Relative transverse activity as an event classifier to investigate collectivity-like phenomena in proton$-$proton collisions at the LHC energies

TL;DR

This study uses PYTHIA simulations to analyze two-particle correlations in proton-proton collisions, revealing that increased underlying event activity can produce long-range, collectivity-like correlations without hydrodynamics.

Contribution

It introduces the relative transverse activity observable $R_{T}$ to classify events and demonstrates that soft QCD processes can generate long-range correlations similar to collectivity.

Findings

Long-range near-side correlations emerge in charge-independent $R_{2}$ for high $R_{T}$ events.

Charge-dependent correlators do not show long-range structures, indicating different underlying mechanisms.

The correlation peak narrows with increasing $R_{T}$, suggesting increased underlying event activity influences correlation structures.

Abstract

Two-particle number ($R_{2}$) and transverse-momentum ($P_{2}$) correlation functions are studied in proton$-$proton collisions at $\sqrt{s}=13$ TeV simulated with PYTHIA 8, in the low-transverse-momentum region where soft QCD processes dominate the particle production. Events are classified using the relative transverse activity observable $R_{\mathrm{T}}$, which provides a differential characterization of predominantly soft underlying-event (UE) activity. A finite long-range near-side component emerges in the charge-independent correlator $R_{2}^{\mathrm{CI}}$ for UE-dominated events ($2.5 < R_{\mathrm{T}} \leq 5.0$), whereas no corresponding long-range structure is observed in the charge-dependent correlators. This behavior suggests that the ridge-like feature is primarily driven by charge-independent QCD dynamics rather than short-range charge-balancing effects. In contrast, the transverse-momentum correlator $P_{2}^{\mathrm{CI}}$ remains dominated by localized, jet-like structures across all $R_{\mathrm{T}}$ intervals. The near-side correlation peak exhibits a systematic narrowing in $\Delta\varphi$ for the charge-independent case with increasing $R_{\mathrm{T}}$, while the charge-dependent correlators remain localized in both $\Delta\eta$ and $\Delta\varphi$ and show only mild broadening. These results indicate that enhanced UE activity can give rise to collectivity-like long-range correlation structures within a non-hydrodynamic framework and provide a PYTHIA baseline for interpreting small-system measurements at the LHC.