  We have studied the J/psi suppression in 158 GeV/c Pb+Pb collisions at CERN
SPS. J/psi production is assumed to be a two step process, (i) formation of c
bar{c} pair, which is accurately calculable in QCD and (ii) formation of J/psi
meson from the c bar{c} pair, which can be conveniently parameterized. In a
pA/AA collision, as the c bar{c} pair pass through the nuclear medium, it gains
relative square momentum. As a result, some of the c bar{c} pairs can gain
enough momentum to cross the threshold to become open charm meson, leading to
suppression in pA/AA collisions. The model without any free parameter could
describe the of NA50 data on centrality dependence of the ratio's; J/psi over
Drell-Yan, J/psi over minimum bias and also the Drell-Yan over minimum bias.
The model was used to predict J/psi suppression at RHIC energy. At RHIC energy,
hard processes may be important. With hard processes included, J/psi's are
strongly suppressed, in agreement with other model calculations. We also show
that centrality dependence of J/psi over minimum bias ratio can be used to
determine the fraction of hard processes in the collision.
