OPAL

The Omni Purpose Apparatus for LEP Detector is designed to "provide precise measurements of charged particles and of electromagnetic energy over nearly the full solid angle" [1]. During LEP1 (1989-1995), OPAL collected millions of events to make precision measurements of the Z[2]. During LEP2 (1996-2000), the physics goal was to search for new physics through W⁺W^- pair production.

Detector Design

The detector provides acceptance for Z⁰ decays over 4π in solid angle. The main detector components are a system of central tracking chambers inside a solenoidal field of 0.435 T, a time-of-flight counter, a lead glass electromagnetic calorimeter with a presampler, a hadron calorimeter in the form of an instrumented magnet return yoke, and an outer muon chamber. A forward calorimeter serves as a luminosity moniter.

Central Tracking Detector

The Central Tracking Detector is divided into a precision vertex chamber, a large jet chamber, and a z chamber.

The vertex detector is a cylindrical drift chamber that surrounds has a length of 1 m, an inner radius of 88 mm, and an outer radius of 235 mm. There are two layers of 36 cells each: an inner layer of axial wires and an outer layer of stereo cells. Both types of cells have radial wire planes consisting of 200 μm gold plated Cu-Be potential wires and 20 μm gold plated W-Rh andoe wires. In the axial cells, there are 12 anode wires with radial spacing of 5.3 mm. The stereo cells have 6 anode wires with a spacing of 5 mm with a stereo angle of 4 °. The cathode planes use 125 μm Cu-Be wires with a 1 mm spacing in both cells. In the r-φ plane, the vertex chamber provides a resolution of 55 μm.

The jet chamber is absed on the design of the jet chamber in the JADE experiment at PETRA. The inner diameter is 0.5 m and the outer diameter is 3.7 m. The chamber is divided into 24 sectors divided by cathode wire planes. Each sector has a radial wire plane with 159 sensing wires parallel to the beam. For 43° < θ < 137°, 159 points will be measured along each track, and at least 8 points are achieved over 98% of the solid angle. The anode wires are spaced 10 mm apart and alternated with potential wires which are set at -2.38 kV. The jet chamber provides a resolution of 135 μm in the r-φ plane and 6 cm in the z direction. The dE/dx resoultion is 3.8% in a diumuon sample with at least 130 points per track.

The z chamber covers 44° < θ < 136° and 94% of the azimuthal angle. The purpose is to make a precision measurement of the z coordinate of charged particles that leave the jet chamber. There are 24 drift chambers, each 4 m long, 50 cm wide, and 59 mm thick. Each chamber is divided into 8 cells in z with six anode wires separated by 4 mm in the radial plane but perpendicular to the beam direction. The chamber provides a resolution of 300 μm in z and 1.5 cm in r-φ.

All three chambers use the same gas mixture of argon (88.2%), methane (9.8%) and isobutane (2.0%) at 4 bar. The gas system has a recirculation and purifation system to remove oxygen to a level of a few ppm. A laser system monitors drift velocity at an accuracy of &sigma/v < 0.1%.

Combined, three chambers provide a resolution of 75 μm in the r-φ plane and 2mm in the r-z plane. The invariant mass resolution for K⁰ goes to π⁺π^- is 8 MeV.

References

1. The OPAL Collaboration. "The OPAL Detector at LEP." CERN-PPE/90-114. August 14, 1990. 2. http://opal.web.cern.ch/Opal/ <math>Insert formula here</math>