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, (q) over bar, qq, (q) over bar(q) over bar) 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 c (c) over bar pair threshold the dominant production mechanism is related to the two body (or quasi two body) reactions piN --> (D) over bar((D) over bar*)Lambda (c), (Sigma (c)). Estimates within the framework of the Quark-Gluon String model suggest cross sections of a few Pb for piN --> (D) over bar Lambda (c) 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. (C) 2001 Elsevier Science B.V. All rights reserved.