Abstract
We present the first measurement of the two-particle transverse momentum differential correlation function, P_{2}≡⟨Δp_{T}Δp_{T}⟩/⟨p_{T}⟩^{2}, in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV. Results for P_{2} are reported as a function of the relative pseudorapidity (Δη) and azimuthal angle (Δφ) between two particles for different collision centralities. The Δϕ dependence is found to be largely independent of Δη for |Δη|≥0.9. In the 5% most central Pb-Pb collisions, the two-particle transverse momentum correlation function exhibits a clear double-hump structure around Δφ=π (i.e., on the away side), which is not observed in number correlations in the same centrality range, and thus provides an indication of the dominance of triangular flow in this collision centrality. Fourier decompositions of P_{2}, studied as a function of the collision centrality, show that correlations at |Δη|≥0.9 can be well reproduced by a flow ansatz based on the notion that measured transverse momentum correlations are strictly determined by the collective motion of the system.
Highlights
Such flow dominance is manifested, in particular, by an approximate factorization of the measured flow coefficients, VnΔðη1; pT;1; η2; pT;2Þ 1⁄4 hcosðnΔφÞi 1⁄4 hvnðη1; pT;1Þvnðη2; pT;2Þi, observed for pairs of particles at relative pseudorapidity Δη > 0.8, in different transverse momentum bins up to pT ≈ 3–5 GeV=c [16]
Measurements of particle production and their correlations in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have provided very compelling evidence that the produced matter is characterized by extremely high temperatures and energy densities consistent with a deconfined, but strongly interacting quark-gluon plasma
Evidence for the production of the strongly interacting quark-gluon plasma (sQGP) is provided by observations of a large suppression of particle production at momenta pT ≳ 3 GeV=c relative to that observed in pp collisions and a strong suppression of away-side particles observed in twoparticle number correlations, as well as by anisotropic flow studies [1,2,3,4,5,6,7,8,9,10,11]
Summary
Such flow dominance is manifested, in particular, by an approximate factorization of the measured flow coefficients, VnΔðη1; pT;1; η2; pT;2Þ 1⁄4 hcosðnΔφÞi 1⁄4 hvnðη1; pT;1Þvnðη2; pT;2Þi, observed for pairs of particles at relative pseudorapidity Δη > 0.8, in different transverse momentum bins up to pT ≈ 3–5 GeV=c [16]. In particular, that in the presence of anisotropic flow the differential transverse momentum correlator hΔpTΔpTi should feature azimuthal Fourier decomposition coefficients calculable with a simple formula, hereafter called the flow ansatz, in terms of the regular and pT weighted flow coefficients [17].
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