ATLAS measurement of long-range multiplicity correlations in 2.76 TeV Pb+Pb collisions at the LHC

Two-particle pseudorapidity correlations are measured using charged particles from $\sqrt{s_{NN}}$ = 2.76 TeV Pb+Pb collisions collected in 2010 by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 7 $\mu b^{-1}$. 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 $\eta_2$, for $|\eta|$ < 2.4 and transverse momentum $p_T$ > 0.5 GeV. A enhancement is observed along $\eta_-$ ≡ $\eta_1 - \eta_2$ ≈ 0 and a suppression at large $\eta_-$ values. The correlation function also shows a characteristic quadratic dependence along the $\eta_+$ ≡ $\eta_1 + \eta_2$ 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 terms of products of Legendre polynomials $T_n(\eta_1)T_m(\eta_2)$, and corresponding coefficients an,m are measured. These are related to mean-square values of the Legendre coefficients, $a_n$, at the single particle level: $a_{n,m}$ = $\langle a_na_m \rangle$. Significant values are observed for $\langle a_n^2 \rangle$ and mixed terms $\langle a_na_{n+2} \rangle$, which decrease quickly for larger n. The leading coefficient $a_1$ is compared to that estimated from a fit to $C_N(\eta_+)$ for different $\eta_-$ slices, as well as to the asymmetry of the number of participating nucleons between the two colliding nuclei.