Collective effects in O-O and Ne-Ne collisions at $\sqrt{s_{\mathrm{NN}}}$=5.36 TeV from a hybrid approach

TL;DR

This study investigates the emergence of collectivity in small-system collisions by simulating O-O and Ne-Ne at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV with three approaches: SMASH-vHLLE-hybrid (hydrodynamics plus hadronic afterburner), pure SMASH transport, and Angantyr. By comparing hydro-driven and non-collective baselines, the work examines how alpha-clustered nuclear structures influence initial eccentricities and final-state anisotropic flow using multi-particle cumulants with sub-events to suppress non-flow. The results show that the hybrid model yields sizable $v_2$ and $v_3$ in central events and that alpha clustering enhances flow and $R_{\mathrm{AA}}$ relative to Woods-Saxon, whereas SMASH and Angantyr are dominated by non-flow signals, especially in peripheral collisions. These findings illuminate the onset conditions for QGP-like behavior in small systems, emphasize the importance of non-flow mitigation in data interpretation, and provide predictions to guide the 2025 LHC light-ion program and future analyses.

Abstract

Many features of heavy-ion collisions are well described by hybrid approaches, where the droplet of strongly coupled quark gluon plasma (QGP) is modeled by hydrodynamics and the subsequent dilute stage is performed with a hadronic transport model. Conventionally, the formation of a QGP is well established in larger collision systems like Lead and Gold. However, hints of collectivity were found even in proton-proton collisions, raising the question where the onset of QGP formation lays. This study aims at making predictions for the light ions run at the LHC in July 2025, in order to explore the applicability of hybrid approaches in smaller collision systems. We employ three different models, the SMASH-vHLLE-hybrid, the pure hadronic cascade of SMASH and Angantyr to simulate O-O collisions at a center of mass energy of $\sqrt{s_{\mathrm{NN}}}$=5.36 TeV. This setup allows us to compare evolutions with and without a hydrodynamic description on an equal basis, while Angantyr serves as a baseline for no collective effects.

Figures (14)

• Figure 1: Opacity as a function of centrality in O-O and Ne-Ne collisions in the SMASH initial conditions at the switching time from Gotz:2025wnv.
• Figure 2: Comparison of the eccentricity in O-O and Ne-Ne collisions with Woods-Saxon and $\alpha$-clustered configurations in the SMASH initial conditions at $\sqrt{s_{\text{NN}}}$=5.36TeV.
• Figure 3: Momentum rapidity ranges for the sub-event methods. A similar approach to Jia:2017hbm is used.
• Figure 4: Multiplicity distribution in O-O collisions at $\sqrt{s_{\text{NN}}}$=5.36TeV from SMASH transport (red), the SMASH-vHLLE hybrid model (blue) and Angantyr (green). The solid lines represent the $\alpha$-clustered configuration and the vertical lines indicate the lower bound of the 0-5% centrality class.
• Figure 5: Nuclear modification factor $R_{\mathrm{AA}}$ in O-O and Ne-Ne collisions from the different models.