PERFORMANCE OF THE ALEPH UPGRADED SILICON VERTEX DETECTOR

The ALEPH Vertex Detector (VDET) has been upgraded for the second phas e of LEP running. The new version still uses double sided silicon stri p detectors, fabricated with the same technology as the previous one, but the upgraded one is twice as long and has about half passive mater ial in the tracking volume. Furthermore the readout electronics is now radiation hard (MX7-RH chips). An almost complete version of the upgr aded VDET was installed in ALEPH during a three week LEP technical sto p and took data in November 1995 during the LEP run at 130 GeV. The ne w detector worked well showing high signal over noise ratio and good e fficiency. The point resolution measured during this run, using high m omentum muons, 13 mu m in the r - phi view and 21 mu m in the r - z vi ew, is dominated by the alignment precision, due to the low statistics available for this short LEP run. This result is however acceptable, since for lower momentum charged particle, the multiple scattering giv es a significant contribution to the final impact parameter resolution . A better resolution has been achieved in the next run, when an initi al period at the Z peak has been foreseen to calibrate and align the w hole detector.