Flow dominance and factorization of transverse momentum correlations in Pb--Pb collisions at the LHC

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2.76 TeV
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We present first measurement of charged particles differential transverse momentum correlation function, $\Delta p_{\rm T}\Delta p_{\rm T}$, in Pb--Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. The correlations are reported as a function of the relative pseudorapidity, $\Delta\eta$, and azimuthal angle difference, $\Delta\varphi$. The $\Delta\varphi$ dependence is found to be largely independent of $\Delta\eta$ at $\Delta\eta >$ 0.9. $\Delta p_{\rm T}\Delta p_{\rm T}exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations, $R_2$, previously reported. In 0-5\% most central Pb--Pb collisions the two particle transverse momentum correlation function exhibits a clear two hump structure at the “away-side”, an indication of the dominance of triangular flow. Detailed comparison of $R_2$ and $\Delta p_{\rm T}\Delta p_{\rm T}correlation functions utilizing Fourier decomposition show that the $\Delta p_{\rm T}\Delta p_{\rm T}correlations at $\Delta\eta >$ 0.9 can be well reproduced by the “flow ansatz”, assuming that all the correlations are dominated by the anisotropic flow.