The OPAL
Hadron Calorimeter (HCAL)
Barrel and Endcaps are formed by
instrumenting the iron of the magnet return yoke with layers of limited
streamer tubes (LSTs) to form a sampling calorimeter.
The fundamental requirement from the HCAL is to detect hadronic energy
flow adequately over as large a solid angle as possible. Its primary role
is therefore that of a hermetic hadron tagger and veto, particularly for
longlived neutral hadrons. The HCAL system provides an excellent coverage
(97 % of the full solid angle) and plays a crucial part in the search for the
Higgs boson produced in the process e+e- --> H0 Z0, for heavy leptons decaying
into muon pairs plus neutrinos, for supersymmetric particles and for any other
process expected to result in a large non-interacting ``missing'' energy
component from neutrinos.
Besides the analog signals from the
Hadron Tower,
to determine the
deposited energy, the HCAL provides digital signals from strips that runs
the length of each single LSTs (about 58,000 channels). These informations,
complementing those from the external muon identifiers, are useful for the
identification of muons produced within hadron jets, and are essential for
the identification of low energy muons (p < 2 GeV) that are absorbed before
reaching the outer detectors.
The Bologna group has been heavily involved in the planning, construction,
installation and exploitation of this detector.
Since 1990 the group has the responsability for maintaining and upgrading
the Data Acquisition system of the Hadron Towers, the High Voltage system
and the co-responsability of running the detector.
References: 1. G.Artusi et al., Nucl. Instr. and Meth. A279 (1989) 523 2. K.Ahmet et al., Nucl. Instr. and Meth. A305 (1991) 275 3. F.Fabbri et al., CERN/EP 87-134 4. A.H.Ball et al., IEEE Trans. on Nucl. Sci. vol.37.No.2, 1990
The luminosity monitors measure the luminosity of LEP by detecting small-angle positron-electron elastic scattering (Bhabha scattering), and also tag electrons from photon-photon interactions. The OPAL luminosity system consists of a Forward Detector (FD) and, since 1993, of a Silicon-Tungsten calorimeter (Si-W).
The FD system consists of two back-to-back forward calorimeters, tube chambers,
gamma catchers and a far forward monitor.
The Si-W electromagnetic calorimeters were installed for the 1993 LEP running
with the aim to measure the luminosity in OPAL with an experimental uncertainty
of less than 0.1 %, in order to provide an even more precise determination of
the Z0 lineshape. They are sampling calorimeters with silicon detectors as
active elements and 5 mm thick tungsten absorbers.
The necessary precision in the electron (positron) position measurement
was obtained, when building the calorimeters, by requiring a very precise
mechanical alignment among the various silicon detectors (known with an
uncertainty of at most 10 microns) and a good position resolution.
So far the OPAL Si-W luminometer has performed the most precise absolute
luminosity measurement obtained at LEP, quoting an uncertainty of 0.07%.
Since 1989, the Bologna group provides the luminosity measurement, the on-line
calibration of the forward calorimeters and contributes to the maintenance of
the detectors during data taking.
The group was responsible, in 1992, for the design and construction of the Si-W
low voltage system.
References: 1. D.C.Imrie et al., Nucl. Instr and Meth. A238 (1989) 515 2. K.Ahmet et al., Nucl. Instr. and Meth. A305 (1991) 275 3. A.Lee et al., OPAL Technical Note TN070, 1991 4. B.E.Anderson et al., IEEE Trans. Nucl. Sci. 41 (1994) 845 5. Si-W working group, OPAL Technical Note TN221, 1994 6. Si-W working group, OPAL Physics Note PN142, 1994
The OPAL experiment achieves excellent charged particle tracking and momentum measurements using the combined properties of a silicon strip microvertex (Si-mVTX) detector, a vertex drift chamber, a large acceptance high resolution jet-type drift chamber, and a z-readout drift chamber, all contained in a solenoidal magnetic field of 0.435 T.
The first OPAL Si-mVTX detector was installed in June 1991 and consisted of
two concentric layers of single-sided silicon detector wafers with AC coupled
readout strips, at 50 micron pitch, oriented for azimuthal (Phi) coordinate
measurement. The desire for a high spatial resolution strip detector was
primarily motivated by the need to measure or identify particles with typical
decay lengths below 1 cm (such as b flavoured hadrons and tau leptons) and to
search for new particles having similar decay lengths. The excellent single hit
resolution achieved (8 microns including alignment uncertainties) resulted in
a large improvement in tracking for OPAL wich was demonstrated in the precision
measurements obtained (tau and b-hadron lifetimes) as well as improvements in
b quark identification. A new Si-mVTX detector with a three dimensional vertex
reconstruction has been operating successfully in OPAL since the start of the
1993 LEP run. This new detector obtains azimuthal (Phi) and z-coordinate (along
the beam direction) readout using back-to-back single-sided AC coupled silicon
strips. The intrinsic single hit resolution (5 microns in Phi and about 13
microns in z) further improved both the precision and overall quality of
reconstructed tracks enhancing the OPAL potential in many physics analyses.
The detector was upgraded for the LEP2 phase.
The Bologna group has been involved in the commissioning of the power and bias
voltages system that was used from 1991 until 1995.
It also provided technical expertise and practical work in the manufacturing
and soldering of special power and signal cables for the new radiation and "fast
beam dump" monitoring system, installed since the 1996 LEP run.
References: 1. P.P.Allport et al., Nucl. Instr. and Meth. A324 (1993) 34 2. P.P.Allport et al., Nucl. Instr. and Meth. A346 (1994) 476
In 1994 the OPAL collaboration proposed to modify the endcap region of its detector by installing a scintillating tile trigger plane in front of the endcap electromagnetic calorimeter, to be ready for data taking during the 1996 LEP run at higher energies (LEP2 phase). Machine related backgrounds may be considerably worse at LEP2 and OPAL will require a trigger which is as robust, flexible and unrestrictive as possible. So far, in fact, the endcap trigger rely heavily on the tracking drift chambers, which may be vulnerable to increased background. Furthermore, the scintillating tiles will act together with the existing pre-sampler to assist in electron and photon measurements.
The technology of scintillator tiles readout by embedded wavelength-shifting fibres (WLS) has been developed for the SDC pre-shower and calorimeter, for the CDF endplug upgrade, for the ZEUS FCAL/RCAL presampler and by several other groups. The OPAL project consists of an array of scintillating tiles in each endcap between the presampler and the electromagnetic calorimeter. The light is collected from each tile separately via WLS. The project was accepted by the LEPC and the detector has been built at CERN, mostly in 1995 and beginning 1996.
The Bologna group was involved, since 1995, in the development of the simulation
software to estimate the delay and the attenuation in the signal propagation
(photons) through the scintillating material, toward the photomultipliers.
References: 1. OPAL Collab., CERN-LEPC/94-09