Multiplicity dependence of two-particle angular correlations of identified particles in pp collisions at $\mathbf{\sqrt{s} = 13}$ TeV

TL;DR

This work investigates two-particle angular correlations for identified pions, kaons, and protons in pp collisions at $\sqrt{s}=13$ TeV, exploring how these correlations depend on event multiplicity. By constructing CP and a multiplicity-free CR correlation function in ($\Delta y$, $\Delta \varphi$), the analysis disentangles trivial scaling from underlying physics such as quantum statistics, final-state interactions, and resonance decays. The results reveal persistent near-side anticorrelations for same-baryon pairs and complex resonance-driven structures across multiplicities, challenging current hadronization models. Comparisons to MC generators show that while meson correlations are partially captured, baryon-baryon correlations remain poorly described, highlighting missing ingredients in baryon production mechanisms and quantum-statistics effects in small systems.

Abstract

Two-particle angular correlations explore particle production mechanisms and underlying event-wide phenomena present in the systems created in hadronic collisions. These correlations are examined as a function of rapidity and azimuthal-angle differences ($Δy, Δ\varphi$) for pairs of like- and unlike-sign pions, kaons, and (anti-)protons produced in pp collisions at $\sqrt{s}$ = 13 TeV, measured by the ALICE experiment. Two-particle correlation functions are provided, along with $Δy$ and $Δ\varphi$ projections, and are compared to Monte Carlo (MC) model predictions. For the first time, the measurement is performed as a function of the event's charged-particle density. The shapes of the correlation functions are studied in detail for each particle pair. Previous studies conducted for pp collisions at $\sqrt{s}$ = 7 TeV at ALICE have revealed an anticorrelation at small relative angles for baryon-baryon and antibaryon-antibaryon pairs, whose origin remains an open question. In this work, an additional approach is introduced to study the multiplicity dependence of the correlation functions in more detail and reveal qualitative differences in the underlying sources of correlations, such as quantum statistics, final-state interactions, and resonance decays. The puzzling near-side anticorrelation in baryon-baryon measurements is observed across all multiplicity classes and remains a challenge for models of particle production in pp collisions. Furthermore, the multiplicity dependence of the correlations between mesons provides an independent means to explore the sensitivity of current MC models to soft-QCD effects and hadronization dynamics. The presented measurements, together with the baryon results, enrich the experimental picture of two-particle correlations in pp collisions and serve as valuable input for ongoing theoretical developments.

Figures (10)

• Figure 1: The ($\Delta y,\Delta\varphi$) correlation functions for like-sign pions, kaons, and protons pairs (top-to-bottom) in the highest (0--0.17%), the middle (30--40%), and lowest (90--100%) multiplicity classes (left-to-right) of pp collisions at $\sqrt{s} = 13$ TeV using the $C_{\rm{R}}$ definition.
• Figure 2: The ($\Delta y,\Delta\varphi$) correlation functions for unlike-sign pions, kaons, and protons pairs (top-to-bottom) in the highest (0--0.17%), the middle (30--40%), and lowest (90--100%) multiplicity classes (left-to-right) of pp collisions at $\sqrt{s} = 13$ TeV using the $C_{\rm{R}}$ definition.
• Figure 3: $\Delta\varphi$ projections of correlation functions within $\abs{\Delta y} < 1$ for like- and unlike-sign pions, kaons, and protons in multiplicity classes, using the $C_{\rm{P}}$ definition. Statistical uncertainties (bars) and systematic uncertainties (boxes) are both drawn with the color associated with the corresponding multiplicity class.
• Figure 4: $\Delta\varphi$ projections of correlation functions within $\abs{\Delta y} < 1$ for like- and unlike-sign pions, kaons, and protons in multiplicity classes, using the $C_{\rm{R}}$ definition. Statistical uncertainties (bars) and systematic uncertainties (boxes) are both drawn with the colour associated with the corresponding multiplicity class.
• Figure 5: Comparison between $\Delta\varphi$-correlation functions and MC models (PYTHIA8 Monash 2013 in blue, EPOS LHC in red, EPOS4 in green, and PYTHIA 8.315 in magenta) in pp collision at $\sqrt{s} = 13$ TeV by using the $C_{\rm{R}}$ definition for like-sign pions, kaons, and protons pairs (top-to-bottom) for the highest (0--1%), middle (30--40%), and lowest (90--100%) multiplicity classes (left-to-right).