The curious case of the J/ψ flow

10 November 2017

Recently, the ALICE collaboration measured the elliptic flow of J/ψ mesons with unprecedented precision in lead–lead (Pb–Pb) collisions and, for the first time, also in proton–lead (p–Pb) collisions. While the results at low transverse momentum (pT) in Pb–Pb collisions confirm that charm quarks flow with the quark–gluon plasma (QGP), the results at high pT do not agree with model predictions. Furthermore, their similarity to p–Pb collisions suggest that additional J/ψ flow-generation mechanisms are still to be identified.

The elliptic flow (v2) is the azimuthal anisotropy of the final-state particles, generated by the collective expansion of the almond-shaped interaction region of the colliding nuclei in non-central nucleus–nucleus collisions. The J/ψ meson is a bound state of charm and anti-charm quarks, which is created at early times in hard-scattering processes. Effects of the QGP on the production of J/ψ mesons are currently understood in terms of two mechanisms: suppression by dissociation due to the large surrounding colour-charge density and regeneration by recombination of de-confined charm quarks. If charm quarks thermalise in the medium, recombined states should inherit their flow.

A clear positive v2 for J/ψ mesons at forward rapidity is observed in Pb–Pb collisions at a nucleon–nucleon energy of 5.02 TeV for different collision centralities. In semi-central collisions, the J/ψ v2 increases with pT up to 4–6 GeV/c and saturates or decreases thereafter. The J/ψ v2 measurement at mid-rapidity has a larger background and is therefore less precise, but demonstrates potential for future studies at the high-luminosity LHC.

A comparison with available theoretical model calculations shows that the measured values at low pT (below 4 GeV/c) can only be explained through a large contribution from the recombination of thermalised charm quarks. The expected v2 without this contribution (labelled “primordial” v2 in the figure) is much smaller than the measured values. However, the models clearly underestimate the measured azimuthal asymmetry at higher transverse momentum and do not reproduce the overall pT dependence, suggesting that there is another mechanism to produce J/ψ v2. The J/ψ v2 has also been measured in p–Pb collisions at energies of 5.02 and 8.16 TeV at forward (p-travelling) and backward (Pb-travelling) rapidities. Interestingly, the J/ψ v2 in the smaller p–Pb collision system is similar to that in central Pb–Pb collisions at high pT. The possibly missing mechanism could therefore be the same in both collision systems.

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