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OPAL was one of the major experiments at CERN's LEP. OPAL studied particles and their interactions by collecting and analysing electron-positron collision events at LEP, the Large Electron-Positron collider. LEP was the largest particle accelerator in the world. There three other experiments at LEP were: ALEPH, DELPHI and L3. 
The OPAL detector was about 12 m long, 12m high and 12m wide. The detector components were arranged around the beam pipe, in a layered structure like that of an onion. The tracking system of OPAL consisted of a silicon microvertex detector, a vertex detector, a jet chamber, and z-chambers (from the beam pipe out).
The silicon microvertex detector and the vertex chamber were used to locate decay vertices of short-lived particles, and to improve the momentum resolution. The central jet chamber identified particles from how much ionization they caused, and how far they curved in the magnetic field. These chambers worked well to identify tracks in the plane perpendicular to the beam axis. They were complemented by so called "z-chambers" at the outside edge of the jet chamber, to provide precise measurements of the perpendicular coordinates of the tracks.
Further out from the beam pipe, OPAL's calorimeter system was divided into electromagnetic calorimeters (for the electrons identification), hadron calorimeters (for hadrons) and forward calorimeters placed around, and close to, the beam pipe at the two ends of the detector to catch particles thrown forwards by collisions in LEP. Muon detectors formed the end caps of the detector.
In its first phase of operation from 1989 to 1995, electrons and positrons collided in LEP at 91 GeV. The aim was to produce Z bosons. OPAL accumulated millions of these Z events for high-precision measurements. In LEP's second phase from 1996 to 2000, the collider's collision energy was increased to make pairs of W bosons, and to search for possible new particles and new physics.