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Fast Thermalization of the Quark-Gluon Plasma

Periodic Reporting for period 1 - FastTh (Fast Thermalization of the Quark-Gluon Plasma)

Reporting period: 2015-09-01 to 2017-08-31

Understanding the properties of extreme phases of nuclear matter is one of the major challenges in theoretical physics. Matter under such extreme conditions was present in the very early universe - a millisecond after the big bang - and is nowadays produced in heavy ion collisions at the Relativistic Heavy Ion Collider in Brookhaven, USA and the Large Hadron Collider in Geneva, Switzerland in the form of the Quark Gluon Plasma. This extreme phase of matter is an almost perfect fluid due to its extremely low viscosity. The objectives of the project have been to elucidate how such a fluid is created and what its properties are. We have been using cutting edge numerical simulations carried out on European Supercomputers to elucidate the formation of the hot viscous plasma under controlled initial conditions. The project is an important step beyond the state-of-the-art models which assumed certain symmetries that are not present in real-life quark gluon plasmas by introducing a source that breaks these symmetries in a controlled manner. The results we find are highly exciting as they show that the creation of the plasma is very fast even in this more realistic setup. In addition they pave the way for upcoming studies of the fast creation of the Quark-Gluon plasma.
Key scientific insights I have obtained during my postdoctoral career at the Institute of Cosmos Science (ICC) of the University of Barcelona (UB), where I held the FastTh EU Horizon 2020 Marie Skłodowska-Curie Action (MSCA) Individual Fellowship include:
◦ a) first simulation of a non-conformal shockwave collision as a holographic model for heavy ion collisions (all holographic models so far neglected the crucial fact that QCD is not conformal)
◦ b) providing the first estimate for the ratio of bulk over shear viscosity ζ/η = 0.22 , responsible for strong non-conformal effects,
◦ c) showing that the presence of bulk viscosity leads to only a small increase of the hydrodynamization time
◦ d) the existence of a new relaxation channel for the plasma: EoSization
◦ e) new example of applicability of hydrodynamics to a system with large energy gradients undergoing a spinodal instability
All results listed above are beyond the state of art as they follow from a successful implementation of gravitational shock wave collisions to model viscous plasma formation which are more realistic than any other approach has managed to this date. Our simulation show phase separation domains and may be able to generate experimental signals of a first order phase transition. Those are relevant both for the upcoming Billion dollar research program at the Relativistic Heavy Ion Collider, where one searches for signatures of the critical point in the phase diagram of this extreme matter and for the heavy ion collisions at the Large Hadron Collider of CERN.
Spinodal instability
Excitation of the scalar condensate
Typical non-conformal shockwave collision