Hadronic Freeze-Out across Small and Large Collision Systems: Identified-Particle Spectra, Model Dependence, and Sequential Decoupling

Hadronic freeze-out is central to the interpretation of the final-state particle composition and momentum distributions produced in high-energy pp, pA, and AA collisions. Chemical freeze-out, constrained mainly by hadron yields and yield ratios, and kinetic freeze-out, constrained mainly by transverse-momentum spectra and collective-flow observables, describe distinct stages of the collision evolution and should not be conflated. Reported values of temperature, baryochemical potential, strangeness saturation, transverse expansion velocity, nonextensivity parameters, and related quantities can depend appreciably on the selected particle species, fit range, collision-energy and event-class definitions, treatment of resonance feed-down and weak decays, uncertainty model, and the adopted statistical-hadronization, blast-wave, nonextensive, transport, or hybrid framework. Moreover, similarly named parameters may arise from inequivalent equations and need not have the same physical meaning. These sources of model and analysis dependence complicate comparisons among experiments and across collision systems, particularly for strange and multistrange hadrons that may decouple differently from the bulk medium. A focused Research Topic is therefore timely to establish transparent conventions, quantify parameter correlations and model dependence, and identify which freeze-out trends remain stable under alternative descriptions of the same data.Summarizing, the complex multi-stage nature of hadronic freeze-out requires a rigorous standardization of analytical techniques to extract genuine physical trends from model-dependent artifacts. Because extracted thermodynamic and kinematic parameters are highly sensitive to experimental definitions, fitting ranges, and specific theoretical frameworks, direct comparisons across diverse collision systems remain fundamentally challenged. Therefore, addressing comprehensively these systematic uncertainties and parameter correlations is necessary and crucial for mapping the transition from microscopic interactions to macroscopic collective behavior. Establishing these clear benchmarks should isolate robust signals of collective expansion, thermalization limits, and species-dependent decoupling from the bulk medium in high-energy collisions. Hence, this dedicated Research Topic is essential to establish standardized analytical conventions, quantify systematically model-dependent parameter correlations, and ascertain the structural stability of extracted freeze-out trends under alternative theoretical formulations of identical experimental datasets.The aim of this Research Topic is to assemble experimentally anchored and methodologically transparent studies of chemical and kinetic freeze-out across pp, pA, and AA collisions from GeV to TeV energies. The principal objectives are to (i) compare extraction methods using matched data selections, definitions, fit ranges, and uncertainty treatments; (ii) distinguish thermodynamic freeze-out quantities from effective spectral temperatures, inverse slopes, and other fit parameters; (iii) quantify covariance and parameter degeneracies, especially temperature-flow and normalization-shape correlations; (iv) test the stability of extracted trends with collision energy, system size, centrality, multiplicity, rapidity, and particle species; and (v) assess evidence for common or sequential decoupling of non-strange, strange, and multistrange hadrons. Contributions may use published experimental data, validated phenomenological fits, statistical-hadronization analyses, and transport or hybrid calculations. The intended outcome is a set of reproducible cross-system comparisons and methodological recommendations, rather than a single model-dependent compilation of freeze-out parameters.This Research Topic welcomes original research, methodological and benchmark studies, systematic comparisons, and focused reviews that advance a quantitatively controlled understanding of hadronic freeze-out. Suitable contributions include, but are not limited to: • Chemical freeze-out analyses based on identified-hadron yields and ratios, including canonical and grand-canonical treatments, conserved-charge chemical potentials, strangeness saturation, and finite-volume effects. • Kinetic freeze-out analyses based on identified-particle transverse-momentum spectra and flow-sensitive observables, with explicit blast-wave or related model equations and conventions. • Tsallis, Hagedorn, nonextensive, multi-component, or other phenomenological spectral descriptions that clearly distinguish fit parameters from thermodynamic freeze-out quantities. • Common versus sequential or species-dependent decoupling, including non-strange, strange, multistrange, resonance, and light-nucleus observables where justified. • Systematic studies of fit-range dependence, resonance feed-down, weak-decay corrections, acceptance, normalization, centrality or multiplicity definitions, correlated uncertainties, covariance, and parameter degeneracies. • Matched comparisons of measurements from ALICE, ATLAS, CMS, LHCb, STAR, PHENIX, NA61/SHINE, and other relevant experiments. • Transport, hydrodynamic, hybrid, or event-generator studies that connect final-state spectra and yields to decoupling dynamics and explicitly enabled microscopic mechanisms. • Reproducible data-driven or machine-learning approaches, provided that the training data, validation strategy, uncertainty treatment, and physical interpretability are clearly documented. • Collectivity of small-systems (p+p and p+Pb) versus large symmetric systems (Xe+Xe and Pb+Pb), tracking the emergence of collective flow and system-dependent thresholds of freeze-out temperatures. • Multi-parameter global fits, Bayesian parameter estimation, and Pearson correlation matrices evaluating the interdependencies and degeneracies among extracted freeze-out parameters. • Interplay between thermalized bulk medium expansion, parton recombination/coalescence at intermediate pT, and vacuum-like jet fragmentation at high pT.Submissions should state the exact equations and parameter conventions, data source and kinematic selection, fit or calibration range, uncertainty treatment, objective or fit statistic, parameter bounds, convergence checks, covariance or correlations, and stability tests. A satisfactory fit should not be presented as proof of a unique freeze-out mechanism, and effective or inverse-slope temperatures should not be labeled as chemical or kinetic freeze-out temperatures without explicit justification. Collectively, the contributions should clarify robust trends and limitations rather than impose a single universal freeze-out scenario. Topic editors: Fu-Hu Liu, Khusniddin Olimov, Muhammad Ajaz Source: https://www.frontiersin.org/research-topics/85243/hadronic-freeze-out-across-small-and-large-collision-systems-identified-particle-spectra-model-dependence-and-sequential-decoupling