J/Psi production in nuclear collisions: measuring the transport coefficient

Abstract: The observed pT-dependence of nuclear effects for J/Psi produced in heavy ion collisions at RHIC might look puzzling, since the nuclear suppression seems to fade at large pT. We explain this by the interplay of three mechanisms: (i) attenuation of J/Psi in the hot medium created in the nuclear collision; (ii) initial state shadowing of charmed quarks and attenuation of a c-cbar dipole propagating through the colliding nuclei; (iii) a strong Cronin effect for J/Psi caused by saturation of gluons in the colliding nuclei. All three effects are well under control and calculated in a parameter free way, except for the transport coefficient q_0-hat characterizing the medium. This is adjusted to the J/Psi data and found to be in good agreement with the pQCD prediction, but more than an order of magnitude smaller than what was extracted from jet quenching data within the energy loss scenario.