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

Abstract

Two-particle pseudorapidity correlations, measured using charged particles with $p_{\mathrm{T}}>$ 0.5 GeV and $|\eta |<$ 2.4, from $\sqrt{s_{NN}}$ = 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 ${\eta }_{-}\equiv {\eta }_1-{\eta }_2\approx$ 0 and a suppression at large ${\eta }_{-}$ values. The correlation function also shows a 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 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.

arXiv comment4 pages, 3 figures, proceedings of the 7th International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probes 2015), Montreal

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