observed hydrodynamical behavior is an important issue which has been addressed in the project, using kinetic theory and weak coupling techniques. A new perspective on this problem has been acquired, where the essential role of the soft momentum modes is emphasized.

A high energy parton propagating in a quark-gluon plasma plays the role of a "test" particle, probing the properties of the matter though which it propagates. It looses energy by emitting cascades of gluons. Such a cascade propagating in matter have distinguishing features that have been uncovered during the project. In particular, a simple mechanism for the transport of energy to large angle, has been identified. It is akin to wave-turbulence, and may be at the origin of the very specific energy distribution that is observed in experiments. Heavy quarks are also used as "test" particles. In particular, the formation of their bound states is strongly affected by the presence of the quark-gluon plasma. A new formalism to handle the heavy quark dynamics in a plasma has been developed during the project.

One of the major features of the field in its present state is that the details of the dynamics of heavy ion collisions start to be understood to a high degree of precision, thanks to the vast array of measurements that are now available. By focussing on conceptual issues, while keeping a close contact with experiment, the project has contributed to significant advances in the understanding of these collisions, paving the way to a more precise extraction of fundamental information from the data. The project has also revealed deep connections between seemingly unrelated phenomena, which have just began to be explored.

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