Importance of non-flow in mixed-harmonic multi-particle correlations in small collision systems

Peng Huo,Katarína Gajdošová,Jiangyong Jia,You Zhou

doi:10.1016/j.physletb.2017.12.035

Peng Huo, Katarína Gajdošová + Show 2 more

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https://doi.org/10.1016/j.physletb.2017.12.035

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Abstract

Recently CMS Collaboration measured mixed-harmonic four-particle azimuthal correlations, known as symmetric cumulants SC(n,m), in pp and p+Pb collisions, and interpreted the non-zero SC(n,m) as evidence for long-range collectivity in these small collision systems. Using the PYTHIA and HIJING models which do not have genuine long-range collectivity, we show that the CMS results, obtained with standard cumulant method, could be dominated by non-flow effects associated with jet and dijets, especially in pp collisions. We show that the non-flow effects are largely suppressed using the recently proposed subevent cumulant methods by requiring azimuthal correlation between two or more pseudorapidity ranges. We argue that the reanalysis of SC(n,m) using the subevent method in experiments is necessary before they can used to provide further evidences for a long-range multi-particle collectivity and constraints on theoretical models in small collision systems.

Highlights

Measurements of two-particle angular correlation in small collision systems, such as pp or p+A, have revealed the ridge phenomena [1,2,3,4,5]: enhanced production of pairs at small azimuthal angle separation, ∆φ, extended over wide range of pseudorapidity separation ∆η
Using the PYTHIA and HIJING models which do not have genuine long-range collectivity, we show that the CMS results, obtained with standard cumulant method, could be dominated by non-flow effects associated with jet and dijets, especially in pp collisions
We show that the non-flow effects are largely suppressed using the recently proposed subevent cumulant methods by requiring azimuthal correlation between two or more pseudorapidity ranges

Summary

Introduction

Measurements of two-particle angular correlation in small collision systems, such as pp or p+A, have revealed the ridge phenomena [1,2,3,4,5]: enhanced production of pairs at small azimuthal angle separation, ∆φ, extended over wide range of pseudorapidity separation ∆η. One key question concerning the ridge is the timescale for the emergence of the long-range multi-particle collectivity, whether it reflects initial momentum correlation from gluon saturation effects [6] or it reflects a final-state hydrodynamic response to the initial transverse collision geometry [7]. More insights about the ridge is obtained via multi-particle correlation technique, known as cumulants, involving four or more particles [8,9,10,11].

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