Azimuthal correlations of charged hadrons in Pb+Pb collisions at sqrt(s) = 2.76 TeV

Azimuthal correlations of charged hadrons were measured in $\sqrt{ s_{NN}}$ = 2.76 TeV PbPb collisions by the CMS experiment at the LHC. The distributions exhibit anisotropies that are correlated with the event-by-event orientation of the reaction plane. The anisotropy parameter, $v_{2}$, defined as the second Fourier coefficient in the particle azimuthal distributions was extracted using four different methods: the event-plane, two- and four-particle cumulants, and Lee-Yang zeros. As expected, the varying sensitivities of the methods to non-flow correlations and event-by-event fluctuations in the eccentricity of the participant zone result in different extracted $v_2$ values from each method. The anisotropy is studied as a function of transverse momentum, pseudorapidity, and centrality in a broad kinematic range: $0.3 < p_T < 12.0$ GeV/c, $|\eta| < 2.4$, and in 12 centrality classes. The differential $v_{2} (p_T)$ distribution reaches a maximum around $p_T = 3$ GeV/$c$. The integrated $v_{2}$ values increase with centrality up to the 40-50\% centrality class, then slowly decrease in more-peripheral collisions. This trend is strongest in the mid-rapidity range $(|\eta| < 0.8)$ and gradually weakens at larger rapidity.