J/ψ suppression in indium-indium collisions at SPS energies

In 2003, NA60 collected data on dimuon production in indium-indium collisions at 158 GeV per incident nucleon, to study several anomalies previously observed in high-energy heavy-ion collisions at the CERN SPS. Among these anomalies stands the observation, by NA50, that the production yield of J/ψ mesons is suppressed in central Pb-Pb collisions beyond the normal nuclear absorption defined by proton-nucleus data. This behaviour was predicted to occur if the matter produced in these extreme collisions goes through a phase of deconfined partonic matter, where the charmonium states would be dissolved either due to the screening of their binding potential when the medium temperature exceeds certain critical thresholds (thermal transition, QGP) or when the density of interacting partons exceeds certain thresholds (geometrical transition, percolation). Thanks to its exceptionally good vertexing capabilities, NA60 is able to accurately study the centrality dependence of J/ψ production without using the Drell-Yan sample as reference. Our data shows that the J/ψ is anomalously suppressed for In-In collisions with more than ∼ 90 participant nucleons. The comparison with three theoretical predictions shows that none of them properly describes the observed suppression pattern.