Oleksandr Vitiuk

We employ the Zubarev approach of the non-equilibrium statistical operator to investigate the enhancement of the low-$p_T$ part of pion spectra, introducing an effective pion chemical potential to describe the formation of an overpopulated pion state. We test the feasibility of such a state at both the kinetic and chemical freeze-out stages by analyzing the transverse momentum spectra of identified particles measured by the ALICE Collaboration in Pb+Pb collisions at CERN LHC. To this end, a Blast-Wave model and a Blast-Wave- based particle generator, coupled with the SMASH hadronic transport model, are utilized. Bayesian inference methods are applied to extract the most probable sets of thermodynamic parameters at the chemical freeze-out hypersurface. Bayesian model selection indicates a slight preference for the scenario in which the measured particle spectra with an overpopulated pion state emerge from final state interactions and decays of a hadron resonance gas that is produced in full chemical equilibrium at hadronization. However, we demonstrate that the introduction of a nonequilibrium pion chemical potential offers an efficient alternative to the conventional explanation of the low-$p_T$ enhancement, typically attributed to resonance decays at the kinetic freeze-out stage.

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