Open Charm Production and its Lepton Pair Signal in High Energy Nuclear Collisions

Open charm production at the initial and the pre-equilibrium stages in nuclear collisions is studied. The pre-equilibrium contribution to open charm production in $A+A$ collisions at $\sqrt s=200 AGeV$ is shown to be very sensitive to correlations between momenta and space-time coordinates of the minijets produced after the initial collision. A minimal correlation model based on the uncertainty principle is introduced, and leads only to a small pre-equilibrium charm yield as compared with initial yields. Opposite-sign lepton pairs $ee, e\mu$ and $\mu\mu$ from open charm decay in $p+A$ reactions at $\sqrt s=200$ AGeV are proposed as a measure of nuclear shadowing effects. Via an approximate scaling, the ratio of the spectra from $p+A$ collisions to those from $pp$ collisions is shown to reflect the shadowing function well. Finally it is shown that the required measurements are feasible at RHIC.