Nuclear Physics Division

The ALICE collaboration has measured, in heavy ion collisions, the elliptic flow (see previous highlight "The intriguing charm flow") of hadrons containing a c quark, linked either to a light quark (forming a D meson) or to a c antiquark (forming a J/ψ meson). Hadrons containing heavy quarks, i.e. quark c or quark b, are excellent "messengers" for the quark-gluon plasma formed in these collisions. They are produced in the initial phases of the collisions, before the plasma is formed, and thus interact with the plasma constituents throughout its evolution from its rapidly expanding phase to its cooling and transformation into hadrons.

When the heavy nuclei do not collide head-on, the plasma elongates and its expansion leads to dominant elliptical modulation of the hadrons’ momentum distribution, or flow. The ALICE collaboration has found that, at low momentum, the elliptic flow of D mesons is not as large as that of pions, which contain only light quarks, whereas the elliptic flow of J/ψ [1] mesons is lower than both but distinctly observed.

“This pattern indicates that the heavy c quarks are dragged by the quark–gluon plasma’s expansion,” says ALICE spokesperson Luciano Musa, “but likely to a lesser extent than light quarks, and that both D and J/ψ mesons at low momentum are in part formed by the binding, or recombination, of flowing quarks.”

Another measurement carried out by the ALICE team the flow of electrons originating from decays of B hadrons, containing a b quark, indicates that b-quarks are also sensitive to the elongated shape of the plasma of quarks and gluons. Upsilon particles, which consist of a b-quark and its antiquark (and not a c-quark and a c-antiquark, as in the case of J/ψ), do not exhibit significant flow [2] (see previous highlight "Bottomonium elliptic-flow no-show"), probably because of their much higher mass and the low number of b-quarks available for recombination.
These results are illustrated in Figure 1, which shows the elliptic flow of various particles, from the non-heavy quark-bearing counters, through the J/ψ mesons, to the Upsilon mesons, measured by ALICE in lead-lead nucleus collisions at the LHC.