Next: Tracking Performance Up: Measurement of the Tau Previous: Introduction

The SLD Detector

A detailed description of the SLD detector can be found in Ref. [2]. A subset of detector elements relevant to the analyses reported here are described briefly below. These include the vertex detector (VXD)[3], the central (or barrel) drift chamber (CDC)[4], and the lead/liquid-argon calorimeter (LAC)[5].

The VXD consists of 480 charge-coupled devices (CCDs) surrounding a 1 mm thick beryllium beam pipe with an inner radius of 25 mm. Each CCD is an array of square pixels m on a side. The active material is a m thick epitaxial silicon layer on a m thick silicon substrate. The CCDs are mounted on 60 alumina boards 9.2 cm long, arranged in four concentric cylinders at radii ranging from 2.9 cm to 4.1 cm. The inner (outer) cylinder covers a range of polar angles defined by . The CCDs are arranged so that at least two hits are possible over the full azimuth within the polar angle acceptance. On average 2.3 CCDs are traversed by a track from the interaction point. The spatial resolution of the VXD is m transverse to the beam and m along the beam direction[7].

The barrel drift chamber in SLD is a cylinder 1.8 m long with an inner radius of 0.2 m and an outer radius of 1.0 m, filled with a mixture of 75%CO, 21%Argon, and 4%isobutane. There are 640 drift cells arranged in ten superlayers covering radii from 24 cm to 96 cm, where superlayers with axial wires alternate with pairs of stereo layers at angles of mrad. Each cell in a superlayer has eight sense wires spaced radially by 5 mm. Field-shaping wires in each cell provide the desired drift fields. Each sense wire provides a measurement of the drift distance with a spatial resolution averaging m over the entire drift cell. Tracks are reconstructed with high efficiency at polar angles in the range .

The LAC consists of an assembly of rectangular lead plates separated by 2.75 mm gaps and mounted in large insulated vessels filled with liquid argon. A barrel section covers polar angles in the range and endcap sections complete the coverage down to . The LAC is segmented in depth into an electromagnetic (EM) section and a hadronic section (HAD). The EM section is made with 2 mm thick lead plates for a total depth of 21 radiation lengths and 0.84 hadronic interaction lengths, while the HAD section has 6 mm thick plates for an additional depth of 2.0 interaction lengths. The lead plates are segmented and connected so as to form projective towers with an azimuthal segmentation of 33 mrad in the EM section and 66 mrad in the HAD section, and with comparable segmentation in polar angle. The energy resolution for electromagnetic showers has been measured to be .

The SLD event trigger requires any of several combinations of tracking and energy-flow information from the detector elements. A subset of these have a relatively high efficiency for tau-pair events, in particular a requirement of two back-to-back tracks, or a single track plus a minimum energy deposition in the calorimeter.

Next: Tracking Performance Up: Measurement of the Tau Previous: Introduction

Mon Sep 11 11:36:55 PDT 1995