Studying Energy-Energy Correlators in pp Collisions at the LHC with a Jet-Free Event-Topology Method

TL;DR

This work tackles the limitation of jet-based EEC measurements at low $p_T$ by introducing a jet-free EEC framework that uses a leading-hadron axis and Toward/Transverse cones with a data-driven background subtraction. It validates the approach against conventional jet-based EEC measurements in pp at $\,sqrt{s}=13$ TeV using PYTHIA8, demonstrating robust access to the perturbative/non-perturbative transition and flavor-dependent dynamics. The results show that the EEC peak position scales roughly as $R_L^{peak} \\propto 1/\\langle p_{T,tot}^{corr} angle$ with the peak height scaling as $\\langle p_{T,tot}^{corr} angle$, and reveal clear flavor effects: gluon-initiated jets have larger EEC activity and a peak at larger angular separations, while charm-triggered EECs exhibit dead-cone suppression. The jet-free EEC framework thus provides a simple, experimentally robust tool for studying QCD dynamics across collision systems, including potential applications to heavy-ion and proton–nucleus collisions and multiplicity-dependent studies.

Abstract

We present a jet-free approach for measuring energy-energy correlators (EEC) in proton-proton (pp) collisions at the Large Hadron Collider (LHC), employing an event-topology method that does not rely on explicit jet reconstruction. Using the leading charged hadron as a reference axis, the azimuthal plane is divided into Toward and Transverse regions, enabling a robust background subtraction and extending EEC measurements into the low $p_T$ regime where conventional jet-based approaches become unreliable. The method is validated through comparisons with conventional jet reconstruction results. We systematically explore the dependence of the EEC on the leading-particle transverse momentum and parton flavor. The observed scaling between the EEC peak position and the hard scale suggests that this topology-based EEC captures effectively the transition between perturbative and non-perturbative QCD regimes. Distinct differences are found between quark- and gluon-initiated events, reflecting their different color charges and radiation patterns. Extending the analysis to heavy flavor, EECs triggered by leading charm mesons exhibit a suppressed magnitude and a peak shifted toward larger angular separations relative to inclusive charged-particle triggers, providing a direct manifestation of the dead-cone effect. This jet-free EEC framework offers a simple and experimentally robust tool for studying the scale and flavor dependence of the QCD dynamics, with promising applications to proton-nucleus and heavy-ion collisions at the LHC.

Figures (8)

• Figure 1: The azimuthal plane is divided into different cones ($\eta$ is the same). The Toward cone (red) is defined around the leading particle. Two Transverse cones (blue) are made perpendicular to the leading particle.
• Figure 2: The average of sum $p_T$ in the Transverse cone as the function of $N_{ch}$.
• Figure 3: The Toward cone EEC distribution before subtracting the background (black solid circles), the background EEC distribution defined in Eq. \ref{['eqn:sig_EEC_region']} (blue solid triangle), the EEC distribution after the background subtraction (red open squares).
• Figure 4: hadron-based method EEC distribution compare with jet-reconstructed-based EEC distribution.
• Figure 5: The EEC distributions after background subtraction applied in different trigger $p_{T}$ range.