Measurement of two-particle pseudorapidity correlations in Pb plus Pb collisions at √SNN=2.76 TeV with the ATLAS detector

Two-particle pseudorapidity correlations, measured using charged particles with p(T) > 0.5 GeV and vertical bar eta vertical bar < 2.4, from root S-NN, = 2.76 TeV Pb+Pb collisions collected in 2010 by the ATLAS experiment at the LHC are presented. The correlation function C-N(eta(1),eta(2)) is measured for different centrality intervals as a function of the pseudorapidity of the two particles, eta(1) and eta(2). The correlation function shows an enhancement along eta(-) equivalent to eta(1) - eta(2) approximate to 0 and a suppression at large eta(-) values. The correlation function also shows a quadratic dependence along the eta(+) equivalent to eta(1) + eta(2) direction. These structures are consistent with a strong forward-backward asymmetry of the particle multiplicity that fluctuates event to event. The correlation function is expanded in an orthonormal basis of Legendre polynomials, T-n(eta(1))T-m(eta(2)), and corresponding coefficients a(n,m) are measured. These coefficients are related to mean-square values of the Legendre coefficients, a(n), of the single particle longitudinal multiplicity fluctuations: a(n,m) = < a(n)a(m)>. Significant values are observed for the diagonal terms < a(n)(2)> and mixed terms < a(n)a(n+2)>. Magnitude of < a(1)(2)> is the largest and the higher order terms decrease quickly with increase in n. The centrality dependence of the leading coefficient < a(1)(2)> is compared to that of the mean-square value of the asymmetry of the number of participating nucleons between the two colliding nuclei, and also to the < a(1)(2)> calculated from HIJING.