Open charm enhancement by secondary interactions in relativistic nucleus–nucleus collisions?

We calculate open charm production in Pb+Pb reactions at SPS energies within the HSD transport approach — which is based on string, quark, diquark (q,q̄,qq,q̄q̄) and hadronic degrees of freedom — including the production of open charm pairs from secondary ‘meson’–‘baryon’ (or quark–diquark and antiquark–diquark) collisions. It is argued that at collision energies close to the cc̄ pair threshold the dominant production mechanism is related to the two body (or quasi two body) reactions πN→D̄(D̄∗)Λc,(Σc). Estimates within the framework of the Quark–Gluon String model suggest cross sections of a few μb for πN→D̄Λc in the region of 1 GeV above threshold. The dynamical transport calculations for Pb+Pb at 160 A GeV indicate that the open charm enhancement reported by the NA50 Collaboration might be due to such secondary reaction mechanisms.