Relative transverse activity as a probe of collectivity-like long-range correlations in pp collisions at $\sqrt{s}=13$ TeV

Abstract

Understanding the origin of collectivity-like signatures in small collision systems is a central open question in high-energy nuclear physics, and two-particle correlation functions offer unique sensitivity to the underlying-event (UE) dynamics that may drive such behavior in proton--proton (pp) collisions. In this work, the two-particle number ($R_2$) and transverse-momentum ($P_2$) correlation functions are studied in pp collisions at $\sqrt{s}=13$ TeV using PYTHIA 8, for final state charged hadrons within $|\eta |<0.8$ and $0.2<p_{\mathrm{T}}<2.0$ GeV/$c$, with events classified by the relative transverse activity $R_{\mathrm{T}}$ to probe how UE activity shapes correlation structures in the soft-QCD-dominated regime. A collectivity-like long-range near-side component is observed in the charge-independent correlator $R_2^{\mathrm{CI}}$ exclusively for the highest $R_{\mathrm{T}}$ class ($2.5<R_{\mathrm{T}}\le 5.0$), while no corresponding structure appears in the charge-dependent correlators. This indicates that enhanced UE activity, driven by multiple partonic interactions and color reconnection, can generate collectivity-like long-range correlations without hydrodynamic evolution. These findings establish $R_{\mathrm{T}}$ as a differential event classifier to provide a non-hydrodynamic baseline for interpreting such signatures in small-system measurements at the LHC.