Flowing heavy quarks

The mystery of the J/Psi

Charm and bottom quarks serve as probes of the quark-gluon plasma (QGP) formed when lead nuclei collide in the LHC. Within the QGP, quarks and gluons are deconfined over a large volume of the order of 1000 fm3. Heavy quarks are produced before the QGP and they subsequently interact with its constituents. The plasma rapidly expands, cools down and eventually forms hadrons. When lead nuclei do not collide head on, the QGP system is elongated and the expansion leads to a dominant elliptical modulation - elliptic flow v2 - in the momentum distribution of hadrons.

Recent ALICE measurements studied the elliptic flow of hadrons containing charm quarks, either bound to a light quark (D meson, arXiv:2005.11131) or in charm-anticharm pairs (J/?, arXiv:2005.14518). At low momentum, the elliptic flow of D mesons is not as large as that of  pions (that contain only light quarks), while the elliptic flow of J/? is lower than both, but distinctly observed. This pattern indicates that the heavy charm quarks are dragged with the QGP expansion, but likely to a lesser extent than light quarks, and that both D mesons and J/? at low momentum are in part formed by the binding (recombination) of flowing quarks. The flow of electrons originating from decays of B hadrons, containing a bottom quark, is also reported (arXiv:2005.11130) and indicates that bottom quarks are also sensitive to the elongated shape of the QGP. Unlike J/?, Y(1S) mesons (bottom-antibottom) do not exhibit significant flow (Phys.Rev.Lett. 123 (2019) 19, 192301), likely because of its much larger mass and of the small number of bottom quarks available for recombination.