The OPAL Detector

Like the other three LEP detectors the OPAL (Omni Purpose Apparatus for Lep) detector is a symmetric barrel detector. It was designed to give good measurement of both particle momenta and energy and in some cases even particle species. It roughly consists of a long central cylinder, called the barrel, and two flat endcaps at either end of the barrel to give the widest possible coverage of the solid angle.

A diagram of the OPAL detector is shown in figure 2.2. Indicated on the diagram is the OPAL co-ordinate system. The electrons and positrons travel along the beam pipe and are bought into collision within the detector. The z-axis is parallel to the beam pipe and positive in the direction of the electrons' momentum. The x-axis indicated in figure 2.2 is taken as positive in the direction pointing towards the centre of the LEP ring.

The main features of the OPAL detector are shown in figure 2.2, and they can concisely be described as follows:

$\bullet$ A central detector, consisting of vertex and tracking subdetectors and surrounded by a solenoid. The central detector provides measurements of the particle's direction and momentum and their identification by dE$/$dx as well as reconstruction of primary and secondary vertices at and near the interaction region. The solenoid causes bending of the charged particles in the tracking chamber.

$\bullet$ An electromagnetic calorimeter to provide identification of photons and electrons and to measure their energy.

$\bullet$ A hadronic calorimeter to measure hadronic energy. This is implemented by measuring the total absorption using instrumentation which incorporates the magnetic yoke.

$\bullet$ A Muon detector to identify muons by measurement of their position and direction within and behind the hadron calorimeter.

$\bullet$ A forward detector, situated in the very forward direction, and used to measure the received LEP luminosity at OPAL using Bhabha scattering events.

A representation of the cross-section of the OPAL detector can be seen in figure 2.3. A detailed description of each OPAL subdetector is beyond the bounds of this thesis, however, a brief overview of each subdetector is given for completeness. A complete and detailed description of the OPAL detector and all its constituent parts can be found in [14].

Figure 2.2: A schematic representation of the OPAL detector.

Figure 2.3: A detailed schematic of the OPAL cross-section.