image absorption in heavy-ion collisions Original Research Article

We present a new calculation of the π–J/ψ dissociation cross sections within the constituent quark–meson model recently introduced. To discuss the absorption of J/ψ in heavy-ion collisions, we assume the J/ψ to be produced inside a thermalized pion gas, as discussed by Bjorken, and introduce the corrections due to absorption by nuclear matter as well. We fit the absorption length of the J/ψ to the data obtained at the CERN SPS by the NA50 Collaboration for Pb–Pb collisions. Collisions of lower centrality allow us to determine the temperature and the energy density of the pion gas. For both these quantities we find values close to those indicated by lattice gauge calculations for the transition to a quark–gluon plasma. A simple extrapolation to more central collisions, which takes into account the increase of the energy deposited due to the increased nucleon flux, fails to reproduce the break in J/ψ absorption indicated by NA50, thus lending support to the idea that an unconfined quark–gluon phase may have been produced. This conclusion could be sharpened by analysing in a similar way, as a function of centrality, other observables such as strange particle production.