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System size and event shape dependence of particle-identified balance functions in proton-proton collisions at $\sqrt{s} = 13$ TeV using PYTHIA 8 and EPOS models

Subash Chandra Behera, Arvind Khuntia

Abstract

We investigate charge balance functions for pion, kaon, and proton pairs in proton-proton (pp) collisions at $\sqrt{s}=13$ TeV using Monte Carlo models, PYTHIA8 and EPOS-LHC, with transverse spherocity to classify event topology and charged-particle multiplicity to select system size. Simulations with PYTHIA8 and EPOS-LHC reveal that balance-function widths in rapidity and azimuthal angle depend on multiplicity and event shape. In PYTHIA8, widths decrease monotonically with multiplicity, consistent with local charge conservation in a fragmentation-dominated scenario. In contrast, the EPOS-LHC model, especially when using the core corona implementation, exhibits a more intricate response, where the combined effects of hydrodynamic radial flow and longitudinal diffusion result in narrower azimuthal correlations and broader rapidity correlations. These features are characteristic signatures of collective dynamics, similar to those observed in heavy-ion collisions. Events with low spherocity, which are jet-like in nature, exhibit significantly narrower balance function widths compared to isotropic events with high spherocity, illustrating that event-shape selection provides clear sensitivity to the underlying dynamics of particle production in pp collisions. The species dependence and event-shape sensitivity of the balance-function widths provide information about the hadronization dynamics and collectivity in small systems. These results demonstrate that multidimensional, particle species dependent balance function measurements can disentangle the underlying mechanisms of charge correlations and medium-like behavior in high-multiplicity pp collisions.

System size and event shape dependence of particle-identified balance functions in proton-proton collisions at $\sqrt{s} = 13$ TeV using PYTHIA 8 and EPOS models

Abstract

We investigate charge balance functions for pion, kaon, and proton pairs in proton-proton (pp) collisions at TeV using Monte Carlo models, PYTHIA8 and EPOS-LHC, with transverse spherocity to classify event topology and charged-particle multiplicity to select system size. Simulations with PYTHIA8 and EPOS-LHC reveal that balance-function widths in rapidity and azimuthal angle depend on multiplicity and event shape. In PYTHIA8, widths decrease monotonically with multiplicity, consistent with local charge conservation in a fragmentation-dominated scenario. In contrast, the EPOS-LHC model, especially when using the core corona implementation, exhibits a more intricate response, where the combined effects of hydrodynamic radial flow and longitudinal diffusion result in narrower azimuthal correlations and broader rapidity correlations. These features are characteristic signatures of collective dynamics, similar to those observed in heavy-ion collisions. Events with low spherocity, which are jet-like in nature, exhibit significantly narrower balance function widths compared to isotropic events with high spherocity, illustrating that event-shape selection provides clear sensitivity to the underlying dynamics of particle production in pp collisions. The species dependence and event-shape sensitivity of the balance-function widths provide information about the hadronization dynamics and collectivity in small systems. These results demonstrate that multidimensional, particle species dependent balance function measurements can disentangle the underlying mechanisms of charge correlations and medium-like behavior in high-multiplicity pp collisions.

Paper Structure

This paper contains 8 sections, 6 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Analysis Methodology
  3. Model description
  4. Result and discussion
  5. 2D correlations
  6. One-dimensional projections
  7. Balance functions width
  8. Summary

Figures (8)

  • Figure 1: Charged particle multiplicity distributions for pythia8 and epos event generators in pp collisions at $\sqrt{s} = 13$ TeV.
  • Figure 2: The transverse spherocity distributions arvind_spALICE_SP1ALICE_SP2 for different multiplicity classes mult_def in pp collisions at $\sqrt{s} = 13$ TeV using the pythia8 (left) and epos (right) event generators.
  • Figure 3: Two-dimensional balance function from pythia8 and epos-lhc model simulation for $\pi, K$ and $p$ in pp collisions at $\sqrt{s} =$13 TeV. Plots are shown for the integrated spherocity value in high multiplicity collisions, $80 < N_{\mathrm{ch}} < 100$.
  • Figure 4: One-dimensional $B$ projections along $\Delta y$ and $\Delta\phi$ from pythia8 model in pp collisions at $\sqrt{s} =$ 13 TeV. The left column is for pion, the middle column is for kaon, and the right panel is for proton, respectively. $\Delta y$ projections are take in $|\Delta\phi| \leq \pi/2$ and $\Delta\phi$ projections are taken in $|\Delta y| \leq 1.0$ range.
  • Figure 5: One-dimensional projections of $B$ along $\Delta y$ and $\Delta\phi$ from epos-lhc model without core in pp collisions at $\sqrt{s} =$ 13 TeV. The left column is for pion, the middle column is for kaon, and the right panel is for proton, respectively. $\Delta\phi$ projections are take in $|\Delta\phi| \leq \pi/2$ and $\Delta\phi$ projections are taken in $|\Delta y| \leq 1.0$ range.
  • ...and 3 more figures