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| | ===Forward Detectors=== | | ===Forward Detectors=== |
| − | The forward detectors serve as luminosity monitors by measuring small-angle Bhabha scattering. The acceptance is 39 mrad < θ < 120 mrad on either side and full azimuthal acceptance. The calorimeter has 35 layers of lead-scintillator (24 X<sub>0</sub>) with readouts with wavelength shifter to VPTs. The front 4 X<sub>0</sub> serves as a presampler with readouts on only the outer edge, but the rest of the calorimeter has readouts on both the inner and outer edges. The detector provides an energy resolution of 17%/sqrt(E) and a radius resolution of 2 mm. Three planes of proportional tube chambers in front of the calorimeter provide radial resolution of 0.5 mm. | + | The forward detectors serve as luminosity monitors by measuring small-angle Bhabha scattering. The acceptance is 39 mrad < θ < 120 mrad on either side and full azimuthal acceptance. The calorimeter has 35 layers of lead-scintillator (24 X<sub>0</sub>) with readouts with wavelength shifter to VPTs. The front 4 X<sub>0</sub> serves as a presampler with readouts on only the outer edge, but the rest of the calorimeter has readouts on both the inner and outer edges. |
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| − | There are drift chambers in front of the presampler with two layers each and two sense wires per layer. There is also a gamma catcher, which is a lead scintillator sampling calorimeter, to provide a veto on on the neutrino counting channel. The far forward luminosity monitor provides coverage for electrons scattered between 5 and 10 mrad.
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| − | The luminosity was measured in the forward detectors corresponding to a 24.23 nb cross section for Bhabha scattering.
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| − | ===Trigger===
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| − | LEP bunch crossings occure every 22.2 μs or at 45 kHz. The trigger system reduces the rate to 5 Hz for the data acquisition system.
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| − | The detector is divided into 144 overlapping bins, with 6 in θ and 24 in φ. Subdetectors produce trigger signals for each of these bins and also independent signals for each detector. Trigger signals are correlated in the trigger logic to create a θ-φ matrix which is compared to conditions for acceptance in each event.
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| − | The vertex chamber and jet chamber provide the track trigger, which consists of signals for the 144 bins and 6 signals for ≥ 1, 2, 3 barrel tracks and ≥ 1, 2, 3 total tracks. The TOF provides a single signal telling if ≥ 6 sectors (out of 24 in φ with no θ divisions) have fired. There is a threshold for the number of counters in each sector to fire which is between 2 and 5.
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| − | The electromagnetic calorimeter provides 200 signals from the barrel and 24 from each endcap to make the 144 signals and total sums for all three detectors. There are two thresholds for the totals at 6 and 4 GeV and two thresholds for the θ-φ bins at 2.5 and 1 GeV. The hadronic calorimeter provides 92 signals which are discriminated at three levels and then mapped to the 144 matrix signals.
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| − | The muon detector barrel provides a single signal if three out of four layers have fired in at least one φ bin (with no θ divisions). The endcap muon detector produces 4 x 24 θ-φ signals by summing over adjacent strips. There are also signals for total hits in each endcap and for coincidence hits.
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| − | OPAL achieved a typical trigger rate of roughly 1 Hz at the beginning of LEP1 running in 1989 with a readout deadtime of 50 ms per event. About 1.5 million events were recorded with a multi-hadronic Z<sup>0</sup> sample of about 30,000. Event sizes were on average 100 kbyte. At design luminosity, a trigger rate of 4 Hz and data rate of 400 kbyte/s are expected.
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| | ==Experimental Results== | | ==Experimental Results== |
