RECENT RESULTS AND RUNNING EXPERIENCE OF THE NEW ALEPH VERTEX DETECTOR

A description of the new ALEPH Vertex Detectors (VEDET 91), including the new mechanical structure and the improved data acquisition system, is given. The apparatus consists of two complete layers of silicon detectors with double-sided readout which provide full x,y,z information on the impact points of charge particles. The authors present results on signal-to-noise ratio for minimum ionizing particles and efficiency in hit/track matching. Results on position, impact parameter, and momentum resolution are also shown as measured using high-momentum muons. Preliminary results on single hit and impact parameter resolution show that the detectors and VLSI read-out electronics work as expected, and the performance compares well with previous test beam results and Monte Carlo simulations. Preliminary results on precision vertex reconstruction show a significant improvement in the data analysis which will be fully exploited in studying tau , b and c physics when all statistics are available.