The HERA-B detector [HB94/95] (fig. 1.1) is a magnetic spectrometer; its main tasks are the identification of the particles created in the collision, a measurement of their momentum (for this a precise particle track measurement is essential), their energy and the determination of their point of nascence.
TheB0/B0mesons are produced by collision of a proton beam at a fixed target consisting of aluminium resp. copper ribbons which are located such that protons of the beam-halo (i. e. from the outer, less populated beam region) interact at 40 MHz rate. After generation in the ribbons,B-mesons move approx. 9 mm, before they decay. The decay vertex resolution required to study CP-violation is typically 10% of this value. The silicon vertex detector (SVD)[Kn¨o95] which is located directly behind the target, has to find these displaced (w. r. t. the ribbons)B-decay vertices. The SVD will be discussed in further detail in section 1.2.
A central component of the HERA-B detector is thedipole magnetwhich creates a mag-netic field behind the SVD approx. 4.5 m apart from the target. Due to the (momentum-dependent) Lorentz-force tracks of charged particles are bent which enables measurement of the particles’ momentum. A particular challenge is the shielding of the electron beam which also traverses the magnet field. The electrical field may not exceed a few hundred Gauss inside the electron beam pipe. The problem is overcome by use of highly perme-able steel as passive shielding and an active shielding of magnetic coils counteracting the dipole field.
The inner tracker detector [Schm97] measures particle tracks near the proton beam pipe; it covers an angle from 10 mrad up to 20-25 cm distance from the beam pipe and is located along the beam axis from behind the SVD to approx. 13 m behind the target.
It is made of microstrip gas chambers (MSGCs) (chapt. 3) which are positioned in ten super layers each consisting of two to eight layers of chambers with angles of 0◦ and±5◦ w. r. t. the vertical. The four chambers of a layer (one in each quadrant) overlap for better efficiency and for better relative alignment. The inner tracker detector has a total of approx. 135 000 channels to be read out, and - since it has to deliver fast information on hit channels for the level 1 trigger - approx. 18400 trigger output signals.
The outer tracker detector[Kap96] covers the larger angles starting from the outer edge of the inner tracker up to 200 mrad from the beam axis. Proportional drift chambers (see chapt. 3) with circular or hexagonal cross section and a mixture based on CF4 as fill gas will be taken for particle detection. The minimum diameter has been chosen to be 5 mm due to the danger of high voltage flash-over and due to the resolution distortion
near the anode wire. The wire positioning should be precise to approx. 100µm in order not to deteriorate the intrinsic resolution of the chambers. The total number of channels will be 96000.
Thering image ˇCerenkov detector (RICH)[Kriz98] uses the ˇCerenkov-effect: elementary particles emit visible or UV-photons when traveling a medium with a velocity higher than that of light. The radiatior gas chosen is C4F10. The aperture of the light cone depends on the particle velocity and can be used together with a measurement of the particle’s momentum or energy for determination of the particle’s mass and hence for identification;
at HERA-B it is mainly used for the identification of kaons. By proper design of a spherical mirror the cones are projected as rings onto a photon detection plane. The single photon detection is achieved by an array of multi-anode photomultipliers (quantum efficiency max. 20 %); the Hamamatsu H6568 photomultiplier under investigation has a bialkali photocathode with the anode divided into 16 pads of 4 mm×4 mm each. The 12-stage, metal-foil dynode system allows for good single photoelectron resolution. The total number of channels will be in the order of 40000.
The electromagnetic calorimeter (ECAL) [Gol95] which is located 13.25 m from the target serves for the energy measurement of photons in the range of 5 to 200 GeV and for the separation of hadronic particles from leptons; the ECAL contributes to the level 1 trigger. A shashlik-calorimeter with tungsten resp. lead absorber plates alternating with scintillator plates has been chosen; the position resolution is between 1.1 mm close to the beam axis and 10 mm at larger angles. Photons are absorbed inside the alternating structure giving rise to showers of electrons, positrons and secondary photons. The energy delivered to the scintillating layers is transfered to visible light and amplified using photmultipliers or photo-diodes. A total of 6500 channels has to be read out from the ECAL.
Atransition radiation detector (TRD)[Sav96] enables further meson/hadron-lepton sep-aration (in particular pions shall be separated from electrons). When traversing an in-terface of materials of different refractive indices, elementary particles emit transition radiation in the soft x-ray region; the energy distribution depends on the particle type.
In the TRD the transitions are achieved by alternating radiator/chamber-layers (36 lay-ers in total). The photons are created when the particle exits the radiator and are detected by the proportional chambers. For the radiators polyethylene-foam resp. fibres are under investigation; the proportional chambers are filled with a gas mixture based on krypton. A total of 77000 channels has to be read out.
Themuon-chambers[Zai98] are located at the end of the HERA-B-detector and serve for the identification of muons to be used in the level 1 trigger decision. Prior to the muon-chambers an absorber made of iron and concrete filters all particles beside muons created in the target-collision. Similar to the tracker detectors the muon-chambers consist of four super-layers, the first two with 0◦and±15◦-orientation to the vertical, the last with only 0◦ orientation. The muon-chamber-layers are composed of gas-pixel-detectors (10 mm
×10 mm) in the central region and multiwire proportional chambers resp. proportional chambers with cathode-pad readout in the outer region. The fill gas to be used is a mixture based on argon. A total of 29500 channels must be read out.
ring image Cherenkov detector 250 mrad
220 mrad
160 mrad
magnet
silicon vertex detector
TRD calorimeter myon detector
target wires
0 m 5
10 15
20
photon detector
plane mirrors
proton beam
electron beam
proton beam electron beam
spherical mirrors
vertex tank outer tracker
inner tracker
C4F10
Figure1.1:HERA-Bdetector[HB94/95]