The OPAL Detector (an~Omni~Purpose~Apparatus~for~Lep)

The OPAL Detector (an Omni Purpose Apparatus for Lep) \\ \\OPAL, a general purpose detector, was designed to study a wide range of unexplored physics at LEP. \\ \\At LEP1, one of the central issues is the precise determination of the mass, width and couplings to quarks and leptons of the Z$^{0}$ boson. At LEP2 the mass and couplings of the W$^\pm$ bosons are determined. Accurate measurements of these quantities might reveal the mechanisms by which symmetries are broken. Many topics relating to heavy flavours are studied, including the properties of tau leptons, and the spectroscopy, lifetimes and mixing of hadrons containing b-quarks. \\ \\There are very active QCD and Two-Photon groups. Among the topics being studied are the determination of the strong coupling constant, $ \alpha _{S} $, tests of the group structure of QCD, differences between quark- and gluon-induced jets, many aspects of the fragmentation process measurements of many different final states in photon-photon collision, and measurement of structure functions. These studies are continuing at LEP2. \\ \\General searches for new particles, such as the Higgs boson and supersymmetric states, were pursued at LEP1 and now extended at the higher energies of LEP2. \\ \\These studies, as well as searches for more unexpected phenomena require a general purpose detector with good energy resolution and particle identification and which is capable of studying complicated events. \\ \\The main components of the detector in order of increasing distance from the interaction point, are as follows (fig.1): \begin{enumerate}[a)] \item A silicon microvertex detector, installed around the beam pipe, which measures two points (r-$\phi$~and~z) per track. \item Central detectors consisting of a Vertex and Jet detector, both of the multi~$\Delta