Measurement of two-particle pseudorapidity correlations in lead-lead collisions at $\sqrt{s_{NN}}$ = 2.76 TeV with the ATLAS detector

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 $C_{\rm N}(\eta_1,\eta_2)$ 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 $\eta_-\equiv\eta_1- \eta_2\approx 0$ and a suppression at large $\eta_-$ values. The correlation function also shows a characteristic quadratic dependence along the $\eta_+\equiv\eta_1+\eta_2$ 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 $a_{n,m}$ 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 $C_{\rm N}(\eta_+)$ for different $\eta_-$ slices, as well as to the asymmetry of the number of participating nucleons between the two colliding nuclei.