Observation of a threshold effect in the anomalous $J/\psi$ suppression

We report on a search for a phase transition from ordinary nuclear matter to a state of deconfined quarks and gluons as predicted by lattice QCD calculations. A new measurement of charmonium production in Pb-Pb interactions at 158 GeV/c per nucleon agrees with our previous results and confirms the anomalous $J/\psi$ suppression we had already observed on a significantly smaller data sample. New event selection and analysis techniques show that, for peripheral collisions, the $J/\psi$ cross-section per nucleon-nucleon collision agrees with the precise suppression pattern inferred from a wide range of measurements extending from $p-p$ up to S-U collisions. As the collisions become more central, the Pb-Pb cross-section exhibits a clear departure from this normal behaviour. The onset of the anomalous $J/\psi$ suppression reported here is the first clear observation of a threshold effect in heavy ion collisions and can be considered as a strong indication of the production of a deconfined quark-gluon phase in central Pb-Pb collisions.