Opacity estimation of OO collision from CoMBolt-ITA hybrid

Abstract

Understanding the effect of system size on the applicability of the hydrodynamic description in heavy-ion physics remains unclear. Recent measurements of OO collisions at the LHC offer a new opportunity to refine our understanding of collectivity because of their intermediate size relative to heavy-ion and small-system collisions, as well as the relatively good control over their initial state. We use the CoMBolt-ITA hybrid model to describe recent OO measurements at the LHC. The model employs TrENTo for the initial state. A combination of the pre-equilibration and hydrodynamized medium stages is modeled consistently by CoMBolt-ITA, which evolves the Boltzmann distribution of massless collective excitations. The afterburner stage is included by employing UrQMD. Using this approach, we test whether the system lies in the regime where its spatial size approaches the mean free path, corresponding to low opacity, or in the opposite limit, where its size exceeds the mean free path sufficiently to enter the fluid-like evolution regime with high opacity. We find that, in light of the data-model comparison and considering the current status of the model, OO collisions with centralities larger than $60\%$ gradually leave the domain of fluid-like evolution.

Figures (3)

• Figure 1: Charged particle multiplicity in unit pseudorapidity from ALICE preliminary (presented at the Initial Stages Conference 2025) compared to CoMBolt-ITA hybrid (top). Two-particle correlations of elliptic and triangular flow from the ALICE Collaboration ALICE:2025luc (filled points) and CoMBolt-ITA hybrid as a filled band (bottom).
• Figure 2: Collectivity response as a function of opacity for various centralities using three different measures: momentum anisotropy at late times (filled markers), momentum anisotropy at the switching time (open markers), and transverse-momentum elliptic flow after the hadronic cascade (filled band). Case 1, which shows better agreement with data, indicates that OO collisions at centralities below $60\%$ are more consistent with fluid-like evolution.
• Figure 3: Opacity as a function of charged particle multiplicity. Case 1, which was previously shown to have better agreement with ALICE measurements, indicates that OO collisions at centralities below $60\%$ are more consistent with fluid-like evolution.