M.X. Liu

DESIGN AND PERFORMANCE OF THE SLD VERTEX DETECTOR : A 307 MPIXEL TRACKING SYSTEM

This paper describes the design, construction, and initial operation of SLDs upgraded vertex detector which comprises 96 two-dimensional charge-coupled devices (CCDs) with a total of 307 Mpixel. Each pixel functions as an independent particle detecting element, providing space point measurements of charged particle tracks with a typical precision of 4 μm in each co-ordinate. The CCDs are arranged in three concentric cylinders just outside the beam-pipe which surrounds the e+e− collision point of the SLAC Linear Collider (SLC). The detector is a powerful tool for distinguishing displaced vertex tracks, produced by decay in flight of heavy flavour hadrons or tau leptons, from tracks produced at the primary event vertex. The requirements for this detector include a very low mass structure (to minimize multiple scattering) both for mechanical support and to provide signal paths for the CCDs; operation at low temperature with a high degree of mechanical stability; and high speed CCD readout, signal processing, and data sparsification. The lessons learned in achieving these goals should be useful for the construction of large arrays of CCDs or active pixel devices in the future in a number of areas of science and technology.

The SLD VXD3 detector and its initial performance

The SLD collaboration completed construction of a new CCD vertex detector (VXD3) in January 1996 and started data taking in April 1996 with the new system. VXD3 is an upgrade of the original CCD vertex detector, VXD2, which had successfully operated in SLD for three years. VXD3 consists of 96 large area CCDs, each having 3.2 million 20 microns x20 microns pixels. By reducing the detector material and lengthening the lever arm, VXD3 is expected to improve secondary vertex resolution by about a factor of two compared with VXD2. The new three layered structure enables stand alone tracking without any ambiguity and its extended size along the beam direction improves the polar angle coverage to COS THETA {lt}0.85. An overview of this detector system and its initial performance are described.

Performance of the new vertex detector at SLD

During the past year, the SLD collaboration completed the construction and began the operation of a new vertex detector (VXD3) employing 307 million pixels. This detector, based on 96 CCDs of 13 cm/sup 2/ area each, is an upgrade of the original vertex detector of SLD (VXD2), made possible by advances in the technology of CCD detectors. Its improved impact parameter resolution, larger solid angle coverage and virtually error-free track linking will enhance the SLD measurement of the polarization-enhanced forward-backward asymmetry for b and c-quarks, increase the precision of the measurement of the b-fraction in hadronic Z decays, and open the possibility to observe B/sub s//sup 0/-mixing. Full separation of primary, secondary and tertiary vertices is accessible. A description of the mechanics and electronics of VXD3 are presented along with results from the first data.