Abstract

Measurements of the azimuthal anisotropy in lead–lead collisions at sqrt{s_{_text {NN}}} = 5.02 TeV are presented using a data sample corresponding to 0.49 {mathrm {nb}}^{-1} integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for “ultra-central” collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, v_{2}–v_{7}, which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics v_{n} over wide ranges of the transverse momentum, 0.5 <p_{mathrm{T}}< 60 GeV, the pseudorapidity, |eta |< 2.5, and the collision centrality 0–80%. Results from different methods are compared and discussed in the context of previous and recent measurements in Pb+Pb collisions at sqrt{s_{_text {NN}}} = 2.76 mathrm{TeV} and 5.02 mathrm{TeV}. In particular, the shape of the p_{mathrm{T}}dependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the v_{n} and p_{mathrm{T}}values by constant factors over the centrality interval 0–60% and the p_{mathrm{T}}range 0.5 < p_{mathrm{T}}< 5 GeV.

Highlights

  • The azimuthal angular distribution of single produced particles can be expanded in a Fourier series [16,17]: dN = N0 1 + 2vn cos [n (φ − n)], (1)dφ 2π n=1 where N0 is the total particle yield, φ is the azimuthal angle of the produced particles and the vn and n are, respectively, the magnitude of the nth-order azimuthal anisotropy and the orientation of the nth-order symmetry plane

  • This paper presents the first ATLAS measurements of azimuthal anisotropy of charged particles in Pb+Pb collisions at 5.02 TeV collected during LHC running in 2015

  • The azimuthal anisotropy is quantified by the flow harmonics v2– v6 and v2–v7 for measurements based on the 2PC and SP/EP methods, respectively

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Summary

Introduction

The vn coefficients – called flow harmonics – are typically mea-. 997 Page 2 of 35 sured as a function of particle pseudorapidity (η), transverse momentum ( pT), and the degree of overlap between the colliding nuclei (centrality). The first harmonic, v1, is known as directed flow and refers to the sideward motion of participants in ultra-relativistic nuclear collisions, and it carries information from the early stage of the collision. The most extensive studies are related to the second flow harmonic, v2, known as elliptic flow. Elliptic flow is sensitive to the initial spatial asymmetry of the almond-shaped overlapping zone of the colliding nuclei. The higher-order coefficients, n > 2, are important due to their sensitivity to initial-state geometric fluctuations and viscous effects [16–18]

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