  Color screening and regeneration are both hot medium effects on quarkonium
production in high energy nuclear collisions. However, they affect in an
opposite way the finally observed quarkonium spectra. Due to the competition of
the two dynamical effects, the ratio of the integrated quarkonium yield between
nuclear and elementary nucleon collisions loses its sensitivity. Once the
information of quarkonium transverse motion is included, on the other hand, the
ratio of averaged transverse momentum square reveals the nature of the QCD
medium created in high energy nuclear collisions.
