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Eccentricity and elliptic flow in proton-proton collisions from parton evolution

Emil Avsar, Christoffer Flensburg, Yoshitaka Hatta, Jean-Yves Ollitrault, Takahiro Ueda

TL;DR

The paper investigates whether high-multiplicity proton-proton collisions at LHC energies can exhibit elliptic flow similar to heavy-ion collisions. Using the DIPSY Monte Carlo with BFKL-like dipole evolution, it computes event-by-event gluon spatial eccentricities and maps them to v2 via hydrodynamic-inspired relations, accounting for fluctuations and nonflow. It predicts a flow signal of about 4–6% in high-multiplicity pp events at 7–14 TeV, driven by sizable initial eccentricities, and discusses how to disentangle flow from large nonflow contributions using higher-order cumulants. The study suggests that collectivity may emerge in small systems under suitable fluctuations and provides guidance for experimental strategies to extract genuine flow signals.

Abstract

It has been argued that high-multiplicity proton-proton collisions at the LHC may exhibit collective phenomena usually studied in the context of heavy-ion collisions, such as elliptic flow. We study this issue using DIPSY - a Monte Carlo event generator based on the QCD dipole model. We calculate the eccentricity of the transverse area defined by the spatial distribution of produced gluons. The resulting elliptic flow is estimated to be about 6%, comparable to the value in nucleus-nucleus collisions at RHIC and the LHC. Experimentally, elliptic flow is inferred from the azimuthal correlation between hadrons, which receives contributions from collective flow, and from various other effects referred to as "nonflow". We discuss how to identify in experiments the signal of flow in the presence of large nonflow effects.

Eccentricity and elliptic flow in proton-proton collisions from parton evolution

TL;DR

The paper investigates whether high-multiplicity proton-proton collisions at LHC energies can exhibit elliptic flow similar to heavy-ion collisions. Using the DIPSY Monte Carlo with BFKL-like dipole evolution, it computes event-by-event gluon spatial eccentricities and maps them to v2 via hydrodynamic-inspired relations, accounting for fluctuations and nonflow. It predicts a flow signal of about 4–6% in high-multiplicity pp events at 7–14 TeV, driven by sizable initial eccentricities, and discusses how to disentangle flow from large nonflow contributions using higher-order cumulants. The study suggests that collectivity may emerge in small systems under suitable fluctuations and provides guidance for experimental strategies to extract genuine flow signals.

Abstract

It has been argued that high-multiplicity proton-proton collisions at the LHC may exhibit collective phenomena usually studied in the context of heavy-ion collisions, such as elliptic flow. We study this issue using DIPSY - a Monte Carlo event generator based on the QCD dipole model. We calculate the eccentricity of the transverse area defined by the spatial distribution of produced gluons. The resulting elliptic flow is estimated to be about 6%, comparable to the value in nucleus-nucleus collisions at RHIC and the LHC. Experimentally, elliptic flow is inferred from the azimuthal correlation between hadrons, which receives contributions from collective flow, and from various other effects referred to as "nonflow". We discuss how to identify in experiments the signal of flow in the presence of large nonflow effects.

Paper Structure

This paper contains 4 sections, 11 equations, 2 figures.

Table of Contents

  1. Introduction
  2. The model
  3. Eccentricity and the elliptic flow
  4. Discussion

Figures (2)

  • Figure 1: (Color online) Predictions for the eccentricity and the interaction area $S \, [{\rm fm}^2]\,(\times 0.1)$ at 7 TeV versus the charged multiplicity in the interval $|\eta|<0.9$ at $\sqrt{s}=7$ TeV.
  • Figure 2: (Color online) Predictions for $v_2\{2\}$ and $v_2\{4\}$ calculated from Eqs. (\ref{['hydroform2']}) and (\ref{['hydroform3']}) versus the charged multiplicity in the interval $|\eta|<0.9$ at $\sqrt{s}=7$ TeV. The error bars are from statistics only.