Measurement of elliptic and higher order flow from ATLAS experiment at the LHC

TL;DR

This paper measures the azimuthal anisotropy of charged hadron production in Pb+Pb collisions at 2.76 TeV, revealing significant elliptic and higher order flow coefficients that indicate collective behavior of the quark-gluon plasma.

Contribution

It provides a differential measurement of flow coefficients $v_2$ to $v_6$ using two methods, confirming their consistency and collective origin in heavy-ion collisions.

Findings

Significant $v_2$ to $v_6$ observed across various kinematic ranges.

Flow coefficients are consistent between two different analysis methods.

Results support the collective response hypothesis of the quark-gluon plasma.

Abstract

We present a differential measurement of the azimuthal anisotropy of charged hadron production in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=2.76 TeV. This azimuthal anisotropy is expanded into a Fourier series in azimuthal angle, where the coefficient for each term, $v_n$, characterizes the magnitude of the anisotropy at a particular angular scale. We extract $v_2-v_6$ via a discrete Fourier analysis of the two-particle $\Delta\phi-\Delta\eta$ correlation with a large $\Delta\eta$ gap ($|\Delta\eta|>2$), and via an event plane method based on the Forward Calorimeter. Significant $v_2-v_6$ values are observed over a broad range in $\pT$, $\eta$ and centrality, and they are found to be consistent between the two methods in the transverse momentum region $\pT<3-4$ GeV. This suggests that the measured $v_2-v_6$ obtained from two-particle correlations at low $\pT$ with a large $\Delta\eta$ gap are consistent with the collective response of the system to the initial state geometry fluctuations, and is not the result of jet fragmentation or resonance decay.