Exploring QCD Matter through Energy Correlators of Soft Hadrons

Abstract

We propose energy-energy correlators (EECs) of soft hadrons and their azimuthal harmonics as new soft observables to probe the collective flow and correlations beyond flow for large and small systems created in high energy nuclear collisions. We separate the soft EEC into a disconnected contribution determined by the mean energy flow and a connected contribution encoding correlations beyond it. Viscous hydrodynamic simulations demonstrate that the soft EEC of a fluid system is governed by the disconnected contribution, while its elliptic and triangular harmonics respond to the initial geometry and shear viscosity as traditional flow harmonics do. Comparisons between iEBE-VISHNU and PYTHIA for p+p, O+O, and Pb+Pb collisions show that the disconnected contribution dominates at large angles in all systems, whereas connected correlations beyond the mean energy flow grow toward small angles and small systems, most strongly for the harmonics. Soft EECs thus provide a unique window into collective flow and correlations beyond it across collision systems.