Difference between revisions of "OPAL"

Revision as of 11:46, 19 March 2008

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.

Time-of-Flight System

The time-of-flight (TOF) system covers the region |cosθ| < 0.82 with the purpose of providing triggers, aiding in particle identification for charged particles with momentum in the range 0.6-2.5 GeV, and rejecting cosmic rays.

The TOF is a barrel of radius 2.360 m made of 160 scintillation counters. Each counter is 6.840 m long with a trapezoidal cross section. The maximum gap between counters is 2.6 mm. Light from each counter is collected at both ends by 300 mm light guides that go directly to PMTs, which have a gain of 3 x 10⁷ at 1850 V.

Trigger signals are generated within 50 ns from the arrival time. The mean speed of scintillation was measured to be 0.582 c from test beam data at CERN-PS. The mean attenuation length was 2.33 m. The timing resolution from the center was 280 ps and 350 ps from the ends, with a z resolution of 5.5 cm. If additional z information is provided, the time resolution at the ends improves to 220 ps.

Electromagnetic Calorimeter

The electromagnetic calorimeter was designed to measure the energy of positrons, electrons and photons from a few MeV to 100 GeV. The barrel and the endcap together cover 98% of the solid angle, and the forward calorimeter extends the hermeticity even farther. The barrel and endcap are made of lead glass because of its resolution of 5%/sqrt(E).

Because there are approximately 2 X₀ in front of the calorimeters, both the barrel and endcap have presamplers to measure the position and energy of electromagnetic showers. The barrel presampler consists of 16 chambers covering the cylinder of radius 2388 mm and 6623 mm long. Each chamber has two layers with four sections of 24 cells each. Anode wires of stainless steel run down the center of each cell and are 75 μm in diameter. The gas in each cell is a mixture of n-pentane (32%) and CO₂ (68%). Charge is collected from cathode strips that cross both top and bottom of each layer offset from the wire direction by 45°, with top and bottom strips orthogonal. The resolution for a single particle is 1-2 mm depending on angle of incidence and 4-6 mm for showers depending on energy. Resolution in z is 10 cm for a single particle.

The barrel calorimeter is made of 9,440 lead glass blocks of 24.6 X₀ at a radius of 2455 mm. It covers |cosθ| < 0.82 and 2π in azimuthal angle. The blocks are pointed at the interaction point with a tilt slightly away from perfect to prevent neutral particles from escaping along gaps. There are 59 segments in the z direction and 160 in the azimuthal direction. There are 16 different shapes. PMTs are attached to the back of each block and can be operated in an external field of up to 100 G with <1% effect on the gain. For calibration, every counter was exposed to a 50 GeV electron beam twice at CERN-SPS. The gains were measured to 0.1% and nonlinearity was measured to be less than 1%. A Xenon lamp is used to provide ongoing calibration to every counter.

The endcap presampler is 16 wedge shaped sectors with a total of 32 chambers covering 0.83 < |cosθ| < 0.95. Thin multiwire chambers are used in the design. Each sector has a large and a small trapezoid shaped chamber and neighboring sectors overlap. Readout is done simultaneously for groups of 4 wires and strips. The resolution is equal to the intrinsic chamber resolution.

The endcap calorimeter consists of two dome shpaed arrays of 1,132 lead glass blocks each, covering 0.83 < |cosθ| < 0.98. The blocks are mounted parallel to the beam axis and instrumented with special vacuum phototriodes (VPTs) which work in the full field of the magnet. The blocks have a depth of at least 20.5 X₀. The VPTs have a gain of 12.3 and quantum efficiency of 26%. Calibration is done with a laser system to provide equivalent light for 10 and 20 GeV electrons.

Overall, the calorimeter provides a resolution of 0.2% + 6.3%/sqrt(E) before material is added in front. When 2.08 X₀ of aluminum is added in front, the resolution degrades by about 50%. The presampler and lead glass can achieve electron identification efficiency of 80-90% with a pion rejection on the order of 10^-3.

Experimental Results

References

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