Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector

TL;DR

This ATLAS study measures the elliptic flow coefficient v2 for charged particles in Pb+Pb collisions at sqrt(s_NN)=2.76 TeV over |η|<2.5 and 0.5<pT<20 GeV using the event-plane method with the forward calorimeter. Centrality is determined from FCal ΣE_T and corrected via Glauber modeling, with the event-plane resolution obtained from subevents to correct the measured v2. The results reveal a rapid rise of v2 with pT up to about 3 GeV, a decline toward 7–8 GeV, and a weak pT dependence at higher pT, while v2(η) remains nearly constant across |η|<2.5, indicating limited longitudinal variation at LHC energies. These findings constrain hydrodynamic evolution and jet-medium interactions in ultra-relativistic heavy-ion collisions and align with, yet extend, previous RHIC and LHC measurements.

Abstract

This paper describes the measurement of elliptic flow of charged particles in lead-lead collisions at sqrt(s_NN) = 2.76 TeV using the ATLAS detector at the Large Hadron Collider (LHC). The results are based on an integrated luminosity of approximately 7 ub^-1. Elliptic flow is measured over a wide region in pseudorapidity, |eta| < 2.5, and over a broad range in transverse momentum, 0.5 < p_T < 20 GeV. The elliptic flow parameter v_2 is obtained by correlating individual tracks with the event plane measured using energy deposited in the forward calorimeters. As a function of transverse momentum, v_2(p_T) reaches a maximum at p_T of about 3 GeV, then decreases and becomes weakly dependent on p_T above 7 - 8 GeV. Over the measured pseudorapidity region, v_2 is found to be approximately independent of |eta| for all collision centralities and particle transverse momenta, something not observed in lower energy collisions. The results are discussed in the context of previous measurements at lower collision energies, as well as recent results from the LHC.

Figures (6)

• Figure 1: Measured $\mathrm{FCal} \; \Sigma E_{T}$ distribution divided into 10% centrality intervals (black). Proton-proton data at $\sqrt{s}=2.76$ TeV, convolved with a Glauber Monte Carlo calculation with $x=0.088$ (grey), as described in the text.
• Figure 2: Distribution of the azimuthal angle of individual tracks relative to the measured event plane, in eight centrality intervals. These distributions are meant to illustrate the observed correlation relative to the event plane, and are not used in the quantitative estimates of $v_2$. The curve is a fit to $1+\sum_n 2v_n \cos(n\phi)$ up to $n=6$.
• Figure 3: (left) Distribution of the difference between the event planes at positive and negative $\eta$ obtained using Layer 1 FCal towers, both with full and half acceptance. (right) $\mathrm{FCal} \; \Sigma E_{T}$ dependence of the resolution correction for event planes from Layer 1 FCal towers in full acceptance (full symbols) and half acceptance (open symbols).
• Figure 4: Elliptic flow $v_2(p_{\mathrm{T}})$ as a function of $p_{\mathrm{T}}$ for eight 10% centrality intervals, for $p_{\mathrm{T}}$ from 0.5 to 20 GeV, and for three ranges in pseudorapidity ($|\eta|<1$, $1<|\eta|<2$ and $2<|\eta|<2.5$). Error bars show statistical and systematic uncertainties added in quadrature. The arrows indicate where the value of $v_2$ does not fit within the chosen plot scale, due to large statistical fluctuations.
• Figure 5: Pseudorapidity dependence of $v_2(p_{\mathrm{T}},\eta)$ for $0.5 < p_{\mathrm{T}} < 20$ GeV in five $p_{\mathrm{T}}$ intervals and 10% centrality intervals. Error bars show statistical and systematic uncertainties added in quadrature.