Measurement of flow harmonics with multi-particle cumulants in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV with the ATLAS detector

TL;DR

This paper reports on measurements of flow harmonics in Pb+Pb collisions at 2.76 TeV using multi-particle cumulants, revealing detailed flow behavior and fluctuations that challenge existing models.

Contribution

It introduces a comprehensive analysis of flow harmonics using multi-particle cumulants in heavy-ion collisions at the LHC, providing new insights into flow fluctuations.

Findings

Flow coefficients decrease with higher-order cumulants.

Flow fluctuations are significant and not well described by current models.

Higher-order flow harmonics vary with transverse momentum and centrality.

Abstract

ATLAS measurements of the azimuthal anisotropy in lead-lead collisions at $\sqrt{s_{NN}}=2.76$ TeV are shown using a dataset of approximately 7 $\mu$b$^{-1}$ collected at the LHC in 2010. The measurements are performed for charged particles with transverse momenta $0.5<p_T<20$ GeV and in the pseudorapidity range $|\eta|<2.5$. The anisotropy is characterized by the Fourier coefficients, $v_n$, of the charged-particle azimuthal angle distribution for n = 2-4. The Fourier coefficients are evaluated using multi-particle cumulants calculated with the generating function method. Results on the transverse momentum, pseudorapidity and centrality dependence of the $v_n$ coefficients are presented. The elliptic flow, $v_2$, is obtained from the two-, four-, six- and eight-particle cumulants while higher-order coefficients, $v_3$ and $v_4$, are determined with two- and four-particle cumulants. Flow harmonics $v_n$ measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to $v_n$ measurements obtained using different analysis methods and previously reported by the LHC experiments is also shown. Results of measurements of flow fluctuations evaluated with multi-particle cumulants are shown as a function of transverse momentum and the collision centrality. Models of the initial spatial geometry and its fluctuations fail to describe the flow fluctuations measurements.