Collisions are measured by a single multipurpose
detector, the SLAC Large Detector [38]. The SLD, shown in Figure 4.5
combines excellent tracking,
calorimetry and particle identification into a state-of-the-art
high energy physics apparatus.
Quadrant view of the SLD.
The SLD is a cylindrically symmetric detector with a 0.6 Tesla solenoidal magnetic field to momentum analyze charge particles. The detector has a central ``barrel'' system that surrounds the interaction point and two ``endcaps'' which cover the forward angles. During down times, the endcaps may be withdrawn for access to the interior components of the detector. The right-handed coordinate system used by the SLD is a normal spherical polar system:
The analyses discussed in this thesis make use of the SLD calorimetry systems: the liquid argon calorimeter and the luminosity monitor. The following sections will describe all of the major subsystems of the SLD. However, calorimetry will be emphasized.
SLD possesses three major tracking systems, the Vertex Detector, the Drift Chambers and the Warm Iron Calorimeter (WIC) Strips.
Taking advantage of the small beampipe at the interaction
point and the low repetition rate of the SLC, the
SLD Vertex Detector utilizes charge coupled devices
to make high resolution space point measurements of
charged particle tracks. When track points are linked
with tracks reconstructed by the Central Drift
Chamber, secondary vertices from heavy quark and tau
lepton decays can be resolved with high precision.
The SLD Vertex Detector.
The Vertex Detector (shown in Figure 4.6 is comprised of 480 CCDs which surround the interaction point in four concentric barrels. Each CCD contains approximately 400x600 pixels each of size 22x22 microns. The data from 120 Mpixels is read out and condensed to approximately 50-80 Kbytes per event [39].
Before linking tracks with hits in the Vertex Detector, tracks are found and fitted with the Central Drift Chamber (CDC) and the Endcap Drift Chambers (EDC). The drift chambers are gas-wire tracking systems. The CDC contains a cylindrical arrangement of wires which run approximately parallel to the beam line. Ionization from charged particles passing through the chamber drifts to the wires in the presence of large electrostatic fields. The wires are instrumented on both ends, so the z component of the tracks may be found via charge division.
The Endcap Drift Chambers have wires running perpendicular to the beamline for track reconstruction at smaller angles. The chambers are divided into an inner and outer chamber which are separated by the Endcap CRID (see Section 4.2.2.) Backgrounds from the SLC as well as material in front of the EDCs have thus far hampered their ability to find charged tracks.
When the velocity of a particle exceeds
the speed of light in a medium, the
particle emits Cerenkov radiation.
The Cerenkov angle is related to the
velocity of the particle. From the measurement of the
Cerenkov angle, coupled with
momentum information from the drift
chamber, the mass of the particle and
hence the identity of the particle can
be ascertained. The
Cerenkov Ring Imaging Detector (CRID)
is designed to measure
the Cerenkov angle of tracks and
therefore perform particle identification [40].
Figure 4.7: The SLD Cerenkov Ring Imaging Detector (CRID).
The CRID is shown schematically in Figure 4.7. The barrel CRID sits between the CDC and the Liquid Argon Calorimeter (LAC). In the case of the endcap, the CRID is sandwiched between layers of the endcap drift chamber. Particles emit Cerenkov radiation in both a liquid and gas radiator. The photons are converted by photo-ionization in gas. The photo-electrons then drift to the end of the detector where they are measured by proportional wires. The drift time yields information regarding the conversion depth. From this information, photon rings may be reconstructed and the Cerenkov angle measured.