Elliptic flow of J/psi
The QGP behaves like a low-viscosity liquid and exhibits a collective, multi-harmonic azimuthally anisotropic expansion pattern (flow), which can be described in terms of a Fourier series. The various Fourier components v_n originate from pressure gradients, due to the collision geometry in non-central collisions (typically the quadrupole component v_2, called elliptic flow) or to fluctuations in the initial nucleon density (typically triangular flow v_3 and higher order components). Non-zero elliptic flow has been observed for the J/psi meson, providing further evidence that the charm quarks are thermalized and participate in the collective flow of the plasma, as expected in a recombination scenario. The elliptic flow at higher transverse momenta, where a significant contribution from primordial J/psi is expected, is not well described by model calculations and presently not well understood.
The new Pb-Pb data will allow detailed investigations of the flow pattern as a function of collision centrality and transverse momentum, which may give additional information on the interactions of charm quarks with the plasma and the balance between different J/psi production mechanisms.