Speaker
Description
Heavy Ion Collisions (HIC) provided the possibility of researching the phase transitions from hadronic matter to the predicted Quark-Gluon Plasma (QGP) phase based on partonic degrees of freedom. Conditions at HIC – nuclear densities much higher than nuclear density and/or temperatures above 150 MeV – suggest such a form of matter both dominant just after the Big Bang as in the cores of neutron stars. Data from HIC confirm that these conditions, both temperatures, and densities have been reached. Contrary to early concepts, based on QCD asymptotic freedom property, the partonic matter has no properties of an almost ideal state of quarks and gluons. The behavior appears of a fluid with very low kinematic viscosity with strong hydrodynamic flows. This
means an almost perfect fluid state, which means quite powerful interactions between constituents. The success of hydrodynamic description in the heavy-ion collisions suggests
the appearance of very fast local thermalization at 1 fm/c. The demand for local (at least) thermalization has been a cornerstone of the hydrodynamical approach. Recent data of hydrodynamical behavior in small systems, e.g. p-p or p-Pb data put a question mark on the logic chain: hydro=thermalization, thermalization needs time, time
accessible in large systems only, so hydrodynamic behavior confirms the appearance of the QGP state. It was shown, however, that even far from equilibrium is hydrodynamics applicable.
Dissipative processes are needed here to get this result. Dissipative processes are necessary here to get this result. The effect of dissipation is more pronounced at the very early stages of heavy-ion collisions This means also that the hadrodynamic behavior does not confirm the QGP state. So there is still an open question about the signature of the QGP. The lecture aims to describe mechanisms that change the composition of the fluid, i.e. particle production and/or chemical reactions. This will be exemplified with the strangeness behavior as it has a special role as the QGP signature. Strangeness production will be connected with entropy growth. Then the viscosity due to the strangeness production and its influence on the state of the system would be estimated. Cooling beyond the flow of matter will be taken into account here
