Small System Collectivity and Freezeout Dynamics in High Energy Collisions

The formation of a Quark-Gluon Plasma (QGP) was long considered a unique phenomenon occurring only in large-volume heavy-ion (AA) collisions. However, the observation of long-range azimuthal correlations and collective flow patterns in high-multiplicity proton–proton (pp) and proton–nucleus (pA) collisions at the LHC and RHIC has fundamentally challenged this picture, suggesting that tiny droplets of strongly-interacting matter may also be created in the smallest collision systems. At the heart of this paradigm shift lies the study of freeze-out dynamics – the thermodynamic and hydrodynamic conditions under which the produced medium transitions into observable hadrons. Phenomenological analyses provide powerful frameworks for extracting key freeze-out parameters (kinetic temperature, effective temperature, radial flow velocity, and chemical potentials) from transverse momentum spectra. These parameters serve as critical discriminants between initial-state effects and genuine final-state collective behavior. Despite significant progress, the exact origin of small-system collectivity and its connection to the strongly-coupled QGP observed in heavy-ion collisions remain open and yields vigorously debated questions. This Research Topic invites contributions that address these dynamics, bridging theoretical modelling, phenomenological extraction, and experimental constraints to chart the frontiers of QCD under extreme conditions.The primary goal of this Research Topic is to advance our understanding of the origin of collective phenomena in small collision systems and to clarify the underlying freeze-out dynamics that govern the transition from the produced medium to observable hadrons. Although compelling evidence for collective flow has been established in high-multiplicity proton–proton and proton–nucleus collisions, the fundamental question remains whether these signatures arise from initial-state effects, such as gluon saturation or colour correlations, or from genuine final-state hydrodynamic expansion of a short-lived strongly-interacting medium. This Research Topic aims to bridge this gap by bringing together experimental measurements, phenomenological extractions, and theoretical interpretations, with contributions that systematically explore how freeze-out parameters—such as kinetic temperature, radial flow velocity, and chemical composition—evolve as a function of collision system size, energy, and event multiplicity. A key objective is to establish robust criteria for distinguishing initial-state geometry from final-state collective behavior, thereby determining the minimal conditions required for thermalization and QGP-like expansion. Ultimately, this Research Topic aspires to provide a comprehensive mapping of freeze-out conditions across the full spectrum of collision systems, from the smallest pp to the largest AA systems, fostering cross-disciplinary discussions that stimulate new theoretical developments and guide future experimental searches for the onset of collectivity in high-energy QCD.The scope of this Research Topic encompasses a broad range of experimental, phenomenological, and theoretical studies on collective flow and freeze-out dynamics in small collision systems. Specific themes include: - High-precision measurements of long-range correlations and azimuthal anisotropies in high-multiplicity proton–proton and proton–nucleus collisions- Systematic extraction of freeze-out parameters as functions of collision system size, energy, and event multiplicity- Rigorous comparisons between initial-state saturation models and final-state hydrodynamical or transport descriptions-The development of novel observables to distinguish genuine collective expansion from non-equilibrium initial-state effects. Studies exploring the multiplicity threshold for the emergence of thermalized behavior and the minimal conditions for QGP-like formation are particularly encouraged. We welcome both original research articles and comprehensive review papers. Extended versions of previously published conference proceedings are acceptable provided they include substantial new material or analysis. Topic editors: Waqas Muhammad, Khusniddin Olimov Source: https://www.frontiersin.org/research-topics/84734/small-system-collectivity-and-freezeout-dynamics-in-high-energy-collisions