  We have analyzed the NA58 data on $J/\psi$ suppression in Pb+Pb collisions.
$J/\psi$ 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, a $c\bar{c}$ pair gain relative square
momentum as it passes through the nuclear medium and some of the $c\bar{c}$
pairs can gain enough momentum to cross the threshold to become an open charm
meson, leading to suppression in pA/AA collisions. A new prescription is
proposed for the gain in momentum square, consistent with Krammer process. The
model without any free parameter could explain the $E_T$ dependence of $J/\psi$
over Drell-Yan ratio.
