J.G. Cramer, D.J. Prindle and G. Odyniec^*

Much can be learned about the dynamics of ultra-relativistic nuclear collisions from the production of strange and charmed particles. The most interesting of these are multi-strange baryons ( and baryons with |S|=2 and |S|=3, respectively) and mesons having open charm (D mesons with |C|=1), because these are formed in the 'hot' early stages of the collisions and are expected to provide important information on possible formation of the conjectured quark-gluon plasma.

We have submitted a proposal to the National Science Foundation requesting funds for the design, fabrication, installation, testing and operation of a silicon-drift vertex detector to be added as an upgrade to CERN experiment NA49. This addition will give NA49, presently the 'flagship' experiment of the CERN heavy ion program, new capabilities which would make it the only CERN experiment capable of detecting charmed D mesons in heavy ion collisions and would also allow NA49 to efficiently detect and analyze multi-strange baryons.

The detector subsystem we propose, which we call NA49-VSD (Vertex Silicon Detector), will track charged particles very close to the target. Thanks to its excellent position and two-track resolution, this will permit separation of short-lived decay tracks from tracks originating at the primary vertex. This new capability of NA49 will allow the detailed study of multiple strangeness and charm production in the SPS ultra-relativistic energy regime, (33 TeV Pb beams on fixed targets). Together with the global characterization of the events already provided by the experiment, this should lead to unambiguous and definitive constraints on the reaction dynamics.

The NA49-VSD system, consisting of 20 p-type silicon drift detectors grouped into 5 planes (4 detectors/plane) will be read out by 3560 electronics channels. Entire thickness of the silicon wafers corresponds to only 1.5% of a radiation length. Therefore its contribution to multiple scattering is negligible.

The design of the detector, its construction, and testing will take place in FY96 and FY97. The detector will be ready for initial data taking for strangeness analysis in Fall-97 and will be refined for the more difficult open charm study in Fall-98.