Abstract
We discuss how the non-equilibrium process of pion production within the Zubarev approach of the non-equilibrium statistical operator leads to a theoretical foundation for the appearance of a non-equilibrium pion chemical potential for the pion distribution function for which there is experimental evidence in experiments at the CERN LHC.
Highlights
The thermal statistical model [1,2,3,4,5,6] for chemical freeze-out of hadron species gives a successful description of a set of particle ratios produced in heavy-ion collisions (HIC) at different center of mass energies ranging from the energies provided by the Schwerionensynchrotron (SIS-18) at GSI Darmstadt over those of the Alternating Gradient Synchrotron (AGS) at BNL Brookhaven and the Super Proton Synchrotron (SPS) at CERN Geneva up to the highest energies at the Relativistic Heavy Ion Collider (RHIC) at BNL and√the Large Hadron Collider (LHC) at CERN
There are different models to describe the low momentum enhancement of pions observed in HIC at SPS, RHIC, and LHC energies
As an origin for the high phase space density of pions, one may think of an initial state in the form of a color glass condensate state which gets converted to a pion gas by particle number conserving process as described, e.g., [54,55]
Summary
The thermal statistical model [1,2,3,4,5,6] for chemical freeze-out of hadron species gives a successful description of a set of particle ratios produced in heavy-ion collisions (HIC) at different center of mass energies ranging from the energies provided by the Schwerionensynchrotron (SIS-18) at GSI Darmstadt over those of the Alternating Gradient Synchrotron (AGS) at BNL Brookhaven and the Super Proton Synchrotron (SPS) at CERN Geneva up to the highest energies at the Relativistic Heavy Ion Collider (RHIC) at BNL and√the Large Hadron Collider (LHC) at CERN. The effect can be seen as a precursor of pion Bose–Einstein condensation due to high phase space occupation at low momenta and has been parametrized by adopting a pion chemical potential very close to the pion mass [14,15]. This concept is based on the assumption that the total pion number is dynamically fixed on a time scale between the pion chemical freeze out tπ,cfo and the thermal freeze-out (or freeze-out) tfo, tπ,cfo < t < tfo, where at tπ,cfo the pion number becomes frozen and at tfo the momentum distributions stop to change [16]. The long-time scale evolution is given by the time dependence of these thermodynamic parameters approaching thermodynamic equilibrium
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