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Nonequilibrium phenomenology of identified particle spectra in heavy-ion collisions at energies available at the CERN Large Hadron Collider

Oleksandr Vitiuk1, David Blaschke1,2,3, Benjamin Dönigus4, and Gerd Röpke5

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Phys. Rev. C 113, 044902 – Published 1 April, 2026

DOI: https://doi.org/10.1103/db8g-55dw

Abstract

We employ the Zubarev approach of the statistical operator to investigate the enhancement of the low-pT region of pion spectra, introducing an effective pion chemical potential, to describe the overpopulation of low-energy pion states. We test a corresponding freeze-out approach by analyzing the transverse-momentum spectra of identified particles measured recently with high precision by the ALICE Collaboration in Pb+Pb collisions at CERN LHC. A blast-wave model and a blast-wave-based particle generator, coupled to a 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. Both models for the overpopulated pion states, the hadronic transport model and the thermal model with a nonzero pion chemical potential, provide a satisfactory description of the observed pion spectra. However, both approaches contain approximations which can be improved within a systematic nonequilibrium approach. We demonstrate that the introduction of a nonequilibrium pion chemical potential offers an efficient alternative to the conventional explanation of the low-pT enhancement, typically attributed to resonance decays with subsequent thermalization. A similar discussion also holds for the kaon spectra.

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Phys. Rev. C 113, 044902– Published 1 April, 2026
  • Received 25 March 2025
  • Revised 16 January 2026
  • Accepted 27 February 2026
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