Abstract
The inclusive J/ψ elliptic (v2) and triangular (v3) flow coefficients measured at forward rapidity (2.5 < y < 4) and the v2 measured at midrapidity (|y| < 0.9) in Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 5.02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750 μb−1 at forward rapidity and 93 μb−1 at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum pT and collision centrality. At midrapidity, the J/ψ v2 is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/ψ v3 with a significance of more than 5σ at forward rapidity in the pT range 2 < pT< 5 GeV/c. The forward rapidity v2, v3, and v3/v2 results at low and intermediate pT (pT ≲ 8 GeV/c) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher pT. At low and intermediate pT, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high pT, path-length dependent effects seem to dominate. The J/ψ v2 measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson vn measurements can be described based on those for charged pions and J/ψ flow.
Highlights
Pb-Pb collisions at sNN = 5.02 TeV using the ALICE detector at the LHC are reported
Flow coefficients for all particles show, in the low pT range, an increasing trend with pT mainly attributed to the radial hydrodynamic expansion of the QGP, reach a maximum in the pT range 3–5 GeV/c depending on the particle mass and species, and drop towards higher pT
No clear pT or centrality dependence is found for this contribution, and the corresponding systematic uncertainty is estimated to be less than 1%
Summary
The vn coefficients are obtained using the scalar product (SP) method [2, 61]. This is a two-particle correlation technique based on the scalar product between the unit flow vector for a given harmonic n, un = einφ, of the particle of interest (here a dilepton) and the. The flow coefficients are extracted from sequential fits to the dilepton invariant mass distribution, m , and the vn as a function of m , which include the superposition of a J/ψ signal and a background contribution, using the function vn(m ) = α(m ) vnJ/ψ + [1 − α(m )] vnbkg(m ). The combinatorial background is estimated using an event mixing technique, where pairs are built from different events with similar collision centrality, flow-vector orientation, and longitudinal position of the event vertex, and subtracted from the same-event dielectron invariant mass distribution. Due to the smaller signal-to-background ratio, the difference between mixed and same event background flow is taken into account by considering in the fit function an additional term which accounts for the flow of the correlated background and imperfections of the mixed event procedure This term is parameterized using a second order polynomial and acts as a correction to the background flow obtained from the mixed event procedure
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