Abstract
We present a differential measurement of the azimuthal anisotropy of charged hadron production in Pb+Pb collisions at = 2.76 TeV. This azimuthal anisotropy is expanded into a Fourier series in azimuthal angle, where the coefficient for each term, vn, characterizes the magnitude of the anisotropy at a particular angular scale. We extract v2–v6 via a discrete Fourier analysis of the two-particle Δϕ–Δη correlation with a large Δη gap (|Δη| > 2), and via an event-plane method based on the forward calorimeter. Significant v2–v6 values are observed over a broad range in , η and centrality, and they are found to be consistent between the two methods in the transverse momentum region GeV. This suggests that the measured v2–v6 obtained from two-particle correlations at low with a large Δη 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.
Highlights
Among the many striking results obtained at RHIC, one important observation is the novel “ridge”-like and “cone”-like structures of the two-particle correlation in relative azimuthal angle ∆φ = φa − φb and pseudorapidity ∆η = ηa − ηb in Au+Au collisions [1]
The event plane Ψn is estimated using the transverse energy flow measured in towers from the Forward Calorimeter (FCal) within 3.3 < |η| < 4.8
The higher-order harmonic coefficients v2 − v6 have been extracted both by correlating tracks with the event plane determined at forward rapidity and by using the two-particle correlation method with a large pseudorapidity gap (|∆η| > 2)
Summary
The event plane Ψn is estimated using the transverse energy flow measured in towers from the Forward Calorimeter (FCal) within 3.3 < |η| < 4.8. The FCalP(N) method greatly increases the pseudorapidity separation between the tracks and the EP from about 3 units to about 5 units, it is much less affected by the so-called “non-flow” correlations, which stem primarily from jet fragmentation and resonance decays. This is especially useful for measuring the pseudorapidity dependence of vn. The similar pT dependence motivates us to find a simple scaling relation between different vn: vn1/n ∝ v21/2 This is qualitatively understood in a ”blast wave” scenario, where vn ∝ βn, with β being the radial flow velocity [6]. The v2 clearly continue to drop out to 10-12 GeV, and only vary slowly beyond that, but at a level that is consistent with pQCD calculations [9]
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