Charmonium suppression in heavy-ion collisions

Data and their interpretations are reviewed for $J/\psi$ and $\psi^{\prime}$ production in heavy ion collisions with particular emphasis on the recently reported discontinuity in $Pb$-$Pb$ reactions. The experimental set-up is described and the various results for $J/\psi$ and $\psi^{\prime}$ suppressions including their $p_T$-dependence are presented. The suppressions in $pA$ collisions are discussed in terms of final state absorption of a premeson and the $p_T$ distributions as parton scattering in the initial state. Both phenomena scale with the trajectory length $L$. The data for charmonium suppression and $< p_T^2 >$ in heavy ion collisions are presented as a function of $L$ and discussed in the light of theories assuming deconfined matter and/or absorption by comovers.