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| | Overall, the calorimeter provides a resolution of 0.2% + 6.3%/sqrt(E) before material is added in front. When 2.08 X<sub>0</sub> 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<sup>-3</sup>. | | Overall, the calorimeter provides a resolution of 0.2% + 6.3%/sqrt(E) before material is added in front. When 2.08 X<sub>0</sub> 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<sup>-3</sup>. |
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| − | ===Hadron Calorimeter===
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| − | The hadron calorimeter covers 97% of the solid angle to measure the energy of hadrons that pass through the electromagnetic calorimeter using the iron of the magnetic return yoke as an absorber. This material provides 4 interaction lengths or more and is segmented into 8 layers of 100 mm thickness, which are alternated with 9 layers of chambers in the barrel. In the endcap, there are 8 layers of chambers and 7 layers of iron.
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| − | The active layers are similar to those used in the presampler. Signals are read through large pads on one side and through aluminum strips that run parallel to the anode wires on the other side. The 57,000 strips help provide single particle tracking for muon identification and shower profiles. The pads are grouped into towers for energy measurements. The chambers use a mixture of isobutane (75%) and argon (25%), which is changed daily. The anode voltages are kep between 4.65 kV and 4.85 kV.
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| − | Calibration was done on prototype modules in a pion test beam. The energy resolution measured was 120%/sqrt(E) for incident energy of 10 GeV.
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| − | The pole tip hadron calorimeter covers 0.91 < |cosθ| < 0.99 with 9 iron layers of 80 mm thickness. The gaps between the samples had to be made smaller to prevent perturbations to the magnetic field, so the sampling rate was increased to compensate. The active chambers are similar to the endcap presampler with anode wires at 3.5 kV and a gas mixture of CO<sub>2</sub> (55%) and n-pentane (45%). The chambers are also read out with a combination of pads and strips. One module was placed in a hadron test beam at CERN with 6-50 GeV incident energy. Below 15 GeV resolution was 100%/sqrt(E) and higher for higher energyies because of leakage.
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| − | ===Muon Detector===
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| − | The muon detector covers 93% of solid angle with at least one layer. Over this area, there is a 0.001 probability for a pion to not interact before reaching the muon detector. Muon identification is done by matching a track from the central tracker to a track in the muon detector. The single muon efficiency over the detector is nearly 100% for muons above 3 GeV, while 5 GeV pions will be misidentified 1% of the time.
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| − | The barrel muon detector is made of 110 large drift chambers, which are 1.2 m wide, 90 mm deep, and come in lengths of 10.4, 8,4 and 6.0 m. The chambers each have 2 cells with a 50 μm anode wire running the entire length. The gas is ethane (10%) and argon (90%) with a typical flow rate of 200 ml/min and a drift velocity of 38 mm/μs. The maximum drift time is 8 μs. The φ coordinate is determined to an accuracy of 1.5 mm and the z coordinate to a resolution of 2 mm.
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| − | In the endcap, which covers 0.67 < |cosθ| < 0.985, there are four layers of tubes perpendicular to the beam in 8 quadrants and 4 ptach chambers. The 100 μm anode wires run along the chambers in a gas of argon (25%) and isobutane (75%) at a voltage of 4.3 kV.
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| − | ===Forward Detectors===
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| − | 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.
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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== |
| − | OPAL produced its first result in 1989 using 190 nb<sup>-1</sup> collected during the first 15 days of running at LEP [3]. Using 4350 multi-hadronic events from seven energy points between 89.26 GeV and 93.26 GeV, the collaboration reported a mass m<sub>Z</sub> = 91.01 ± 0.05 ± 0.05 GeV with a width Γ<sub>Z</sub> = 2.60 ± 0.13 GeV and found that there should be 3.1 ± 0.4 neutrino generations.
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| − | Hadronic events for this sample were triggered by using the electromagnetic calorimeter, the TOF, and the jet chamber. An energy sum in the calorimeter of at least 6 GeV in either the barrel or one of the endcaps is required with TOF hits in at least three nonadjacent countes. There must be at least two tracks from the interaction vertex with at least 450 MeV/c transverse momentum. The forward calorimeter could trigger if there was at least 12 GeV in both sides back-to-back or at least 15 GeV in both sides without being back-to-back. The trigger efficiency was found to be > 99.5% using a Monte Carlo simulation.
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| − | Offline cuts were made using the calorimeter and the TOF. A multihadron candidate requires at least 10 different calorimeter clusters with more than 100 MeV in each cluster, the total energy in the calorimeter is greater than 10% of the collision energy, and the ratio | R<sub>bal</sub> | = Σ(E<sub>clus</sub>•cosθ) / ΣE<sub>clus</sub> ≤ 0.65. Monte Carlo studies found that the acceptance for these cuts was 98.2%.
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| − | Monte Carlo calculations were made to estimate the background contamination of the sample for two photon processes, electron-pair and τ-pair decays of the Z<sup>0</sup>. The calculation found these backgrounds to be < 0.1%, <0.1%, and 0.3%, respectively. Using a sample of 819 events from the mass peak, the τ decay, beam-gas, beam-wall, and cosmic ray backgrounds were estimated. Another sample with reconstructed vertex >50 from the interaction vertex was used for just beam-wall and beam-gas backgrounds. The overall background contamination of the sample was 0.3% from the τ-pair decays.
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| − | The dominating systematic error of the measurement is the luminosity determination. Position calibration uncertainties and resolution effects are the main sources of this error, which results in a systematic error of 5% in the cross section measurement.
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| − | The mass of the Z<sup>0</sup> is determined by fitting a Breit-Wigner lineshape to the cross section vs. collision energy spectrum with 3 free parameters: m<sub>Z</sub>, Γ<sub>Z</sub>, and a normalization factor. The three parameters were varied independently.
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| − | To constrain the number of light neutrino flavors, the fit is recalculated by replacing Γ<sub>Z</sub> with Γ<sub>Z</sub><sup>SM</sup> + (N<sub>ν</sub> - 3)•Γ<sub>ν</sub><sup>SM</sup> and varying m<sub>Z</sub> and N<sub>ν</sub>, but constraining the normalization constant. From the fit, the probability of N<sub>ν</sub> = 3 is 47% and the probability of N<sub>ν</sub> = 4 is 8% or that N<sub>ν</sub> < 3.88 at a confidence level of 90%.
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| | ==References== | | ==References== |
| | 1. The OPAL Collaboration. "The OPAL Detector at LEP." CERN-PPE/90-114. August 14, 1990. | | 1. The OPAL Collaboration. "The OPAL Detector at LEP." CERN-PPE/90-114. August 14, 1990. |
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| | 2. http://opal.web.cern.ch/Opal/ | | 2. http://opal.web.cern.ch/Opal/ |
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| − | 3. The OPAL Collaboration. "Measurement of the Z<sup>0</sup> Mass and Width with the OPAL Detector at LEP." CERN-EP/89-133. October 13, 1989.
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