Abstract
Quarkonia, i.e. bound states of heavy quarks (charm/bottom), provide remarkable probes of the hot and strongly-interacting medium, the quark gluon plasma (QGP), which is created during heavy-ion collisions. Heavy quarks are produced during initial hard-scattering processes prior to the QGP formation and their number is conserved throughout the partonic and hadronic phases of the collision. At LHC, experimental observations of quarkonium states in A-A collisions are reproduced through two antagonist mechanisms: the sequential suppression and quarkonium production by (re)combination of deconfined quarks. Recent measurements of a significant J/ψ elliptic flow (v2) in Pb-Pb collisions at TeV suggest a participation of the charm quarks to the collectivity of the medium. However, quarkonium interactions in the medium are not yet fully understood and several points should still be clarified. The ALICE experiment at the LHC measures quarkonia at mid-rapidity in the dielectron decay channel and at forward rapidity in the dimuon one, both down to zero transverse momentum. Latest measurements of quarkonium nuclear modification factor and elliptic flow in Pb-Pb collisions will be presented and compared to lower energy results and theoretical predictions.
Highlights
Measurements of quarkonia in heavy-ion collisions have been performed for several decades
I.e. bound states of heavy quarks, provide remarkable probes of the hot and strongly-interacting medium, the quark gluon plasma (QGP), which is created during heavy-ion collisions
Heavy quarks are produced during initial hard-scattering processes prior to the QGP formation and their number is conserved throughout the partonic and hadronic phases of the collision
Summary
Measurements of quarkonia in heavy-ion collisions have been performed for several decades. Heavy quarks are produced prior to the formation of the QGP and experience the full medium evolution, providing crucial probes of the QGP properties. In the QGP they experience a suppression mechanism by color-screening in the medium and (re)generation by quark (re)combination at a later stage of the collision. Substantial anisotropies are observed in the heavy-flavour sector and if charm quarks interact in the medium, their flow should be transmitted to quarkonium states formed through (re)combination. 2. Apparatus The ALICE detector [1] measures quarkonium down to zero transverse momentum in two rapidity ranges: at mid-rapidity (|y| < 0.9) with the central barrel through the dielectron decay channel and at forward rapidity (2.5 < y < 4) with the muon arm through the dimuon decay channel. The 2015 Pb–Pb data campaign provided unprecedented statistics for quarkonium measurements, with an integrated luminosity reaching ∼ 13 μb−1 in the central barrel and ∼ 225 μb−1 in the muon spectrometer. All quarkonium measurements are inclusive, i.e. they include direct production and contributions from heavier charmonium states and B decays
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