The DELPHI Silicon Tracker at LEP2

The DELPHI Silicon Tracker, an ensemble of microstrips, ministrips and pixels, was completed in 1997 and has accumulated over $70~{\rm pb^{-1}}$ of high energy data. The Tracker is optimised for the LEP2 physics programme. It consists of a silicon microstrip barrel and endcaps with layers of silicon pixel and ministrip detectors. In the barrel part, three dimensional $b$ tagging information is available down to a polar angle of $25^\circ$. Impact parameter resolutions have been measured of $28~\mu {\rm m} \oplus 71/(p~{\rm sin} ^{\frac{3}{2}} \theta)~\mu {\rm m} $ in $R \phi$ and $34~\mu {\rm m} \oplus 69/p~\mu {\rm m}$ in $Rz$, where $p$ is the track momentum in $\rm {GeV/c}$. The amount of material has been kept low with the use of double-sided detectors, double-metal readout, and light mechanics. The pixels have dimensions of 330~$\times$~330~$\m u${\rm m}$^2$ and the ministrips have a readout pitch of 200~$\mu {\rm m}$. The forward part of the detector shows average efficiencies of more than 96\%, has signal-to-noise ratios of up to 40 in the ministrips, and noise levels at the level of less than one part per million in the pixels. Measurements of space points with low backgrounds are provided, leading to a vastly improved tracking efficiency for the region with polar angle less than $25^\circ$.