Two-particle correlations in pseudorapidity in a hydrodynamic model

Abstract

Two-particle pseudorapidity correlations of hadrons produced in Pb+Pb collisions at $\sqrt{{s}_{NN}}=2.76$ TeV at the CERN Large Hadron Collider are analyzed in the framework of a model based on viscous 3+1-dimensional hydrodynamics with the Glauber initial condition. Based on our results, we argue that the correlation from resonance decays, formed at a late stage of the evolution, produce significant effects. In particular, their contribution to the event averages of the coefficients of the expansion in the Legendre basis explain 60--70% of the experimental values. We propose an accurate way to compute these coefficients, independent of the binning in pseudorapidity, and test a double expansion of the two-particle correlation function in the azimuth and pseudorapidity, which allows us to investigate the pseudorapidity correlations between harmonics of the collective flow. In our model, these quantities are also dominated by the nonflow effects from the resonance decays. Finally, our method can be used to compute higher-order cumulants for the expansion in orthonormal polynomials [A. Bzdak and P. Bo\ifmmode \dot{z}\else \.{z}\fi{}ek, arXiv:1509.02967 [hep-ph] (2015)] which offers a suitable way of eliminating the nonflow effects from the correlation analyses.