Abstract
The polarization of inclusive J/ψ and ϒ(1S) produced in Pb–Pb collisions at sNN=5.02 TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region 2.5<y<4 and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters λθ, λϕ and λθϕ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval 2<pT<10 GeV/c and pT<15 GeV/c for the J/ψ and ϒ(1S), respectively. The polarization parameters for the J/ψ are found to be compatible with zero, within a maximum of about two standard deviations at low pT, for both reference frames and over the whole pT range. The values are compared with the corresponding results obtained for pp collisions at s=7 and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for ϒ(1S) production in Pb–Pb collisions are also consistent with zero.
Highlights
Quarkonia, bound states of charm (c) and anticharm (c) or bottom (b) and antibottom (b) quarks, represent an important tool to test our understanding of quantum chromodynamics (QCD), since their production process involves both perturbative and nonperturbative aspects
Another forward detector, the V0 [49], composed of two scintillator arrays located at opposite sides of the interaction point (IP) and covering the pseudorapidity intervals −3.7 < η < −1.7 and 2.8 < η < 5.1, provides the minimum bias (MB) trigger which is given by a coincidence of signals from the two sides
The statistical uncertainty is given by the average of the statistical uncertainties of the 2D fits, while the root mean square of the results provides the systematic uncertainty on the signal extraction, with the absolute values ranging between 0.002 and 0.039
Summary
Bound states of charm (c) and anticharm (c) or bottom (b) and antibottom (b) quarks, represent an important tool to test our understanding of quantum chromodynamics (QCD), since their production process involves both perturbative and nonperturbative aspects. The creation of the heavy quark-antiquark pair is a process that can be described using a perturbative QCD approach, due to the large value of the charm and bottom quark masses (mc ∼ 1.3 GeV/c2, mb ∼ 4.2 GeV/c2 [1]). Models based on Non-Relativistic QCD (NRQCD) [2] give the most successful description of the production cross section, as measured at high-energy hadron colliders (Tevatron, RHIC, LHC) [3,4,5,6,7,8,9,10,11,12,13,14]. In the NRQCD approach, the non-perturbative aspects are parameterized via long-distance matrix elements (LDME), corresponding to the possible intermediate color, spin and angular momentum states of the evolving quark-antiquark pair.
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