  We show that in high energy nucleus-nucleus collisions the transverse energy
or multiplicity distribution P_C, associated to the production of a rare,
unabsorbed event C, is universally related to the standard or minimum bias
distribution P by the equation $P_C(\nu)={\nu\over<\nu>}P(\nu)$, with $\sum
P(\nu)=1$ and $\nu\equiv E_T$ or n. Deviations from this formula are discussed,
in particular having in view the formation of the plasma of quarks and gluons.
This possibility can be distinguished from absortion or interaction of
comovers, looking at the curvature of the $J/\Psi$ over Drell-Yan pairs as a
function of E_T.
