Two- and Multi-particle Azimuthal Correlations in Small Collision Systems with the ATLAS Detector

Adam Trzupek

doi:10.1016/j.nuclphysa.2017.05.019

Abstract

The recent ATLAS results on two- and multi-particle azimuthal correlations of charged particles are presented for s=5.02 TeV and 13 TeV pp, sNN=5.02 TeV p+Pb and sNN=2.76 TeV low-multiplicity Pb+Pb collisions. To suppress the “non-flow” contribution from the correlations, a template fitting procedure is used in the two-particle correlations (2PC) measurements, while for multi-particle correlations the cumulant method is applied. The correlations are expressed in the form of Fourier harmonics vn(n=2,3,4) measuring the global azimuthal anisotropy of produced particles. The measurements presented hereafter confirm the evidence for collective phenomena in high-multiplicity p+Pb and low-multiplicity Pb+Pb collisions. For pp collisions the results on four-particle cumulants do not demonstrate a similar collective behaviour.

Highlights

The large azimuthal anisotropy observed in particles produced in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), is one of the main signatures of the formation of strongly interacting Quark-Gluon Plasma [1, 2]
Significant azimuthal anisotropy was observed at the LHC for the first time in pp and p+Pb collisions [3, 4]
Results obtained for different collision systems or pT-ranges are compared in a common event activity variable, Nch(pT > 0.4), defined as the mean number of charged particles with pT > 0.4 GeV in events in the Mref interval used for the cumulants calculations

Summary

Introduction

The large azimuthal anisotropy observed in particles produced in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), is one of the main signatures of the formation of strongly interacting Quark-Gluon Plasma [1, 2]. Significant azimuthal anisotropy was observed at the LHC for the first time in pp and p+Pb collisions [3, 4]. This observation leads to puzzling questions related to the origin of this phenomenon as well as indicates a possible presence of the Quark-Gluon Plasma in collisions involving light nuclei. The 5trie.o0cne2nmTteeAVtTh[oL6d]A., SFaro[er5]tshhreeoswsuanmltsfeoosrnypsFtpeomucrosilealrinscdiooanedsffidaicttiieo√nntassll=vynf5,o.b0ra2lsoeTwde-Vomnuaalntidnpol1ivc3eiltTyteeV√mspaNlnNadte=pfi2+t.tP7inb6gTctoewVlloisP-ipboa+nrstPicbalteco√clolsirNsriNeolna=smeasurements of multi-particle cumulants and corresponding flow harmonics are presented [7]

The 2PC method

Multi-particle cumulants

Summary in