Energy-energy correlator of soft sector from small to large systems

by Yiyang Peng (北京大学理论物理研究所)

Monday Nov 23, 2026, 10:00 AM → 11:45 AM Europe/Zurich

We study energy–energy correlators (EEC) of soft hadrons across high-energy collision systems. These observables can reveal how QCD systems reorganize, with increasing size and multiplicity, from microscopic few-body correlations to collective many-body dynamics and thermalization. Over the full angular range, they are sensitive to both short-range correlations and the global angular structure of bulk energy flow. Here we make a first attempt to study this change by calculating EEC for different systems and multiplicity classes, using viscous hydrodynamics and PYTHIA 8 as two limiting baselines for hydrodynamic evolution and non-hydrodynamic particle production, respectively. We separate the contribution determined by the mean energy flow from the connected component. The hydrodynamic results are dominated by the former and retain sensitivity to geometry and shear viscosity. PYTHIA exhibits much stronger connected correlations, particularly at low multiplicity; their magnitude falls approximately as the inverse total soft energy, although this scaling varies with angle. Together, these results show how soft-sector EECs can track the changing balance between microscopic correlations and collective energy flow as the system grows.