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Dynamics of the Quark-Gluon Plasma: A Journey into new phases of the Strong Interaction

Final Report Summary - QGPDYN (Dynamics of the Quark-Gluon Plasma: A Journey into new phases of the Strong Interaction)

The Early Universe in the first 10 microseconds after the explosion of the Big Bang consisted of a plasma of quarks and gluons (QGP), a soup of the fundamental constituents of matter. Such a state of matter that forms at temperatures above a thousand of billion degrees has not only permeated the Early Universe but should be present in the inner core of compact stars around the galaxies, and nowdays can be re-created in the laboratory using high-energy collisions between heavy nuclei at the Large Hadron Collider (LHC) at CERN and at the RHIC facility at the Brookhaven National Laboratory (BNL).
The QGPDyn project is concerned with understanding the dynamics of such collisions to suggest possible observations and interpret the experimental results. The aim is to study the properties of the QGP and more generally of the Hot QCD Matter understanding the strong interaction at high temperature. The QGPDyn is based on the development of a relativistic Boltzmann-like transport approach able to describe in a unified framework various aspects of the dynamics of the QGP taking into account the non-equilibrium dynamics that play a role in the early stage of the collisions and more generally the non-equilibrium dynamics of particles at energies quite large than the local temperature of the plasma as well as the evolution of heavy quark mass (charm and bottom).
We have provided a first realistic dynamical approach showing that the matter produced in ultra-relativistic collisions in a strong out-of-equilibrium state thermalizes in a very short time scale of less than 1 fm/c (10^-23 s), as conjectured by hydrodynamical approaches. Our research has also identified a strong enhacement in ultra-central collisions of the correlation between the anisotropies in particle production and the initial eccentricities of the created matter. This will be a powerful method to have a new insight into the initial state created in ultra-relativistic heavy-ion collisions and will allow to determine the temperature dependence of the shear viscosity of η/s which can provide a signature of the phase transition of ordinary matter to QGP. We have also shown that hadron production and in particular the baryon to meson ratio is modified with respect to the vacuum and is consistent with a quark coalescence mechanism. This provides a further evidence of the creation of a matter with quarks as degrees of freedom.
One of the strength of the project was to treat in the same framework also the dynamics of Heavy Quarks (HQ). It was a puzzle to describe both the HQ transverse momentun spectra and their asimmetry in the azimuthal emission. We have solved this showing that charm quarks does not have a Brownian motion in the QGP, as commonly assumed, and that the interaction is significantly temperature dependent. The outcome has been that the transport coefficient for HQ extracted by the phenomenology is in agreement with the most recent calculation in Quantum ChromoDynamics (QCD) solved on lattice. This results implies a thermalization time for charm quarks of about 3-5 fm/c comparable to the QGP lifetime. This further indicates that the matter created in ultra-relativistic collisions is the one defined by the QCD at high temperature, which gives a significant support to the fact that a QGP is created in such collisions and it is possible a quantitative determination of its properties.
An important last achievement has been the indentification of charm quarks as a new probe for the initial strong magnetic field (10^18 Gauss) created in heavy-ion collisions. This supplies a complementary quantitative access to the study of CP (Charge-Parity) violation in strong interactions at high temperatures by mean of an effect know as “Chiral Magnetic”. This could provide an explanation of the asymmetry between matter and anti-matter that gave rise to the matter dominance in our Universe.