Abstract
ATLAS measurements of the azimuthal anisotropy in lead–lead collisions at sqrt{s_{mathrm {NN}}}=2.76 TeV are shown using a dataset of approximately 7 upmu b^{-1} collected at the LHC in 2010. The measurements are performed for charged particles with transverse momenta 0.5<p_{mathrm {T}}<20 GeV and in the pseudorapidity range |eta |<2.5. The anisotropy is characterized by the Fourier coefficients, mathrm {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 mathrm {v}_n coefficients are presented. The elliptic flow, mathrm {v}_2, is obtained from the two-, four-, six- and eight-particle cumulants while higher-order coefficients, mathrm {v}_3 and mathrm {v}_4, are determined with two- and four-particle cumulants. Flow harmonics mathrm {v}_n measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to mathrm {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.
Highlights
Where n is the order of the Fourier harmonic, referred to as flow harmonic, φ is the azimuthal angle of the outgoing particle, n defines the azimuthal angle of the nth-order symmetry plane of the initial geometry [15], and the angled brackets denote an average over charged particles in an event
The event-plane v2 is systematically smaller than v2{2} since it is less affected by short-range two-particle correlations, which are partially removed from v2{EP} due to the separation between the phase-space region where the event plane angle is determined (3.2 < |η| < 4.9) and the phase space where charged-particle momenta are reconstructed (|η| < 2.5)
F( 3) and F( 4) obtained from the eccentricity distributions predicted by the Glauber and Monte Carlo (MC)-KLN models are shown
Summary
The anisotropy of charged-particle azimuthal angle distributions in heavy-ion collisions has been a subject of extensive experimental studies at RHIC [1,2,3,4,5,6] and more recently at the LHC [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. The final-state anisotropy is a global property of particle production that arises from the initial spatial asymmetry of the collision region in a plane transverse to the beam axis for heavy-ion collisions with a non-zero impact parameter. It is characterized by the coefficients, vn, of the Fourier expansion of the measured azimuthal angle distributions [1,26]: vn ≡ ein(φ− n) = cos [n(φ − n)] , (1). This leads to a problem of disentangling all-particle flow and contributions from particle correlations unrelated to the initial geometry, known as non-flow correlations These non-flow effects include correlations due to energy and momentum conservation, resonance decays, quantum interference phenomena and jet production. In order to suppress non-flow correlations, methods that use genuine multi-particle correlations, estimated using cumulants, were proposed [27,28,29,30]
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