  Hadron production at high-$p_T$ displays a strong suppression pattern in a
wide rapidity region in heavy ion collisions at RHIC energies. This finding
indicates the presence of strong final state effects for both transversally and
longitudinally traveling partons, namely induced energy loss. We have developed
a perturbative QCD based model to describe hadron production in $pp$ collision,
which can be combined with the Glauber -- Gribov model to describe hadron
production in heavy ion collisions. Investigating $AuAu$ and $CuCu$ collisions
at energy $\sqrt{s}=200$ $A$GeV at mid-rapidity, we find the opacity of the
strongly interacting hot matter to be proportional to the participant nucleon
number. Considering forward rapidities, the suppression pattern indicates the
formation of a longitudinally contracted dense deconfined zone in central heavy
ion collisions. We determine parameters for the initial geometry from the
existing data.
